Biomass

Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households.^[1] Some people use the terms biomass and biofuel interchangeably, while others consider biofuel to be a liquid or gaseous fuel used for transportation, as defined by government authorities in the US and EU.^[a]^[b] The European Union's Joint Research Centre defines solid biofuel as raw or processed organic matter of biological origin used for energy, such as firewood, wood chips, and wood pellets.^[c]

Wood pellets

In 2020 biomass produced 58 EJ (exajoules) of energy, compared to 172 EJ from crude oil, 157 EJ from coal, 138 EJ from natural gas, 29 EJ from nuclear, 16 EJ from hydro and 15 EJ from wind, solar and geothermal combined.^[2]^[d] Approximately 86% of modern bioenergy is used for heating applications, with 9% used for transport and 5% for electricity.^[e] Most of the global bioenergy is produced from forest resources.^[f]

The IEA's Net Zero by 2050 scenario^[g] calls for traditional bioenergy to be phased out by 2030, with modern bioenergy's share increasing from 6.6% in 2020 to 13.1% in 2030 and 18.7% in 2050.^[3] The IPCC (Intergovernmental Panel on Climate Change) believes that bioenergy has a significant climate mitigation potential if implemented correctly,^[h]^[i] with most of the IPCC's pathways including substantial contributions from bioenergy in 2050 (average at 200 EJ).^[j]^[k]^[4] Some researchers criticize bioenergy projects with low emission savings, high initial carbon intensities, and/or long waiting times before positive climate impacts occur.^[5]

The most promising raw material feedstocks for the future are lignocellulosic (non-edible) biomass, such as coppices or perennial energy crops, agricultural residues, and biological waste, which also have the shortest delay before producing climate benefits. Heat production from biomass combustion is typically more "climate friendly" than electricity production due to the more efficient conversion from chemical to heat energy, and it is also harder to replace with heat from alternative renewable sources that may be more costly or limited by the maximum temperature of the steam they can produce.^[l] Power plants using biomass as fuel can provide a stable power output, unlike the intermittent power from solar or wind farms.^[m]^[n] Solid biofuel is generally more climate friendly than liquid biofuel because its production is more energy efficient.

Biomass conversion

Raw biomass can be upgraded into better and more practical fuel simply by compacting it (e.g. wood pellets), or by different conversions broadly classified as thermal, chemical, and biological:^[28]

Thermal conversion

Thermal upgrading produces solid, liquid or gaseous fuels, with heat as the dominant conversion driver. The basic alternatives are torrefaction, pyrolysis, and gasification, these are separated principally by how far the chemical reactions involved are allowed to proceed. The advancement of the chemical reactions is mainly controlled by how much oxygen is available, and the conversion temperature.

Biomass plant in Scotland.

Torrefaction is a mild form of pyrolysis where organic materials are heated to 400–600 °F (200–300 °C) in a no–to–low oxygen environment.^[29]^[30] The heating process removes (via gasification) the parts of the biomass that has the lowest energy content, while the parts with the highest energy content remain. That is, approximately 30% of the biomass is converted to gas during the torrefaction process, while 70% remains, usually in the form of compacted pellets or briquettes. This solid product is water resistant, easy to grind, non-corrosive, and it contains approximately 85% of the original biomass energy.^[31] Basically the mass part has shrunk more than the energy part, and the consequence is that the calorific value of torrefied biomass increases significantly, to the extent that it can compete with coals used for electricity generation (steam/thermal coals). The energy density of the most common steam coals today is 22–26 GJ/t.^[32] There are other less common, more experimental or proprietary thermal processes that may offer benefits, such as hydrothermal upgrading (sometimes called "wet" torrefaction.)^[ap] The hydrothermal upgrade path can be used for both low and high moisture content biomass, e.g. aqueous slurries.^[33]

Pyrolysis entails heating organic materials to 800–900 °F (400–500 °C) in the near complete absence of oxygen. Biomass pyrolysis produces fuels such as bio-oil, charcoal, methane, and hydrogen. Hydrotreating is used to process bio-oil (produced by fast pyrolysis) with hydrogen under elevated temperatures and pressures in the presence of a catalyst to produce renewable diesel, renewable gasoline, and renewable jet fuel.^[34]

Gasification entails heating organic materials to 1,400–1700 °F (800–900 °C) with injections of controlled amounts of oxygen and/or steam into the vessel to produce a carbon monoxide and hydrogen rich gas called synthesis gas or syngas. Syngas can be used as a fuel for diesel engines, for heating, and for generating electricity in gas turbines. It can also be treated to separate the hydrogen from the gas, and the hydrogen can be burned or used in fuel cells. The syngas can be further processed to produce liquid fuels using the Fischer-Tropsch synthesis process.^[28]^[35]

Chemical conversion

A range of chemical processes may be used to convert biomass into other forms, such as to produce a fuel that is more practical to store, transport and use, or to exploit some property of the process itself. Many of these processes are based in large part on similar coal-based processes, such as the Fischer-Tropsch synthesis.^[36] A chemical conversion process known as transesterification is used for converting vegetable oils, animal fats, and greases into fatty acid methyl esters (FAME), which are used to produce biodiesel.^[28]

Biological conversion

As biomass is a natural material, many biological processes have developed in nature to break down the biomass molecules, and many of these conversion processes can be harnessed. In most cases, microorganisms are used to perform the conversion process: anaerobic digestion, fermentation, and composting. Fermentation converts biomass into bioethanol, and anaerobic digestion converts biomass into renewable natural gas. Bioethanol is used as a vehicle fuel. Renewable natural gas—also called biogas or biomethane—is produced in anaerobic digesters at sewage treatment plants and at dairy and livestock operations. It also forms in and may be captured from solid waste landfills. Properly treated renewable natural gas has the same uses as fossil fuel natural gas.^[28]

IRENA argues that the success of large-scale international bioenergy trade require biomass conversion in order to transport high density commodities at low costs.^[aq]

Climate impact

Currently there is a lively debate going on about the real carbon intensity of a number of bioenergy pathways, especially from forestry. The critics are especially concerned about short-term or medium-term climate effects. Critics have emerged both among researchers^[ar] and environmental activists.^[as]^[at] At the same time, bioenergy supporters in influential research organizations like the IPCC, IEA, and EU's Joint Research Centre argue that bioenergy is climate friendly when implemented correctly and at an appropriate scale. For instance in 2022, IEA stated that "bioenergy is an important pillar of decarbonisation in the energy transition as a near zero-emission fuel", and that "more efforts are needed to accelerate modern bioenergy deployment to get on track with the Net Zero Scenario [....] while simultaneously ensuring that bioenergy production does not incur negative social and environmental consequences."^[37]

Carbon accounting principles

Different carbon accounting methodologies have a significant impact on the calculated results and therefore on the scientific arguments. Generally, the purpose of carbon accounting is to determine the carbon intensity of an energy scenario, i.e. whether it is carbon positive, carbon neutral or carbon negative. Carbon positive scenarios are likely to be net emitters of CO₂, carbon negative projects are net absorbers of CO₂, while carbon neutral projects balance emissions and absorption perfectly.^[au]

As a consequence of both natural causes and human practices, carbon continually flows between carbon pools, for instance the atmospheric carbon pool, the forest carbon pool, the harvested wood products carbon pool, and the fossil fuels carbon pool. When the carbon level in pools other than the atmospheric carbon pool increase, the carbon level in the atmosphere decrease, which helps mitigate global warming.^[av] If a researcher measures the amount of carbon moving from one pool to another, they can gain insight and recommend practices that maximize the amount of carbon stored in carbon pools other than the atmospheric carbon pool. Three concepts are especially important, namely carbon debt, carbon payback time and carbon parity time:

Carbon debt accrues when biomass is removed from growing sites, for instance forests. It is counted when the trees are felled because the UNFCCC (the UN organization that countries report their emissions to) has decided that emissions should be counted already at this point in time, instead of at the combustion event.^[aw]

Carbon payback time is the time it takes before this carbon is "paid back" to the forest, by having the forest re-absorb an equivalent amount of carbon from the atmosphere.

Carbon parity time is the time it takes for one energy scenario to reach carbon parity with another scenario (i.e. store the same amount of carbon as another scenario.)^[ax] One of these scenarios can for instance be a bioenergy scenario, with carbon counted as stored in the part of the forest that was not harvested, and carbon counted as lost for the amount of forest that were harvested (cf. the UNFCCC rule mentioned above.) However, the amount of carbon that resides in woody construction materials and biofuels made from this harvest can be "counted back" into the bioenergy scenario's carbon pools for the amount of time it takes before this carbon decays naturally or are burnt for energy. The alternative scenario can for instance be a forest protection scenario, with carbon counted as stored in the whole forest – a forest that is bigger than in the bioenergy scenario because no trees were harvested at all, and in addition also continued to grow (while waiting for the carbon stored in the bioenergy scenario to catch up to its own carbon level.)^[ay] However, the implied "lock-in" of carbon in the forest also means that this carbon no longer is available for production of woody construction materials and biofuels, which means that these have to be replaced by other sources. In most cases, the most realistic sources are fossil sources, which means that the forest protection scenario here will be "punished" by having the fossil fuel emissions it is responsible for subtracted from its carbon pool. (Note that this fossil carbon is often instead technically speaking counted as added to the bioenergy carbon pool (instead of subtracted from the no-bioenergy carbon pool), and called "displaced" or "avoided" fossil carbon.)

A net carbon debt for the bioenergy scenario is calculated when the net amount of carbon stored in the forest protection scenario's carbon pool is larger than the net amount of carbon stored in the bioenergy scenario's carbon pools. A net carbon credit for the bioenergy scenario is calculated when the net amount of carbon stored in the forest protection scenario's carbon pool is smaller than the net amount of carbon stored in the bioenergy scenario's carbon pools.^[38] The carbon parity time then is the time it takes for the bioenergy scenario to go from debt to credit.^[az]

To recap, a project or scenario can be assessed solely on its own merits, specifically the time it takes to pay back removed carbon (carbon payback time.) However, it is common to include alternative scenarios (also called "reference scenarios" or "counterfactuals") for comparison.^[ba] When there is more than one scenario, carbon parity times between these scenarios can be calculated. The alternative scenarios range from scenarios with only modest changes compared to the existing project, all the way to radically different ones (i.e. forest protection or "no-bioenergy" counterfactuals.) Generally, the difference between scenarios is seen as the actual carbon mitigation potential of the scenarios.^[bb] In other words, quoted emission savings are relative savings; savings relative to some alternative scenario the researcher suggest. This gives the researcher a large amount of influence over the calculated results.

Carbon accounting system boundaries

System boundaries for carbon accounting: Option 1 (black) limits the carbon calculation to stack emissions, option 2 (green) limits the calculation to the forest carbon stock, option 3 (blue) limits the calculation to forest and stack emissions combined (the supply chain) and option 4 (red ) includes both stack emissions, forest and the bioeconomy (carbon storage in wood products and displaced fossil fuels.)^[bc]

In addition to the choice of alternative scenario, other choices has to be made as well. The so-called "system boundaries" determine which carbon emissions/absorptions that will be included in the actual calculation, and which that will be excluded. System boundaries include temporal, spatial, efficiency-related and economic boundaries:^[bc]

Temporal system boundaries

The temporal boundaries define when to start and end carbon counting. Sometimes "early" events are included in the calculation, for instance carbon absorption going on in the forest before the initial harvest. Sometimes "late" events are included as well, for instance emissions caused by end-of-life activities for the infrastructure involved, e.g. demolition of factories. Since the emission and absorption of carbon related to a project or scenario changes with time, the net carbon emission can either be presented as time-dependent (for instance a curve which moves along a time axis), or as a static value; this shows average emissions calculated over a defined time period.

The time-dependent net emission curve will typically show high emissions at the beginning (if the counting starts when the biomass is harvested.) Alternatively, the starting point can be moved back to the planting event; in this case the curve can potentially move below zero (into carbon negative territory) if there is no carbon debt from land use change to pay back, and in addition more and more carbon is absorbed by the planted trees. The emission curve then spikes upward at harvest. The harvested carbon is then being distributed into other carbon pools, and the curve moves in tandem with the amount of carbon that is moved into these new pools (Y axis), and the time it takes for the carbon to move out of the pools and return to the forest via the atmosphere (X axis). As described above, the carbon payback time is the time it takes for the harvested carbon to be returned to the forest, and the carbon parity time is the time it takes for the carbon stored in two competing scenarios to reach the same level.^[bd]

The static carbon emission value is produced by calculating the average annual net emission for a specific time period. The specific time period can be the expected lifetime of the infrastructure involved (typical for life cycle assessments; LCA's), policy relevant time horizons inspired by the Paris agreement (for instance remaining time until 2030, 2050 or 2100),^[39] time spans based on different global warming potentials (GWP; typically 20 or 100 years),^[be] or other time spans. In the EU, a time span of 20 years is used when quantifying the net carbon effects of a land use change.^[bf] Generally in legislation, the static number approach is preferred over the dynamic, time-dependent curve approach. The number is expressed as a so-called "emission factor" (net emission per produced energy unit, for instance kg CO₂e per GJ), or even simpler as an average greenhouse gas savings percentage for specific bioenergy pathways.^[bg] The EU's published greenhouse gas savings percentages for specific bioenergy pathways used in the Renewable Energy Directive (RED) and other legal documents are based on life cycle assessments (LCA's).^[bh]^[bi]

Spatial system boundaries

The spatial boundaries define "geographical" borders for carbon emission/absorption calculations. The two most common spatial boundaries for CO₂ absorption and emission in forests are 1.) along the edges of a particular forest stand and 2.) along the edges of a whole forest landscape, which include many forest stands of increasing age (the forest stands are harvested and replanted, one after the other, over as many years as there are stands.) A third option is the so-called increasing stand level carbon accounting method:

– In stand level carbon accounting, the researcher may count a large emission event when the stand is harvested, followed by smaller, annual absorption amounts during the accumulation phase that continues until the stand has reached a mature age and is harvested again.

– Likewise, in increasing stand level accounting, the researcher counts a large emission event when the stand is harvested, followed by absorption of smaller quantities of carbon each year during the accumulation period. However, one year after the first harvest, a new stand is harvested. The researcher do not count the carbon that was absorbed in this second stand after the first, neighbouring stand was harvested, only the large emission at the harvest event of the second stand. The next year the same procedure repeats for the third stand; the carbon that was absorbed by this stand after the harvest of the first and second stand is not counted, while a large emission amount is counted when the third stand is harvested. In other words, in increasing stand level accounting the whole carbon account is composed of a number of individual stand-level accounts, each with its own, individual starting point.

– In landscape level accounting, the researcher counts a large emission event when the first stand is harvested, followed by absorption of smaller quantities of carbon each year during the accumulation period for this particular stand. Like with increasing stand level accounting, a new stand is harvested the second and third year etc., and these emission events are all counted. Unlike with increasing stand level accounting however, the researcher also counts the carbon that is absorbed by all stands after the harvest of the first stand in the forest landscape. In other words, instead of calculating carbon emissions from a lot of different starting points, forest landscape accounting uses only one, common starting point for the whole forest landscape, namely the year the first stand was harvested.^[40]

So, the researcher has to decide whether to focus on the individual stand, an increasing number of stands, or the whole forest landscape.

According to Lamers et al., the stand level spatial boundary choice is typical for early carbon modeling, and it leads to carbon cycles that resemble sawteeth (dramatic increases in emissions at harvest, followed by slow declines as the forest stand absorb carbon.) The key benefit of stand-level analysis is its simplicity, and this is the primary reason for it still being part of today's carbon analyses. However, while the study of single stands provide easily comprehensible results (for example on the carbon effects of different harvesting choices), real-world timber/woody biomass supply areas consist of several stands of different maturity, for instance 80. Over a time period of 80 years then, all stands are successively harvested and replanted. To accurately calculate the carbon flow over such large areas the spatial boundary of the calculation has to increase from stand level to landscape level, as the forest "landscape" contains all the individual forest stands.^[41] Cowie et al. argue that landscape level accounting is more representative of the way the forestry sector manages to produce a continuous supply of wood products.^[bj] The IPCC recommends landscape-level carbon accounting as well (see Short-term urgency below).

Further, the researcher has to decide whether emissions from direct/indirect land use change should be included in the calculation. Most researchers include emissions from direct land use change, for instance the emissions caused by cutting down a forest in order to start some agricultural project there instead. The inclusion of indirect land use change effects is more controversial, as they are difficult to quantify accurately.^[bk]^[bl] Other choices involve defining the likely spatial boundaries of forests in the future. For instance, is increased harvesting and perhaps even forest expansion more realistic than forest protection in a situation with high demand for forest products? Or alternatively, is smaller forests perhaps more realistic than forest protection in a situation with low demand for forest products and high demand for new land or new areas for housing and urban development? Lamers & Junginger argue that from a nature conservation and carbon strategy evaluation perspective, forest protection is a valid option. However, protection is unlikely for forest plantations – in the absence of demand for forest products (e.g., timber, pulp or pellets), "[...] options such as conversion to agriculture or urban development may be more realistic alternatives [...]."^[bm] Cowie et al. argue that privately owned forests are often used to create income and therefore generally sensitive to market developments. Forest protection is an unrealistic scenario for most of the privately owned forests, unless forest owners can be compensated for their loss of income.^[bn] According to the EU's Joint Research Centre, 60% of the European forests are privately owned.^[42] In the US, over 80% is privately owned in the east, and over 80% publicly owned in the west.^[43]

Efficiency-related system boundaries

Bioenergy displacement factors for substituted fossil fuels.^[44]

Woody materials displacement factors for substituted fossil based materials.^[45]

The efficiency-related boundaries define a range of fuel substitution efficiencies for different biomass-combustion pathways. Different supply chains emit different amounts of carbon per supplied energy unit, and different combustion facilities convert the chemical energy stored in different fuels to heat or electrical energy with different efficiencies. The researcher has to know about this and choose a realistic efficiency range for the different biomass-combustion paths under consideration. The chosen efficiencies are used to calculate so-called "displacement factors" – single numbers that shows how efficient fossil carbon is substituted by biogenic carbon.^[bo] If for instance 10 tonnes of carbon are combusted with an efficiency half that of a modern coal plant, only 5 tonnes of coal would actually be counted as displaced (displacement factor 0.5). Schlamadinger & Marland describes how such low efficiency lead to high parity times when bioenergy and coal-based forest protection scenarios are compared, and on the other hand how an efficiency identical to the coal scenario lead to low parity times.^[46] Generally, fuel burned in inefficient (old or small) combustion facilities gets assigned lower displacement factors than fuel burned in efficient (new or large) facilities, since more fuel has to be burned (and therefore more CO₂ released) in order to produce the same amount of energy.^[bp]

Likewise, since the production of wood based construction materials demand lower fossil fuel input than the production of fossil based construction materials (e.g. cement or steel), the wood based construction materials get assigned displacement factors when substitution of cement and steel based construction materials is realistic, i.e. when they have the same utility in construction. The more fossil fuel emissions avoided by using utility-equivalent wood construction products, the higher the assigned displacement factors.^[bq] Additionally, the carbon stored in wood products during the products' service life, and the fossil carbon that is displaced when the wood products are combusted for energy at the end of their service life, can both be included in the displacement factor calculations. However, so far this is not common practice.^[br] (52% of the harvested forest biomass in the EU is used for materials.)^[47]

Sathre & O'Connor examined 21 individual studies and found displacement factors of between −2.3 and 15 for construction wood products, with the average at 2.1, which means that for each tonne of biogenic carbon produced, on average 2.1 tonnes of fossil carbon is displaced.^[48] For wood based biofuels, the displacement factors varied between roughly 0.5 and 1, "[...] depending largely on the type of fossil fuel replaced and the relative combustion efficiencies."^[49] The authors write that when construction wood products are combusted for energy at the end of their service life, the displacement effect is sometimes added to the calculation, "[...] as the GHG benefits of both material substitution and fuel substitution accrue."^[50] In another meta study on construction wood products, where this additional end-of-life combustion substitution effect was excluded, the authors found somewhat lower displacement factors. The combustion-specific displacement factors were similar but with a wider range (see charts on the right.)^[51]

The displacement factor varies with the carbon intensity of both the biomass fuel and the displaced fossil fuel. If or when bioenergy can achieve negative emissions (e.g. from afforestation, energy grass plantations and/or bioenergy with carbon capture and storage (BECCS),^[bs] or if fossil fuel energy sources with higher emissions in the supply chain start to come online (e.g. because of fracking, or increased use of shale gas), the displacement factor will start to rise. On the other hand, if or when new baseload energy sources with lower emissions than fossil fuels start to come online, the displacement factor will start to drop. Whether a displacement factor change is included in the calculation or not, depends on whether or not it is expected to take place within the time period covered by the relevant scenario's temporal system boundaries.^[bt]

Economic system boundaries

The economic boundaries define which market effects to include in the calculation, if any. Changed market conditions can lead to small or large changes in carbon emissions and absorptions from supply chains and forests,^[bu] for instance changes in forest area as a response to changes in demand. Miner et al. describe how researchers have begun to examine forest bioenergy in a broader, integrated framework that also addresses market impacts. Based both on empirical data and modeling, these studies have determined that increased demand often leads to investments in forestry that increase forest area and incentivize improvements in forest management. Depending on circumstances, this dynamic can increase forest carbon stocks. Where growth rates are relatively high and the investment response strong, net GHG benefits from increased use of trees for energy can be realized within a decade or two, depending on the fossil fuel being displaced and the timing of the investment response. Where tree growth is slow and the investment response is lacking, many decades may be required to see the net benefits from using roundwood for energy. The investment response has been found to be especially important in places such as the US South, where economic returns to land have been shown to directly affect gains and losses in forest area.^[52] Abt et al. argue that the US South is the world's largest timber producer, and that the forest is privately owned and therefore market driven.^[53] Further, EU's Joint Research Centre argue that macroeconomic events/policy changes can have impacts on forest carbon stock.^[bv] Like with indirect land use changes, economic changes can be difficult to quantify however, so some researchers prefer to leave them out of the calculation.^[bw]

System boundary impacts

The chosen system boundaries are very important for the calculated results.^[bx] Shorter payback/parity times are calculated when fossil carbon intensity, forest growth rate and biomass conversion efficiency increases, or when the initial forest carbon stock and/or harvest level decreases.^[54] Shorter payback/parity times are also calculated when the researcher choose landscape level over stand level carbon accounting (if carbon accounting starts at the harvest rather than at the planting event.) Conversely, longer payback/parity times are calculated when carbon intensity, growth rate and conversion efficiency decreases, or when the initial carbon stock and/or harvest level increases, or the researcher choose stand level over landscape level carbon accounting.^[by]

Critics argue that unrealistic system boundary choices are made,^[bz] or that narrow system boundaries lead to misleading conclusions.^[ca] Others argue that the wide range of results shows that there is too much leeway available and that the calculations therefore are useless for policy development.^[cb] EU's Join Research Center agrees that different methodologies produce different results,^[cc] but also argue that this is to be expected, since different researchers consciously or unconsciously choose different alternative scenarios/methodologies as a result of their ethical ideals regarding man's optimal relationship with nature. The ethical core of the sustainability debate should be made explicit by researchers, rather than hidden away.^[cd]

Climate impacts expressed as varying with time

Time-dependent net emission estimates for forest bioenergy pathways, compared against coal and natural gas alternative scenarios. Plus signs represents positive climate effects, minus signs negative climate effects.^[19]

According to EU's Joint Research Centre, the use of boreal stemwood harvested exclusively for bioenergy have a positive climate impact only in the long term, while the use of wood residues have a positive climate impact also in the short to medium term.^[ce] See chart on the right for an overview over expected emission reductions from different forest bioenergy pathways, including stemwood, residues and new plantations, compared against energy generation from coal and natural gas in the alternative scenarios. Stems from short-rotation coppices or short-rotation forests also have positive climate effects in the short to medium term (see below.)

Short carbon payback/parity times for forest residues

Short carbon payback/parity times are produced when the most realistic no-bioenergy scenario is a traditional forestry scenario where "good" wood stems are harvested for lumber production, and residues are burned or left behind in the forest or in landfills. The collection of such residues provides material which "[...] would have released its carbon (via decay or burning) back to the atmosphere anyway (over time spans defined by the biome's decay rate) [...]."^[55] In other words, payback and parity times depend on the decay speed. The decay speed depends on a.) location (because decay speed is "[...] roughly proportional to temperature and rainfall [...]"^[56]), and b.) the thickness of the residues.^[cf] Residues decay faster in warm and wet areas, and thin residues decay faster than thick residues. Thin residues in warm and wet temperate forests therefore have the fastest decay, while thick residues in cold and dry boreal forests have the slowest decay. If the residues instead are burned in the no-bioenergy scenario, e.g. outside the factories or at roadside in the forests, emissions are instant. In this case, parity times approach zero.^[cg]

Madsen & Bentsen examined emissions from both forest residues and coal, combusted in the same, real Northern European CHP (combined heat and power) plant, and found that the carbon parity time was 1 year.^[ch] The low parity time was mainly the result of the use of residues, the generally high conversion efficiencies of CHP plants compared to regular power plants (in this case 85.9%), and longer transport distance for coal.^[ci] The authors note that most bioenergy emission studies use hypothetical rather than real field data, and that 16 times more biomass is combusted in CHP plants than in pure electricity plants in the EU.^[cj] In other words, it is heat-related payback/parity times such as these that are the most relevant for the current situation.

Holmgren studied climate effects from actual forestry practices in a whole country over a 40-year time period (Sweden 1980–2019), and found that at the national landscape level, no carbon debt accrued at any point in time during this period. The actual forestry practice was compared to two alternative forest protection scenarios. The counted emissions caused by the initial harvest in the actual forestry scenario did not lead to a carbon debt because 1.) the initial harvest-related carbon emission was outweighed by carbon absorption caused by growth elsewhere in the forest (a trend that is expected to continue in the future), and 2.) because a national forest protection policy would cause large initial emissions from the national wood-based products and energy infrastructure when it is converted to work with fossil fuels.^[ck] The conversion is described as a "[...] one-off transformation, representing major and required modifications to energy systems, infrastructure, industrial processing, building sector, manufacturing of consumer products and other economic activities towards fossil-based production if a no-harvest scenario were to be implemented."^[57] Of course, if the bioenergy scenario's initial harvest-related emission event is outweighed by 1.) forest growth elsewhere, and 2.) infrastructure conversion emissions (in the forest protection scenario), no carbon debt accrues at all, and the payback and parity times reduce to zero. The author argue that since forest protection most likely will cause fossil carbon to be emitted instead of biogenic carbon, the practical effect of forest protection is simply a transfer of carbon from the underground fossil carbon pool via combustion to the atmospheric carbon pool, and then via photosynthesis to the forest carbon pool. However, when carbon is stored in forests instead of underground fossil reservoirs, it is more unstable, that is, easier to convert to CO₂ because of natural disturbances.^[58] A conservative displacement factor of 0.78 tonnes of fossil carbon displaced per tonne of biogenic carbon produced is used for both harvested wood products (HWP) and energy combined.^[cl] The author criticizes studies that limit carbon accounting to the carbon flows within the forests themselves and leave out fossil displacement effects, and argues that this narrow system boundary essentially works as "[...] a justification for continued fossil emissions elsewhere with no net gain for the global climate."^[59] In Sweden, the biomass that is available for energy is mainly used in heating facilities (7.85 Mtoe used for heating, 0.84 Mtoe for electricity.)^[60]

In the US, Walker et al. found parity times of 10 years or less when using forest residues in New England to replace coal in a regular, utility-scaled electricity plant.^[61] Likewise, Miner et al. argue that in the eastern parts of the US, all kinds of forest residues can be used for bioenergy with climate benefits within 10 years compared to a coal-based alternative scenario, and within 20 years compared to a natural gas-based alternative scenario.^[cm]

Carbon parity times for wood-pellet electricity from different feedstocks (Hanssen et al. 2017).^[62]

Hanssen et al. compared a bioenergy scenario that included continued pellet production in the Southeast USA to three alternative fossil fuel mix scenarios, all seen as more realistic scenarios than forest protection: 1.) Use all harvested biomass to produce paper, pulp or wood panels, 2.) quit the thinning practice, i.e., leave the small trees alone, so more of their growth potential is realized, and 3.) leave the residues alone, so they decay naturally over time, rather than being burned almost immediately in power plants. Three different levels of demand (low, average, high) was included for each alternative scenario. Parity times ranged from 0–21 years in all demand scenarios, and 0–6 years in the average demand scenarios (see chart on the right). The authors used landscape level carbon accounting, rotation time was 25 years, and market effects were included.^[cn]

Carbon parity times for various residues-based energy systems, compared to alternative scenarios.^[co]

Time-dependent global warming mitigation potentials for forest residues, cereal straw and biogas slurry.^[63]

Time-dependent emission levels from decaying forest residues with different thicknesses: stumps (30 cm), thinnings (10 cm) and branches (2 cm). Dotted lines = North Finland, solid lines = South Finland.^[cp]

Lamers & Junginger examined a number of studies on (sub)-boreal forest residues (including stumps in some cases), and found carbon parity times of between 0 and 16 years. The bioenergy scenario was compared against an alternative reference scenario where the residues either were left in the forests to decay naturally, or was incinerated at the roadside. The parity time was 0 years compared to a scenario where the residues was burned at roadside and electricity instead produced by coal plants. However, parity times increased to 3–24 years when roadside burning was exchanged with natural decay, and coal exchanged with oil. Parity times increased further to 4–44 years when oil was replaced with natural gas. All bioenergy scenarios used landscape level carbon accounting.^[64]

Zanchi et al. agree that there are climate benefits from the beginning when using easily decomposable forest residues for bioenergy. They also write that "[...] new bioenergy plantations on lands with low initial C [carbon] stocks, such as marginal agricultural land, has the clearest advantages in terms of emission reductions."^[65] The reason is that newly planted areas (which now has a large growing stock of trees or other plants), absorb much more carbon than earlier. Such areas build up a carbon credit instead of a carbon debt, where the credit is used later (at harvest) to acquire "debt free" biomass. In general, "early" carbon accounting like this, which starts at the planting event rather than at the harvest event (cf. Temporal system boundaries above), is seen as uncontroversial for new bioenergy plantations on land areas with very little vegetation. On the other hand, for areas where there already is a large amount of vegetation in place, "late" carbon accounting is often preferred. In this case, carbon accounting starts at harvest, with no build-up of a prior carbon credit. With this type of carbon accounting, the calculated results show that there are short to medium term negative impacts when trees are felled exclusively for bioenergy (so-called "additional fellings"). The situation gets worse if residues are left to rot on the forest floor. There is also a risk for negative impacts if areas with large amounts of biomass such as forests are clear-cut in order to make room for low-productivity forest plantations.^[66]

The assessment of such "additional fellings" from "new" bioenergy plantations after the first rotation is complete, depends on the chosen carbon accounting method. If the "early" carbon accounting continues, there will be a build-up of a carbon credit also after the first rotation, i.e. from the moment in time when the trees have been replanted. If the researcher at that time change to "late" carbon accounting, no carbon credit will be calculated, and at the end of the second rotation (at harvest) a large carbon debt will be created instead, causing payback and parity times to increase dramatically.

Long carbon payback/parity times for forest residues

EU's Joint Research Centre provides time-dependent emission estimates for electricity production on a large scale from residue-based wood pellets, cereal straw and biogas from slurry, compared against a no-bioenergy scenario with emissions equal to EU's current electricity mix. Conversion efficiencies are 34%, 29% and 36% for wood pellets, straw and biogas, respectively. If not used for electricity production, the forest residues would have been left to decay on the forest floor, the straw residues would also have been left behind in the fields, and the raw manure would have been used as organic fertilizer. The results show that if these biomass types instead were used to produce electricity, a global warming mitigation effect would start after approximately 50, 10 and 5 years of use, for wood, straw and biogas respectively. The main cause for the long parity time for wood pellets is the comparison with electricity from EU's electricity mix (which includes electricity from solar, wind and fossil fuels with lower emissions than coal). Also, the forest residue category includes stumps.^[cq]

The JRC also found parity^[cr] times ranging from 0 to 35 years for harvest residues (including branches, thinnings and stumps), when compared to some other alternative scenarios. In Finland, parity times for stumps were 22 years compared against oil, and 35 years compared against natural gas, with stand level carbon accounting. In Canada, parity time increased from 16 to 74 years when the harvested biomass was used to produce ethanol instead of wood pellets, and compared against a gasoline-based alternative scenario instead of a coal-based alternative scenario.^[cs] Ethanol production from whole trees removed from old-growth forests in Oregon, USA, (categorized as residues because the trees were felled to prevent wildfire) increased parity time dramatically, with the worst-case scenario at 459 years. The authors used stand level carbon accounting starting with the harvest event, assumed an additional, controlled burning every 25 years, and compared this to a scenario with no wildfire-preventive fellings and a severe wildfire every 230 years.^[67] The trees in question were huge western hemlock and coast Douglas fir trees which both take hundreds of years to mature and can withstand wildfires due to very thick stems. Since energy-intensive ethanol production caused a low displacement factor of only 0.39, a long parity time was calculated.^[68] Generally, the JRC's reported parity times were influenced by displacement factor, alternative scenario, residue size and climate type. See chart above.

Short carbon payback/parity times for stemwood

If an existing natural forest is clear-cut in order to make room for forest plantations, the implied carbon change create a significant carbon debt roughly equal to the amount of carbon residing in the felled trees (fossil based forestry operations create an additional, small debt.) But for new plantations on "empty" land like agricultural or marginal land, with no standing vegetation, no carbon is removed. In this case, a carbon credit is instead soon built up as the trees mature. When those trees later are felled, the amount of carbon that resides in the trees is subtracted from the built up carbon credit (not the carbon amount in the standing trees), so in this case no carbon debt is created. With no carbon debt created at harvest, carbon payback/parity times will be zero or very low, for residues and stemwood alike.^[ct]

Jonker et al. calculated both carbon payback and carbon parity times for stemwood with rotation times of 20–25 years harvested from southeastern forests in the US, using both stand level, increasing stand level, and landscape level carbon accounting. With stand-level carbon accounting, the authors found carbon payback times of 5, 7 and 11 years in the high, medium and low yield scenario, respectively. With increasing stand level accounting, the payback times were 12, 13 and 18 years in the high, medium and low yield scenario, respectively. With landscape level accounting, the payback time was below 1 year for all yield scenarios.^[40] The authors also calculated parity times for a scenario where wood pellets from stems only (no residue collection) were used for co-firing in an average, coal-based electricity plant. The conversion efficiency was 41%, which together with an efficient supply chain leads to a relatively high displacement factor of 0.92. The alternative scenario was a no-bioenergy scenario where the stemwood was instead used for lumber production, so no co-firing at all in this case (electricity from coal exclusively.) When using the increasing stand level accounting principle, the authors calculated parity times of 17, 22 and 39 years for the high, medium and low yield scenario, respectively. When using the landscape level accounting principle, the authors calculated parity times of 12, 27 and 46 years for the high, medium and low yield scenario, respectively. A different alternative scenario was a forest protection scenario where no biomass was extracted from the forest at all; not for lumber, and not for bioenergy. The forest was simply left to itself and therefore regrew rather slowly. Landscape level parity times for this scenario was 3, 3, and 30 years for the high, medium and low yield scenario, respectively (stand level or increasing stand level parity times were not provided.)^[69]

The authors note that "the result of the carbon balances clearly demonstrate that the choice of carbon accounting method has a significant impact on the carbon payback and carbon offset parity point calculations."^[70] They argue that the short parity times are caused by the fast growth rates (10–12 tonnes dry mass per hectare per year) in softwood plantations in the southeastern USA. Other researchers have often based their calculations on the slow growth rates typical for hardwood in natural boreal forests, which generates much higher payback and parity times. The authors also argue that for established softwood plantations, there is no carbon debt caused by land use change. Also, the displacement factor is higher here than in some other studies, due to the efficient supply chain and the high conversion efficiency achieved when wood pellets are used for co-firing in regular coal plants rather than in small-scale bioenergy plants; the latter often assumed to be the case in other studies. In effect, these favourable system boundaries cause the parity time to reduce to one or two rotations. The carbon debt is small before the parity point, and the subsequent carbon credit rises high after the parity point has been passed: "It is also clear that the absolute size of the temporary negative carbon balance is limited, whereas the positive carbon balance after break-even soon reaches levels many times greater."^[71] The authors argue that the no-bioenergy and the forest protection scenario is unrealistic in the study area, since the forests here are privately owned and there is a large wood processing industry already in place. In this situation (without viable alternative scenarios) the authors argue that the most relevant temporal metric is the carbon payback time of below 1 year for all yield scenarios, based on the landscape level carbon accounting principle.^[72] Abt et al. also argue that in the southeastern USA, forest protection scenarios are unrealistic since the forests are privately owned.^[53]

Carbon parity times for stemwood harvested exclusively for bioenergy, compared against various alternative fossil-based scenarios.^[73]

EU's Joint Research Centre reviewed a number of studies and found that if stemwood is harvested for both bioenergy and wood products, continued harvesting works better for the climate than forest protection given a 40 years timeframe.^[cu] The reason is the larger displacement effect of wood products compared to bioenergy. If wood products are used for energy when reaching their end of life (so-called "cascading"), the displacement effect grows even larger, and under optimal conditions, parity times can reduce from several centuries to zero. The JRC therefore argue that studies that fail to include the wood for material displacement effect may come to misleading conclusions.^[cv] On the other hand, if a forest is harvested exclusively for bioenergy, there is no displacement effects happening for wood products, which means a lower displacement factor and therefore a net increase in calculated CO₂ emissions "[...] in the short-and medium term (decades) [...]" when compared to fossil fuels, except when it is harvested from new plantations on marginal, agricultural or grazing land. In this case there is an immediate net increase in carbon at the site, as planting without prior tree felling increases the amount of biomass there.^[cw] Again, when there is no carbon debt, the payback and parity times reduce to zero.^[cx]

Long carbon payback/parity times for stemwood

Zanchi et al. found that parity times can reach 175 years with a coal-based alternative scenario and 300 years with a natural gas-based alternative scenario if spruce stems in the Austrian Alps are harvested exclusively for bioenergy. The main reason is the long rotation time for these trees (90 years). Generally, trees take 70–120 years to mature in boreal forests.^[74] Critics reply that stems that meet quality requirements are used to produce high-value products such as sawn wood and engineered wood products such as cross laminated timber, rather than low-value products such as wood pellets.^[cy] In a different scenario where forests of this type is clear-cut and used 50/50 for bioenergy and solid wood products, and then subsequently replaced with short rotation forest, parity times varies between 17 and 114 years for the coal alternative scenario, with the shortest parity time achieved by the forest with the shortest rotation time and highest yield (10 years rotation time with a yield of 16 tonnes per hectare per year.) Parity times increased to between 20 and 145 years when compared to an oil-based electricity alternative case, and between 25 and 197 years when compared to a natural gas-based electricity alternative case. For an afforestation vs. fossil fuel mix scenario, a parity time of 0 years was reported.

The authors note that these scenarios are "illustrative examples" and that "results are strongly influenced by the assumptions made." The authors assumed that residues were left un-collected on the forest floor, where they decay and therefore produce emissions. If these residues instead are collected and used for bioenergy, the parity times decrease by 100 years. The extra emissions produced by the longer supply routes for fossil fuels compared to wood fuel were not included in the calculation.^[cz] Extra emissions from pests, windthrows and forest fires (normally expected to increase when unmanaged forests age), were also not included in the calculation. Market effects were not included. On the other hand, landscape level carbon accounting was used, and the assumed conversion efficiency for bioenergy and coal were the same.^[75]

Like other scientists, the JRC staff note the high variability in carbon accounting results, and attribute this to different methodologies.^[da] In the studies examined, the JRC found carbon parity times of 0 to 400 years (see chart on the right) for stemwood harvested exclusively for bioenergy, depending on different characteristics and assumptions for both the forest/bioenergy system and the alternative fossil system, with the emission intensity of the displaced fossil fuels seen as the most important factor, followed by conversion efficiency and biomass growth rate/rotation time. Other factors relevant for the carbon parity time are the initial carbon stock and the existing harvest level; both higher initial carbon stock and higher harvest level means longer parity times.^[76] Liquid biofuels have high parity times because about half of the energy content of the biomass is lost in the processing.^[db]

Climate impacts expressed as static numbers

Net emissions from various biofuel pathways (heat production). Stippled lines show net emissions for EU coal, light fuel oil, most relevant fossil fuel alternative, and natural gas. Dotted areas show emission savings percentages compared to the most relevant fossil fuel alternative (white 70–80%, green 80–85%, blue 85–100%.^[77]

Net emissions from various biofuel pathways (transportation). The stippled line show net emissions for the most relevant fossil fuel alternative. Dotted areas show emission savings percentages also compared to the most relevant fossil fuel alternative (white 50–60%, green 60–70%, blue 70–100%.^[77]

Net emissions from various biofuel pathways (electricity production). Stippled lines show net emissions for EU coal (black), most relevant fossil fuel alternative (green), electricity mix (red) and natural gas (blue). Dotted areas show emission savings percentages compared to the most relevant fossil fuel alternative (white 70–80%, green 80–85%, blue 85–100%.^[78]

Greenhouse gas emissions from wood pellet production and transport from the US to the EU (Hanssen et al. 2017).^[dc]

EU's Joint Research Centre has examined a number of bioenergy emission estimates found in literature, and calculated greenhouse gas savings percentages for bioenergy pathways in heat production, transportation fuel production and electricity production, based on those studies (see charts on the right). The calculations are based on the attributional LCA accounting principle. It includes all supply chain emissions, from raw material extraction, through energy and material production and manufacturing, to end-of-life treatment and final disposal. It also includes emissions related to the production of the fossil fuels used in the supply chain. It excludes emission/absorption effects that takes place outside its system boundaries, for instance market related, biogeophysical (e.g. albedo), and time-dependent effects. Because market related calculations are excluded, the results are only seen as valid for small-scale energy production.^[79] Also, the bioenergy pathways have typical small-scale conversion efficiencies. Solid biofuels for electricity production have 25% efficiency in most cases, and 21–34% in a few cases. Biogas for electricity production have 32–38%. Heat pathways have 76–85%. The forest residue category include logs and stumps, which increases carbon intensity especially in forests with slow decay rates.^[80]

The charts have vertical bars that represent the emission range found for each bioenergy pathway (since emissions for the same pathway vary from study to study.) The higher end of the range represents emission levels found in studies that assume for instance long transport distances, low conversion efficiencies and no fossil fuel displacement effect. The lower end of the range represents emission levels found in studies that assume optimized logistics, higher conversion efficiencies, use of renewable energy to supply process-heat and process-electricity, and include displacement effects from the substitution of fossil fuels.^[dd] The bars can be compared with emission levels associated with multiple alternative energy systems available in the EU. The dotted, coloured areas represent emission savings percentages for the pathways when compared to fossil fuel alternatives.^[78] The authors conclude that "[m]ost bio-based commodities release less GHG than fossil products along their supply chain; but the magnitude of GHG emissions vary greatly with logistics, type of feedstocks, land and ecosystem management, resource efficiency, and technology."^[81]

Because of the varied climate mitigation potential for different biofuel pathways, governments and organizations set up different certification schemes to ensure that biomass use is sustainable, for instance the RED (Renewable Energy Directive) in the EU and the ISO standard 13065 by the International Organization for Standardization.^[82] In the US, the RFS (Renewables Fuel Standard) limit the use of traditional biofuels and defines the minimum life-cycle GHG emissions that are acceptable. Biofuels are considered traditional if they achieve up to 20% GHG emission reduction compared to the petrochemical equivalent, advanced if they save at least 50%, and cellulosic if the save more than 60%.^[de]

Consistent with the charts, the EU's Renewable Energy Directive (RED) states that the typical greenhouse gas emissions savings when replacing fossil fuels with wood pellets from forest residues for heat production varies between 69% and 77%, depending on transport distance: When the distance is between 0 and 2500 km, emission savings is 77%. Emission savings drop to 75% when the distance is between 2500 and 10 000 km, and to 69% when the distance is above 10 000 km. When stemwood is used, emission savings varies between 70% and 77%, depending on transport distance. When wood industry residues are used, savings varies between 79% and 87%.^[df]

Based on a similar methodology, Hanssen et al. found that greenhouse gas emissions savings from electricity production based on wood pellets produced in the US southeast and shipped to the EU, varies between 65% and 75%, compared to EU's fossil fuel mix.^[dg] They estimate that average net GHG emission from wood pellets imported from the US and burnt for electricity in the EU amounts to approximately 0.2 kg CO₂ equivalents per kWh, while average emissions from the mix of fossil fuels that is currently burnt for electricity in the EU amounts to 0.67 kg CO₂-eq per kWh (see chart on the right). Ocean transport emissions amounts to 7% of the displaced fossil fuel mix emissions per produced kWh.^[dh]

Likewise, IEA Bioenergy estimates that in a scenario where Canadian wood pellets totally replace coal in a European coal plant, the ocean transport related emissions (for the distance Vancouver – Rotterdam) amounts to approximately 2% of the plant's total coal-related emissions.^[83] The lower percentage here is caused by the alternative scenario being a particular coal plant, not EU's fossil fuel mix. Cowie et al. argue that calculations from actual supply chains show low emissions from intercontinental biomass transport, for instance the optimized wood pellet supply chain from the southeastern USA to Europe.^[di] Lamers & Junginger argue that future EU import of wood pellets "[...] will likely continue to be dominated by North America, especially from the South-East USA [...]."^[84] In 2015, 77% of the imported pellets came from the USA.^[dj]

While regular forest stands have rotation times spanning decades, short rotation forestry (SRF)^[r] stands have a rotation time of 8–20 years, and short rotation coppicing (SRC)^[q] stands 2–4 years.^[85] 12% of the EU forests is coppice forests.^[dk] Perennial grasses have a rotation time of one year in temperate areas, and 4–12 months in tropical areas.^[86] Food crops like wheat and maize also have rotation times of one year.

Because short rotation energy crops only have managed to grow/accumulate carbon for a short amount of time before they are harvested, it is relatively easy to pay back the harvest-related carbon debt, provided that there is no additional large carbon debt from land use change to deal with (for instance created by clear-cutting a natural forest in order to use this land area for energy crops), and no better climate-related use for the areas in question. Schlamadinger & Marland write that "[...] short-rotation energy crops will provide much earlier and larger C [carbon] mitigation benefits if implemented on previously unforested land than if an initial forest is harvested to provide space for the plantation."^[87] EU's Joint Research Centre state: "In case that there is no raw material displacement from other sectors such as food, feed, fibers or changes in land carbon stocks due to direct or indirect land use change, the assumption of carbon neutrality can still be considered valid for annual crops, agriresidues, short-rotation coppices and energy grasses with short rotation cycles. This can also be valid for analysis with time horizons much longer than the feedstock growth cycles."^[88] Other researchers argue that the small carbon debts associated with energy crop harvests means short carbon payback and parity times, often less than a year.^[dl] IRENA argues that short-rotation energy crops and agricultural residues are carbon neutral since they are harvested annually.^[dm] IEA writes in its special report on how to reach net zero emissions in 2050 that the "[...] energy‐sector transformation in the NZE [Net Zero Emissions scenario) would reduce CO₂ emissions from AFLOU [Agriculture, Forestry and Other Land Use] in 2050 by around 150 Mt CO₂ given the switch away from conventional crops and the increase in short rotation advanced‐bioenergy crop production on marginal lands and pasture land."^[89]

Since the long payback and parity times calculated for some forestry projects is seen as a non-issue for energy crops (except in the cases mentioned above), researchers instead calculate static climate mitigation potentials for these crops, using LCA-based carbon accounting methods. A particular energy crop-based bioenergy project is considered carbon positive, carbon neutral or carbon negative based on the total amount of CO₂ equivalent emissions and absorptions accumulated throughout its entire lifetime: If emissions during agriculture, processing, transport and combustion are higher than what is absorbed (and stored) by the plants, both above and below ground, during the project's lifetime, the project is carbon positive. Likewise, if total absorption is higher than total emissions, the project is carbon negative. In other words, carbon negativity is possible when net carbon accumulation more than compensates for net lifecycle greenhouse gas emissions.

Miscanthus × giganteus is a perennial energy grass.

The most climate friendly energy crops seems to be perennial energy grasses, because of low energy inputs and large amounts of carbon stored in the soil. Researchers argue that the mean energy input/output ratios for the perennial crop miscanthus is 10 times better than for annual crops, and that greenhouse gas emissions are 20-30 times better than for fossil fuels.^[dn] Miscanthus chips for heating saved 22.3 tonnes of CO₂ emissions per hectare per year in the UK, while maize for heating and power saved 6.3. Rapeseed for biodiesel saved 3.2.^[do] Other researchers have similar conclusions.^[dp]

Typically, perennial crops sequester more carbon than annual crops because the root buildup is allowed to continue undisturbed over many years. Also, perennial crops avoid the yearly tillage procedures (plowing, digging) associated with growing annual crops. Tilling helps the soil microbe populations to decompose the available carbon, producing CO₂.^[dq]^[dr] Soil organic carbon has been observed to be greater below switchgrass crops than under cultivated cropland, especially at depths below 30 cm (12 in).^[90] A meta-study of 138 individual studies, done by Harris et al., revealed that the perennial grasses miscanthus and switchgrass planted on arable land on average store five times more carbon in the ground than short rotation coppice or short rotation forestry plantations (poplar and willow).^[ds] McCalmont et al. compared a number of individual European reports on Miscanthus × giganteus carbon sequestration, and found accumulation rates ranging from 0.42 to 3.8 tonnes per hectare per year,^[dt] with a mean accumulation rate of 1.84 tonne,^[du] or 25% of total harvested carbon per year.^[dv]

Fundamentally, the below-ground carbon accumulation works as a greenhouse gas mitigation tool because it removes carbon from the above-ground carbon circulation (the circulation from plant to atmosphere and back into new plants.) The circulation is driven by photosynthesis and combustion: First, a plant absorbs CO₂ and assimilates it as carbon in its tissue both above and below ground. When the above-ground carbon is harvested and then burned, the CO₂ molecule is formed yet again and released back into the atmosphere. Then, an equivalent amount of CO₂ is absorbed back by next season's growth, and the cycle repeats.

Carbon negative (miscanthus) and carbon positive (poplar) production pathways.^[dw]

This above-ground circulation has the potential to be carbon neutral, but of course the human involvement in operating and guiding it means additional energy input, often coming from fossil sources. If the fossil energy spent on the operation is high compared to the amount of energy produced, the total CO₂ footprint can approach, match or even exceed the CO₂ footprint originating from burning fossil fuels exclusively, as has been shown to be the case for several first-generation biofuel projects.^[dx]^[dy]^[dz] Transport fuels might be worse than solid fuels in this regard.^[ea]

The problem can be dealt with both from the perspective of increasing the amount of carbon that is stored below ground, and from the perspective of decreasing fossil fuel input to the above-ground operation. If enough carbon is stored below ground, it can compensate for the total lifecycle emissions of a particular biofuel. Likewise, if the above-ground emissions decreases, less below-ground carbon storage is needed for the biofuel to become carbon neutral or negative.

Relationship between above-ground yield (diagonal lines), soil organic carbon (X axis), and soil's potential for successful/unsuccessful carbon sequestration (Y axis). Basically, the higher the yield, the more land is usable as a GHG mitigation tool (including relatively carbon-rich land).^[91]

Whitaker et al. argue that a miscanthus crop with a yield of 10 tonnes per hectare per year store enough carbon to compensate for both agriculture, processing and transport related emissions. The chart on the right displays two carbon negative miscanthus production pathways, and two carbon positive poplar production pathways, represented in gram CO₂-equivalents per megajoule. The bars are sequential and move up and down as atmospheric CO₂ is estimated to increase and decrease. The grey/blue bars represent agriculture, processing and transport related emissions, the green bars represents soil carbon change, and the yellow diamonds represent total final emissions.^[dw] The second chart displays the mean yields necessary to achieve long-term carbon negativity for soils with different amounts of existing carbon. The higher the yield, the more likely carbon negativity becomes. Other researchers make the same claim about carbon negativity for miscanthus in Germany, with a yield of 15 dry tonnes per hectare per year, and carbon storage of 1.1 tonnes per hectare per year.^[eb]

Successful storage is dependent on planting sites, as the best soils are those that are currently low in carbon.^[ec] For the UK, successful storage is expected for arable land over most of England and Wales, with unsuccessful storage expected in parts of Scotland, due to already carbon rich soils (existing woodland). Also, for Scotland, the relatively lower yields in this colder climate makes carbon negativity harder to achieve. Soils already rich in carbon include peatland and mature forest. The most successful carbon storage in the UK takes place below improved grassland.^[ed] However, since the carbon content of grasslands vary considerably, so does the success rate of land use changes from grasslands to perennial.^[ee] Even though the net carbon storage below perennial energy crops like miscanthus and switchtgrass greatly exceeds the net carbon storage below regular grassland, forest and arable crops, the carbon input is simply too low to compensate for the loss of existing soil carbon during the early establishment phase.^[92] Over time however, soil carbon may increase, also for grassland.^[93]

Researchers argue that after some initial discussion, there is now (2018) consensus in the scientific community that "[...] the GHG [greenhouse gas] balance of perennial bioenergy crop cultivation will often be favourable [...]", also when considering the implicit direct and indirect land use changes.^[ef]

Albedo and evapotranspiration

Plants change the color of the surface of the earth, and this has an effect on the surface reflectivity (albedo). Lighter colors tend to reflect heat, and darker colors tend to absorb heat. Research show that afforestation have a net warming effect in snowy, boreal areas (also after carbon absorption caused by afforestation have been accounted for), because the color of the trees is darker than the color of the snow. Forest albedo has a slight cooling effect globally.^[eg]

Plants causes more evapotranspiration and therefore increased local humidity. The increased humidity causes more of the incoming solar energy to be spent evaporating water rather than heating the ground, thereby creating a cooling effect. In tropical forests, evapotranspiration can also create low-hanging clouds that reflects sunlight, adding to the albedo effect. Forests release small particles called organic carbon, both via combustion and directly from live trees. The particles reflect sunlight, so have a cooling effect on their own, but also helps create clouds, since water vapor condense around the particles. In both cases, the reflection creates a cooling effect.^[eh]

If annual crops across the central USA were replaced by perennial grasses, it would cause significant global cooling, mostly from evapotranspiration effects but also from albedo. The albedo effect alone was six times larger than the grasses' fossil fuel displacement effect. The reason for the albedo effect in this case was that perennial grasses keep the surface green for a longer period of time during the year, compared to annual crops.^[ei]^[94]

Environmental impact

Surface power production densities

The environmental impact caused by biomass or other renewable energy production depends to some extent on its land use requirements. To calculate land use requirements, it is essential to know the relevant surface power production densities (e.g. power production per square metre). Vaclav Smil estimates that the average lifecycle surface power densities for modern biofuels, wind, hydro and solar power production are 0.3 W/m², 1 W/m², 3 W/m² and 5 W/m², respectively (power in the form of heat for biofuels, and electricity for wind, hydro and solar).^[95] Lifecycle surface power density includes land used by all supporting infrastructure, manufacturing, mining/harvesting and decommissioning. Van Zalk et al. estimates 0.08 W/m² for biofuel, 0.14 W/m² for hydro, 1.84 W/m² for wind, and 6.63 W/m² for solar (median values, with none of the renewable sources exceeding 10 W/m²). Fossil gas has the highest surface density at 482 W/m² while nuclear power at 240 W/m² is the only high-density and low-carbon energy source.^[96] The average human power consumption on ice-free land is 0.125 W/m² (heat and electricity combined),^[97] although rising to 20 W/m² in urban and industrial areas.^[98]

The reason for the low power density for some of the biofuels is a combination of low yields and only partial utilization of the plant (for instance, ethanol is typically made from sugarcane's sugar content or corn's starch content, while biodiesel is often made from the oil content in rapeseed or soybean).

Wheat fields in the US.

When used for ethanol production, miscanthus plantations with a yield of 15 tonnes per hectare per year generate 0.40 W/m².^[99] Corn fields generate 0.26 W/m² (yield 10 t/ha).^[100] In Brazil sugarcane fields typically generate 0.41 W/m².^[100] Winter wheat (USA) generates 0.08 W/m² and German wheat generates 0.30 W/m².^[101] When grown for jet fuel, soybean generates 0.06 W/m², while palm oil generates 0.65 W/m².^[102] Jathropa grown on marginal land generate 0.20 W/m².^[102] When grown for biodiesel, rapeseed generate 0.12 W/m² (EU average).^[103] Liquid biofuel production require large energy inputs compared to solid biofuel production.^[ej] When these inputs are compensated for (i.e. when used energy is subtracted from produced energy), power density drops further down: Rapeseed based biodiesel production in the Netherlands have the highest energy efficiency in the EU with an adjusted power density of 0.08 W/m², while sugar beets based bioethanol produced in Spain have the lowest, at only 0.02 W/m².^[104]

Eucalyptus plantation in India.

Using solid biomass for energy purposes is more efficient than using liquids, as the whole plant can be utilized. For instance, corn plantations producing solid biomass for combustion generate more than double the amount of power per square metre compared to corn plantations producing for ethanol, when the yield is the same: 10 t/ha generates 0.60 W/m² and 0.26 W/m² respectively, without compensating for energy input.^[105] It has been estimated that large-scale plantations with pines, acacias, poplars and willows in temperate regions achieve yields of 5–15 dry tonnes per hectare per year, which means a surface power production density of 0.30–0.90 W/m².^[106] For similarly large plantations, with eucalyptus, acacia, leucaena, pinus and dalbergia in tropical and subtropical regions, yields are typically 20–25 t/ha, which means a surface power production density of 1.20–1.50 W/m². This yield put these plantations' power densities in-between the densities of wind and hydro.^[106] In Brazil, the average yield for eucalyptus is 21 t/ha, but in Africa, India and Southeast Asia, typical eucalyptus yields are below 10 t/ha.^[107]

Oven dry biomass in general, including wood, miscanthus^[108] and napier^[109] grass, have a calorific content of roughly 18 GJ/t.^[110] When calculating power production per square metre, every t/ha of dry biomass yield increases a plantation's power production by 0.06 W/m².^[ek] As mentioned above, the world average for wind, hydro and solar power production is 1 W/m², 3 W/m² and 5 W/m² respectively. In order to match these surface power densities, plantation yields must reach 17 t/ha, 50 t/ha and 83 t/ha for wind, hydro and solar respectively. This seems achievable for the tropical plantations mentioned above (yield 20–25 t/ha) and for elephant grasses, e.g. miscanthus (10–40 t/ha), and napier (15–80 t/ha), but unlikely for forest and many other types of biomass crops. To match the world average for biofuels (0.3 W/m²), plantations need to produce 5 tonnes of dry mass per hectare per year. When instead using the Van Zalk estimates for hydro, wind and solar (0.14, 1.84, and 6.63 W/m² respectively), plantation yields must reach 2 t/ha, 31 t/ha and 111 t/ha in order to compete. Only the first two of those yields seem achievable, however.

In the case of old combustion facilities, yields need to be adjusted to compensate for the amount of moisture in the biomass (evaporating moisture in order to reach the ignition point is wasted energy unless the resulting steam can be harnessed for energy).^[el] The moisture of biomass straw or bales varies with the surrounding air humidity and eventual pre-drying measures, while pellets have a standardized (ISO-defined) moisture content of below 10% (wood pellets) and below 15% (other pellets).^[em] Likewise, for wind, hydro and solar, power line transmission losses amounts to roughly 8% globally and should be accounted for.^[en] If biomass is to be utilized for electricity production rather than heat production, yields has to be roughly tripled in order to compete with wind, hydro and solar, as the current heat to electricity conversion efficiency is only 30-40%.^[111] When simply comparing the surface power production densities of biofuel, wind, hydro and solar, without regard for cost, this effectively pushes both hydro and solar power out of reach of even the highest yielding plantations, power density wise.^[eo]

Biodiversity

Gasparatos et al. reviews current research about the side effects of all kinds of renewable energy production and argue that in general there is a conflict between "[...] site/local-specific conservation goals and national energy policy/climate change mitigation priorities [...]." The authors argue that for instance biodiversity should be seen as an equally "[...] legitimate goal of the Green Economy as curbing GHG emissions."^[112] Oil palm and sugar cane are examples of crops that have been linked to reduced biodiversity.^[113] Other problems are pollution of soil and water from fertiliser/pesticide use,^[114] and emission of ambient air pollutants, mainly from open field burning of residues.^[115]

Classification scheme for win-win (green), trade-off (orange), and lose-lose (red) scenarios caused by additional bioenergy pathways in the EU.^[ep]

Short term climate and biodiversity impacts for 3 alternative bioenergy pathways in the EU (forest residues, afforestation and conversion to forest plantation.) Short term is here defined as a period of 0–20 years, medium term 30-50 years, and long term over 50 years.^[116]

The authors note that the extent of the environmental impact "[...] varies considerably between different biomass energy options."^[113] For impact mitigation, they recommend "[...] adopting environmentally-friendly bioenergy production practices, for instance limiting the expansion of monoculture plantations, adopting wildlife-friendly production practices, installing pollution control mechanisms, and undertaking continuous landscape monitoring."^[117] They also recommend "[...] multi-functional bioenergy landscapes."^[117] Other measures include "[...] careful feedstock selection, as different feedstocks can have radically different environmental trade-offs. For example, US studies have demonstrated that 2nd generation feedstocks grown in unfertilized land could provide benefits to biodiversity when compared to monocultural annual crops such as maize and soy that make extensive use of agrochemicals."^[117] Miscanthus and switchgrass are examples of such crops.^[118]

Since biodiversity has been defined by the EU as an important policy goal, EU's Joint Research Centre has examined ways to ensure that increased use of bioenergy does not negatively effect biodiversity in European forests.^[eq] Only bioenergy pathways that provides additional bioenergy resources compared to the existing forestry practices were considered, namely 1.) increased use of logging residues, 2.) afforestation of unused land areas and 3.) conversion of natural forests to more productive forest plantations.^[er] The authors divided the results into four categories, depending on their potential for climate and biodiversity mitigation: 1.) Win-win scenarios (green quadrant in the chart to the right) have positive consequences for both the climate and for biodiversity, 2.) win-lose scenarios (yellow quadrant) are trade-off scenarios with positive consequences for the climate but negative consequences for biodiversity, 3.) lose-win scenarios (yellow quadrant) are trade-off scenarios with negative consequences for the climate but positive consequences for biodiversity, and 4.) lose-lose scenarios (red quadrant) have negative consequences for both the climate and for biodiversity (see chart on the right.)

Long term, increased bioenergy may have a positive impact on biodiversity because "[...] climate change in itself is a major driver of biodiversity loss." However, this is hard to quantify, so as a conservative measure, the authors chose to only recommend bioenergy pathways with consequences for biodiversity seen as positive in the short term.^[es] The same goes for climate effects; only bioenergy pathways with positive short-term consequences were recommended (short-term is defined as a period of 0–20 years, medium-term 30–50 years, and long-term over 50 years.) The alternative scenario for all bioenergy scenarios was a fossil fuel mix ("fossil sources"), i.e. not coal exclusively.^[119] No market effects were considered, so the results are only seen as valid for small-scale bioenergy deployment.^[et]

Win-win scenarios include increased use of whole trees from coppice forests, increased use of thin forest residues from boreal forests with slow decay rates, and increased use of all kinds of residues from temperate forests with faster decay rates. Win-win scenarios also include afforestation of former agricultural land with mixed or naturally regenerating forests.^[eu] Win-lose scenarios (good for the climate, bad for biodiversity) include afforestation on ancient, biodiversity-rich grassland ecosystems which were never forests, and afforestation of former agricultural land with monoculture plantations.^[ev] Lose-win scenarios (bad for the climate, good for biodiversity) include natural forest expansion on former agricultural land.^[ew] Lose-lose scenarios include increased use of thick forest residues like stumps from some boreal forests with slow decay rates, and conversion of natural forests into forest plantations.^[ex] Some of the negative consequences in the trade-off scenarios (yellow quadrants) can be minimized by implementing the RED II sustainability criteria, for instance no-go areas for biomass harvesting.^[ey] However, as the European forests age, the authors expect a moderate harvest level increase because of "forest age dynamics" and in order to avoid emissions caused by forest fires, pests and windstorms.^[ez] In general, scientists can describe the situation as they see it and provide policy options, but ultimately it should be up to the politicians to prioritize between climate and biodiversity mitigation in the trade-off scenarios because this prioritization is based on ethical value choices, not science.^[fa]

Pollution

The traditional use of wood in cook stoves and open fires produces pollutants, which can lead to severe health and environmental consequences. However, a shift to modern bioenergy contribute to improved livelihoods and can reduce land degradation and impacts on ecosystem services.^[fb] According to the IPCC, there is strong evidence that modern bioenergy have "large positive impacts" on air quality.^[120] Likewise, the IEA argue that traditional bioenergy is inefficient and that the phasing out of this energy source both have large health benefits and large economic benefits.^[fc] When combusted in industrial facilities, most of the pollutants originating from woody biomass reduce by 97-99%, compared to open burning.^[121] A study of the giant brown haze that periodically covers large areas in South Asia determined that two thirds of it had been principally produced by residential cooking and agricultural burning, and one third by fossil-fuel burning.^[122]

Local protests

While bioenergy is generally agreed to mitigate greenhouse gas emissions on a global scale, environmental activists argue that increased biomass demand can create significant social and environmental pressure in the locations where the biomass is produced.^[123] The impact is primarily related to the low surface power density of biomass. The low surface power density has the effect that much larger land areas are needed in order to produce the same amount of energy, compared to for instance fossil fuels.

Feasibility assessments to replace coal in German power plants with bush biomass harvested in Namibia, which experiences bush encroachment on over 30 million hectares, have caused protests from environmental organisations. The organisations argue that the trees and bushes store carbon, and that burning them releases more CO₂ upfront than burning coal.^[124] Namibian researchers argue that bush encroaching causes lower income for farmers, lower biodiversity, lower groundwater level and displacement of wildlife.^[125] Long-distance transport of biomass have been criticised as wasteful and unsustainable,^[126] and there have been protests against forest biomass export in Sweden^[127] and Canada.^[128]

In Mississippi a company producing wood pellets for UK power plants was fined $2.5m for exceeding volatile organic compounds pollution for a number of years.^[129] In some cases, large areas of natural forests have been logged illegally (e.g. in Romania^[130] and Siberia^[131]) and the remaining forest has been put on fire to cover up illegal operations.^[132]

The forest biomass debate

Smokestack emissions from forest biomass compared to coal

Smokestack emissions per produced energy unit depend on moisture content in the fuel, chemical differences between fuels and conversion efficiencies. Moisture content in wood pellets is usually below 10%, as defined in the ISO standard 17225-2:2014.^[133] The coal type anthracite typically contains below 15% moisture, while bituminous contains 2–15%, sub-bituminous 10–45%, and lignite 30–60%.^[134] The most common coal type in Europe is lignite.^[135]

Coal port in Russia.

When combusted in combustion facilities with the same heat-to-electricity conversion efficiency, oven dry wood emits slightly less CO₂ per unit of heat produced, compared to oven dry coal.^[fd] However, many biomass-only combustion facilities are relatively small and inefficient, compared to the typically much larger coal plants. Further, raw biomass (for instance wood chips) can have higher moisture content than coal (especially if the coal has been dried). When this is the case, more of the wood's inherent energy must be spent solely on evaporating moisture, compared to the drier coal, which means that the amount of CO₂ emitted per unit produced heat will be higher.

Some researchers (e.g. the research group Chatham House) therefore argue that "[...] the use of woody biomass for energy will release higher levels of emissions than coal [...]."^[136] Likewise, the Manomet Center for Conservation Sciences argues that for smaller scale utilities, with 32% conversion efficiency for coal, and 20-25% for biomass, coal emissions are 31% less than emissions from wood chips. The assumed moisture content for wood chips is 45%. Assumed moisture content for coal is not provided.^[137]

Hektor et al. argue that the moisture problem is efficiently mitigated by modern combustion facilities.^[el] Cowie et al. argue that stack emissions for biomass and coal is the same when biomass is co-fired with coal in large power plants, and that torrefied biomass has a higher conversion efficiency than low-rank coals.^[fe] Wood pellets combusted at Drax in the UK (the world's largest biomass power plant) have 7% moisture, and when combusted the plant has a higher conversion efficiency than what is average for coal plants in the UK (38.6 vs. 35.9%). Stack emissions were 2% higher than the UK average for coal in 2015.^[ff] When emissions from the wood pellet supply chain is included (the pellets are shipped to the UK from the USA), Drax claims that emissions are reduced by over 80%, compared to coal.^[fg]

Wood pellet mill in Germany.

The bioenergy consultant group FutureMetrics argue that wood pellets with 6% moisture content emits 22% less CO₂ for the same amount of produced heat, compared to sub-bituminous coal with 15% moisture, when both fuels are combusted in facilities with the same conversion efficiency (here 37%).^[fh] Likewise, they state that "[...] dried wood at MC's [moisture content] below 20% have the same or less CO₂ emission per MMBTU [million British thermal units] as most coal. Wood pellets at under 10% MC result in less CO₂ emission than any coal under otherwise equal circumstances."^[138] However, when raw wood chips are used instead (45% moisture content), this wood biomass emits 9% more CO₂ than coal in general, for the same amount of produced heat.^[138]

Taking into account the existing, small-scale biomass combustion facilities, IEA Bioenergy estimate that forest biomass on average produce 10% more CO₂ than coal,^[139] and the IPCC estimates 16%.^[fi] Both research groups argue however that focusing on gross emissions misses the point, what counts is the net climate effect from emissions and absorption, taken together.^[fj]^[fk] IEA Bioenergy concludes that the additional CO₂ from biomass "[...] is irrelevant if the biomass is derived from sustainably managed forests."^[139]

Sustainable forestry and forest protection

In the context of CO₂ mitigation, the key measure regarding forest sustainability is the size of the forest carbon stock: "The core objective of all sustainable management programmes in production forests is to achieve a long-term balance between harvesting and regrowth. [...] [T]he practical effect of maintaining a balance between harvesting and regrowth is to keep long-term carbon stocks stable in managed forests."^[140] The IPCC defines sustainable forestry in a similar manner, while including ecological, economic and social criteria.^[fl]

Globally, the forest carbon stock has decreased 0.9% and tree cover 4.2% between 1990 and 2020, according to FAO.^[141] IPCC states that there is disagreement about whether the global forest is shrinking or not, and quote research indicating that tree cover has increased 7.1% between 1982 and 2016.^[fm] The IPCC writes: "While above-ground biomass carbon stocks are estimated to be declining in the tropics, they are increasing globally due to increasing stocks in temperate and boreal forests [...]."^[142]

Some researchers seem to want more than "just" sustainably managed forests; they want to realize the forests full carbon storage potential. For instance the EASAC writes: "There is a real danger that present policy over-emphasises the use of forests in energy production instead of increasing forest stocks for carbon storage."^[143] Further, they argue that "[...] it is the older, longer-rotation forests and protected old-growth forests that exhibit the highest carbon stocks."^[144] Chatham House argues that old trees have a very high carbon absorption rate, and that felling old trees means that this large potential for future carbon absorption is lost. In addition, they argue that there is a loss of soil carbon due to the harvest operations.^[145]

In Europe, 25% of all forests are protected,^[146] including 89% of the primary/old-growth forests.^[147] The new version of the Renewable Energy Directive (RED II), introduced in 2021, extended its sustainability criteria from liquid biofuel production to also include solid (and gaseous biofuels), which is more likely to be produced from forest biomass.^[fn]

Old-growth spruce forest in France.

Stephenson et al. agree that old trees absorb more CO₂ than young trees, because of the larger leaf area in full grown trees.^[148] However, the old forest (as a whole) will eventually stop absorbing CO₂ because CO₂ emissions from dead trees cancel out the remaining living trees' CO₂ absorption.^[fo] The old forest (or forest stands) are also vulnerable for natural disturbances that produces CO₂. The IPCC writes: "When vegetation matures or when vegetation and soil carbon reservoirs reach saturation, the annual removal of CO₂ from the atmosphere declines towards zero, while carbon stocks can be maintained (high confidence). However, accumulated carbon in vegetation and soils is at risk from future loss (or sink reversal) triggered by disturbances such as flood, drought, fire, or pest outbreaks, or future poor management (high confidence)."^[149] Summing up, IPCC writes that "[...] landscapes with older forests have accumulated more carbon but their sink strength is diminishing, while landscapes with younger forests contain less carbon but they are removing CO₂ from the atmosphere at a much higher rate [...]."^[150]

EU's Joint Research Centre write that the measured effects of harvest and replanting on soil carbon is "[...] slight in the short term, with carbon decreases concentrated in the forest floor and near the soil surface and carbon increases occurring in the deep mineral soil layers."^[151] The JRC also argues that "[w]hole-tree harvesting for biomass production has little long-term effect on soil carbon stocks if surface soil layers containing organic material (O horizon) are left on site, nutrients are managed, and the site is allowed to regenerate [...]."^[151] The IPCC state that the current scientific basis is not sufficient to provide soil carbon emission factors.^[fp]

Plantation forest in Hawaii.

The IPCC argues that the net climate effect from conversion of unmanaged to managed forest can be positive or negative, depending on circumstances. The carbon stock is reduced, but since managed forests grow faster than unmanaged forests, more carbon is absorbed. Positive climate effects are produced if the harvested biomass is used efficiently.^[fq] There is a tradeoff between the benefits of having a maximized forest carbon stock, not absorbing any more carbon, and the benefits of having a portion of that carbon stock "unlocked", and instead working as a renewable fossil fuel replacement tool, for instance in sectors which are difficult or expensive to decarbonize.^[fr]^[fs] When put to work, this carbon moves from the forest carbon pool into forest products and energy carriers, then via combustion into the atmosphere, and then back to the forest via photosynthesis. For each roundtrip, it displaces more and more of the fossil fuel carbon that is normally used in heat production, industry production and electricity production. After some roundtrips, the amount of displaced carbon far exceeds the amount of locked-away carbon: "The biomass produced cumulatively across subsequent rotations can far exceed the biomass produced in the no-bioenergy scenario, thus constituting ‘additional biomass', delivering cumulative net GHG savings that exceed the GHG cost of forest carbon stock reduction [...]."^[152] Said differently: "If the forest is allowed to continue to grow, biomass energy will be replaced with fossil fuels and wood products will be replaced with alternate materials."^[153] Miner argue that "in the long term, using sustainably produced forest biomass as a substitute for carbon-intensive products and fossil fuels provides greater permanent reductions in atmospheric CO₂ than preservation does."^[154]

Summing up the above, IEA Bioenergy writes: "As the IPCC has pointed out in several reports, forests managed for producing sawn timber, bioenergy and other wood products can make a greater contribution to climate change mitigation than forests managed for conservation alone, for three reasons. First, the sink strength diminishes as conservation forests approach maturity. Second, wood products displace GHG-intensive materials and fossil fuels. Third, carbon in forests is vulnerable to loss through natural events such as insect infestations or wildfires, as recently seen in many parts of the world including Australia and California. Managing forests can help to increase the total amount of carbon sequestered in the forest and wood products carbon pools, reduce the risk of loss of sequestered carbon, and reduce fossil fuel use."^[155]

Forest area increase in the EU 1990–2020.^[156]

The IPCC argues that sustainable forest management "[...] aimed at providing timber, fibre, biomass and non-timber resources can provide long-term livelihood for communities, reduce the risk of forest conversion to non-forest uses (settlement, crops, etc.), and maintain land productivity, thus reducing the risks of land degradation [...]."^[150] The connection between economic opportunities in forestry and increased forest size is emphasized by other researchers as well.^[ft]^[fu] However, Cowie et al. argue that in some situations, "[...] such as high latitudes where forest productivity is very low, greater abatement may result from retaining and enhancing forest carbon stocks than harvesting forests for wood products including bioenergy, especially if the GHG savings from bioenergy use are small [...]."^[152] They also argue that forests that produce income for private forest owners are unlikely to be protected. When forest products are in demand and forests therefore are managed for timber production, the most realistic no-bioenergy scenario is not forest protection but continued timber production without residues collection and utilization. In this case, the residues will instead decay on their own or be incinerated, which in both cases produce emissions without any fossil fuel displacement effect. The most realistic no-bioenergy scenarios in case of low demand for forest products is land use change to natural forests (with increased risk for wildfires), or clear-cutting to prepare for agriculture or urbanization.^[fv]

Possibly strengthening the arguments above, data from FAO show that most wood pellets are produced in regions dominated by sustainably managed forests. Europe (including Russia) produced 54% of the world's wood pellets in 2019, and the forest carbon stock in this area increased from 158.7 to 172.4 Gt between 1990 and 2020. In the EU, above-ground forest biomass increases with 1.3% per year on average, however the increase is slowing down because the forests are maturing.^[157] In 2020, the forested area covered 39.8% of EU's total land area.^[156] Likewise, North America produced 29% of the worlds pellets in 2019, while forest carbon stock increased from 136.6 to 140 Gt in the same period. Carbon stock decreased from 94.3 to 80.9 Gt in Africa, 45.8 to 41.5 Gt in South and Southeast Asia combined, 33.4 to 33.1 Gt in Oceania,^[fw] 5 to 4.1 Gt in Central America, and from 161.8 to 144.8 Gt in South America. Wood pellet production in these areas combined was 13.2% in 2019.^[fx] However, Chatham House argues that "[f]orest carbon stock levels may stay the same or increase for reasons entirely unconnected with use for energy."^[158]

Short-term urgency

Some research groups still argue that even if the European and North American forest carbon stock is increasing, it simply takes too long for harvested trees to grow back. EASAC for instance argues that since the world is on track to pass by the agreed target of 1.5 degrees temperature increase already in a decade or so, bioenergy from sources with high payback and parity times make it harder to achieve that goal. They therefore suggest that the EU should adjust its sustainability criteria so that only renewable energy with carbon payback times of less than 10 years is defined as sustainable,^[fy] for instance wind, solar, biomass from wood residues and tree thinnings that would otherwise be burnt or decompose relatively fast, and biomass from short rotation coppicing (SRC).^[159]

Cowie et al. argue that "[...] a 10-year payback time as a criterion for identifying suitable mitigation options is inconsistent with the long-term temperature goal of the Paris Agreement, which requires that a balance between emission and removals is reached in the second half of this century [...]."^[fz] They also argue that emissions from bioenergy is fundamentally different from emissions from fossil fuels, since the former are circular and the latter linear.^[ga] Biomass is compatible with the current energy infrastructure, so it works today, while proposed alternatives with low emissions "[...] may be restricted by immature development, high cost or dependence on new infrastructure."^[gb]

Chatham House argues that there could be tipping points along the temperature scale where warming accelerates.^[gc] Cowie et al. argues that tipping points are an uncertainty, but a global tipping point seems unlikely "[...] if warming does not exceed 2°C [...]."^[gd] The IPCC argue that while there are "[...] arguments for the existence of regional tipping points, most notably in the Arctic [...]", there is "[...] no evidence for global-scale tipping points in any of the most comprehensive models evaluated to date in studies of climate evolution in the 21st century."^[160]

An important presupposition for the "tree regrowth is too slow" argument is the view that carbon accounting should start when trees from particular, harvested forest stands are combusted, and not when the trees in those stands start to grow (see Temporal system boundaries, above.)^[ge] It is within this frame of thought it becomes possible to argue that the combustion event creates a carbon debt that has to be repaid through regrowth of the harvested stands.^[gf]

When instead assuming that carbon accounting should start when the trees start to grow, it becomes impossible to argue that the emitted carbon constitutes debt. FutureMetrics for instance argue that the harvested carbon is not a debt but "[...] a benefit that was earned by 30 years of management and growth [...]."^[161] Likewise, Lamers & Junginger argue that owners of existing intensively managed, even-aged forests probably will consider the plantation establishment year as the logical start year for carbon accounting, and that harvesting redeems a carbon credit rather than creating a new debt. However, from a policy maker's perspective, [...] the main question is rather whether he/she should incentivize harvest for bioenergy or not."^[162] In other words, "[...] what is important to climate policy is understanding the difference in future atmospheric GHG levels, with and without switching to woody biomass energy. Prior growth of the forest is irrelevant to the policy question [...]."^[163] If this line of reasoning later is applied to new forest plantations planted on "empty" land areas as well (for instance agricultural or marginal lands), the onset of carbon accounting will shift from the planting event to the harvest event, for instance after the second rotation.

As mentioned in Spatial system boundaries above, some researchers limit their carbon accounting to particular forest stands, ignoring the carbon absorption that takes place in the rest of the forest.^[gg] Other researchers include the whole forest landscape when doing their carbon accounting. FutureMetrics for instance argues that the whole forest continually absorbs CO₂ and therefore immediately compensates for the relatively small amounts of biomass that is combusted in biomass plants from day to day.^[gh] Likewise, IEA Bioenergy criticizes EASAC for ignoring the carbon absorption that is happening in the forest landscape, noting that there is no net loss of carbon if the annual harvest is smaller than the forest's annual growth.^[gi]

IPCC argue along similar lines: "While individual stands in a forest may be either sources or sinks, the forest carbon balance is determined by the sum of the net balance of all stands."^[164] IPCC also state that the only universally applicable approach to carbon accounting is the one that accounts for both carbon emissions and carbon removals (absorption) for managed lands (e.g. forest landscapes.)^[gj] When the total is calculated, natural disturbances like fires and insect infestations are subtracted, and what remains is the human influence.^[gk]

Roundwood and residues

Researchers also discuss the use of roundwood vs. logging residues. Roundwood is defined by the EU's Joint Research Centre as all woody material removed from the forest, and logging residues is the parts that would most likely remain in the forest in the case of no demand from bioenergy. 20% of the felled biomass is currently left in the forest as logging residues.^[gl] Residues include tree tops, branches and stumps, but also pre-commercial thinnings (small, thin, young trees cleared away for increased productivity of the whole forest stand), salvage loggings and trees cleared away for fire hazard control.^[aa] Stemwood is a type of roundwood; according to the JRC's definition the stem of the tree is cut at a height of 15 cm above ground, and extends in a straight manner up to a point where the diameter of the stem should be minimum 9 cm. See footnote for full definitions of roundwood, stemwood, fuelwood, salvage loggings, pulpwood and sawnwood.^[gm] In general, residues and cascaded wood (wood products that are combusted for energy at the end of their service life) is seen as maximizing "the positive climate impact of bioenergy".^[gn] In Europe, approximately 20% stemwood is used for bioenergy, with the rest from logging residues, processing residues and post consumer wood. At least half of the stemwood is sourced from short rotation coppice forests, which have low payback/parity times and provides ecosystem services.^[go]

Sankey diagram that shows the flow of biomass from forest to wood products, paper and energy in Sweden.^[165]

Chatham House argue that it would be better if some of the biomass defined as roundwood (most notably stems) was not harvested and used for wood pellets, as this would increase the growing carbon stock in the forest.^[166] They also argue that "[...] trees that would not qualify as high-quality sawtimber could nevertheless be used for pulp, panels or laminated products."^[167] In other words, it would be better if this low-value biomass was used as raw material for other products than for wood pellets, since carbon is stored for a longer period of time in the former case. Chatham House also argues that all available sawmill residue is already being used for pellet production, so there is no room for expansion. For the bioenergy sector to significantly expand in the future, more of the harvested pulpwood must go to pellet mills.^[166]

Cowie et al. argue that approximately 20% "[...] roundwood (also referred to as stemwood), such as small stems from forest thinning [...]" is used for wood pellets in the USA. However, the use of stemwood from short-rotation forests have short parity times, and in long-rotation forests, the stemwood used for wood pellets usually consists of by-products from sawnwood production (thinnings or irregular/bent/damaged stem sections from larger trees.) Sawnwood production provides over 90% of foresters income and is the main reason forestry exist.^[cy]^[gp] Without a market for the low-quality stem sections or thinnings, they would have been left in the forest to decay, or been incinerated at roadside. Cowie et al. also argue that using thinnings for bioenergy strengthens the carbon displacement effect of harvested wood products, since the thinning practice help produce more sawnwood.^[gq]

Likewise, FutureMetrics argues that it makes no sense for foresters to sell sawlog-quality roundwood to pellet mills, since they get a lot more money for this part of the tree from sawmills. Foresters make 80-90% of their income from sawlog-quality roundwood and only 10-15% from pulpwood, defined as a.) the upper part of the stem that is too thin or too bent to be used for sawnwood production, plus branches, and b.) tree thinnings. This low-value biomass is mainly sold to pulp mills for paper production, but in some cases also to pellet mills for pellet production.^[168] Pellets are typically made from sawmill residues in areas where there are sawmills, but also from pulpwood in areas without sawmills.^[gr] Lamers & Junginger argue that the "[...] higher economic value for timber and cellulose [pulp] products makes large-scale use of whole-trees for energy purposes highly unlikely wherever there is regional competition for the fiber."^[gs] According to EU's Joint Research Centre, both the bioenergy sector, the wood panel sector and the pulp sector "[...] are all dependant on the demand for sawnwood, and they compete for the same feedstocks."^[gt]

Short-term vs long-term climate benefits

According to Cowie et al., "[...] the perceived attractiveness of specific forest bioenergy options is influenced by the priority given to near-term versus longer term climate objectives."^[169] IPCC for instance states that forest carbon emission avoidance strategies always give a short-term mitigation benefit, but argue that the long-term benefits from sustainable forestry activities are more important:

Relative to a baseline, the largest short-term gains are always achieved through mitigation activities aimed at emission avoidance [...]. But once an emission has been avoided, carbon stocks on that forest will merely be maintained or increased slightly. [...] In the long term, sustainable forest management strategy aimed at maintaining or increasing forest carbon stocks, while producing an annual yield of timber, fibre, or energy from the forest, will generate the largest sustained mitigation benefit.^[164]

Similarly, addressing the issue of climate consequences for modern bioenergy in general, the IPCC states: "Life-cycle GHG emissions of modern bioenergy alternatives are usually lower than those for fossil fuels [...]."^[170] Consequently, most of IPCC's GHG mitigation pathways include substantial deployment of bioenergy technologies.^[4] Limited or no bioenergy pathways leads to increased climate change or shifting bioenergy's mitigation load to other sectors.^[i] In addition, mitigation cost increases.^[gu]

IEA Bioenergy argue that an exclusive focus on the short-term make it harder to achieve efficient carbon mitigation in the long term, and compare investments in new bioenergy technologies with investments in other renewable energy technologies that only provide emission reductions after 2030, for instance the scaling-up of battery manufacturing or the development of rail infrastructure.^[gv] The National Association of University Forest Resources Programs recommends a time horizon of 100 years in order to produce a realistic assessment of cumulative emissions.^[gw]

By country

References

Quotes and comments

^ "Biofuels are transportation fuels such as ethanol and biodiesel that are made from biomass materials." EIA 2021b.
^ In EU legislation, biofuel is defined as: "Liquid or gaseous fuel for transport produced from biomass." See European Commission 2018a.
^ "Solid biofuels cover organic, non-fossil material of biological origin which may be used as fuel for heat and electricity production. [...] Primary solid biofuels are defined as any plant matter used directly as fuel or converted into other forms before combustion. This covers a multitude of woody materials generated by industrial process or provided directly by forestry and agriculture (firewood, wood chips, bark, sawdust, shavings, chips, sulphite lye also known as black liquor, animal materials/wastes and other solid biofuels). This category excludes charcoal. Wood pellets are agglomerates produced either directly by compression or by the addition of a binder in a proportion not exceeding 3% by weight. Such pellets are cylindrical, with a diameter not exceeding 25 mm and a length not exceeding 100 mm. The term ‘other agglomerates' is the term used for agglomerates that are not pellets, such as briquettes or log agglomerates. Wood pellets and other agglomerates are often reported jointly, with other agglomerates being usually a minor part. Black liquor is a by-product from chemical and semi-chemical wood pulp industry." Camia et al. 2021, pp. 20–21.
^ In 2020, the world produced a total of 24.6 EJ of electrical energy from all renewables except bioenergy. The individual contributions consists of 15.5 EJ from hydro, 5.8 EJ from wind, 3 EJ from solar and 0.3 EJ from geothermal (all values converted from TWh with IEA's unit converter.)
^ In 2020, 9.5 EJ of heat energy for industrial applications was consumed, and 5 EJ of heat for buildings. 3.7 EJ of liquid fuels for transportation was produced (ethanol 2.2 EJ, biodiesel 1.5 EJ), and 2.2 EJ in the form of electricity.
^ ^a ^b "The forestry sector is the largest contributor to the bioenergy mix globally. Forestry products including charcoal, fuelwood, pellets and wood chips account for more than 85% of all the biomass used for energy purposes. One of the primary products from forests that are used for bioenergy production is woodfuel. Most of the woodfuel is used for traditional cooking and heating in developing countries in Asia and Africa. Globally, 1.9 billion m³ of woodfuel was used for energy purposes." WBA 2019, p. 3. In the EU, 60% of all renewable energy comes from biomass. 75% of all biomass is used in the heating and cooling sector. See JRC 2019, p. 1.
^ "The International Energy Agency (IEA, 2012) defines traditional use of biomass as: '…the use of wood, charcoal, agricultural residues and animal dung for cooking and heating in the residential sector' and notes that 'it tends to have very low conversion efficiency (10% to 20%) and often relies on unsustainable biomass supply.'" IRENA 2014, p. 7.
^ "Bioenergy has a significant greenhouse gas (GHG) mitigation potential, provided that the resources are developed sustainably and that efficient bioenergy systems are used. Certain current systems and key future options including perennial cropping systems, use of biomass residues and wastes and advanced conversion systems are able to deliver 80 to 90% emission reductions compared to the fossil energy baseline. However, land use conversion and forest management that lead to a loss of carbon stocks (direct) in addition to indirect land use change (d+iLUC) effects can lessen, and in some cases more than neutralize, the net positive GHG mitigation impacts." IPCC 2012, p. 214.
^ ^a ^b "For example, limiting deployment of a mitigation response option will either result in increased climate change or additional mitigation in other sectors. A number of studies have examined limiting bioenergy and BECCS. Some such studies show increased emissions (Reilly et al. 2012). Other studies meet the same climate goal, but reduce emissions elsewhere via reduced energy demand (Grubler et al. 2018; Van Vuuren et al. 2018), increased fossil carbon capture and storage (CCS), nuclear energy, energy efficiency and/or renewable energy (Van Vuuren et al. 2018; Rose et al. 2014; Calvin et al. 2014; Van Vuuren et al. 2017b), dietary change (Van Vuuren et al. 2018), reduced non-CO₂ emissions (Van Vuuren et al. 2018), or lower population (Van Vuuren et al. 2018)." IPCC 2019e, p. 637.
^ "Bioenergy is a versatile renewable energy source that can be used in all sectors, and it can often make use of existing transmission and distribution systems and end-user equipment. But there are constraints on expanding the supply of bioenergy, and possible trade-offs with sustainable development goals, including avoiding conflicts at local level with other uses of land, notably for food production and biodiversity protection. To navigate these risks, our Roadmap to Net Zero by 2050 combined for the first time the IEA's global energy system modelling with the International Institute for Applied Systems Analysis (IIASA)'s Global Biosphere Management Model to provide insights on bioenergy's supply, land use and net emissions. We aimed to ensure that the peak level of total primary bioenergy demand – including losses from the conversion of biomass into useful fuels – falls within the lowest estimates of global sustainable bioenergy potential in 2050, namely around 100 exajoules (EJ). Bioenergy demand in our global net zero pathway – the Net-Zero Emissions by 2050 (NZE) Scenario – is lower than all comparable scenarios from the Intergovernmental Panel on Climate Change (IPCC) that are aligned with 1.5 °C. Those IPCC scenarios use a median of 200 EJ of bioenergy in 2050." IEA 2021a.
^ IEA estimates high levels of sustainable bioenergy in 2050, but set their NZE target to only 100 EJ for conservative reasons: "The level of bioenergy use in the NZE [Net Zero Emissions scenario] [...] in 2050 is around 100 EJ. The global sustainable bioenergy potential in 2050 has been assessed to be at least 100 EJ (Creutzig, 2015) and recent assessments estimate a potential between 150‐170 EJ when integrating relevant UN Sustainable Development Goals (Frank, 2021; IPCC, 2019; IPCC, 2014; Wu, 2019). However, there is a high degree of uncertainty over the precise levels of this potential. Using modelling developed in co‐operation with IIASA, here we examine the implications for achieving net‐zero CO₂ emissions by 2050 if the available levels of sustainable bioenergy were to be lower." IEA 2021b, p. 90.
^ "[...] CHP is an alternative for a wide range of production processes as the temperature and pressure of delivered steam can be adjusted to the specific requirements of industrial processes. There are other renewable alternatives for process heat generation (e.g., solar thermal, heat pumps or geothermal technologies). These are, however, either more costly or their deployment is constrained by the maximum temperature of the steam they can deliver. Therefore, biomass CHP plays a critical role for the manufacturing industry to raise its renewable energy share." IRENA 2014, p. 24.
^ "Biomass-based electricity can provide balancing power needed to maintain power stability and quality as the contribution from solar and wind power increases (Arasto et al., 2017; Lenzen et al., 2016; Li et al., 2020), complementing other balancing options such as battery storage, reservoir hydropower, grid extensions and demand-side management (Göransson & Johnsson, 2018). Beyond its value as a dispatchable resource for electricity generation, biomass is an important option for renewable heating in buildings and industrial processes. In 2019, bioenergy contributed almost 90% of renewable industrial heat consumption and two-thirds of the total modern renewable heating and cooling in buildings and industrial processes (IEA, 2020; IRENA/IEA/REN21, 2020). It is one of the options available to reduce emissions from heavy industries such as iron and steel production (Mandova et al., 2018, 2019) and cement production (IEA, 2018). Furthermore, carbon-based transportation fuels will remain important in the coming decades, as electrification of the transport sector will take time (IEA-AMF/IEA Bioenergy, 2020). Biofuels can contribute to reducing fossil fuel use and associated GHG emissions while there remain vehicles that use carbon-based fuels. In the longer term, biofuels will likely be used in sectors where the substitution of carbon-based fuels is difficult, such as long-distance aviation and marine transportation." Cowie et al. 2021, p. 1212.
^ "There are two distinct categories of renewable power generators: dispatchable and variable. Dispatchable renewable power generators control their output within a specific range, just like conventional fossil power plants. These generators can provide baseload power if needed. Reservoir hydropower plants, biomass (including biogas) power plants, geothermal power plants, and concentrated solar power (CSP) plants with thermal storage (such as in molten salt) all generate dispatchable renewable power. Integrating these renewable sources into power systems does not pose additional challenges. [...] The output of variable renewable energy (VRE) sources, such as wind and solar photovoltaic (PV) is much less controllable." IRENA 2015, p. 7.
^ According to IRENA, "[...] biomass energy comes from two different sources. One is primary bioenergy, which uses farmland or forests to produce biomass, the other is biomass residue, which is generated as a by-product of food or wood products throughout their supply-consumption chain." IRENA 2014, p. 5.
^ "There are two broad categories for woody biofuels: primary sources, such as logging residues, stumps, and low-quality logs, and secondary sources, i.e., by-products from the forest industries such as bark, saw dust, and black liqueur." Eggers et al. 2020, p. 2.
^ ^a ^b "Some fast growing tree species can be cut down to a low stump (or stool) when they are dormant in winter and go on to produce many new stems in the following growing season. This practice is well established in the UK and Europe, having been a traditional method of woodland management over several hundred years for a variety of purposes including charcoal, fencing and shipbuilding." Forest Research 2022c.
^ ^a ^b "While short rotation coppicing (SRC) cuts the tree back to a stool to promote the growth of multiple stems, on a regular cycle of roughly 2-4 years, it is also possible to practice something more closely akin to conventional forestry, though on a shorter timescale. Short rotation forestry (SRF) consists of planting a site and then felling the trees when they have reached a size of typically 10-20 cm diameter at breast height. Depending on tree species this usually takes between 8 and 20 years, and is therefore intermediate in timescale between SRC and conventional forestry. This has the effect of retaining the high productivity of a young plantation, but increasing the wood to bark ratio." Forest Research 2022a.
^ "Woody energy crops: Short‐rotation plantings of woody biomass for bioenergy production, such as coppiced willow and miscanthus." IEA 2021b, p. 212.
^ For instance is the promising crop Miscanthus × giganteus only grown on 30.000 hectares in the EU. See ETIP Bioenergy 2021. 30.000 hectares produces approximately 0.01 EJ annually, given the EU average peak yield of 22 tonnes dry matter per hectare per year (approximately 15 tonnes during spring harvest). See Anderson et al. 2014, p. 79. The energy content in miscanthus biomass is 18 GJ/t. Ghose 2011, p. 263.
^ "We will establish the amount of land that could be used in the UK for perennial energy crop production and for short rotation forestry (SRF). Existing biomass support schemes (Renewables Obligation, Contracts for Difference, RHI & RTFO) already support the use of perennial energy crops such as short rotation coppice and Miscanthus grown specifically for bioenergy purposes and as a material. However, only a small land area (~10,000 hectares) is cultivated with perennial energy crops in the UK at present, and this is mainly used for heat and electricity generation. Currently, there is little to no use of perennial energy crops for low carbon fuels supported under the RTFO due to a lack of commercial-scale processing capacities to convert these resources cost-efficiently into fuel. [...] The CCC's 6th Carbon Budget report highlighted the significant potential for perennial energy crops and SRF to contribute towards our carbon budget targets by increasing soil and biomass carbon stocks while also delivering other ecosystem benefits. In their balanced pathway, the CCC suggests that up to 708,000 hectares of land could be dedicated to energy crop production, which has led to an increased interest in the role of perennial energy crops and SRF as biomass feedstocks to deliver GHG savings in the land use and energy sectors. The Defra land use net zero programme, which is currently building a spatial understanding of the land use trade-offs across a number of policy areas, will help determine the potential scale of future availability of domestically grown biomass and their potential for delivering GHG savings in a landscape where land use change will need to be optimised for multiple benefits. This programme will inform our understanding and evidence on the availability and mix of biomass feedstocks for uses across sectors." Department for Business, Energy & Industrial Strategy 2021, pp. 15–16.
^ Brauch et al. write that in theory "[...] energy farming on current agricultural (arable and pasture) land could, with projected technological progress, contribute over 800 EJ, without jeopardizing the future world's food supply." The authors also write that "[...] a significant part of the technical potential (around 200 EJ in 2050) for biomass production may be developed at low production costs in the range of US$2/GJ [...] assuming this land is used for perennial crops. Another 100 EJ of biomass could be produced with lower productivity and higher costs at marginal and degraded lands." Brauch et al. 2009, p. 384.
^ "According to FAO, the issue is not the volume of available land, which is enough to supply growing demand, but securing the substantial financial investments to actually deploy these potential areas, plus the disparate distribution of land resource by country. For example, 60% of the world's unexploited prime land is held by only thirteen countries. [...] These thirteen countries are Madagascar, Mozambique, Canada, Angola, Kazakhstan, the Democratic Republic of the Congo, China, the Sudan, Australia, Argentina, Russia, the US and Brazil (in ascending order)." IRENA 2014, p. 40.
^ "Estimates of marginal/degraded lands currently considered available for bioenergy range from 3.2–14.0 Mkm2, depending on the adopted sustainability criteria, land class definitions, soil conditions, land mapping method and environmental and economic considerations (Campbell et al. 2008; Cai et al. 2011; Lewis and Kelly 2014)." IPCC 2019c, p. 193.
^ Given the EU average peak yield of 22 tonnes dry matter per hectare per year (approximately 15 tonnes during spring harvest). See Anderson et al. 2014, p. 79. The energy content in miscanthus biomass is 18 GJ/t, see Ghose 2011, p. 263. The calculation goes like this: 44990100 hectares times 15 tonnes per hectare times 18 GJ per tonne is 12147327000 GJ, or approximately 12 EJ.
^ "Plants convert CO₂ from the atmosphere into biomass. Carbon stored in biomass is called biogenic carbon. Some of this carbon stays above ground and some in the ground. When plants die, decomposition starts. As plant material decays, the stored carbon is released as CO₂ back into the atmosphere." IRENA 2014, p. 45.
^ ^a ^b "Wood from thinnings may, to some extent, be assimilated to harvest residues (especially pre-commercial thinnings). If not collected for bioenergy it would be left in the forest to decay, or combusted at roadside. On the other hand, depending on the wood quality, the use of thinnings wood for bioenergy may compete with other uses, such as pulp and paper or engineered wood. Salvage loggings can also be assimilated to harvest residues. Damaged, dying or dead trees affected by injurious agents, such as wind or ice storms or the spread of invasive epidemic forest pathogens, insects and diseases would remain in the forest and decay or combusted at roadside. Wood removed for prescribed fire hazard control as well can be considered residual wood." JRC 2014, pp. 42–43, table 3.
^ "This study estimated quantities of logging residues that can physically be recovered from harvest sites and utilized for electricity production in the US South. [...] Although almost all physically available logging residues could be recovered with a relatively short hauling distance, a mail survey indicated that only 4 percent of mills utilized this feedstock." Pokharel et al. 2019, p. 543.
^ "Currently, logging residue extraction, i.e., the harvest of tops and branches left during final felling, occurs on less than 20% of the harvested area in northern Sweden, and about 60% in southern Sweden. Stump harvest occurs to a limited extent today, but is expected to increase to about 5%–10% of the annual clear-felled area in the coming years. Similarly, logging residues constitute the main primary source of woody biofuels in most countries, but in the near future stumps and roundwood may play a more prominent role. Biofuel harvest from early thinnings in dense young forests are currently done to insignificant levels, but will increase, as for stumps, if prize levels rise. Therefore, there is considerable potential for increased extraction rates of primary woody biofuels, especially in northern Sweden, where current extraction rates are relatively low due to longer transport distances and lower harvestable volume per hectare compared to southern Sweden. The situation is similar in other European countries, with large un-used potentials for woody biomass for energy use." Eggers et al. 2020, p. 2.
^ "Logging residues are increasingly being extracted for bioenergy purposes." Dahlberg et al. 2011, p. 1220
^ van den Born et al. distinguish between logging residues in general and dead wood, with the logging residues potential at 14 EJ, and the dead wood potential at 1 EJ annually. For the logging residues potential, see van den Born et al. 2014, p. 20, table 4.2. Regarding the dead wood potential, the authors write: "A biomass pool is dead wood that remains in the forest, either standing or lying, and is transferred to the soil. It is often too costly to harvest dead wood. Besides, it is useful in increasing biodiversity (the proportion of dead wood is a sustainability criteria, (EEA, 2012). The global quantity of dead wood is estimated roughly at 67 Gt of biomass, which is about 11% of the total biomass (FAO, 2010), and about 20 times the annual wood harvest. [...] Dead wood: the global stock of dead wood is estimated at about 1200 EJ of biomass (FAO, 2010). This large pool has built up over a long period of time and in the entire forest area. Assuming an average rotation of 50 to 100 years, this implies a biomass pool of 10 to 20 EJ yr-1 [EJ per year]. When primary forests are excluded because they have not been used (based on FAO, 2010), about 7 to 14 EJ yr-1 of dead biomass remains. Forests with large quantities of dead wood are located in Russia and in parts of Africa. A limitation to the use of salvaged wood is the high costs of access and transport (Niquidet et al., 2012). A conservative estimate of accessible planted forests reduces the pool of available dead wood to about 2 EJ yr-1 biomass (Table 4.2). When an additional assumption is made that half of the dead wood needs to remain in forests to maintain biodiversity (Verkerk et al, 2012), the estimate is about 1 EJ yr-1 biomass available annually for energy production." (p. 15, 19-20)
^ "Biomass for bioenergy is usually a by-product of sawlog and pulpwood production for material applications (Dale et al., 2017; Ghaffariyan et al., 2017; Spinelli et al., 2019; Figure 1). Logs that meet quality requirements are used to produce high-value products such as sawnwood and engineered wood products such as cross laminated timber, which can substitute for more carbon-intensive building materials such as concrete, steel and aluminium (Leskinen et al., 2018). Residues from forestry operations (tops, branches, irregular and damaged stem sections, thinnings) and wood processing residues (e.g. sawdust, bark, black liquor) are used for bioenergy (Kittler et al., 2020), including to provide process heat in the forest industry (Hassan et al., 2019). These biomass sources have high likelihood of reducing net GHG emissions when substituting fossil fuels (Hanssen et al., 2017; Matthews et al., 2018), and their use for bioenergy enhances the climate change mitigation value of forests managed for wood production (Cintas, Berndes, Hansson, et al., 2017; Gustavsson et al., 2015, 2021; Schulze et al., 2020; Ximenes et al., 2012). Part of the forest biomass used for bioenergy consists roundwood (also referred to as stemwood), such as small stems from forest thinning. For example, roundwood was estimated to contribute around 20% of the feedstock used for densified wood pellets in the United States in 2018 (US EIA, 2019)." Cowie et al. 2021, pp. 1215–1216.
^ "The most crucial feedstock for the wood pellet sector is currently sawmill residues (85% of the mix), roundwood (13%), and recovered wood (2%). This mix is likely to change in the coming years with the forecasted expansion of the wood pellet industry. [...] Experience from North America shows that it is possible to use more forest residues as fiber furnish. Although it yields pellets with higher ash content, it is often a lower-cost raw material than, for example, roundwood and wood chips. This practice is increasingly common in both the US South (mainly for pellets exported to Europe) and Canada (mainly exported to Europe and Asia). In Western Canada, the sawmill residue share of the total feedstock has fallen from 97% in 2010 to 72% in 2020, with the balance being forest residues and roundwood." Wood Resources International 2022.
^ Recalculated from a total production of 43678925 tonnes wood pellets (FAO 2020), with 17 GJ/t energy content.
^ Recalculated from a total production of 265212933 m³ wood chips (FAO 2020), with 3.1 GJ/m³ energy content.
^ "In 2017, 55.6 EJ of biomass was utilized for energy purposes [...]. One of the most promising sectors for growth in bioenergy production is in the form of residues from agriculture sector. Currently, the sector contributes less than 3% to the total bioenergy production." WBA 2019, p. 3.
^ The Netherlands Environmental Assessment Agency estimated in 2014 that the total amount of agricultural residues amounts to 78 EJ, with 51 EJ from straw alone (pp. 12-13, table 3.4). "The large production of rice and the relatively low residue flow to the soil makes rice residues the residue with the highest potential for bioenergy, followed by residues from oilcrops, cereals, corn and sugarcane."(p. 19) Because a certain amount should be left in the fields for soil quality purposes, the total amount of agricultural residues that can be sustainably harvested amounts to 24 EJ. van den Born et al. 2014, pp. 2–21.
^ "One of the most promising sectors for growth in bioenergy production is in the form of residues from agriculture sector. Currently, the sector contributes less than 3% to the total bioenergy production. Data shows that utilizing the residues from all major crops for energy can generate approx. 4.3 billion tonnes (low estimate) to 9.4 billion tonnes (high estimate) annually around the world. Utilizing standard energy conversion factors, the theoretical energy potential from residues can be in the range of 17.8 EJ to 82.3 EJ. The major contribution would be from cereals – mainly maize, rice and wheat." WBA 2019, p. 3.
^ "In reality, most residues are not utilised for energy because they are difficult to collect or used for specific purposes, such as land conservation, manure and straw incorporation in the field to maintain soil organic matter. This is accounted for in the residue recovery rates. The historical and projected annual crop production growth by region and the residue coefficients are provided in Annex A. About a quarter of the residue generated for each crop is assumed to be recoverable, reflecting an assessment that half the residue could be collected sustainably and half of that amount could be collected economically. After the recoverable fraction of residues is estimated, the amount of residue used for animal feed is calculated separately. This is deducted from the total residue volume." IRENA 2014, p. 9.
^ "At present, traditional methods of space heating and cooking, such as burning firewood, account for 35 EJ, or two-thirds of total biomass use. By 2030, this would give way to modern biomass consumption, including substantially larger shares for power and transport applications. Power and district heating would reach 36 EJ (one-third of total biomass use in 2030) and transport 31EJ (almost 29%), while heat for industry and buildings would reach up to 41 EJ, of which only 6 EJ would be from less sustainable traditional uses. While global biomass potential is sufficient to meet growing demand, different types of biomass resources are distributed unevenly. Global biomass supply potential in 2030 is estimated to range from 97 EJ to 147 EJ per year. Approximately 40% of this total would originate from agricultural residues and waste (37-66 EJ). The remaining supply potential is shared between energy crops (33-39 EJ) and forest products, including forest residues (24-43 EJ). In geographic terms, the largest supply potential — estimated at 43-77 EJ per year — exists in Asia and Europe. North and South America together account for another 45-55 EJ per year." IRENA 2021.
^ "Advanced (second and third generation) biofuels are biofuels produced from feedstock that do not compete directly with food and feed crops, such as wastes and agricultural residues (i.e. wheat straw, municipal waste), non-food crops (i.e. Miscanthus and short rotation coppice) and algae." European Commission 2018.
^ "Recent studies by Reza et al. and Smith et al. have reported of the fate of inorganics and heteroatoms during HTC [hydrothermal carbonisation] of Miscanthus and indicate significant removal of the alkali metals, potassium and sodium, along with chlorine. [...] Analysis of ash melting behaviour in Smith et al., showed a significant reduction in the slagging propensity of the resulting fuel, along with the fouling and corrosion risk combined. [...] Consequently HTC offers the potential to upgrade Miscanthus from a reasonably low value fuel into a high grade fuel, with a high calorific value, improved handling properties and favourable ash chemistry. [...] HTC at 250 °C can overcome slagging issues and increase the ash deformation temperature from 1040 °C to 1320 °C for early harvested Miscanthus. The chemistry also suggests a reduction in fouling and corrosion propensity for both 250 °C treated fuels." Smith et al. 2018, pp. 547, 556.
^ "The success of large-scale international bioenergy trade will require the transport of high density commodities at low costs. Transport costs can be decreased by introducing pre-treatment into the supply chain. Pre-treatment, including torrefaction, pelletisation and pyrolysis, increases energy density from 2-8 MJ/m³ of raw biomass up to 11-20 MJ/m³ for pre-treated biomass. By optimising the supply chain through incorporating pretreatment, logistics costs could be significantly reduced compared with the raw materials-based supply chain." IRENA 2014, p. 53.
^ "A critical factor in the use of forest biomass in energy provision is the ‘payback time', during which atmospheric concentrations of carbon dioxide (CO₂) will be increased as a result of using biomass. EASAC concludes that the European Commission should consider the extent to which large-scale forest biomass energy use is compatible with UNFCCC targets (of limiting warming to 1.5 °C above pre-industrial levels), and whether a maximum allowable payback period should be set in its sustainability criteria." EASAC 2017, p. 2.
^ "The UK's plan to burn more trees to generate “renewable” electricity has come under fire from green groups and sustainable investment campaigners over the controversial claim that biomass energy is carbon-neutral. A letter to the government signed by more than a dozen green groups including Greenpeace and Friends of the Earth warns ministers against relying too heavily on plans to capture carbon emissions to help tackle the climate crisis. The plans are being pioneered by Drax Group, which claims that burning wood pellets is carbon-neutral because trees absorb as much carbon dioxide when they grow as they emit when they are burnt. Capturing the carbon emissions from biomass power plants would then effectively create “negative carbon emissions”, according to Drax. The green groups have disputed these claims and warned that the plans “will be costly” and “will not deliver negative emissions” after accounting for the full carbon footprint of biomass in the power sector." Ambrose 2021.
^ "By definition, clear-cutting trees and combusting their carbon emits greenhouse gases that heat up the earth. But policymakers in the U.S. Congress and governments around the world have declared that no, burning wood for power isn't a climate threat—it's actually a green climate solution. [...] [T]he [...] basic argument is that the carbon released while trees are burning shouldn't count because it's eventually offset by the carbon absorbed while other trees are growing. That is also currently the official position of the U.S. government, along with many other governments around the world. In documentaries, lawsuits and the teenage activist Greta Thunberg's spirited Twitter feed, critics of the industry have suggested an alternative climate strategy: Let trees grow and absorb carbon, then don't burn them. [...] Cutting down a tree and burning it clearly releases more carbon than leaving the tree alone; replanting the tree can only pay back the carbon debt in the long run, and an even longer run if the replanted tree is eventually reharvested. But biomass defenders say that focusing on one tree or even one clear-cut is far too narrow a way to think about forest carbon, because as long as the carbon absorbed by forests equals the carbon released from forests, the climate doesn't care. [...] The industry's position is that wood pellets actually help expand forests, by making it more lucrative for the private landowners who control most U.S. forest land to stay in the forestry business. The opponents argue that what wood pellets make more lucrative is deforestation. [...] “We can't say, ‘Oh, we can sacrifice forest over here, because it's growing over there. We need to stop sacrificing forest.” Grunwald 2021.
^ The IEA defines carbon neutrality and carbon negativity like so: "Carbon neutrality, or 'net zero,' means that any CO₂ released into the atmosphere from human activity is balanced by an equivalent amount being removed. Becoming carbon negative requires a company, sector or country to remove more CO₂ from the atmosphere than it emits."IEA 2020.
^ "Schlamadinger & Marland describe how the atmospheric carbon pool changes depending on what is happening in other carbon pools (living biomass, soils and forest litter, wood and wood products, fossil fuels displaced by biomass fuels, fossil fuels used for forest management activities and for biomass conversion processes, and fossil fuels required to manufacture wood products or their substitutes.)" Schlamadinger & Marland 1996, p. 275. See also Sathre & O'Connor 2010, p. 104
^ Some researchers would like to move the counting to the combustion event, but Cowie et al. and others argue against this: "The UNFCCC reporting requirements specify that CO₂ emissions associated with biomass combustion are counted in the land use sector, that is, where the harvest takes place; they are therefore reported as zero in the energy sector to avoid double-counting (Goodwin et al., 2019). This reporting approach is accurate, has no gaps and does not assume that bioenergy is carbon neutral (Haberl at al., 2012; Marland, 2010), although it has sometimes been described as such (e.g. Norton et al., 2019; Searchinger et al., 2009). [...] While the UNFCCC reporting approach is theoretically sound, incomplete coverage of the Kyoto Protocol created a gap in accounting: if an Annex I party (i.e. country with a Kyoto Protocol commitment) imported forest biomass from a country with no Kyoto Protocol commitment, any associated stock change in the forest of the exporting country was not accounted. [...] Several authors (Brack, 2017; Hudiburg et al., 2019; Norton et al., 2019) propose changing the UNFCCC accounting rules by which biomass is treated as having zero emissions at the point of combustion. However, accounting for CO₂ emissions from bioenergy within the energy sector would require revision of the established GHG accounting framework to adjust the land sector values to remove the component related to biomass used for energy, to avoid double-counting of emissions, which would be very difficult to achieve, as explained by Camia et al. (2021). It would create a disincentive for countries to utilize biomass to displace fossil fuels, adversely affecting all types of bioenergy systems irrespective of their potential to provide climate benefits (Pingoud et al., 2010). Rather than changing the accounting convention solely for bioenergy, a flux-based ‘atmospheric flow approach' (Rüter et al., 2019) could potentially be applied to all wood products. However, if carbon fluxes from all wood products were to be reported at the time and place of emission, emissions due to forest harvest for export would not be reported by the country where the harvest takes place, thereby removing incentives for maintaining forest carbon stocks and potentially leading to deforestation because the country where the harvest takes place would report no emissions. Furthermore, reporting only at the time and place of emission would create a disincentive for use and trade in all sustainable wood products, including use for construction and bioenergy (Apps et al., 1997; Cowie et al., 2006; UNFCCC, 2003). [...] With respect to the treatment of bioenergy in UNFCCC reporting and accounting, we disagree with proposals to count emissions at the point of combustion, which could have adverse climate impacts. We recommend that complete and transparent reporting and accounting be applied consistently across the whole land sector, to ensure recognition of the interactions between terrestrial carbon stocks and biomass use for energy and other purposes, and to incentivize land use and management systems that deliver climate benefits." Cowie et al. 2021, pp. 1220–1222.
^ A graphical explanation of carbon payback and parity times, with carbon debt shown as a curve that moves along a time axis, is available here: EASAC 2017, p. 23.
^ "The potential carbon debt caused by harvest and the resulting time spans needed to reach pre-harvest carbon levels (payback) or those of a reference case (parity) have become important parameters for climate and bioenergy policy developments." Lamers & Junginger 2013, p. 373.
^ Lamers & Junginger state that the carbon debt "[...] can be indicated to the site itself (absolute) or against a baseline (relative)." The absolute carbon balance approach (payback time) is chosen to define the time until a site reaches its own pre-harvest carbon level, and the relative carbon balance approach (parity time) is chosen to define the time until an alternative land or biomass use scenario "[...] reaches the same carbon volume as its counterfactual (reference case)." The reference or alternative scenarios can be for example "[...] material use of biomass (e.g. pulp and paper), land protection (no harvest) or conversion to agriculture." According to the authors, "[t]his provides insight whether it is more beneficial from a net carbon perspective to keep biogenic carbon sequestered in plants (subjected to natural disturbances such as insects or wildfire) or use it for energy purposes." Lamers & Junginger 2013, p. 375.
^ EU's Joint Research Centre defines "counterfactual" like so: "The impacts of each bioenergy pathway are evaluated against a counterfactual, i.e. a reference use of the biomass or of the land (thus the results should be interpreted as conditional to the chosen reference)." Camia et al. 2021, p. 83.
^ "It is important to notice that the definition of the reference system (both the energy system and the counterfactual biomass use) is as important as the definition of the bioenergy systems since the stated goal of the study is to assess the mitigation potential of the new systems as compared to the reference one." Camia et al. 2018, p. 100.
^ ^a ^b "Critical methodology decisions include the definition of spatial and temporal system boundaries [...] and reference (counterfactual) scenarios [...]. Focus on stack emissions (Option 1) neglects the key differences between fossil and biogenic carbon [...]. Focus on the forest only (Option 2) captures the effects of biomass harvest on forest carbon stocks [...] but omits the climate benefits of displacing fossil fuels. Option 3, the biomass supply chain, overlooks the interactions between biomass and other forest products [...]. Option 4 covers the whole bioeconomy, that is, the forest, the biomass supply chain and all bio-based products from managed forests, and thus provides a more complete assessment of the climate effects of forest bioenergy. In order to quantify the net climate effect of forest bioenergy, assessments should take a whole systems perspective. While this increases the complexity and uncertainty of the assessments, it provides a sound basis for robust decision-making. Biomass for bioenergy should be considered as one component of the bioeconomy (Option 4 [...]). Studies should therefore assess the effects of increasing biomass demand for bioenergy on carbon stocks of the whole forest, and also include the broader indirect impacts on emissions (potentially positive or negative) due to policy- and market-driven influences on land use, use of wood products and GHG-intensive construction materials, and fossil fuel use, outside the bioenergy supply chain. The bioenergy system should be compared with a realistic counterfactual(s) that includes the reference land use and energy systems [...]. This approach is consistent with consequential LCA [...]. The temporal boundary should recognize: forest carbon dynamics, for example, modelling over several rotations; the trajectory for energy system transition; and short- and long-term climate objectives. Matthews et al. (2018) suggest criteria that could be used to identify woody biomass with greater climate benefits when assessed from a full life cycle, whole system perspective." Cowie et al. 2021, pp. 1213, 1219–1220.
^ A simplified curve, complete with carbon payback and parity times, is available here: EASAC 2017, p. 23.
^ "The GWP is a measure of the effect of the pulse emission of a unit (mass) of a certain gas over its lifetime on the radiative properties of the atmosphere for a certain period of time. In the methodology designed by the IPCC [IPCC 2006], the GWP of CO₂- is taken as the reference value and assigned the value of 1. The reasoning of the authors is that biogenic CO₂- has indeed the same radiative effect of fossil CO₂ on the atmosphere but, while fossil CO₂- can only be reabsorbed by oceans and biosphere (according to the formulation using Bern CC equation, as given by [IPCC 2006]), biogenic-CO₂- has an additional factor which is the reabsorption of the CO₂- via re-growth of vegetation on the same piece of land. By this mathematical formulation, they have been able to assign various values of a so-called GWP_bio- over the typical time horizons of 20, 100 and 500 years and depending on the timing of biomass re-growth. Technically, this factor can then be simply used in a classical LCA and applied as correction factor to the amount of the biogenic-CO₂ emitted by the combustion of biomass." JRC 2014, p. 45.
^ "Annualised emissions from carbon stock changes caused by land-use change, e_l, shall be calculated by dividing total emissions equally over 20 years." European Parliament, Council of the European Union 2018, p. Annex VI.
^ See for instance the European Union's official emission savings percentages for different fuels here: European Parliament, Council of the European Union 2018, p. ANNEX VI. Note that these estimates do not include the average net emissions which results from an eventual land use change prior to planting.
^ "The Renewable Energy Directive (RED), as well as the Fuel Quality Directive (FQD) and the proposal for a RED-Recast (EP 2009, EP 2009b and EC 2016) apply a simplified attributional LCA methodology to assess GHG emissions savings for a series of liquid biofuels pathways used in the transport sector. A similar methodology is also extended to biomass used for power, heat and cooling generation (EC 2016). The RED evaluates the supply-chains GHG emissions of various bioenergy pathways and compares them to each other on a common basis (GHG emission savings with respect to a fossil fuel comparator) to promote the pathways that perform best on this relative scale and to exclude the pathways with the worst technologies and GHG performances." Camia et al. 2018, p. 89.
^ "Two main modelling principles are in use in LCA practice: Attributional (A-LCA) and Consequential (C-LCA) modelling, with the former being more widely used for historical and practical reasons. [...] Attributional modelling makes use of historical, fact-based, average, measureable data of known (or at least knowable) uncertainty, and includes all the processes that are identified to relevantly contribute to the system being studied. In attributional modelling, the system is hence modelled “as it is” or “as it was” (or as it is forecasted to be) (EC, 2010). Attributional modelling is also referred to as “accounting”, “book-keeping”, “retrospective”, or “descriptive”. [...] [P]urely attributional LCA studies of bioenergy systems are unable to capture properly all of the complexities linking bioenergy, climate, bioenergy and ecosystem services (e.g. market-mediated effects, biogeophysical, time-dependent effects). [...] The results of these types of assessment are static in time and do not account for biogenic-C flows. It has become established practice in A-LCA to assume that any emission of biogenic CO₂ (release to the atmosphere of the carbon contained in biological resources) is compensated by photosynthesis during the re-growth of the biomass feedstock. This assumption originates from an interpretation of the rules for reporting national GHG inventories to the United Nations Framework Convention on Climate Change (UNFCCC). Biogenic-C flow are accounted for in the land use, land-use change, and forestry (LULUCF) chapter at the time the biomass commodity is harvested and are therefore not accounted for in the energy sector at the time the biomass is burnt (JRC, 2013). It remains valid for system-level analysis, when the changes in biomass carbon stocks are accounted in the land-use sector rather than in the energy sector (EC, 2016c)." Camia et al. 2018, pp. 89–91.
^ "Some studies of forest bioenergy consider carbon dynamics at the individual stand level [...]. Stand-level assessments represent the forest system as a strict sequence of events (e.g. site preparation, planting or natural regeneration, thinning and other silvicultural operations, final felling). Results are strongly influenced by the starting point: commencing the assessment at harvest shows upfront emissions, followed by a CO₂ removal phase, giving a delay before forest bioenergy contributes to net reductions in atmospheric CO₂, particularly in long-rotation forests. This delay has been interpreted as diminishing the climate benefit of forest bioenergy [...]. In contrast, commencing at the time of replanting shows the opposite trend: a period of CO₂ removal during forest growth, followed by a pulse emission returning the CO₂ to the atmosphere. Thus, stand-level assessments give inconsistent results and can be misleading as a basis to assess climate impacts of forest systems [...]. Furthermore, when considering only the stand level, it is difficult to identify whether the forest is sustainably managed or subject to unsustainable practices that cause declining productive capacity and decreasing carbon stocks. [...] The alternative to stand level is landscape-scale assessment, that considers the total area of managed forests. Stand- and landscape-level assessments respond to different questions. Stand-level assessment provides detailed information about plant community dynamics, growth patterns and interactions between carbon pools in the forest. But the stand-level perspective overlooks that forests managed for wood production are generally a series of stands of different ages, harvested at different times to produce a continuous supply of wood products. Across the whole forest landscape, that is, at the scale that forests are generally managed, temporal fluctuations observed at stand level are evened out and the forest carbon stock fluctuates around a trend line that can be increasing or decreasing, or roughly stable, depending on the age class distribution and weather patterns (Cowie et al., 2013). Landscape-level assessment provides a more complete representation of the dynamics of forest systems, as it can integrate the effects of all changes in forest management and harvesting taking place in response to—experienced or anticipated—bioenergy demand, and it also incorporates the effects of landscape-scale processes such as fire [...]. In a forest managed such that annual carbon losses due to harvest plus other disturbances and natural turnover equal the annual growth in the forest, there is no change in forest carbon stock when considered at landscape level [...]. To conclude, impacts of bioenergy policy should be assessed at the landscape scale because it is the change in forest carbon stocks at this scale, due to change in management to provide bioenergy along with other forest products, that determines the climate impact. Understanding of stand-level dynamics is critical to forest management and is useful to inform assessments at the landscape scale." Cowie et al. 2021, pp. 1217–1218.
^ "Bioenergy from dedicated crops are in some cases held responsible for GHG emissions resulting from indirect land use change (iLUC), that is the bioenergy activity may lead to displacement of agricultural or forest activities into other locations, driven by market-mediated effects. Other mitigation options may also cause iLUC. At a global level of analysis, indirect effects are not relevant because all land-use emissions are direct. iLUC emissions are potentially more significant for crop-based feedstocks such as corn, wheat and soybean, than for advanced biofuels from lignocellulosic materials (Chum et al. 2011; Wicke et al. 2012; Valin et al. 2015; Ahlgren and Di Lucia 2014). Estimates of emissions from iLUC are inherently uncertain, widely debated in the scientific community and are highly dependent on modelling assumptions, such as supply/demand elasticities, productivity estimates, incorporation or exclusion of emission credits for coproducts and scale of biofuel deployment (Rajagopal and Plevin 2013; Finkbeiner 2014; Kim et al. 2014; Zilberman 2017). In some cases, iLUC effects are estimated to result in emission reductions. For example, market-mediated effects of bioenergy in North America showed potential for increased carbon stocks by inducing conversion of pasture or marginal land to forestland (Cintas et al. 2017; Duden et al. 2017; Dale et al. 2017; Baker et al. 2019). There is a wide range of variability in iLUC values for different types of biofuels, from –75–55 gCO₂ MJ–1 (Ahlgren and Di Lucia 2014; Valin et al. 2015; Plevin et al. 2015; Taheripour and Tyner 2013; Bento and Klotz 2014). There is low confidence in attribution of emissions from iLUC to bioenergy." IPCC 2019i, p. 194.
^ One often cited example of indirect land use change is the land use change from forest to agriculture that happened in Brazil after the US started to use some of its harvested corn for ethanol production rather than animal feed. The resulting lower supply of animal feed on the global market was seen as an opportunity by Brazilian farmers, who subsequently cut down forests in order to plant soya beans destined for the animal feed market. See Bird et al. 2010, p. 5, and also Searchinger et al. 2008, pp. 1238–1240 for the original research article.
^ The authors also note that depending on the biome, forest protection implies exposure to natural disturbances, such as wildfires, droughts, or insect infestations. While wildfires have been studied and are often included in the carbon calculations, droughts, insect outbreaks, and other related climate change impact factors on forest are much harder to predict. These natural disturbances "[...] may have severe carbon implications" however. The authors conclude that "[...] forest protection assumptions postulate that the carbon and thus the land will not be used for human economic activities for centuries; an assumption generally questionable in our land-constrained world." Lamers & Junginger 2013, pp. 378–379.
^ "Assuming the forest would remain unharvested in the no-bioenergy scenario is not a realistic reference in situations where landholders use the land to generate income, unless landholders can obtain equivalent income from payments for carbon sequestration or other ecosystem services (Srinivasan, 2015). In cases where a no-harvest scenario is a valid reference case, there are challenges in quantifying future carbon stocks: carbon sequestration rate in unharvested forests, especially in the longer term, is uncertain in many cases due to a paucity of relevant data (e.g. Derderian et al., 2016) and uncertain effects of climate change. Furthermore, accumulated carbon is vulnerable to future loss through disturbances such as storm, drought, fire or pest outbreaks. Where more than one alternative is plausible, it is informative to analyse several alternative reference land-use scenarios (Koponen et al., 2018)." Cowie et al. 2021, p. 1218.
^ According to Nabuurs et al., displacement factors takes into consideration the difference in CO₂ emissions per unit of primary energy produced, differences in efficiency of energy conversion (e.g. conversion from primary energy to electricity) and in some cases also the emission differences in the supply chains. See Nabuurs, Arets & Schelhaas 2017, p. 4. See also Cowie et al. 2021, p. 1214.
^ "Wood and coal have similar CO₂ emission factors, as the ratio of heating values between the two fuels is similar to the ratio of carbon content [...]. Where biomass is co-fired with coal in large power plants, the conversion efficiency may decrease a few percent, although there is usually no significant efficiency penalty when the co-firing ratio is below 10% [...]. Conversion efficiencies depend on fuel properties including moisture content and grindability in addition to heating value [...]. For low rank coal, biomass co-firing (especially torrefied biomass) can increase the boiler efficiency and net power plant efficiency [...]. Smaller biomass-fired plants can have lower electric conversion efficiency than large coal-fired plants, but as they are typically combined heat and power plants, they also displace heat production from other sources, that could otherwise have generated fossil fuel emissions [...]. Large dedicated biomass units (converted from coal) can operate with roughly the same level of thermal efficiency as delivered historically from coal [...]. Cowie et al. 2021, p. 1214.
^ Sathre & O'Connor found that in general, wood products require less production energy and less use of fossil fuels than what is needed to produce a functionally equivalent amount of metals, concrete, or bricks. They write that a displacement factor of wood product substitution is a measure of the amount of GHG emissions that is avoided when wood is used instead of some other material. In other words, a displacement factor shows the efficiency with which the use of biomass reduces net GHG emissions. The authors also write that a higher displacement factor indicates that more GHG emissions are avoided per unit of wood used. Likewise, a negative displacement factor means that emissions are greater when using the wood product. Sathre & O'Connor 2010, pp. 104–111.
^ Myllyviita et al. regrets that most researchers do not include wood storage in their calculations: "If the aim of DFs [displacement factors] is to describe the overall climate effects of wood use, DFs should include all the relevant GHG flows, including changes in forest and HWP [harvested wood products] carbon stock and post-use of HWPs, however, based on this literature review this is not a common practice." Myllyviita et al. 2021, p. 1.
^ "Bioenergy with carbon capture and storage (BECCS) plays a critical role in the NZE Scenario by offsetting emissions from sectors where full decarbonisation is extremely difficult to achieve. In 2050, around 10% of total bioenergy is used in facilities equipped with carbon capture, utilisation and storage, and around 1.3 billion tonnes of CO₂ is captured using BECCS. Around 45% of this CO₂ is captured in biofuels production, 40% in the electricity sector, and the rest in heavy industry, notably cement production." IEA 2021a.
^ "In case that there is no raw material displacement from other sectors such as food, feed, fibers or changes in land carbon stocks due to direct or indirect land use change, the assumption of carbon neutrality can still be considered valid for annual crops, agriresidues, short-rotation coppices and energy grasses with short rotation cycles. This can also be valid for analysis with time horizons much longer than the feedstock growth cycles. [...] The timeframe of the comparison too plays a relevant role in the performances of the reference system. If the timeframe chosen is short, the current emissions from the reference system can be considered appropriate and constant. In the case of a long-term analysis, though, also the changes in the fossil reference system have to be accounted for. For instance, practically in all of the studies analyzed the reference system (coal or NG) is kept constant and unchanged for the whole duration of the analysis (even centuries), while, according to EU policies, by 2050 the EU should be decarbonized, implying that future savings might be much smaller than current ones. In this case [...] it may happen that the payback time is never reached. [...] On the other hand, if the reference fossil system gets ‘dirtier', as in the case of most of the unconventional fossil energy (shale gas, bituminous coal etc.) the fossil fuel parity may be reached sooner than with a constant reference fossil fuel." JRC 2014, pp. 23, 51–52. Note that the JRC use the term "payback time" in the sense of "parity time" as defined in Carbon accounting principles above. See JRC 2014, p. 16.
^ "Studies of real forest landscapes show that the net GHG effects of bioenergy incentives are more variable than suggested by studies that do not consider economic factors and varying conditions in the forest and wood products sector." Cowie et al. 2021, p. 1218.
^ EU's Joint Research Centre describes how the economic boundaries can expand to reach macro-economic size: "Large scale techno-economic modeling: This type of analysis includes a macroeconomic model that estimates the developments of the wood market in terms of imports, quantity of wood used for wood products and for bioenergy etc. as response to a given decision. The market model is coupled with a forest model that can model changes in carbon stocks in all the pools of forests (including living and dead wood, soil-C etc.) and eventually the carbon stocked in wood products. These two models can then be combined with several scenarios for the substitution of wood products in which a typical LCA (biogenic-CO₂ emissions are set to zero) is applied to calculate the GHG savings due to the use of biomass compared to the alternative materials / feedstocks. The combination of these calculations would provide a clear and quantitative forecast of possible carbon savings or emissions due to different policy scenarios and over different time horizons." JRC 2014, p. 69.
^ See for instance Camia et al. 2021, pp. 86, 100.
^ "Wide variation in published estimates of payback time for forest bioenergy systems reflects both inherent differences between these systems and different methodology choices [...]. Critical methodology decisions include the definition of spatial and temporal system boundaries [...] and reference (counterfactual) scenarios [...]. Misleading conclusions on the climate effects of forest bioenergy can be produced by studies that focus on emissions at the point of combustion, or consider only carbon balances of individual forest stands, or emphasize short-term mitigation contributions over long-term benefits, or disregard system-level interactions that influence the climate effects of forest bioenergy." Cowie et al. 2021, pp. 1213, 1221.
^ Jonker et al. examined the carbon intensity for southeastern forests in the US, and concluded that due to the large number of possible methodological choices and reference systems, the calculations produce a wide range of payback and parity times, from below 1 year payback time with landscape level carbon accounting to 27 years with stand level accounting, and parity times of 2 –106 years depending on system boundaries and the choice of alternative scenarios. The authors consider landscape-level carbon accounting more appropriate for the examined situation. Under this precondition, the issue of carbon payback time is basically nonexistent. If comparison against a protection scenario is deemed realistic and policy relevant, and assuming that wood pellets directly replace coal in an average coal power plant, the carbon parity time is 12–46 years; i.e. one or two rotations. Switching to intensively managed plantations yields the most drastic reduction in parity time (below 18 years in 9 of 12 cases). The authors conclude that the choice of carbon accounting method has a significant impact on the carbon payback and parity times. Jonker, Junginger & Faaij 2013, pp. 371–387.
^ "Studies reporting long carbon debt payback times in general assume that the biomass is utilized for electricity production with low conversion efficiencies and that the woody biomass originates from the dedicated harvest of trees for energy from long rotation forestry. Looking at the current use of bioenergy in the EU, there is little evidence that such supply chains dominate." Madsen & Bentsen 2018, p. 1.
^ "Misleading conclusions on the climate effects of forest bioenergy can be produced by studies that focus on emissions at the point of combustion, or consider only carbon balances of individual forest stands, or emphasize short-term mitigation contributions over long-term benefits, or disregard system-level interactions that influence the climate effects of forest bioenergy. Payback time calculations are influenced by subjective methodology choices and do not reflect the contribution of bioenergy within a portfolio of mitigation measures, so it is neither possible nor appropriate to declare a generic value for the maximum acceptable payback time for specific forest bioenergy options. To answer the key question ‘what are the climate implications of policies that promote bioenergy?' assessment should be made at the landscape level, and use a full life cycle approach that includes supply chain emissions, changes in land carbon stocks and other variables influenced by the policies studied. Effects on land cover, land management and the wood products and energy sectors need to be considered, including indirect impacts at international level. The bioenergy system should be compared with reference scenarios (counterfactuals) that describe the most likely alternative land use(s) and energy sources that would be displaced by the bioenergy system, and the probable alternative fates for the biomass being utilized. A no-harvest counterfactual is not realistic in most current circumstances, but markets that pay for carbon sequestration and other ecosystem services could change incentives for harvest in the future."Cowie et al. 2021, pp. 1221–1222.
^ Bentsen examined 245 individual studies and found that the carbon payback time of apparently comparable forest bioenergy supply scenarios vary by up to 200 years, which provides ample room for confusion and dispute about the climate benefits of forest bioenergy. He concludes that the outcome of carbon debt studies lie in the assumptions, and that methodological rather than ecosystem and management related assumptions determine the findings. The findings are therefore seen as inadequate for informing and guiding policy development. Bentsen 2017, p. 1211.
^ "There is a large variability in the literature results for fossil fuel parity times. This is due to differences in the characteristics of the forest system considered (growth rate, management), in the carbon pools included, in the system boundaries definition and in the reference baseline used in the analysis." JRC 2014, p. 75.
^ EU's Joint Research Centre recommend that "[...] policymakers and scientists alike recognize that diverging values, worldviews, and ethical perceptions of natural resources and their management are a core part of the debate. These will not be solved by more scientific research, because science is a social endeavour where value-choices and judgements are inevitable. Transparency is key and cooperation with policymakers and co-creation of useful results should be welcomed." Camia et al. 2021, p. 93. In a presentation of this report for IEA Bioenergy, the JRC staff expand on this conclusion. They write that the question “Does forest bioenergy mitigate climate change?” really has no answer, as it is depends on "modelling approaches and the assumptions about hypothetical futures", and that researchers "come to equally valid, but opposite answers depending on assumptions chosen." They also write that "the assumptions chosen will align (consciously or unconsciously) with the worldviews and ethical values of the authors." According to the JRC, supporters of bioenergy usually have a more anthropocentric view of the human-nature relationship, while opposers of bioenergy are more aligned with nature conservation values. These norms lead to different concerns and definitions of what sustainability really is. See Mubareka, Giuntoli & Grassi 2021, pp. 8–9.
^ "Most of the forest feedstocks used for bioenergy, as of today, are industrial residues, waste wood, residual wood (thinnings, harvest residues, salvage loggings, landscape care wood etc.) for which, in the short to medium term, GHG savings may be achieved. On the other hand, in the case of stemwood harvested for bioenergy purposes only, if all the carbon pools and their development with time are considered in both the bioenergy and the reference fossil scenario, there is an actual increase in CO₂ emissions compared to fossil fuels in the short-term (few decades). In the longer term (centuries) also stemwood may reach the fossil fuel parity points and then generate GHG savings if the productivity of the forest is not reduced because of bioenergy production. [...] The results attained are strongly correlated with the following parameters: the fossil fuel replaced, efficiency of the biomass utilization, the future growth rate of the forest, the frequency and intensity of biomass harvests and the initial landscape carbon stock." JRC 2014, p. 75.
^ "Increased removal of FWD [fine woody debris], low stumps, CWD (course woody debris): It depends strongly on the decay rates considered. For instance, (Giuntoli et al., 2015) and (Giuntoli et al., 2016) found that residues with decay rates of 11.5%/year would mitigate climate change compared to natural gas heating and natural gas electricity after about 20 years, but residues with decay rate lower than 2.7%/year would take more than 86 years to payback compared to natural gas heating, or more than a century compared to the current EU power mix. FWD are thus likely to achieve carbon mitigation in a short term. However, decay rates for low stumps have been reported to range between 0.7%/year up to even 11%/year (Persson and Egnell, 2018), depending on climatic conditions and species. Considering a representative decay rate for temperate/boreal forests of between 3 and 6%/year would mean stumps would be unlikely to achieve climate mitigation before 50 years. This is substantiated also by the work of (Laganière et al., 2017). However, we indicate a range of uncertainty across other climate change levels. CWD are very likely to exhibit low decay rates and to have very long payback times." Camia et al. 2021, p. 143. See also JRC 2014, pp. 16–17, 43–44.
^ Lamers & Junginger examined a number of studies and argue that parity times for residues "[...] mostly vary depending on the respective fossil fuel used in the reference scenario [...]." However, the second most important influencing factor "[...] is the size/diameter of the residue and the forest biome, i.e. conditions affecting the decay rate." The shortest parity times were found for forest residues which would otherwise be burned at the factory or roadside. This immediate carbon release in the alternative scenario causes an immediate carbon benefit and a net zero parity time for the bioenergy scenario. The longest parity times were for stump harvest in the cold boreal forests of northern Finland, when compared to a natural decay scenario for the stumps, and instead production of electricity from natural gas. For stemwood, parity times vary to some degree by forest biome with significantly shorter periods for highly productive regions, such as the temperate moist forests of the South-Eastern USA. In the boreal or sub-boreal forests, parity times against a forest protection scenario are about twice as large, but there are variations between studies. Under specific conditions, for instance where insect infestation has killed a large amount of merchantable timber stock, "[...] bioenergy harvest can reach parity times as low a zero." The high share of fast decaying tree biomass in the protection scenario shortens parity times. Parity times against regular timber harvest (business as usual) vary greatly with the fossil fuel alternative scenario, the shortest being coal and oil compared to natural gas. Afforestation on the other hand has a parity time of zero years if the land area in question would not be sequestering large amounts of carbon otherwise. Lamers & Junginger 2013, p. 379.
^ "Here, we analyze carbon debt and payback time of substituting coal with forest residues for combined heat and power generation (CHP). The analysis is, in contrast to most other studies, based on empirical data from a retrofit of a CHP plant in northern Europe. The results corroborate findings of a carbon debt, here 4.4 kg CO₂eq GJ⁻¹. The carbon debt has a payback time of one year after conversion, and furthermore, the results show that GHG emissions are reduced to 50% relative to continued coal combustion after about 12 years. The findings support the use of residue biomass for energy as an effective means for climate change mitigation. [...] Dehue points out that there is no universally applied definition of ‘carbon debt' and ‘carbon debt payback time', leading authors to apply different definitions in an inconsistent manner. A definition often referred to is by Mitchell et al., where the terms ‘carbon debt', ‘carbon debt repayment', and ‘carbon offset parity point' are introduced. However, this definition only applies to bioenergy scenarios where the source of woody biomass comes from dedicated harvest and forest regrowth is included in the modelling. In contrast, bioenergy sources from wood waste and forest residues are resources that are generated independently of a bioenergy demand. The method that is used here is in line with the typical approach to carbon debt and payback time analyses, allowing for a comparison with other studies." Madsen & Bentsen 2018, pp. 1–2.
^ "The study covers the direct emissions from the extraction and processing, transportation and the combustion of the fuels. It therefore excludes the embodied emissions in the used fuels or materials, e.g., the emissions related to produce diesel used for transportation of biomass or coal. The system boundary also exclude emissions related to distribution and use of the produced heat and electricity together with emissions that are related to the end of life of the CHP plant. Furthermore, GHG emission related to indirect effects, e.g., indirect land use change or indirect wood use change, of biomass consumption are not considered. The carbon debt concept is adopted from Mitchell et al., but applied to waste and residue resources, as e.g., demonstrated by Sathre et al. (Equation (1)). NE [equals] E_bio − (E_fossil + E_decay) where NE is the annual net GHG emission to the atmosphere, E_bio is the direct GHG emissions from the bioenergy supply chain including emissions from biomass combustion, E_fossil the direct GHG emissions from the counterfactual fossil supply chain, including emissions from fossil fuel combustion, and E_decay the GHG emissions from the counterfactual decay of forest residues. The payback time is determined as the time, where the time integrated NE [equals] 0. (Figure 2). The conceptual carbon emission profile corresponds to modelled profiles for the use of stumps or branches for energy. The payback time is understood as the point in time, where the bioenergy scenario starts to reduce the atmospheric GHG emissions relative to the counterfactual reference scenario. [...] In order to set up a reference scenario, the realistic alternative use of the biomass must be determined. Based on a literature review and interview with some of the biomass suppliers to the plant, the most likely alternative is decomposition on the forest floor, either as logs or as branches. [...] The total emissions per produced GJ are almost identical for both scenarios. Emissions from the biomass scenario are slightly higher by 4.4 kg CO₂eq GJ⁻¹, which represents the carbon debt, equaling 3.2% of the total emissions. [...] Emissions from processing are roughly identical for both scenarios; however, transport emissions are approximately three times higher in the reference scenario than the biomass scenario. This is in line with earlier research and is mainly attributable to longer transport distances for coal. The carbon debt incurred in the transition from coal to biomass is primarily related to the higher carbon intensity of biomass when compared to coal due to a lower carbon to oxygen ratio in biomass. Lower supply chain emissions in the biomass scenario, on the other hand, reduces the carbon debt." Madsen & Bentsen 2018, pp. 2–3, 5, 7. In simpler terms, the calculation starts with the total bioenergy-related emissions, then the coal-related emissions are subtracted (including the emissions from decaying forest residues).
^ "Buchholz et al. conducted a meta-analysis of 59 carbon debt studies, and showed that the majority (47 studies) was based on hypothetical data and only a dozen were based on field data. [...] Data from EUROSTAT show that less than 1% of electricity production in the EU comes from solid biomass fired power plants. Solid biomass is more prevalent in combined heat and power (CHP) and heat production that are plants feeding into district heating systems. 16.3% of heat production to district heating in the EU comes from solid biomass, while the majority comes from natural gas and coal. [...] Contrary to most other studies, which are based on hypothetical scenarios, this analysis benefits from the use of data from an existing power plant retrofit in northern Europe, which is considered to be representative for the use of biomass for CHP in the EU." Madsen & Bentsen 2018, pp. 1–2.
^ See Holmgren 2021, pp. 10–26. The climate mitigation effect of the established forestry practice was determined by counting the specific annual changes over 10 and 40 years in this scenario's aggregated carbon pools. First, the net annual carbon increase in the national forest carbon pool was calculated: Annual total forest growth minus natural losses minus harvest removals (harvest removals includes both stemwood and residues). Subsequently, carbon used for harvested wood products, and the residues that is left in the forest but not yet decayed, are added to their respective pools (the HWP pool and the dead biomass pool). Then the carbon emitted from the decaying residues (including stumps, roots and branches) is subtracted. Finally, the displaced fossil carbon is added to the displaced fossil carbon pool (the fossil carbon is seen as "held back" in the fossil carbon reservoirs underground since forest biogenic carbon, which is already counted as emissions at harvest, has been used in its place.) As mentioned, and unlike in other studies, the study boundaries here included fossil fuel displacment effects, including from "[...] solid wood products and fibre products, representing normal wood utilization in Sweden where different parts of the tree is used for either of these product categories [...]." A displacement factor of 0.78 tonne fossil carbon displaced per 1 tonne biogenic carbon produced (which "[...] corresponds to displacement effects for the integrated mix of solid wood products, fiber products and bioenergy applied in several studies [...]") (p. 12) is used for both harvested wood products and bioenergy combined, and the forest residues half-life decay rate is set to 10 years (p. 13). In the two forest protection scenarios, the forest increases by 64% and 91% over 40 years respectively, while the actual forestry practice only achieve an increase of 44% (p. 17). However, since the forest in the forest protection scenarios is left to itself, the natural carbon losses (pests, fires etc.) increases by 4% and 6% for the two scenarios, respectively (p. 14). As mentioned above, substantial emissions are caused when the national forest carbon products and energy infrastructure is converted to fossil carbon products and energy infrastructure. The difference compared to the actual forestry practice over these 40 years was calculated by: 1.) Adding the extra amount of carbon that would have been absorbed from the atmospheric carbon pool and stored in the protected forest pool, compared to the amount of carbon stored in the actually managed forest. 2.) Subtracting the projected loss of carbon in the harvested wood products pool during the same time period (since each year some harvested wood products would be cycled out of this carbon pool and into the atmospheric carbon pool because of combustion or rotting when reaching end-of-life, while no new harvested wood products would have entered this pool. 3.) Subtracting the substantial amount of fossil carbon that would have to be moved from the underground fossil reservoirs and into fossil carbon products and energy carriers, in order to a.) replace the forest carbon products and energy carriers, and b.) convert the national forest carbon products and energy infrastructure into a fossil carbon products and energy infrastructure. These subtractions are highest at the beginning of the 40-year period, and (together with the carbon absorption going on elsewhere in the forest) more than compensate for the carbon debt caused by harvest-related carbon emissions (p. 14-17).
^ Recalculated from Holmgren's 0.5 tonne CO₂e per m³ (Holmgren 2021, p. 12), 40% spruce and 40% pine in Swedish forests (Swedish Wood, p. Fig. 5), and 320 vs. 390 kg dry mass per m³ for spruce and pine, respectively. Here, an average value of 350 kg/m³ was assumed, together with 50% carbon content, and 3.67 kg CO₂e per kg carbon. The actual forestry practice mitigated in total 3.54 Gt CO₂e, with the displacement-specific mitigation effects estimated at 1.84 Gt CO₂e. If the displacement effects were left out of the calculation, one of the forest protection scenarios (the one with only a 4% increase in natural losses (e.g. wildfires and diseases), had more carbon stored in the forest after 10 and 40 years compared to the actual forestry practice (0.74 vs. 0.55 and 2.41 vs. 1.84 Gt CO₂e, respectively.) The other forest protection scenario (the one with a 6% increase in natural losses) had less carbon absorbed in the forest compared to the actual forestry practice after 40 years (1.56 vs. 1.84 Gt CO₂e) but more after 10 years (0.64 vs. 0.55 Gt CO₂e.) In addition to the forest protection scenarios, the author also included a 10% reduced harvest scenario, which performed similar to the actual forestry scenario.
^ Miner et al. writes that in the eastern parts of the USA, bioenergy from forest residues that otherwise would have been left to decay naturally, typically accomplishes net GHG benefits within a decade when displacing coal-based electricity, and within two decades when displacing natural gas-based electricity. Miner et al. 2014, p. 599.
^ See Hanssen et al. 2017, pp. 1407–1410, and table S1 in the supporting information document, link available at the bottom of the article.
^ "The studies analyzed report payback times in the range of 0 – 74 years for harvest residues. The main factors affecting these values are mostly similar to the ones described for stemwood. The ratio of fossil carbon displacement is the main parameter. If the residues are used with high efficiency to displace coal (such as in co-firing), the payback times are rather short, if any. In case the residues are heavily processed to produce liquid biofuel the payback time increases dramatically. Also the size of the residue plays a relevant role, as well as the geographic and local conditions that influence the bacterial decomposition rates. Wood from thinnings may, to some extent, be assimilated to harvest residues (especially pre-commercial thinnings). If not collected for bioenergy it would be left in the forest to decay, or combusted at roadside. On the other hand, depending on the wood quality, the use of thinnings wood for bioenergy may compete with other uses, such as pulp and paper or engineered wood. Salvage loggings can also be assimilated to harvest residues. Damaged, dying or dead trees affected by injurious agents, such as wind or ice storms or the spread of invasive epidemic forest pathogens, insects and diseases would remain in the forest and decay or combusted at roadside. Wood removed for prescribed fire hazard control as well can be considered residual wood." JRC 2014, pp. 42–43, table 3. Note that the JRC use the term "payback time" in the sense of "parity time" as defined in Carbon accounting principles above. See JRC 2014, p. 16.
^ "Total GHG emission per energy content from the production of energy from harvest residues. Norway spruce stumps (diameter 30 cm), young stand delimbed thinning wood (diameter 10 cm) and branches (diameter 2 cm). Emissions over a 100 year period after start of the practice in Northern Finland (dotted line) and Southern Finland (solid line) and the entire fuel cycle emissions of some fossil fuels. The total emission estimates of forest bioenergy include emissions resulting from the changes in carbon stocks and the emissions from production chain including collecting, transporting, chipping and combusting the forest residues." JRC 2014, p. 42.
^ "Three systems were designed to represent three different power production scales (see Figure 7.15): i) large-scale power plant of 80 MWel. fuelled with wood pellets from forest logging residues (FRel); ii) medium-scale power plant of 15 MWel. fuelled with cereal straw bales (STel); iii) small-scale internal combustion engine of 300 kWel. fuelled with biogas produced from anaerobic digestion of cattle slurry, employing an open or gas-tight tank for digestate storage (Biogas OD/CD). [...] The results are explicit in time, Near-Term Climate Forcers (i.e. ozone precursors and aerosols) are included, an instantaneous, absolute climate metric is used and biogenic-C flows are explicitly accounted for (see Table 7.1 for all methodological details). These results reveal additional details compared to the analysis in section 7.3.1. For instance, they indicate with clarity that power generation from cereal straws and cattle slurry can provide, by 2100, global warming mitigation compared to the current European electricity mix in all of the systems and scenarios considered. Power generation from forest logging residues is an effective mitigation solution only in situations in which the decay rates of the residues on the forest floor were above 5.2% /yr. Even with faster-decomposing feedstocks, bioenergy temporarily causes a climate change worsening compared to the fossil system. Strategies for bioenergy deployment should thus take into account the potential increase in global warming rate and temporary increase in temperature anomaly. Further details on the methodology and on the results of the case studies can be found in Giuntoli et al. (2016)." Camia et al. 2018, pp. 100–104. The supporting documentation show that the "residues" category includes not only small-diameter residues like branches, but also logs and stumps. See JRC 2018 and JRC 2015, p. 92.
^ The JRC here actually use the term "payback time", but define this term in the same way the term "parity time" is defined above: "[...] [A]t the payback time the fossil fuel parity is reached (i.e. the bioenergy system and the fossil counterfactual have emitted the same amount of CO₂ in the atmosphere). After the fossil fuel parity time, the bioenergy system starts to provide CO₂ savings." See JRC 2014, p. 16. Normally, the time it takes for a bioenergy scenario to store as much carbon as a no-bioenergy scenario (i.e. when their net emission level is the same) is known as the carbon parity time: "Eventually carbon levels in the forest return to the level at which they would have been if they had been left unharvested. (Some of the literature employs the term ‘carbon payback period' to describe this longer period, but it is more commonly used to mean the time to parity with fossil fuels; this meaning is used in this paper.)" Chatham House 2017, p. 27.
^ In the JRC's chart above, landscape level carbon accounting is assumed for this scenario. However, the original research article does not actually say which accounting method is used, only that biomass is sourced "[...] from 5.25 million hectares within the GLSL forest region in Ontario." McKechnie et al. 2010, p. 791.
^ Lamers & Junginger argue that payback times are mainly determined by plant growth rates, i.e. the forest biome (e.g. climate zone), tree species, site productivity and management. Parity times are primarily influenced by the choice and construction of the reference scenario and fossil carbon displacement efficiencies. The authors write that using "[...] small residual biomass (harvesting/processing), deadwood from highly insect-infected sites, or new plantations on highly productive or marginal land offers (almost) immediate net carbon benefits." The actual climate mitigation potential however is determined by the effectiveness of the fossil fuel displacement. Lamers & Junginger 2013, p. 373.
^ The JRC's example here is from a UK coniferous forest study: "In the case wood is used for bioenergy only the total emissions of the bioenergy system would be −5.5 tCO₂/ha*y (5.1 tCO₂/ha*y from displacement of fossil fuel and 0.4 tCO₂/ha*y due to the sink of the forest system), that, compared to the missed growth of the forest (14 tCO₂/ha*y) [14 tonnes CO₂ per hectare per year] results in net emissions of 8.5 tCO₂/ha*y. This result shows that, in a 40 years timeframe, CO₂ emissions are lower for the suspended management forest than for the forest managed for bioenergy only. The second case is if the wood is used for materials as well as bioenergy (bioenergy from residues). In this case the total emissions of the material and bioenergy system would be −22.8 tCO₂/ha*y, (-6 tCO₂/ha*y in carbon stock of the forest and products and −16.8 tCO₂/ha*y from displacement of products) to which the missed growth of the forest has to be subtracted (14 tCO₂/ha*y) resulting in net GHG savings of 8.8 tCO₂/ha*y. Therefore managing the forest for products determines higher GHG savings than suspending the management." JRC 2014, p. 26.
^ "In fact, wood products have multiple climate mitigation benefits: they increase the anthropogenic carbon pools, they are often much less GHG and energy intensive than similar materials of fossil origin (e.g. concrete, metals etc.) and, finally, bioenergy can be obtained from these products at the end of life to replace fossil fuels and guarantee additional substitution. [...] [W]hen wood is used in a cascade utilization, then climate mitigation can be achieved in much shorter times than when wood is used purely for energy. Moreover, with the proper measures (longer storage, substitution of C-intensive materials and fossil fuels), the payback time can be even shortened to zero, as compared to centuries indicated for energy-only use. Studies that fail to consider the wood for material displacement may come to misleading conclusions." JRC 2014, pp. 59, 61. Note that the JRC use the term "payback time" in the sense of "parity time" as defined in Carbon accounting principles above. See JRC 2014, p. 16.
^ "The reviewed studies indicate that the use of stemwood from dedicated harvest for bioenergy would cause an actual increase in GHG emissions compared to those from fossil fuels in the short-and medium term (decades), while it may start to generate GHG savings only in the long-term (several decades to centuries), provided that the initial assumptions remain valid. The harvest of stemwood for bioenergy purposes is not common today, however, it is becoming a more common practice that is expected to expand in the future. [...] The GHG saving can be immediate if in the counterfactual scenario the wood would be burnt at roadside. This feedstock is expected to provide most of the additional increment of biomass for bioenergy by 2020. Also in the case of new plantations on agricultural or grazing land the GHG savings can be immediate (in absence of iLUC)." JRC 2014, pp. 16–17.
^ "The initial landscape conditions and land-use history are also fundamental in determining the amount of time required for forests to recover the initial additional emissions of the bioenergy system over the fossil one. While Recently Disturbed and Old-Growth landscapes required very long payback times, Post- Agricultural and Rotation Harvest landscapes were capable of recovering the additional emission in relatively short time periods, often within 1 year [Mitchell 2012]. This is a conclusion also of Zanchi et al. [Zanchi 2011]. The reason is that planting a short-rotation forest on unused agricultural land does not start with high carbon stocks so causes an increase in average carbon stocks." JRC 2014, pp. 40–41. Note that the JRC use the term "payback time" in the sense of "parity time" as defined in Carbon accounting principles above. See JRC 2014, p. 16.
^ ^a ^b "The source of forest biomass is a key determinant of climate change effects of bioenergy (Matthews et al., 2018). Concerns have been raised that bioenergy demand could lead to widespread harvest of forests solely for bioenergy, causing large GHG emissions and forgone carbon sequestration (Brack, 2017; Norton et al., 2019; Searchinger et al., 2018). However, long-rotation forests are generally not harvested for bioenergy products alone: Biomass for bioenergy is usually a by-product of sawlog and pulpwood production for material applications (Dale et al., 2017; Ghaffariyan et al., 2017; Spinelli et al., 2019; Figure 1). Logs that meet quality requirements are used to produce high-value products such as sawnwood and engineered wood products such as cross laminated timber, which can substitute for more carbon-intensive building materials such as concrete, steel and aluminium (Leskinen et al., 2018). Residues from forestry operations (tops, branches, irregular and damaged stem sections, thinnings) and wood processing residues (e.g. sawdust, bark, black liquor) are used for bioenergy (Kittler et al., 2020), including to provide process heat in the forest industry (Hassan et al., 2019). These biomass sources have high likelihood of reducing net GHG emissions when substituting fossil fuels (Hanssen et al., 2017; Matthews et al., 2018), and their use for bioenergy enhances the climate change mitigation value of forests managed for wood production (Cintas, Berndes, Hansson, et al., 2017; Gustavsson et al., 2015, 2021; Schulze et al., 2020; Ximenes et al., 2012). Part of the forest biomass used for bioenergy consists of roundwood (also referred to as stemwood), such as small stems from forest thinning. For example, roundwood was estimated to contribute around 20% of the feedstock used for densified wood pellets in the United States in 2018 (US EIA, 2019)." Cowie et al. 2021, pp. 1215–1216.
^ Hektor write that supply chain emissions for biomass are "[...] in most cases less than half of the corresponding emissions from fossil fuels." Hektor, Backéus & Andersson 2016, p. 4.
^ "There is a large variability in the results of forest bioenergy fossil fuel parity times calculations. This large variability depends on the many different characteristics of the systems compared and non-consistent modeling assumptions and approaches. The first, most important assumption is on the fossil fuel displaced. Then, concerning both the bioenergy system and the reference fossil system the following characteristics heavily impact the results: efficiency in the final use, future growth rate of the forest, the frequency and intensity of biomass harvests, the initial forest carbon stock, the forest management practices assumed." JRC 2014, p. 17.
^ "The reviewed studies show payback times ranging from 0 to almost 500 years. This large variability depends on the many different characteristics and assumptions on both the forest/bioenergy system and the reference fossil system. The most straight forward relation is with the fossil fuel used as a reference in the fossil scenario. Obviously, the more carbon intensive the fossil fuel replaced is, the shorter is the payback time. [...] A further correlation exists with the efficiency of the biomass utilization. The less efficient the bioenergy system is, the longer are the payback times. In case of electricity production, in biomass only plants, the electrical efficiency of biomass conversion is lower than the fossil, while thermal conversion energetic efficiency is similar for biomass and fossil fuels. In co-firing plants, biomass generally achieves the same efficiency as coal. An intensive processing, such as for liquid biofuel substitution via lignocellulosic ethanol, causes much longer payback times because of the loss of energy in the biofuels production (about half of the energy content of the biomass is lost in the processing [...]. The slower the forest growth rate is, the longer is the payback time. The forest growth rate depends on the latitude (boreal, temperate, tropical), but also on specific characteristics of the trees species, the microclimate and the soil fertility." JRC 2014, p. 34. Note that the JRC use the term "payback time" in the sense of "parity time" as defined in Carbon accounting principles above. See JRC 2014, p. 16.
^ See Hanssen et al. 2017: Figure S3 at page 3 in the Supporting Information document, link to document available at the bottom of the article.)
^ "The modeller assigns independent inputs to specific parameters, called here decision variables, thus producing a range of results deriving from different configurations of the same supply-chains associated to a single commodity. For instance, the distance at which biomass resources are transported influences significantly the overall impact of bio-based commodities. Conversion efficiencies can be both a source of variability (e.g. range of possible efficiencies for a single engine) and important decision variables (e.g. modelling different conversion technologies).[...] Another source of variation is linked to methodological choices in each assessment (e.g. allocation basis, background processes, etc.); for the benefit of the users, critical methodological choices are reported explicitly in the database. [...] The range of results associated with each pathway is dependent on several factors of variability:1. Transport distances of the feedstock or of the final product, 2. End-use conversion efficiencies, 3. Utilities, 4. Process characteristics, 5. Background data, 6. LCA Methodology. The GHG emissions reported in this work, therefore, should not be interpreted as a universal property associated to the product/commodity, since the changes in methodological choices and background data can largely influence the absolute value of GHG emission. However, the relative benchmarking among similar products and commodities can provide important information. The results [...] show that, most bioenergy pathways emit less GHG along their supply chain than fossil fuel pathways. However, the various pathways can achieve very different GHG emission levels. For instance, using dairy cattle slurry to produce biogas or biomethane can guarantee the highest GHG emissions mitigation due to the emission credits assigned for the avoided methane emissions associated to the use of raw manure as organic fertilizer (Giuntoli et al., 2017). In order for commodities to achieve the highest ambitions in terms of GHG emission savings (>85% savings), generally, high resource efficiency along the supply chain is required, and in particular: 1. Optimized logistics with short or efficient transport options (e.g. biomass feedstock is traded within EU neighbouring countries), 2. High efficiency of final conversion, 3. Use of renewable energy sources to supply process-heat and process-electricity, 4. Optimal process design (e.g. digestate residue from anaerobic digestion is stored in gas-tight tanks), 5. Use of wastes, residual or low-input feedstocks, 6. Assignment of credits to co-products (substitution method). Nonetheless, the results show that even with current technologies, significant optimizations are available to reduce the impacts of each supply chain (lower boundary of floating columns)." Camia et al. 2018, pp. 95, 98.
^ "In addition to the EU, the US has also amended its Renewables Fuel Standard 1 (RFS1) to include minimum life-cycle GHG emissions in the RFS2. RFS2 distinguishes between the production of conventional and advanced biofuels, which are defined based on their GHG abatement potential. All biofuels which can save up to 20% GHG in their life cycle compared to the petroleumbased equivalents are categorised as conventional. Conventional biofuel production is limited to 15 billion gallons to 2022. Advanced biofuels production accounts for the remainder 21 billion gallons. A biofuel can be considered advanced if it saves at least 50% GHG. Cellulosic biofuels require a 60% GHG emission reduction compared to the petrochemical equivalent (EPA, 2012). These emissions include ILUC GHG emissions." IRENA 2014, p. 47.
^ The estimates are for the "medium case" considered (case 2a); a pellet mill that uses wood for processing heat, but sources electricity from the grid. Estimates (for forest residue based pellets) reduce to 50–58% when fossil fuels is used for processing heat (case 1), but increase to 84-92% when electricity is sourced from a CHP biomass power plant (case 3a). See European Parliament, Council of the European Union 2018, p. Annex VI.
^ "[...] GHG emission reductions of wood-pellet electricity compared to fossil EU grid electricity are 71% (for small roundwood and harvest residues), 69% (for commercial thinnings) or 65% (for mill residues), as shown in more detail in Fig. S3. The GHG reduction percentage of wood-pellet electricity from mill residues was [...] 75% [...]." Hanssen et al. 2017, pp. 1415–1416.
^ See Hanssen et al. 2017: Figure S3 and Table S1 at pages 3–4 in the Supporting Information document, link to document available at the bottom of the article.)
^ "It is commonly perceived that bioenergy supply chain emissions are substantial, particularly when biomass is transported internationally, and could negate the climate benefits of fossil fuel substitution. However, fossil energy use along domestic forest biomass supply chains, from harvest, processing and transport, is generally small compared to the energy content of the bioenergy product and, with efficient handling and shipping, even when traded internationally [...]. The European Commission's Joint Research Centre determined that shipping pellets between North America and Europe increases supply chain emissions by 3–6 g CO₂/MJ, from around 3–15 g CO₂/MJ for wood chips or pellets dried using bioenergy and transported 500 km by truck (Giuntoli et al., 2017). For context, the EU average emission factors for hard coal are 96 and 16 g CO₂/MJ for combustion and supply respectively (Giuntoli et al., 2017). This underscores the importance of assessing actual supply chains. For example, the international pellet supply chain between the southeast United States and Europe has been intentionally designed to minimize trucking and associated handling costs, with pellet mills and large end users such as power plants located near rail lines, waterways and ports, thereby minimizing transport emissions and increasing net climate benefits (Dwivedi et al., 2014; Favero et al., 2020; Kline et al., 2021)."Cowie et al. 2021, p. 1219.
^ "In 2015 the net imports (i.e. imports minus exports) of wood pellets amounted to 3% of the total wood for energy mix (around 16 Mm³). The UK accounted for 97% of EU net imports of wood pellets (JFSQ). The United States was by far the most important source of EU wood pellets imports, with a 77% share (United Nations, 2020)." Camia et al. 2021, pp. 7, 42.
^ "Following the findings of the COST Action EuroCoppice (FP1301)17, coppice forests cover more than 19 Mha in the EU, corresponding to about 12% of the total forest area in 2015. The large majority (17 Mha) are in the EU Mediterranean countries, where about 32% of the forest area is reported as coppice (Unrau et al. 2018)." Camia et al. 2021, p. 33.
^ "The initial landscape conditions and land-use history are also fundamental in determining the amount of time required for forests to recover the initial additional emissions of the bioenergy system over the fossil one. While Recently Disturbed and Old-Growth landscapes required very long payback times, Post- Agricultural and Rotation Harvest landscapes were capable of recovering the additional emission in relatively short time periods, often within 1 year [Mitchell 2012]. This is a conclusion also of Zanchi et al. [Zanchi 2011]. The reason is that planting a short-rotation forest on unused agricultural land does not start with high carbon stocks so causes an increase in average carbon stocks." JRC 2014, pp. 40–41. Note that the JRC use the term "payback time" in the sense of "parity time" as defined in Carbon accounting principles above. See JRC 2014, p. 16. Likewise, Liu et al. found an average carbon payback time of 0.97 years for miscanthus planted on marginal, eroded soils in the Loess Plateau in China. Liu et al. 2016, p. 1.
^ "If the total biogenic carbon released during biomass decay and/or combustion is sequestered, the system continues to be in balance. As a result, the amount of CO₂ in the atmosphere does not increase. [...] When short-rotation energy crops or agricultural residues are used as fuel, they result in a balanced carbon cycle because they grow/renew themselves annually. In comparison, the rapid expansion of palm oil plantations in Indonesia and Malaysia, for example, has led to major problems associated with bioenergy. Logging rain forests or peat bogs for palm oil plantations has a negative effect. Plantations which were partly built on carbon-rich peat soils in the region resulted in drainage. The subsequent oxidation of peat and natural or anthropogenic fires results in substantial CO₂ emissions. Peat digging also has a negative effect, which results in an increase in CO₂ emissions in the atmosphere." IRENA 2014, p. 45.
^ "Perennial Miscanthus has energy output/input ratios 10 times higher (47.3 ± 2.2) than annual crops used for energy (4.7 ± 0.2 to 5.5 ± 0.2), and the total carbon cost of energy production (1.12 g CO₂-C eq. MJ⁻¹) is 20–30 times lower than fossil fuels." McCalmont et al. 2017, p. 489.
^ "The results in Fig. 3c show most of the land in the UK could produce Miscanthus biomass with a carbon index that is substantially lower, at 1.12 g CO₂-C equivalent per MJ energy in the furnace, than coal (33), oil (22), LNG (21), Russian gas (20), and North Sea gas (16) (Bond et al., 2014), thus offering large potential GHG savings over comparable fuels even after accounting for variations in their specific energy contents. Felten et al. (2013) found Miscanthus energy production (from propagation to final conversion) to offer far higher potential GHG savings per unit land area when compared to other bioenergy systems. They found Miscanthus (chips for domestic heating) saved 22.3 ± 0.13 Mg [tonnes] CO₂-eq ha⁻¹ yr⁻¹ [CO₂ equivalents per hectare per year] compared to rapeseed (biodiesel) at 3.2 ± 0.38 and maize (biomass, electricity, and thermal) at 6.3 ± 0.56." McCalmont et al. 2017, p. 500.
^ "The costs and life-cycle assessment of seven miscanthus-based value chains, including small- and large-scale heat and power, ethanol, biogas, and insulation material production, revealed GHG-emission- and fossil-energy-saving potentials of up to 30.6 t CO₂eq C ha⁻¹ y⁻¹ and 429 GJ ha⁻¹ y⁻¹, respectively. Transport distance was identified as an important cost factor. Negative carbon mitigation costs of –78€ t−1 CO₂eq C were recorded for local biomass use. The OPTIMISC results demonstrate the potential of miscanthus as a crop for marginal sites and provide information and technologies for the commercial implementation of miscanthus-based value chains. [...] The overall biomass transport distance was assumed to be 400 km when bales were transported to the bioethanol plant or to the plant producing insulation material as well as in the value chain 'Combined heat and power (CHP) bales.' For the value chains 'CHP pellets' and 'Heat pellets' the bales were transported 100 km to a pelleting plant and from there the pellets were transported 400 km to the power plants. The average farm-to-field distance was assumed to be 2 km. This transport distance is also assumed for the value chain 'heat chips' in which a utilization of the chips as a biomass fuel on the producing farm was assumed. Because of the higher biomass requirements of the biogas plant an average transport distance of 15 km from field to plant was assumed." Lewandowski et al. 2016, pp. 2, 7.
^ "Any soil disturbance, such as ploughing and cultivation, is likely to result in short-term respiration losses of soil organic carbon, decomposed by stimulated soil microbe populations (Cheng, 2009; Kuzyakov, 2010). Annual disturbance under arable cropping repeats this year after year resulting in reduced SOC levels. Perennial agricultural systems, such as grassland, have time to replace their infrequent disturbance losses which can result in higher steady-state soil carbon contents (Gelfand et al., 2011; Zenone et al., 2013)." McCalmont et al. 2017, p. 493.
^ "Tillage breaks apart soil aggregates which, among other functions, are thought to inhibit soil bacteria, fungi and other microbes from consuming and decomposing SOM (Grandy and Neff 2008). Aggregates reduce microbial access to organic matter by restricting physical access to mineral-stabilised organic compounds as well as reducing oxygen availability (Cotrufo et al. 2015; Lehmann and Kleber 2015). When soil aggregates are broken open with tillage in the conversion of native ecosystems to agriculture, microbial consumption of SOC and subsequent respiration of CO₂ increase dramatically, reducing soil carbon stocks (Grandy and Robertson 2006; Grandy and Neff 2008)." IPCC 2019a, p. 393.
^ "A systematic review and meta-analysis were used to assess the current state of knowledge and quantify the effects of land use change (LUC) to second generation (2G), non-food bioenergy crops on soil organic carbon (SOC) and greenhouse gas (GHG) emissions of relevance to temperate zone agriculture. Following analysis from 138 original studies, transitions from arable to short rotation coppice (SRC, poplar or willow) or perennial grasses (mostly Miscanthus or switchgrass) resulted in increased SOC (+5.0 ± 7.8% and +25.7 ± 6.7% respectively)." Harris, Spake & Taylor 2015, p. 27.
^ "[...] it seems likely that arable land converted to Miscanthus will sequester soil carbon; of the 14 comparisons, 11 showed overall increases in SOC [soil organic carbon] over their total sample depths with suggested accumulation rates ranging from 0.42 to 3.8 Mg C ha⁻¹ yr⁻¹. Only three arable comparisons showed lower SOC stocks under Miscanthus, and these suggested insignificant losses between 0.1 and 0.26 Mg ha⁻¹ yr⁻¹." McCalmont et al. 2017, p. 493.
^ "The correlation between plantation age and SOC can be seen in Fig. 6, [...] the trendline suggests a net accumulation rate of 1.84 Mg C ha⁻¹ yr⁻¹ with similar levels to grassland at equilibrium." McCalmont et al. 2017, p. 496.
^ Given the EU average peak yield of 22 tonnes dry matter per hectare per year (approximately 15 tonnes during spring harvest). See Anderson et al. 2014, p. 79. 15 tonnes also explicitly quoted as the mean spring yield in Germany, see Felten & Emmerling 2012, p. 662. 48% carbon content; see Kahle et al. 2001, table 3, page 176.
^ ^a ^b See Whitaker et al. 2018, p. 156, Fig. 3, or Fig. 3 in Appendix S1 (Supplementary Materials)
^ "The environmental costs and benefits of bioenergy have been the subject of significant debate, particularly for first‐generation biofuels produced from food (e.g. grain and oil seed). Studies have reported life‐cycle GHG savings ranging from an 86% reduction to a 93% increase in GHG emissions compared with fossil fuels (Searchinger et al., 2008; Davis et al., 2009; Liska et al., 2009; Whitaker et al., 2010). In addition, concerns have been raised that N₂O emissions from biofuel feedstock cultivation could have been underestimated (Crutzen et al., 2008; Smith & Searchinger, 2012) and that expansion of feedstock cultivation on agricultural land might displace food production onto land with high carbon stocks or high conservation value (i.e. iLUC) creating a carbon debt which could take decades to repay (Fargione et al., 2008). Other studies have shown that direct nitrogen‐related emissions from annual crop feedstocks can be mitigated through optimized management practices (Davis et al., 2013) or that payback times are less significant than proposed (Mello et al., 2014). However, there are still significant concerns over the impacts of iLUC, despite policy developments aimed at reducing the risk of iLUC occurring (Ahlgren & Di Lucia, 2014; Del Grosso et al., 2014)." Whitaker et al. 2018, p. 151.
^ "The impact of growing bioenergy and biofuel feedstock crops has been of particular concern, with some suggesting the greenhouse gas (GHG) balance of food crops used for ethanol and biodiesel may be no better or worse than fossil fuels (Fargione et al., 2008; Searchinger et al., 2008). This is controversial, as the allocation of GHG emissions to the management and the use of coproducts can have a large effect on the total carbon footprint of resulting bioenergy products (Whitaker et al., 2010; Davis et al., 2013). The potential consequences of land use change (LUC) to bioenergy on GHG balance through food crop displacement or 'indirect' land use change (iLUC) are also an important consideration (Searchinger et al., 2008)." Milner et al. 2016, pp. 317–318.
^ "While the initial premise regarding bioenergy was that carbon recently captured from the atmosphere into plants would deliver an immediate reduction in GHG emission from fossil fuel use, the reality proved less straightforward. Studies suggested that GHG emission from energy crop production and land-use change might outweigh any CO₂ mitigation (Searchinger et al., 2008; Lange, 2011). Nitrous oxide (N₂O) production, with its powerful global warming potential (GWP), could be a significant factor in offsetting CO₂ gains (Crutzen et al., 2008) as well as possible acidification and eutrophication of the surrounding environment (Kim & Dale, 2005). However, not all biomass feedstocks are equal, and most studies critical of bioenergy production are concerned with biofuels produced from annual food crops at high fertilizer cost, sometimes using land cleared from natural ecosystems or in direct competition with food production (Naik et al., 2010). Dedicated perennial energy crops, produced on existing, lower grade, agricultural land, offer a sustainable alternative with significant savings in greenhouse gas emissions and soil carbon sequestration when produced with appropriate management (Crutzen et al., 2008; Hastings et al., 2008, 2012; Cherubini et al., 2009; Dondini et al., 2009a; Don et al., 2012; Zatta et al., 2014; Richter et al., 2015)." McCalmont et al. 2017, p. 490.
^ "Significant reductions in GHG emissions have been demonstrated in many LCA studies across a range of bioenergy technologies and scales (Thornley et al., 2009, 2015). The most significant reductions have been noted for heat and power cases. However, some other studies (particularly on transport fuels) have indicated the opposite, that is that bioenergy systems can increase GHG emissions (Smith & Searchinger, 2012) or fail to achieve increasingly stringent GHG savings thresholds. A number of factors drive this variability in calculated savings, but we know that where significant reductions are not achieved or wide variability is reported there is often associated data uncertainty or variations in the LCA methodology applied (Rowe et al., 2011). For example, data uncertainty in soil carbon stock change following LUC has been shown to significantly influence the GHG intensity of biofuel production pathways (Fig. 3), whilst the shorter term radiative forcing impact of black carbon particles from the combustion of biomass and biofuels also represents significant data uncertainty (Bond et al., 2013)." Whitaker et al. 2018, pp. 156–157.
^ "Miscanthus is one of the very few crops worldwide that reaches true CO₂ neutrality and may function as a CO₂ sink. [...] Related to the combustion of fuel oil, the direct and indirect greenhouse gas emissions can be reduced by a minimum of 96% through the combustion of Miscanthus straw [...]. Due to the C‐sequestration [carbon storage] during Miscanthus growth, this results in a CO₂‐eq mitigation potential of 117%". Emmerling & Pude 2017, pp. 275–276. Emmerling & Pude paraphrase Felten et al. 2013. For yield, carbon sequestration and GHG calculations, see Felten et al. 2013, pp. 160, 166, 168.
^ "Whilst these values represent the extremes, they demonstrate that site selection for bioenergy crop cultivation can make the difference between large GHG [greenhouse gas] savings or losses, shifting life‐cycle GHG emissions above or below mandated thresholds. Reducing uncertainties in ∆C [carbon increase or decrease] following LUC [land use change] is therefore more important than refining N₂O [nitrous oxide] emission estimates (Berhongaray et al., 2017). Knowledge on initial soil carbon stocks could improve GHG savings achieved through targeted deployment of perennial bioenergy crops on low carbon soils (see section 2). [...] The assumption that annual cropland provides greater potential for soil carbon sequestration than grassland appears to be over‐simplistic, but there is an opportunity to improve predictions of soil carbon sequestration potential using information on the initial soil carbon stock as a stronger predictor of ∆C [change in carbon amount] than prior land use." Whitaker et al. 2018, pp. 156, 160.
^ "Fig. 3 confirmed either no change or a gain of SOC [soil organic carbon] (positive) through planting Miscanthus on arable land across England and Wales and only a loss of SOC (negative) in parts of Scotland. The total annual SOC change across GB in the transition from arable to Miscanthus if all nonconstrained land was planted with would be 3.3 Tg C yr⁻¹ [3.3 million tonnes carbon per year]. The mean changes for SOC for the different land uses were all positive when histosols were excluded, with improved grasslands yielding the highest Mg C ha⁻¹ yr⁻¹ [tonnes carbon per hectare per year] at 1.49, followed by arable lands at 1.28 and forest at 1. Separating this SOC change by original land use (Fig. 4) reveals that there are large regions of improved grasslands which, if planted with bioenergy crops, are predicted to result in an increase in SOC. A similar result was found when considering the transition from arable land; however for central eastern England, there was a predicted neutral effect on SOC. Scotland, however, is predicted to have a decrease for all land uses, particularly for woodland due mainly to higher SOC and lower Miscanthus yields and hence less input." Milner et al. 2016, p. 123.
^ "In summary, we have quantified the impacts of LUC [land use change] to bioenergy cropping on SOC [soil organic carbon] and GHG balance. This has identified LUC from arable, in general to lead to increased SOC, with LUC from forests to be associated with reduced SOC and enhanced GHG emissions. Grasslands are highly variable and uncertain in their response to LUC to bioenergy and given their widespread occurrence across the temperate landscape, they remain a cause for concern and one of the main areas where future research efforts should be focussed." Harris, Spake & Taylor 2015, pp. 33, 37 The authors note however that "[t]he average time since transition across all studies was 5.5 years (Xmax 16, Xmin 1) for SOC" and that "[...] the majority of studies considered SOC at the 0–30 cm profile only [...]." Harris, Spake & Taylor 2015, pp. 29–30. Low carbon accumulation rates for young plantations are to be expected, because of accelerated carbon decay at the time of planting (due to soil aeration), and relatively low mean carbon input to the soil during the establishment phase (2-3 years).
^ "In 2015, a workshop was convened with researchers, policymakers and industry/business representatives from the UK, EU and internationally. Outcomes from global research on bioenergy land‐use change were compared to identify areas of consensus, key uncertainties, and research priorities. [...] Our analysis suggests that the direct impacts of dedicated perennial bioenergy crops on soil carbon and nitrous oxide are increasingly well understood and are often consistent with significant life cycle GHG mitigation from bioenergy relative to conventional energy sources. We conclude that the GHG balance of perennial bioenergy crop cultivation will often be favourable, with maximum GHG savings achieved where crops are grown on soils with low carbon stocks and conservative nutrient application, accruing additional environmental benefits such as improved water quality. The analysis reported here demonstrates there is a mature and increasingly comprehensive evidence base on the environmental benefits and risks of bioenergy cultivation which can support the development of a sustainable bioenergy industry." Whitaker et al. 2018, p. 150.
^ "In tropical regions, afforestation may be beneficial since beside sequestering carbon it can lead to cloud formation resulting in a net cooling. In boreal regions, however, low surface albedo of afforested areas might have a warming climatic forcing that 'may exceed the cooling forcing from sequestration' [Thompson 2009]. Bright et al. have defined a possible way of integrating the impact on albedo in the LCA of a forest biofuel [Bright 2012]. [...] In their paper an example for the clear-cut of a boreal forest is reported. In that specific case the increase in albedo that follows the clear-cut harvest may offset about half of the total CO₂ emissions (that include also the biogenic emissions due to carbon stock changes) in a 100 years timeframe. Also Schwaiger and Bird [Schwaiger 2010] have attempted to integrate albedo effects into the bioenergy GHG calculations. They have considered an afforestation project in a south European mountainous area and used average yearly meteorological data. They have concluded that afforestation in the case study area accumulates up to 624 t CO₂ eq./ha, while the change in albedo due to crown cover is equivalent to emissions of roughly 401 t CO₂ eq./ha by the end of the first rotation period (90 years). The net effect, thus, varies around a neutral level with the cumulative result of a slight cooling in the long term. [...] With a similar approach Bright et al. [Bright 2011] have come to the conclusion that for a boreal forest, the albedo effect of the forest management in addition to the fossil fuel replacement leads to a near-neutral climate system. At a global level Bala et al. [Bala 2007] have simulated the climate impacts of deforestation, including the climate forcing of the CO₂ emitted and the albedo changes. They found that global-scale deforestation has a net cooling influence on Earth's climate because the warming carbon-cycle effects of deforestation are overwhelmed by the net cooling associated with changes in albedo and evapotranspiration. Latitude-specific deforestation experiments indicate that afforestation projects in the tropics would be clearly beneficial in mitigating global-scale warming, but would be counterproductive if implemented at high latitudes and would offer only marginal benefits in temperate regions [Betts 2000]." JRC 2014, pp. 57–58.
^ "Black carbon, like other aerosol particles, interacts with clouds, changing their reflectivity and lifetime, with effects on local and global climate. In addition, when calculating the climate effect of BC, it is important to realize that it is often mixed with organic carbon (OC) which is also produced during combustion and which reflects sunlight much more strongly than it absorbs it. A low OC-to-BC ratio means a predominantly absorbing aerosol that will contribute to warming. A high OC-to-BC ratio means a predominantly reflecting (or scattering) aerosol that will contribute to cooling. The ratio depends on the emission source: it can be lower than 1 in the case of emissions from diesel engines, but will be much higher in the case of, for example, smoldering wood (Table 9). [...] In other studies [Kulmala 2004] the analysis has been further expanded to include the emissions of organic carbon (mainly terpenes) from boreal forests, that, besides having an intrinsic cooling effect, act as condensation nuclei for cloud formation, thus enhancing the cloud albedo effect and resulting in additional climate cooling to that of the carbon sink. Spraklen et al. [Spraklen 2008] have quantified the relevance of the cooling effect of organic aerosols emissions and compared it to the warming effect of land surface albedo changes. Using a global atmospheric model they have shown that changes in cloud albedo cause a radiative forcing sufficiently large to result in boreal forests having an overall cooling impact on climate. This is the result of emissions of organic vapours and increased cloud formation due to the increased amount of condensation nuclei (doubled). They conclude that the combination of climate forcings related to boreal forests may result in an important global homeostasis [optimization, stableization]. In cold climatic conditions, the snow–vegetation albedo effect dominates and boreal forests warm the climate, whereas in warmer climates they may emit sufficiently large amounts of organic vapour modifying cloud albedo and acting to cool climate." JRC 2014, pp. 56–58.
^ "Georgescu et al. [Georgescu 2011] have shown that the bio-geo-physical effects that result from hypothetical conversion of annual to perennial bioenergy crops across the central United States would have a significant global climate cooling effect, beside the local cooling related mainly to local increases in transpiration, due to higher albedo. They concluded that the reduction in radiative forcing from albedo alone is equivalent to a carbon emission reduction of 78 t C/ha, which is six times larger than the annual biogeochemical effects that arise from offsetting fossil fuel use." JRC 2014, pp. 58.
^ "An intensive processing, such as for liquid biofuel substitution via lignocellulosic ethanol, causes much longer payback times because of the loss of energy in the biofuels production (about half of the energy content of the biomass is lost in the processing [...]." JRC 2014, p. 34. Note that the JRC use the term "payback time" in the sense of "parity time" as defined in Carbon accounting principles above. See JRC 2014, p. 16.
^ See for instance the estimate of 0.60 W/m² for the 10 t/ha yield above. The calculation is: Yield (t/ha) multiplied with energy content (GJ/t) divided by seconds in a year (31 556 926) multiplied with the number of square metres in one hectare (10 000).
^ ^a ^b Hektor et al. argue that flue gas condensation devices combined with natural drying of biomass makes it possible to achieve similar or better combustion efficiency than coal: "When burning moist biomass, energy is 'lost' in the evaporation of water. However, modern technology makes it possible to recover a large portion of that energy by flue gas condensation devices." The author also recommends "[...] simpler measures, such as natural drying [...]", and argue that "[...] state-of-the art technologies are nowadays generally included in new applications of biomass energy [...]" and that "[...] taking these factors into consideration, biomass would have about the same gross CO₂ emissions per generated amount of energy as coal [...]." Hektor, Backéus & Andersson 2016, p. 4. See also OECD/IEA 2004, p. 20.
^ "The raw material for non-woody pellets can be herbaceous biomass, fruit biomass, aquatic biomass or biomass blends and mixtures. These blends and mixtures can also include woody biomass. They are usually manufactured in a die with total moisture content usually less than 15 % of their mass." ISO 2014b.
^ Transmission loss data from the World Bank, sourced from IEA. The World Bank 2010.
^ Additionally, Smil estimates that newly installed photovoltaic solar parks reaches 7–11 W/m² in sunny regions of the world. Smil 2015, p. 191.
^ "Pathways in the first and fourth quadrants are relatively clear situations in which trade-offs are not evident, and should thus clearly be a target for governance measures; in the sense that pathways in quadrant 1 should be incentivised, while pathways in quadrant 4 should be discouraged. Forest bioenergy pathways which fit within the first quadrant are the ones that are very likely to contribute to climate change mitigation in a short-medium term, and at the same time are likely to improve the condition of local ecosystems and biodiversity (or at least do not affect paths of ecosystem restoration). Pathways in the fourth quadrant are the ones that are unlikely to contribute to climate change mitigation in the short-medium term and at the same time are likely to further degrade ecosystems' condition. Conversely, pathways in quadrants 2 and 3 are the ones for which trade-offs between climate mitigation and biodiversity can be identified or assumed. Pathways in quadrant 2 are the ones that even though they are likely to mitigate climate change, they are also likely to negatively impact local biodiversity. For these pathways, safeguards or mitigation strategies should be investigated, and if available, should be considered mandated as contingent to the promotion of bioenergy. This case is also the only case in which the trade-off mentioned above (global climate change mitigation vs. local degradation) could influence the final evaluation of the pathway. Pathways in the third quadrant are likely to improve local ecosystem condition, but might not mitigate climate change in the short term. In these cases, bioenergy production might be seen as a by-product of restoration operations. In both cases in quadrants 2 & 3, trade-offs that cannot be resolved will need to be weighted and discussed during the decision-making process." Camia et al. 2021, p. 107.
^ "In May 2020, the EU Biodiversity Strategy for 2030 (COM/2020/380) was adopted. In the communication, under section 2.2.5 (“Win-win solutions for energy generation”), the Commission committed to publishing this report on the use of forest biomass for energy production in order to inform the EU climate and energy policies that govern the sustainable use of forest biomass for energy production and the accounting of associated carbon impacts, namely the Renewable Energy Directive, the Emissions Trading Scheme (ETS), and the Regulation on land use, land use change and forestry (LULUCF). [...] [T]he study would take stock of the available data related to the use of woody biomass for bioenergy; assess the uses of woody biomass in the EU with a focus on bioenergy; provide suggestions on how to improve the knowledge base on forests in a harmonised way; and expand the evidence basis by highlighting pathways that minimise trade-offs between climate mitigation and biodiversity conservation." Camia et al. 2021, p. 5.
^ "In this study, we assess three categories of interventions and their potential impacts: removal of logging residues, afforestation and conversion of natural forests to plantations. These three interventions were chosen because they are considered as practices that aim to supply ‘additional' biomass, i.e. growing biomass that would not be produced in the absence of bioenergy demand, or using biomass, such as residues and wastes, which would otherwise decompose or be burned on site. We acknowledge that, until now, many of these responses have not been triggered as a direct consequence of bioenergy expansion, but they are high on the agenda of potential climate mitigation strategies and could occur, in the EU or outside, as a direct or indirect effect of increased EU demand for forest biomass for wood products and bioenergy." Camia et al. 2021, pp. 6–7.
^ "Assessing the impact of forest bioenergy on ecosystems' condition in general, and in particular on biodiversity, is complicated because bioenergy pathways can exert multiple pressures on ecosystems and biodiversity and at the same time alleviate others. This creates an intricate matrix of trade-offs and synergies between forest bioenergy production and biodiversity and the condition of forests. [...] At the local level, intensified forest management to produce additional biomass can increase pressures on forest ecosystems. Similarly, land use change associated to afforestation can drive positive or negative impacts on local biodiversity. Additionally, the supply chain to produce bioenergy commodities is associated to the emission of pollutants which may contribute to acidification, eutrophication, and further climate change. Nevertheless, at the global level, climate change in itself is a major driver of biodiversity loss, therefore the overall benefit to the ecosystems and biodiversity might still be higher from global climate change mitigation if compared with the local level effects mentioned above. The trade-off between potential long-term advantages from climate change mitigation and short term, local ecosystems' degradation is very difficult to quantify. Therefore, under the precautionary principle, we exclude it from this analysis, assuming that we should not evaluate hypothetical long-term benefits versus short-term effects on ecosystems. Instead, we focus our analysis on potential pressures on local biodiversity and ecosystems from land use changes and forest management intensification in order to highlight potential pathways causing negative environmental trade-offs, or “bio-perversities” (Lindenmayer et al., 2012)." Camia et al. 2021, pp. 102–103.
^ "Secondly, concerning the assessment of carbon emissions: the impacts reported here are based on a ‘ceteris paribus' perspective, which is apt to capture only small-scale changes and not suitable to capture the overall impact of large-scale deployment of bioenergy, since it excludes market-mediated effects on other sectors." Camia et al. 2021, p. 148.
^ "Win-win management practices that benefit climate change mitigation and have either a neutral or positive effect on biodiversity include removal of slash (fine, woody debris) below thresholds defined according to local conditions, and afforestation of former arable land with mixed forest or naturally regenerating forests. [...] [C]oppice forests are particularly important in Mediterranean countries, they provide many ecosystem services, have relevant socio-economic functions in many rural areas and are mainly utilised for bioenergy. However, in large areas coppices are no longer managed or completely abandoned, resulting in old or overgrown declining stands. In these cases, it is suggested to encourage active forest management, that would enhance the capacity of these ecosystems to store carbon and supply services. Depending on local considerations the preferred option could be active conversion to high forest, or coppice restoration (see Section 5.9.2). [...] [W]e find that collecting slash within the limits of locally recommended thresholds could generate energy without damaging forest ecosystems and at the same time likely contributing to reducing GHG emissions. Similarly, afforesting former agricultural land with mixed species plantations or with naturally regenerating forests would enhance the terrestrial sink even before producing biomass for energy and thus would contribute to climate change mitigation, while at the same time improving ecosystems' conditions. [...] Collecting slash within the limits of locally recommended thresholds could be used to generate energy without damaging forest ecosystems while likely contributing to reducing GHG emissions. Afforesting former agricultural land with mixed species plantations or with naturally regenerating forests would enhance the terrestrial sink even before producing biomass for material and energy uses and thus would contribute to climate change mitigation, while at the same time improving ecosystems' conditions. [...] [C]ollection and use of low stumps within locally established thresholds in climate areas with high decay rates could potentially provide carbon emissions mitigation without damaging local biodiversity; local conditions should be evaluated in these cases." Camia et al. 2021, pp. 8–149.
^ "Although not extensively captured in the case studies, there is clear consensus in the literature that afforestation of primary, ancient grassland ecosystems which were never forests, may have very detrimental effects on local biodiversity; some authors compare these effects to the destructive effects of deforestation (Abreu et al., 2017; Bond, 2016; Bond et al., 2019; Feurdean et al., 2018; Veldman et al., 2015a, 2015b). Semi-natural grasslands and anthropogenic heathlands are ecosystems where closed canopy forest did not historically develop because of natural processes such as fire or mega fauna, or because of extensive management by local people. Local biodiversity adapted to open spaces has evolved in those ecosystems, and afforestation or tree planting of closed canopy forests is considered as a significant threat for local biodiversity, as highlighted by IPBES (2018a, b). Bubová et al. (2015) reviewed how abandonment of traditional grassland management followed by natural forest succession or active afforestation, is the main driver for the decline of butterfly diversity in Europe.[...] [P]athways in quadrant 2 may provide a significant contribution to climate change that would benefit global ecosystems and biodiversity even if local ecosystems are damaged in the process. However, this is a very uncertain trade-off and would be contrary to the precautionary principle, as explained in section 5.7. In this quadrant, for instance, we can find afforestation of former agricultural land with monoculture plantations: this intervention is likely to lead to carbon benefits in the short-term, but the impacts on local ecosystem should be evaluated carefully, for instance in the framework of landscape mosaic management and climate change resilience. Afforestation of natural grasslands or anthropogenic heathlands could also produce carbon benefits in the medium term, but the cost for local biodiversity, especially for species adapted to open spaces, could be devastating. Indeed, these practices are already discouraged within the Pan-European Guidelines for Afforestation and Reforestation, but they are still popular around the world (Veldman et al., 2015b, 2015a). Further in this quadrant, operations which should be already discouraged by sustainable management guidelines are classified: removing slash in very high quantities could be detrimental for local biodiversity." Camia et al. 2021, pp. 125–147.
^ "The overall carbon impact of afforestation operations needs to be properly calculated including changes in biogenic C-stocks and sinks, the substitution benefits of the newly produced wood, and eventual market-mediated indirect land use change effects. Generally, the overall carbon impact of afforestation is found to be positive, albeit the time scale required might be long (Agostini et al., 2014; Giuntoli et al., 2020b). Nonetheless, not always newly planted forests show a higher C-stock [carbon stock] than existing ecosystems, especially when considering the carbon in soil organic matter. Several studies in our review have tried to provide insights. Bárcena et al. (2014) found increased SOC [soil organic carbon] with afforestation on former cropland and heathland in Northern Europe, however afforestation on former grassland actually decreased SOC levels even for mature forests (>30 years). Laganière et al. (2010) found very similar results from their global meta-analysis, with afforestation on former cropland leading to a significant increase in SOC, but no significant changes in SOC for former pastures and natural grasslands. Furthermore, they also found that the tree species (and thus plantation features) influence the final result, with broadleaves forests generating the highest SOC increase and coniferous forests having the same SOC as the former land use. Li et al. (2012), similarly, found increased SOC for new forests on former cropland and pastureland, but a stable or slightly decreased SOC in former grassland. [...] Pathways in quadrant 3 are probably unlikely to be driven by bioenergy demand, however, they might be definitely valuable for conservation interventions and produce biomass for bioenergy." Camia et al. 2021, pp. 125–147.
^ "Lose-lose pathways include removal of coarse woody debris, removal of low stumps, and conversion of primary or natural forests into plantations. [...] Bubová et al. (2015) reviewed how abandonment of traditional grassland management followed by natural forest succession or active afforestation, is the main driver for the decline of butterfly diversity in Europe. [...] Generally, the overall carbon impact of afforestation is found to be positive, albeit the time scale required might be long (Agostini et al., 2014; Giuntoli et al., 2020b). Nonetheless, not always newly planted forests show a higher C-stock than existing ecosystems, especially when considering the carbon in soil organic matter. Several studies in our review have tried to provide insights. Bárcena et al. (2014) found increased SOC with afforestation on former cropland and heathland in Northern Europe, however afforestation on former grassland actually decreased SOC [soil organic carbon] levels even for mature forests (>30 years). Laganière et al. (2010) found very similar results from their global meta-analysis, with afforestation on former cropland leading to a significant increase in SOC, but no significant changes in SOC for former pastures and natural grasslands. Furthermore, they also found that the tree species (and thus plantation features) influence the final result, with broadleaves forests generating the highest SOC increase and coniferous forests having the same SOC as the former land use. Li et al. (2012), similarly, found increased SOC for new forests on former cropland and pastureland, but a stable or slightly decreased SOC in former grassland. [...] [S]everal pathways are categorized in the lose-lose quadrant and should be discouraged. For instance, the removal of CWD [course woody debris] and low stumps can be detrimental to forest ecosystems while at the same time likely not contributing to reducing carbon emissions in the short or even medium term compared to fossil sources. [...] Further, as expected, the conversion of natural and old growth forests to plantations aiming to provide wood for bioenergy would be extremely negative for local biodiversity, and at the same time it would provide no carbon mitigation in the short-medium term and should be thus discouraged. Similar considerations are valid also for the conversion of naturally regenerating forests to high-intensity management plantations: the impact on local biodiversity is highly negative while, even though wood production might increase, the benefits in terms of carbon mitigation are only accrued in the medium to long term. [...] Depending on local conditions, determining the decay rates on the forest floor, the removal of Coarse Woody Debris and low stumps can be detrimental to forest ecosystems while at the same time likely not contribute to reducing carbon emissions in the short or even medium term compared to fossil sources." Camia et al. 2021, pp. 8–147.
^ "[W]e are of the opinion that several negative impacts associated with the pathways reviewed in this study could be effectively minimised through swift and robust implementation of the RED II sustainability criteria related to forest biomass, which will be further operationalised through the upcoming EU operational guidance on the evidence for demonstrating compliance with the forest biomass criteria. [...] More specifically, RED II indicates specific no-go areas for agricultural biomass, meaning that biomass for bioenergy cannot be directly produced from land that was, at any time after 2008, classified as highly biodiverse grasslands, primary forest, highly biodiverse forest, or protected areas. However, these criteria do not apply to forest biomass (except for the protected areas criterion). Expanding such land criteria to forest biomass would introduce additional safeguards to ensure that forest biomass for energy is not associated with the afforestation pathways that have the most negative impacts, i.e. those on high-nature value grasslands or anthropogenic heathlands, and it would also forbid the sourcing of wood from plantations established on converted old-growth, primary forest for energy feedstock." Camia et al. 2021, pp. 10–11.
^ "Irrespective from market drivers, a moderate future increase in the production of harvested wood products at EU level may be expected because of forest age dynamics (Grassi et al. 2018 and Korosuo et al. 2020) and, in some circumstance, to reduce risks (or as consequence) of forest fires, pests and windstorms. The residues and the industrial by-products associated with these harvested wood products – along with wood from silvicultural operations specifically aimed at enhancing the quality of trees and the growth of the forest stands - may be meaningfully used for energy production, also contributing to the economic viability of forestry which is an integral element of Sustainable Forest Management." Camia et al. 2021, p. 93.
^ "[...] [A]s scientists, we need to clearly understand our role in this debate: we can gather and synthesise evidence highlighting problems and possible solutions as honest brokers of policy options, but we cannot identify the ‘right' policy tool or the ‘right' policy principle to follow because those issues are within the realm of the political arena and no amount of scientific research will appease ethical disputes. [...] If the question is ‘Is forest bioenergy sustainable?' the answer might be positive or negative depending on who attempts to answer it, and how. [...] [T]o a large extent there are no right or wrong answers, and the definition of ‘good enough' solutions is the role of policymaking, not science. [...] As illustrated schematically in Figure 28, the various tools within the EU legal framework provide incentives towards different management goals for European forests, from incentivising forest bioeconomy to protecting the carbon sink and forest ecosystems. The resulting balance of these different pulling drivers will eventually define both the contribution of forests wood-based products to EU climate mitigation, as well as the resulting state of forests' health (Wolfslehner et al., 2020). As mentioned also in Section 5.2.1, it is natural that different stakeholders with different worldviews, including within the scientific community, have a preference for one driver or another. At the same time, many different equilibrium points are possible and acceptable within the socio-economic context of each Member State. [...] Differences in ethical values on the interaction between humans and nature clearly play a role in defining what ‘sustainable management' means. We think that if we want to de-toxify the debate surrounding the sustainability of forest bioenergy, these divergences in values should be acknowledged and discussed explicitly also within the scientific community." Camia et al. 2021, pp. 6, 83, 91, 166.
^ "Traditional biomass (fuelwood, charcoal, agricultural residues, animal dung) used for cooking and heating by some 2.8 billion people (38% of global population) in non-OECD countries accounts for more than half of all bioenergy used worldwide (IEA 2017; REN21 2018) (Cross-Chapter Box 7 in Chapter 6). Cooking with traditional biomass has multiple negative impacts on human health, particularly for women, children and youth (Machisa et al. 2013; Sinha and Ray 2015; Price 2017; Mendum and Njenga 2018; Adefuye et al. 2007) and on household productivity, including high workloads for women and youth (Mendum and Njenga 2018; Brunner et al. 2018; Hou et al. 2018; Njenga et al. 2019). Traditional biomass is land-intensive due to reliance on open fires, inefficient stoves and overharvesting of woodfuel, contributing to land degradation, losses in biodiversity and reduced ecosystem services (IEA 2017; Bailis et al. 2015; Masera et al. 2015; Specht et al. 2015; Fritsche et al. 2017; Fuso Nerini et al. 2017). Traditional woodfuels account for 1.9–2.3% of global GHG emissions, particularly in ‘hotspots' of land degradation and fuelwood depletion in eastern Africa and South Asia, such that one-third of traditional woodfuels globally are harvested unsustainably (Bailis et al. 2015). Scenarios to significantly reduce reliance on traditional biomass in developing countries present multiple co-benefits (high evidence, high agreement), including reduced emissions of black carbon, a short-lived climate forcer that also causes respiratory disease (Shindell et al. 2012). A shift from traditional to modern bioenergy, especially in the African context, contributes to improved livelihoods and can reduce land degradation and impacts on ecosystem services (Smeets et al. 2012; Gasparatos et al. 2018; Mudombi et al. 2018)." IPCC 2019a, p. 375.
^ "In the NZE Scenario, bioenergy rapidly shifts to 100% sustainable sources of supply, and sustainable use. There is a complete phase-out of the traditional use of solid biomass for cooking, which is inefficient, often linked to deforestation, and whose pollution was responsible for 2.5 million premature deaths in 2020. The traditional use of solid biomass – estimated at around 40% of total bioenergy supply, or around 25 EJ, today – falls to zero by 2030 in the NZE Scenario, in line with achieving UN Sustainable Development Goal 7 on universal access to affordable, reliable, sustainable and modern energy for all. [...] Sustainable use of bioenergy in the NZE Scenario not only avoids negative impacts such as increased deforestation and competition with food production – it also delivers benefits beyond the energy sector. Shifting from traditional use of biomass to modern bioenergy can avoid undue burdens on women often tasked with collecting wood for fuel, bring health benefits from reduced air pollution and proper waste management, and reduce methane emissions from inefficient combustion and waste decomposition. More generally, sustainable bioenergy can provide a valuable source of employment and income for rural communities in emerging economies." IEA 2021a.
^ See EPA 2020, p. 1. The emission factors are based on the higher heating value (HHV) of the different fuels. The HHV value reflects the actual chemical energy stored in the fuel (mainly carbon and hydrogen molecules), without taking into account 1.) the energy that is lost by producing steam (when the fuel's hydrogen content reacts with oxygen at combustion), and 2.) the energy that is lost in combustion by evaporating the fuel's moisture content. The fuel's lower heating value (LHV) is the energy that remains after the necessary amount of energy has been spent to vaporize all this water. See OECD/IEA 2004, p. 20.
^ "Some scientific papers state that burning biomass for energy produces higher emissions of CO₂ per kWh of electricity at the smoke-stack compared with burning coal due to lower energy density of wood and/or less efficient conversion to electricity (e.g. Brack, 2017; Norton et al., 2019; Searchinger et al., 2018; Sterman et al., 2018; Walker et al., 2013), leading to the assertion that ‘biomass is worse for the climate than coal' (Johnston & van Kooten, 2015; McClure, 2014; PFPI, 2011; RSBP, 2012; Tsanova, 2018; Yassa, 2017). However, this interpretation neglects several significant factors. First, stack emissions will not necessarily increase when there is a shift to biomass fuels. The CO₂ emission factor (g CO₂ per GJ of fuel) is solely dependent on the chemical composition of the fuel. Wood and coal have similar CO₂ emission factors, as the ratio of heating values between the two fuels is similar to the ratio of carbon content (ECN, undated; Edwards et al., 2014; US EPA, 2018; van Loo & Koppejan, 2008). Where biomass is co-fired with coal in large power plants, the conversion efficiency may decrease a few percent, although there is usually no significant efficiency penalty when the co-firing ratio is below 10% (van Loo & Koppejan, 2008). Conversion efficiencies depend on fuel properties including moisture content and grindability in addition to heating value (Mun et al., 2016; Shi et al., 2019; Zuwała & Lasek, 2017). For low rank coal, biomass co-firing (especially torrefied biomass) can increase the boiler efficiency and net power plant efficiency (Liu et al., 2019; Thrän et al., 2016). Smaller biomass-fired plants can have lower electric conversion efficiency than large coal-fired plants, but as they are typically combined heat and power plants, they also displace heat production from other sources, that could otherwise have generated fossil fuel emissions (e.g. Madsen & Bentsen, 2018). Large dedicated biomass units (converted from coal) can operate with roughly the same level of thermal efficiency as delivered historically from coal (Koss, 2019)." Cowie et al. 2021, p. 1214.
^ Total stack emissions for wood pellets were 0.897 Mt CO₂/MWh_el vs. 0.877 Mt CO₂/MWh_el for coal. Buchholz & Gunn 2017, pp. 4, 6, 9.
^ "Drax's biomass delivers carbon savings of more than 80% compared to coal – this includes emissions from our supply chain." Drax 2020.
^ See FutureMetrics 2015a, pp. 1–2. Chatham House notes that modern CHP plants (Combined Heat and Power) achieve much higher efficiencies, above 80%, for both fossil fuels and biomass. Chatham House 2017, p. 16.
^ The individual emission rates are: Wood 112 000 kg CO₂eq per TJ, anthracite 98 300, coking coal 94 600, other bituminous 94 600, sub-bituminous 96 100, lignite 101 000. IPCC 2006a, pp. 2.16–2.17.
^ "Estimating gross emissions only, creates a distorted representation of human impacts on the land sector carbon cycle. While forest harvest for timber and fuelwood and land-use change (deforestation) contribute to gross emissions, to quantify impacts on the atmosphere, it is necessary to estimate net emissions, that is, the balance of gross emissions and gross removals of carbon from the atmosphere through forest regrowth [...]." IPCC 2019a, p. 368.
^ "It is incorrect to determine the climate change effect of using biomass for energy by comparing GHG emissions at the point of combustion [...] the misplaced focus on emissions at the point of combustion blurs the distinction between fossil and biogenic carbon, and it prevents proper evaluation of how displacement of fossil fuels with biomass affects the development of atmospheric GHG concentrations." IEA Bioenergy 2019, pp. 3–4.
^ "Sustainable Forest Management (SFM) is defined as ‘the stewardship and use of forests and forest lands in a way, and at a rate, that maintains their biodiversity, productivity, regeneration capacity, vitality and their potential to fulfill, now and in the future, relevant ecological, economic and social functions, at local, national, and global levels, and that does not cause damage to other ecosystems' [...]. This SFM definition was developed by the Ministerial Conference on the Protection of Forests in Europe and has since been adopted by the Food and Agriculture Organization [of the United Nations (FAO)]." IPCC 2019a, p. 351. Further, IPCC writes: "Sustainable forest management can prevent deforestation, maintain and enhance carbon sinks and can contribute towards GHG emissions-reduction goals. Sustainable forest management generates socio-economic benefits, and provides fibre, timber and biomass to meet society's growing needs. IPCC 2019a, p. 348.
^ "The trends of productivity shown by several remote-sensing studies (see previous section) are largely consistent with mapping of forest cover and change using a 34-year time series of coarse resolution satellite data (NOAA AVHRR) (Song et al. 2018). This study, based on a thematic classification of satellite data, suggests that (i) global tree canopy cover increased by 2.24 million km² between 1982 and 2016 (corresponding to +7.1%) but with regional differences that contribute a net loss in the tropics and a net gain at higher latitudes, and (ii) the fraction of bare ground decreased by 1.16 million km² (corresponding to –3.1%), mainly in agricultural regions of Asia (Song et al. 2018), see Figure 4.5. Other tree or land cover datasets show opposite global net trends (Li et al. 2018b), but high agreement in terms of net losses in the tropics and large net gains in the temperate and boreal zones (Li et al. 2018b; Song et al. 2018; Hansen et al. 2013)." IPCC 2019a, p. 367.
^ "In the previous version of the Renewable Energy Directive (EU, 2009) in force until 2020, sustainability criteria were defined only for biomass used for the production of biofuels and bioliquids. With the REDII, to be transposed by countries by June 2021, new criteria are defined also cover solid and gaseous biomass fuels used in large installations for the production of power and heating or cooling." Camia et al. 2021, p. 78.
^ "Second, our findings are similarly compatible with the well-known age-related decline in productivity at the scale of even-aged forest stands. [...] We highlight the fact that increasing individual tree growth rate does not automatically result in increasing stand productivity because tree mortality can drive orders-of-magnitude reductions in population density. That is, even though the large trees in older, even-aged stands may be growing more rapidly, such stands have fewer trees. Tree population dynamics, especially mortality, can thus be a significant contributor to declining productivity at the scale of the forest stand." Stephenson et al. 2014, p. 3.
^ "Recent studies indicate, that effects of forest management actions on soil C [carbon] stocks can be difficult to quantify and reported effects have been variable and even contradictory (see Box 4.3a)." Because the "current scientific basis is not sufficient", the IPCC will not currently provide soil carbon emission factors for forest management.IPCC 2019f, p. 4.6.
^ "SFM [sustainable forest management] applied at the landscape scale to existing unmanaged forests can first reduce average forest carbon stocks and subsequently increase the rate at which CO₂ is removed from the atmosphere, because net ecosystem production of forest stands is highest in intermediate stand ages (Kurz et al. 2013; Volkova et al. 2018; Tang et al. 2014). The net impact on the atmosphere depends on the magnitude of the reduction in carbon stocks, the fate of the harvested biomass (i.e. use in short – or long-lived products and for bioenergy, and therefore displacement of emissions associated with GHG-intensive building materials and fossil fuels), and the rate of regrowth. Thus, the impacts of SFM on one indicator (e.g., past reduction in carbon stocks in the forested landscape) can be negative, while those on another indicator (e.g., current forest productivity and rate of CO₂ removal from the atmosphere, avoided fossil fuel emissions) can be positive. Sustainably managed forest landscapes can have a lower biomass carbon density than unmanaged forest, but the younger forests can have a higher growth rate, and therefore contribute stronger carbon sinks than older forests (Trofymow et al. 2008; Volkova et al. 2018; Poorter et al. 2016)." IPCC 2019a, p. 351.
^ "Bioenergy provides only 5% of total electricity generation in 2050, but it is an important source of low-emissions flexibility to complement variable generation from solar PV and wind. In the industry sector, where solid bioenergy demand reaches 20 EJ in 2050, it is used to meet high temperature heat needs that cannot be easily electrified such as paper and cement production. In 2050, bioenergy meets 60% of energy demand in the paper sector and 30% of energy demand for cement production." IEA 2021a.
^ The IEA estimates that replacing short rotation coppice forests with hydrogen production (for heat processing purposes) would cost 4.5 trillion USD: "The additional wind, solar, battery and electrolyser capacity, together with the electricity networks and storage needed to support this higher level of deployment would cost more than USD 5 trillion by 2050. This is USD 4.5 trillion more than would be needed if the use of bioenergy were to be expanded as envisaged in the NZE [Net Zero Emissions scenario], and would increase the total investment needed in the NZE by 3%. While it might therefore be possible still to achieve net‐zero emissions in 2050 without expanding land use for bioenergy, this would make the energy transition significantly more expensive. " IEA 2021b, p. 94.
^ "Research demonstrates that demand for wood helps keep land in forest and incentivizes investments in new and more productive forests, all of which have significant carbon benefits. [...] Failing to consider the effects of markets and investment on carbon impacts can distort the characterization of carbon impacts from forest biomass energy." NAUFRP 2019, p. 2.
^ Favero et al. focus on a potential future increase in demand and argues: "Increased bioenergy demand increases forest carbon stocks thanks to afforestation activities and more intensive management relative to a no-bioenergy case [...] higher biomass demand will increase the value of timberland, incentivize additional investment in forest management and afforestation, and result in greater forest carbon stocks over time". Favero, Daigneault & Sohngen 2020, p. 6.
^ "Some studies assess unharvested forest as one (and sometimes the only) reference scenario [...] and attribute extra GHG emissions to the bioenergy system based on forgone sequestration in comparison with natural regeneration. Others use a historical baseline reference point, without considering the dynamic nature of carbon stocks under a no-bioenergy scenario [...]. For biomass obtained as a co-product from forests managed for timber production, the relevant reference is commonly management for timber only, with thinning and harvest residues decomposing (or burned) on-site [...]. In some situations, the most likely reference land use could involve land use change. For example, markets for wood products can be an important incentive for private landowners to retain land as managed forest rather than converting to other uses [...]; the reference scenario in this situation may involve: regeneration of natural forest, possibly subject to higher incidence of wildfire; replacement of forest stands with agriculture; or urbanization, each with different impacts on the land carbon stock [...]. Assuming the forest would remain unharvested in the no-bioenergy scenario is not a realistic reference in situations where landholders use the land to generate income, unless landholders can obtain equivalent income from payments for carbon sequestration or other ecosystem services [...]." Cowie et al. 2021, p. 1218.
^ According to FAO, tree cover in Australia is increasing, but carbon stock is only provided for Oceania as a whole. FAO 2020, p. 136.
^ Wood chips, mainly used in the paper industry, have similar data; Europe (including Russia) produced 33% and North America 22%, while forest carbon stock increased in both areas. West, Central and East Asia combined produced 18%, and the forest carbon stock in this areas increased from 31.3 to 43.3 Gt. Wood chips production in the areas of the world were carbon stock is decreasing, was 26.9% in 2019. For wood pellet and wood chips production data, see FAOSTAT 2020. For carbon stock data, see FAO 2020, p. 52, table 43.
^ "The potentially very long payback periods for forest biomass raise important issues given the UNFCCC's aspiration of limiting warming to 1.5 °C above preindustrial levels to ‘significantly reduce the risks and impacts of climate change'. On current trends, this may be exceeded in around a decade. Relying on forest biomass for the EU's renewable energy, with its associated initial increase in atmospheric carbon dioxide levels, increases the risk of overshooting the 1.5°C target if payback periods are longer than this. The European Commission should consider the extent to which large-scale forest biomass energy use is compatible with UNFCCC targets and whether a maximum allowable payback period should be set in its sustainability criteria." EASAC 2017, p. 34.
^ "Some authors (e.g. Booth, 2018; Brack, 2017; Norton et al., 2019) propose that forest bioenergy should only receive support under renewable energy policies if it delivers net reduction in atmospheric CO₂ within about a decade, due to the urgent need to reduce GHG emissions. However, besides the subjectivity of payback time analysis raised above, applying a 10-year payback time as a criterion for identifying suitable mitigation options is inconsistent with the long-term temperature goal of the Paris Agreement, which requires that a balance between emission and removals is reached in the second half of this century (Tanaka et al., 2019). Furthermore, it reflects a view on the relationship between net emissions, global warming and climate stabilization that contrasts with the scenarios presented in the SR1.5: The report shows many alternative trajectories towards stabilization temperatures of 1.5 and 2°C warming that reach net zero at different times and require different amounts of CDR (IPCC, 2018). The IPCC report did not determine that individual mitigation measures must meet specific payback times, but rather that a portfolio of mitigation measures is required that together limits the total cumulative global anthropogenic emissions of CO₂." Cowie et al. 2021, p. 1213.
^ "Furthermore, applying a payback time criterion when evaluating forest bioenergy, and determining the contribution of bioenergy to meeting the Paris Agreement temperature goal, is complicated by the fact that bioenergy systems operate within the biogenic carbon cycle (see Section 3), which implies a fundamentally different influence on atmospheric CO₂ concentrations over time compared to fossil fuel emissions (Cherubini et al., 2014). [...] [C]omparing GHG emissions from biomass and fossil fuels at the point of combustion ignores the fundamental difference between fossil fuels and biomass fuels. Burning fossil fuels releases carbon that has been locked up in the ground for millions of years. Fossil fuel emissions transfer carbon from the lithosphere to the biosphere–atmosphere system, causing temperature increases that are irreversible on timescales relevant for humans (Archer et al., 2009; Solomon et al., 2009; Ter-Mikaelian, Colombo, & Chen, 2015). In contrast, bioenergy operates within the biosphere–atmosphere system, and burning biomass emits carbon that is part of the continuous exchange of carbon between the biosphere and the atmosphere (Smith et al., 2016)." Cowie et al. 2021, pp. 1213–1215.
^ "The IPCC emphasizes the need for transformation of all sectors of society to achieve the ‘well below 2°C' goal of the Paris Agreement (IPCC, 2018). This will entail technology and infrastructure development to generate a portfolio of emissions reduction and CDR strategies. Such investments may include, for example, scaling-up battery manufacturing to support electrification of car fleets, building rail infrastructure and district heating networks and changing the management and harvesting of forests and other lands to provide biomass for biobased products. The mobilization of mitigation options such as these can initially increase net GHG emissions while providing products and services with low, neutral or net negative emissions in the longer term (Cuenot & Hernández, 2016; Hausfather, 2019). The contribution of specific options to mitigation will depend on technology readiness level, costs, resource availability and inertia of existing technologies and systems. Options assessed as having low net GHG emissions per unit energy provided may be restricted by immature development, high cost or dependence on new infrastructure. Other options, including bioenergy, have greater near-term mitigation potential due to being compatible with existing infrastructure and cost competitive in many applications. Strategy development needs to recognize the complementarity of many mitigation options, and balance trade-offs between short- and long-term emissions reduction objectives. Critically, strategies based on assessments of individual technologies in isolation from their broader context, and that apply a strong focus on emissions reduction in the short term, can make long-term climate goals more difficult to achieve (e.g. Berndes at al., 2018; Smyth et al., 2014). Mitigation options available in the near term need to be evaluated beyond the direct effect on GHG emissions, considering also their influence on systems transition and implementation of other mitigation options (see Section 2)." Cowie et al. 2021, p. 1214.
^ "Some have argued that the length of the carbon payback period does not matter as long as all emissions are eventually absorbed. This ignores the potential impact in the short term on climate tipping points (a concept for which there is some evidence) and on the world's ability to meet the target set in the 2015 Paris Agreement to limit temperature increase to 1.5°C above pre-industrial levels, which requires greenhouse gas emissions to peak in the near term. This suggests that only biomass energy with the shortest carbon payback periods should be eligible for financial and regulatory support." EASAC 2017, p. 4.
^ "Risks related to climate tipping points are sometimes raised in relation to the timing of GHG savings: crossing thresholds, for example, associated with forest dieback or thaw of permafrost, could lead to large, irreversible changes in the global climate system (e.g. Grimm et al., 2013). A recent study found a low probability of crossing a tipping point in the global climate system if warming does not exceed 2°C (Fischer et al., 2018). Also, critical threshold values and irreversibility of specific tipping points are uncertain (Collins et al., 2013), and the universal application of critical threshold values is questioned in relation to ecosystem function (Hillebrand et al., 2020). Nevertheless, uncertainties and risks associated with climate tipping points are additional considerations in evaluations of different trajectories towards temperature stabilization. Rather than connecting the timing of GHG savings to specific but uncertain climate tipping points, evaluation of bioenergy options is preferably based on a holistic assessment that considers how bioenergy can contribute to resilience and adaptation to changes in climate along with other environmental stressors." Cowie et al. 2021, p. 1214.
^ "Harvesting immediately reduces the standing forest carbon stock compared with less (or no) harvesting (Bellassen and Luyssaert, 2014; Sievänen et al., 2014) and it may take from decades to centuries until regrowth restores carbon stocks to their former level—especially if oldgrowth forests are harvested." EASAC 2017, p. 21.
^ "Following this argument, the carbon dioxide (and other greenhouse gases) released by the burning of woody biomass for energy, along with their associated life-cycle emissions, create what is termed a ‘carbon debt' – i.e. the additional emissions caused by burning biomass instead of the fossil fuels it replaces, plus the emissions absorption foregone from the harvesting of the forests. Over time, regrowth of the harvested forest removes this carbon from the atmosphere, reducing the carbon debt. The period until carbon parity is achieved (i.e. the point at which the net cumulative emissions from biomass use are equivalent to those from a fossil fuel plant generating the same amount of energy) is usually termed the ‘carbon payback period'. After this point, as regrowth continues biomass may begin to yield ‘carbon dividends' in the form of atmospheric greenhouse gas levels lower than would have occurred if fossil fuels had been used. Eventually carbon levels in the forest return to the level at which they would have been if they had been left unharvested. (Some of the literature employs the term ‘carbon payback period' to describe this longer period, but it is more commonly used to mean the time to parity with fossil fuels; this meaning is used in this paper.)" Chatham House 2017, p. 27.
^ "It has been argued that carbon balances should not be assessed at the stand level since at landscape level depletion of carbon in one stand may be compensated by growth in a stand elsewhere. For scientific analysis of the impact on climate forcing, however, it is necessary to compare the effects of various bioenergy harvest options against a baseline of no bioenergy harvest (or other credible counterfactual scenarios) for the same area of forest. Such studies provide information on the impacts of changes at the stand level, which can then be integrated with other factors (economic, regulatory and social) that may influence effects at landscape level." EASAC 2017, p. 23.
^ "It is important to realize that our 3650 ton per year CHP plant does not receive 3650 tons in one delivery and does not release 3650 tons of wood's worth of carbon in one lump either. In fact, the forest products industry can be characterized as a just-in-time manufacturing system. For our CHP plant, 10 tons per day are sustainably harvested and delivered off of our 3650 acre FSC or SFI certified forest. So the carbon released into the atmosphere that day is from 10 tons of wood. The atmosphere “sees” new carbon. But during that same day on our 3650 acre plot, 10 new tons of wood grow and sequester the amount of carbon that was just released." FutureMetrics 2011b, p. 2.
^ "Forests are generally managed as a series of stands of different ages, harvested at different times, to produce a constant supply of wood products. When considered at plot level, long-rotation forests take many years to regrow after harvest, and the EASAC statement indicates this as a time gap between releasing forest carbon and its reabsorption from the atmosphere. However, across the whole forest estate or landscape, the temporal fluctuations are evened out since other stands continue to grow and sequester carbon, making the time gap as indicated by EASAC less relevant. If annual harvest does not exceed the annual growth in the forest, there is no net reduction in forest carbon." IEA Bioenergy 2019: "The use of forest biomass for climate change mitigation: response to statements of EASAC" IEA Bioenergy 2019, p. 2.
^ The forest landscape works as a proxy for calculating specifically human GHG emissions: "In the AFOLU [Agriculture, Forestry and Other Land Use] sector, the management of land is used as the best approximation of human influence and thus, estimates of emissions and removals on managed land are used as a proxy for anthropogenic emissions and removals on the basis that the preponderance of anthropogenic effects occurs on managed lands (see Vol. 4 Chapter 1). This allows for consistency, comparability, and transparency in estimation. Referred to as the Managed Land Proxy (MLP), this approach is currently recognised by the IPCC as the only universally applicable approach to estimating anthropogenic emissions and removals in the AFOLU sector (IPCC 2006, IPCC 2010)." IPCC 2019j, p. 2.67.
^ "The natural disturbance component is subtracted from the total estimate of [...] emissions and removals, yielding an estimate of the emissions and removals associated with human activity on managed land." See IPCC 2019j, p. 2.72. "The 2006 IPCC Guidelines are designed to assist in estimating and reporting national inventories of anthropogenic greenhouse gas emissions and removals. For the AFOLU Sector, anthropogenic greenhouse gas emissions and removals by sinks are defined as all those occurring on ‘managed land'. Managed land is land where human interventions and practices have been applied to perform production, ecological or social functions. [...] This approach, i.e., the use of managed land as a proxy for anthropogenic effects, was adopted in the GPG–LULUCF and that use is maintained in the present guidelines. The key rationale for this approach is that the preponderance of anthropogenic effects occurs on managed lands. By definition, all direct human-induced effects on greenhouse gas emissions and removals occur on managed lands only. While it is recognized that no area of the Earth's surface is entirely free of human influence (e.g., CO₂ fertilization), many indirect human influences on greenhouse gases (e.g., increased N deposition, accidental fire) will be manifested predominately on managed lands, where human activities are concentrated. Finally, while local and short-term variability in emissions and removals due to natural causes can be substantial (e.g., emissions from fire, see footnote 1), the natural ‘background' of greenhouse gas emissions and removals by sinks tends to average out over time and space. This leaves the greenhouse gas emissions and removals from managed lands as the dominant result of human activity. Guidance and methods for estimating greenhouse gas emissions and removals for the AFOLU Sector now include: • CO₂ emissions and removals resulting from C stock changes in biomass, dead organic matter and mineral soils, for all managed lands; • CO₂ and non-CO₂ emissions from fire on all managed land; • N₂O emissions from all managed soils; • CO₂ emissions associated with liming and urea application to managed soils; • CH4 emissions from rice cultivation; • CO₂ and N₂O emissions from cultivated organic soils; • CO₂ and N₂O emissions from managed wetlands (with a basis for methodological development for CH4 emissions from flooded land in an Appendix 3); • CH4 emission from livestock (enteric fermentation); • CH4 and N₂O emissions from manure management systems; and • C stock change associated with harvested wood products." See IPCC 2006b, p. 1.5.
^ "Only part of the biomass from felled trees is removed from forests during harvest operations, on average around 80 percent for the EU as a whole during the period 2004 to 2013 (estimate also based on Pilli et al. 2017). The remainder is left as logging (primary) residues." Camia et al. 2021, p. 34.
^ "Salvage loggings are any harvesting activity consisting of recovering timber that can still be used, at least in part, from lands affected by natural disturbances (source: EU 2013.); with natural disturbances denominating damages caused by any factor (biotic or abiotic) that adversely affects the vigour and productivity of the forest and that is not a direct result of human activities (FAO 2018). Salvage logging is part of the removals. It includes both the removal of dead trees (belonging to what is reported as natural losses) and living trees (part of the growing stock) to prevent the spread of diseases or pests. Roundwood includes all wood removed with or without bark, including wood removed in its round form, or split, roughly squared or in other form (e.g. branches, roots, stumps and burls (where these are harvested)) and wood that is roughly shaped or pointed. It is a general term referring to wood fuel, including wood for charcoal and industrial roundwood. All roundwood is also referred to as primary wood or primary woody biomass. Fuelwood is roundwood that will be used as fuel for energy purposes such as cooking, heating, or power production. It includes wood harvested from main stems, branches and other parts of trees (where these are harvested for fuel), round or split, and wood that will be used for the production of charcoal (e.g. in pit kilns and portable ovens), wood pellets and other agglomerates. It also includes wood chips to be used for fuel that are made directly (i.e. in the forest) from roundwood. It excludes wood charcoal, pellets, and other agglomerates. Industrial roundwood corresponds to all roundwood except fuelwood. It includes sawlogs and veneer logs; pulpwood, round and split; and other industrial roundwood. As described in Chapter 3, industrial roundwood, although normally intended to be used for manufacturing of woodbased products, can sometimes end up as fuel." Pulpwood is defined like so: "Roundwood that is primarily intended for the production of pulp, particleboard or fibreboard. It includes: roundwood (with or without bark) in its round form or as splitwood or wood chips made directly (i.e. in the forest) from roundwood." Stemwood is defined like so: The wood of the stem(s) of a tree, i.e. the above ground main growing shoot(s). Stemwood includes wood in main axes and in major branches where there is at least X m of ‘straight' length to Y cm top diameter. (Source: Camia et al. 2018). Stemwood, within the context of this study, is the over bark biomass of the stem from 15 cm height (thus excluding the stump) up to a minimum top diameter of 9 cm." Sawnwood is defined like so: "Wood that has been produced from roundwood, either by sawing lengthways or by a profile-chipping process and that exceeds 6 mm in thickness. It includes planks, beams, joists, boards, rafters, scantlings, laths, boxboards and 'lumber', etc., in the following forms: un-planed, planed, end jointed, etc." Camia et al. 2021, pp. 21, 68.
^ "In general, prioritizing residues and a cascade use of wood remains a key overarching principle for maximizing the positive climate impact of bioenergy [...]." Camia et al. 2021, p. 92.
^ "Further characterising the primary woody biomass used, we estimate that roughly 20% of the total wood used for energy production is made up of stemwood, while 17% is made up of other wood components (treetops, branches, etc.). Based on available knowledge, at least half of the stemwood used for energy is assumed to be derived from coppice forests, which are particularly important in Mediterranean countries. Coppice forests, for the most part, provide many ecosystem services, and this management system has relevant socio-economic functions in many rural areas. However, in large areas coppices are no longer managed, resulting in old or overgrown declining stands; it is suggested to encourage active coppice restoration or conversion into high forest, depending on local conditions, to enhance the capacity of these ecosystems to store carbon and supply wood and other services. [...] Following what illustrated in section 3.4, for the year 2015 about 20% of this biomass can be broadly estimated as stemwood from primary wood (of which at least half is likely from coppice forests), while a larger part would come from either primary other wood components (tree tops, branches, that would have anyway emitted CO₂ in their decaying processes if left in the forest as residues, about 17%) or from secondary sources (by-products of wood processing industries, bark, post-consumer wood, about 49%); the remaining 14%, being reported as uncategorized, cannot be attributed (see Figure 8). Based on this analysis, it could be preliminary concluded that the large majority of forest bioenergy currently used in the EU is based on residues and the widely recommended “cascade” approach (EU 2015). However, the increase in woody biomass used for energy production from 2005 to 2018 (about 34%, dashed blue line) seems mainly associated to an increase in fuelwood (see Figure 10). More importantly, the large uncertainty in the bioenergy input mix highlighted in chapter 3 prevents to assign a high confidence to the conclusion above." Camia et al. 2021, pp. 7, 88.
^ Sathre & O'Connor cite research on woody construction materials and write that "[...] over 90% of revenue is gained from the main wood product, with less than 10% gained from other biomass co-products." They also write that "[...] the average economic value added per hectare of forestland is over 40 times greater for main products made from sawlogs than for harvest residues." In other words, "[...] it is unlikely that trees will be harvested solely to produce these low-value products; instead, trees are harvested to produce high-value main products, and by-products are generated simultaneously." Sathre & O'Connor 2010, p. 111.
^ The existence of a bioenergy market can improve the financial viability of forest thinning (Cintas et al., 2016), which stimulates production of high-quality timber with the aforementioned climate benefits from product substitution. In addition, extracting (otherwise unutilized) lower quality biomass (e.g. resulting from pest and disease impacts or overstocking) can reduce the frequency and severity of wildfires and associated loss of forest carbon and release of non-CO₂ GHGs, further enhancing the climate benefit (Agee & Skinner, 2005; Evans & Finkral, 2009; Mansuy et al., 2018; Regos et al., 2016; Sun et al., 2018; Verkerk et al., 2018). On the other hand, the mitigation value of forest bioenergy could be diminished if policies supporting bioenergy reduce timber availability for material applications (Favero et al., 2020), thereby reducing the wood products pool and increasing use of GHG-intensive materials; if excessive removal of residues reduces forest productivity (Achat et al., 2015; Helmisaari et al., 2011); or if reforestation displaces food production and results in deforestation elsewhere to provide new cropland. Cowie et al. 2021, pp. 1216–1217.
^ "In many locations sawmill residuals from structural lumber production are abundant and they supply much of the raw material needed to produce wood pellets. In other locations, there are insufficient sawmill residuals. In those locations, the pellet mills, just like the pulp mills, use the non-sawlog portions of the tree." FutureMetrics 2017, p. 8.
^ Lamers & Junginger write that the vast majority of wood pellets imported to Europe are based on processing and harvesting residues "[...] with an increasing though still minor share from low-grade roundwood." They state that generally, "[...] the higher economic value for timber and cellulose products makes large-scale use of whole-trees for energy purposes highly unlikely wherever there is regional competition for the fiber." The technical potential for additional use of harvest residues is high in areas with large forests and harvest residues is therefore "[...] more likely to be used as feedstock when process-based wood waste streams become scarce." Lamers & Junginger 2013, p. 382.
^ "Synergies as well as competition within the wood-based economy are evident. Similar to the energy sector, the wood-based panel and pulp industries are likewise largely based on forest industry by-products. Therefore, the energy sector, wood-based panel, and pulp industries are all dependant on the demand for sawnwood, and they compete for the same feedstocks." Camia et al. 2021, pp. 57–58. In Germany, "[...] two thirds of the distributed sawmill by-products are delivered to panel and pulp industry; only 12% are distributed directly to energy producers (11% pellets, 1% directly to power plants)." Hoefnagels et al. 2017, p. 50.
^ "Limitations on bioenergy and BECCS can result in increases in the cost of mitigation (Kriegler et al. 2014; Edmonds et al. 2013). Studies have also examined limiting CDR, including reforestation, afforestation, and bioenergy and BECCS (Kriegler et al. 2018a,b). These studies find that limiting CDR can increase mitigation costs, increase food prices, and even preclude limiting warming to less than 1.5°C above pre-industrial levels (Kriegler et al. 2018a,b; Muratori et al. 2016)." IPCC 2019e, p. 638.
^ "Concern about near-term emissions is not a strong argument for stopping investments that contribute to net emissions reduction beyond 2030, be it the scaling-up of battery manufacturing to support electrification of car fleets, the development of rail infrastructure, or the development of biomass supply systems and innovation to provide biobased products displacing fossil fuels, cement and other GHG-intensive products. We assert that it is critical to focus on the global emissions trajectory required to achieve climate stabilization, acknowledging possible trade-offs between short- and long-term emissions reduction objectives. A strong focus on short-term carbon balances may result in decisions that make long-term climate objectives more difficult to meet."IEA Bioenergy 2019, p. 4.
^ "Comparisons between forest biomass emissions and fossil fuel emissions at the time of combustion and for short periods thereafter do not account for long term carbon accumulation in the atmosphere and can significantly distort or ignore comparative carbon impacts over time. [...] The most common timeframe for measuring the impacts of greenhouse gases is 100 years, as illustrated by the widespread use of 100-year global warming potentials. This timeframe provides a more accurate accounting of cumulative emissions than shorter intervals." NAUFRP 2019, pp. 1–2.

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Shortened footnotes

^ EIA 2021a.
^ "Energy Statistics Data Browser – Data Tools". IEA. Retrieved 2022-12-27.
^ ^a ^b IEA 2021a.
^ ^a ^b IPCC 2019b, p. B 7.4.
^ Norton et al. 2019, pp. 1256–1263.
^ ^a ^b ^c IEA 2021d.
^ "Household air pollution and health: fact sheet". WHO. 8 May 2018. Retrieved 2020-11-21.
^ "Goal 7: Ensure access to affordable, reliable, sustainable and modern energy for all". SDG Tracker. from the original on 2 February 2021. Retrieved 12 March 2021.
^ ETIP Bioenergy 2022.
^ IRENA 2014, p. 20-21.
^ IEA 2021c.
^ IRENA 2014, p. 9.
^ Brauch et al. 2009, p. 384.
^ IRENA 2014, pp. 1, 5.
^ IRENA 2014, p. 8.
^ MAGIC 2021.
^ WBA 2016, p. 4.
^ JRC 2019, p. 3.
^ ^a ^b JRC 2014, p. 75.
^ Camia et al. 2021, p. 7.
^ Camia et al. 2018, p. 6.
^ ^a ^b van den Born et al. 2014, p. 20, table 4.2.
^ ETIP Bioenergy 2020.
^ IEA 2019.
^ van den Born et al. 2014, p. 2, 21.
^ IRENA 2014, p. 21.
^ IEA Bioenergy 2017, pp. 1, 22.
^ ^a ^b ^c ^d EIA 2022.
^ Basu et al. 2013, pp. 171–176.
^ Koukoulas 2016, p. 12.
^ Wild 2015, p. 72.
^ Smil 2015, p. 13.
^ Renewable Energy 2021, pp. 473–483.
^ EIA 2021.
^ Akhtar, Krepl & Ivanova 2018.
^ Liu et al. 2011.
^ "Bioenergy – Energy system overview". September 2022.
^ Zetterberg & Chen 2014, p. 785.
^ C2ES 2021.
^ ^a ^b Jonker, Junginger & Faaij 2013, pp. 378–381.
^ Lamers & Junginger 2013, p. 375.
^ Camia et al. 2018, p. 29.
^ Nelson, Liknes & Butler 2010, pp. 1–2.
^ Myllyviita et al. 2021, p. 7-8.
^ Myllyviita et al. 2021, p. 9-11.
^ Schlamadinger & Marland 1996, pp. 283–285.
^ Camia et al. 2018, p. 34, 45.
^ Sathre & O'Connor 2010, p. 104.
^ Sathre & O'Connor 2010, p. 109.
^ Sathre & O'Connor 2010, p. 110.
^ Myllyviita et al. 2021, p. 5-11.
^ Miner et al. 2014, p. 602.
^ ^a ^b Abt et al. 2021, p. 28.
^ JRC 2014, p. 41, table 2.
^ Lamers & Junginger 2013, p. 380.
^ Bird et al. 2010, p. 26.
^ Holmgren 2021, p. 13.
^ Holmgren 2021, pp. 16, 24, 26.
^ Holmgren 2021, p. 25.
^ Chatham House 2020, p. 1, table 12.
^ Walker et al. 2013, p. 153.
^ Hanssen et al. 2017, p. 1416.
^ Camia et al. 2018, p. 104.
^ Lamers & Junginger 2013, p. 379-380.
^ Zanchi, Pena & Bird 2011, p. 768.
^ Zanchi, Pena & Bird 2011, pp. 761, 768.
^ Mitchell, Harmon & O'Connell 2009, pp. 648, 651.
^ Mitchell, Harmon & O'Connell 2016.
^ Jonker, Junginger & Faaij 2013, pp. 381–387, table 5.
^ Jonker, Junginger & Faaij 2013, p. 386.
^ Jonker, Junginger & Faaij 2013, p. 381.
^ Jonker, Junginger & Faaij 2013, p. 385, 388.
^ JRC 2014, pp. 35–37.
^ JRC 2014, p. 29.
^ Zanchi, Pena & Bird 2011, p. 761-772.
^ JRC 2014, p. 41.
^ ^a ^b Camia et al. 2018, p. 97.
^ ^a ^b Camia et al. 2018, p. 96.
^ Camia et al. 2018, p. 87-91.
^ JRC 2018.
^ Camia et al. 2018, p. 105.
^ IRENA 2014, pp. 57–58.
^ IEA Bioenergy 2006, pp. 1, 4 (table 4).
^ Lamers & Junginger 2013, p. 382.
^ Forest Research 2022a.
^ Forest Research 2022b.
^ Schlamadinger & Marland 1996, p. 288.
^ JRC 2014, p. 23.
^ IEA 2021b, p. 92.
^ ARS 2022.
^ Milner et al. 2016, p. 323, fig. 2.
^ Agostini, Gregory & Richter 2015, p. 1068.
^ Zang et al. 2017, p. 269, fig. 6.
^ Georgescu, Lobell & Field 2011, pp. 4307–4312.
^ Smil 2015, p. 211, box 7.1.
^ van Zalk & Behrens 2018, pp. 83–91.
^ Smil 2015, p. 170.
^ Smil 2015, p. 2095 (kindle location).
^ Smil 2015, p. 91.
^ ^a ^b Smil 2015, p. 89.
^ Smil 2015, p. 228.
^ ^a ^b Smil 2015, p. 227.
^ Smil 2015, p. 90.
^ Smil 2015, p. 229.
^ Smil 2015, pp. 80, 89.
^ ^a ^b Smil 2015, p. 85.
^ Smil 2015, p. 86.
^ Schwarz 1993, p. 413.
^ Flores et al. 2012, p. 831.
^ Ghose 2011, p. 263.
^ van den Broek 1996, p. 271.
^ Gasparatos et al. 2017, p. 174.
^ ^a ^b Gasparatos et al. 2017, p. 166.
^ Gasparatos et al. 2017, p. 172.
^ Gasparatos et al. 2017, p. 167.
^ Camia et al. 2021, p. 146.
^ ^a ^b ^c Gasparatos et al. 2017, p. 168.
^ Gasparatos et al. 2017, p. 173.
^ Camia et al. 2021, pp. 108, 144.
^ IPCC 2019h, p. 628.
^ Springsteen et al. 2011.
^ Gustafsson et al. 2009, pp. 495–498.
^ Climate Central 2015.
^ Robin Wood 2021.
^ Shikangalah & Mapani 2020, pp. 251–266.
^ IFL Science 2016.
^ Forest Defenders Alliance 2021.
^ STAND.earth 2021.
^ Ward 2021.
^ Ribout 2020.
^ Wyatt 2013, pp. 15–35.
^ The Irish Times 2013.
^ ISO 2014a.
^ Indiana Center for Coal Technology Research 2008, p. 13.
^ Euracoal 2019.
^ Chatham House 2017, p. 2.
^ Manomet 2010, p. 103-104.
^ ^a ^b FutureMetrics 2012, p. 2.
^ ^a ^b IEA Bioenergy 2019, p. 3.
^ Miner 2010, p. 39–40.
^ FAO 2020, p. 16, 52.
^ IPCC 2019a, p. 385.
^ EASAC 2017, p. 33.
^ EASAC 2017, p. 1.
^ Chatham House 2017, p. 3.
^ EU Science Hub - European Commission 2021.
^ Sabatini et al. 2018, p. 1426.
^ Stephenson et al. 2014, pp. 2–3.
^ IPCC 2019b, p. B.1.4.
^ ^a ^b IPCC 2019a, p. 386.
^ ^a ^b JRC 2014, p. 49.
^ ^a ^b Cowie et al. 2021, p. 1217.
^ Schlamadinger & Marland 1996, p. 291.
^ Miner 2010, p. 39.
^ IEA Bioenergy 2019, p. 4–5.
^ ^a ^b Camia et al. 2021, pp. 32–33.
^ Camia et al. 2018, p. 29, 32, 34, 45.
^ Chatham House 2017, p. 7.
^ EASAC 2017, p. 23, 26, 35.
^ IPCC 2013, p. 121.
^ FutureMetrics 2011a, p. 5.
^ Lamers & Junginger 2013, p. 374.
^ Gunn 2011.
^ ^a ^b IPCC 2007, p. 549.
^ IRENA 2019, p. 21.
^ ^a ^b Chatham House 2017, p. 19.
^ Chatham House 2017, pp. 19, 21–22.
^ FutureMetrics 2016, p. 5–9.
^ Cowie et al. 2021, p. 1222.
^ IPCC 2019g, p. 194.

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January 09, 2023

[EIA-2021b-2] "Biofuels are transportation fuels such as ethanol and biodiesel that are made from biomass materials." EIA 2021b.

[EuropeanCommissionGlossary-2018-1-a-3] In EU legislation, biofuel is defined as: "Liquid or gaseous fuel for transport produced from biomass." See European Commission 2018a.

[Camia-2021-20-21-a-4] "Solid biofuels cover organic, non-fossil material of biological origin which may be used as fuel for heat and electricity production. [...] Primary solid biofuels are defined as any plant matter used directly as fuel or converted into other forms before combustion. This covers a multitude of woody materials generated by industrial process or provided directly by forestry and agriculture (firewood, wood chips, bark, sawdust, shavings, chips, sulphite lye also known as black liquor, animal materials/wastes and other solid biofuels). This category excludes charcoal. Wood pellets are agglomerates produced either directly by compression or by the addition of a binder in a proportion not exceeding 3% by weight. Such pellets are cylindrical, with a diameter not exceeding 25 mm and a length not exceeding 100 mm. The term ‘other agglomerates' is the term used for agglomerates that are not pellets, such as briquettes or log agglomerates. Wood pellets and other agglomerates are often reported jointly, with other agglomerates being usually a minor part. Black liquor is a by-product from chemical and semi-chemical wood pulp industry." Camia et al. 2021, pp. 20–21.

[IEAdata2020-a-6] In 2020, the world produced a total of 24.6 EJ of electrical energy from all renewables except bioenergy. The individual contributions consists of 15.5 EJ from hydro, 5.8 EJ from wind, 3 EJ from solar and 0.3 EJ from geothermal (all values converted from TWh with IEA's unit converter.)

[IEAdata2020-b-7] In 2020, 9.5 EJ of heat energy for industrial applications was consumed, and 5 EJ of heat for buildings. 3.7 EJ of liquid fuels for transportation was produced (ethanol 2.2 EJ, biodiesel 1.5 EJ), and 2.2 EJ in the form of electricity.

[bioenergy-statistics-2019-3-a-8] "The forestry sector is the largest contributor to the bioenergy mix globally. Forestry products including charcoal, fuelwood, pellets and wood chips account for more than 85% of all the biomass used for energy purposes. One of the primary products from forests that are used for bioenergy production is woodfuel. Most of the woodfuel is used for traditional cooking and heating in developing countries in Asia and Africa. Globally, 1.9 billion m³ of woodfuel was used for energy purposes." WBA 2019, p. 3. In the EU, 60% of all renewable energy comes from biomass. 75% of all biomass is used in the heating and cooling sector. See JRC 2019, p. 1.

[IRENAprojections-b-9] "The International Energy Agency (IEA, 2012) defines traditional use of biomass as: '…the use of wood, charcoal, agricultural residues and animal dung for cooking and heating in the residential sector' and notes that 'it tends to have very low conversion efficiency (10% to 20%) and often relies on unsustainable biomass supply.'" IRENA 2014, p. 7.

[IPCC-2012-214-a-11] "Bioenergy has a significant greenhouse gas (GHG) mitigation potential, provided that the resources are developed sustainably and that efficient bioenergy systems are used. Certain current systems and key future options including perennial cropping systems, use of biomass residues and wastes and advanced conversion systems are able to deliver 80 to 90% emission reductions compared to the fossil energy baseline. However, land use conversion and forest management that lead to a loss of carbon stocks (direct) in addition to indirect land use change (d+iLUC) effects can lessen, and in some cases more than neutralize, the net positive GHG mitigation impacts." IPCC 2012, p. 214.

[IPCC-2019-637-a-12] "For example, limiting deployment of a mitigation response option will either result in increased climate change or additional mitigation in other sectors. A number of studies have examined limiting bioenergy and BECCS. Some such studies show increased emissions (Reilly et al. 2012). Other studies meet the same climate goal, but reduce emissions elsewhere via reduced energy demand (Grubler et al. 2018; Van Vuuren et al. 2018), increased fossil carbon capture and storage (CCS), nuclear energy, energy efficiency and/or renewable energy (Van Vuuren et al. 2018; Rose et al. 2014; Calvin et al. 2014; Van Vuuren et al. 2017b), dietary change (Van Vuuren et al. 2018), reduced non-CO₂ emissions (Van Vuuren et al. 2018), or lower population (Van Vuuren et al. 2018)." IPCC 2019e, p. 637.

[IEA-net-zero-2050-b-13] "Bioenergy is a versatile renewable energy source that can be used in all sectors, and it can often make use of existing transmission and distribution systems and end-user equipment. But there are constraints on expanding the supply of bioenergy, and possible trade-offs with sustainable development goals, including avoiding conflicts at local level with other uses of land, notably for food production and biodiversity protection. To navigate these risks, our Roadmap to Net Zero by 2050 combined for the first time the IEA's global energy system modelling with the International Institute for Applied Systems Analysis (IIASA)'s Global Biosphere Management Model to provide insights on bioenergy's supply, land use and net emissions. We aimed to ensure that the peak level of total primary bioenergy demand – including losses from the conversion of biomass into useful fuels – falls within the lowest estimates of global sustainable bioenergy potential in 2050, namely around 100 exajoules (EJ). Bioenergy demand in our global net zero pathway – the Net-Zero Emissions by 2050 (NZE) Scenario – is lower than all comparable scenarios from the Intergovernmental Panel on Climate Change (IPCC) that are aligned with 1.5 °C. Those IPCC scenarios use a median of 200 EJ of bioenergy in 2050." IEA 2021a.

[IEA-net-zero-by-2050-90-a-14] IEA estimates high levels of sustainable bioenergy in 2050, but set their NZE target to only 100 EJ for conservative reasons: "The level of bioenergy use in the NZE [Net Zero Emissions scenario] [...] in 2050 is around 100 EJ. The global sustainable bioenergy potential in 2050 has been assessed to be at least 100 EJ (Creutzig, 2015) and recent assessments estimate a potential between 150‐170 EJ when integrating relevant UN Sustainable Development Goals (Frank, 2021; IPCC, 2019; IPCC, 2014; Wu, 2019). However, there is a high degree of uncertainty over the precise levels of this potential. Using modelling developed in co‐operation with IIASA, here we examine the implications for achieving net‐zero CO₂ emissions by 2050 if the available levels of sustainable bioenergy were to be lower." IEA 2021b, p. 90.

[IRENAprojections-d-17] "[...] CHP is an alternative for a wide range of production processes as the temperature and pressure of delivered steam can be adjusted to the specific requirements of industrial processes. There are other renewable alternatives for process heat generation (e.g., solar thermal, heat pumps or geothermal technologies). These are, however, either more costly or their deployment is constrained by the maximum temperature of the steam they can deliver. Therefore, biomass CHP plays a critical role for the manufacturing industry to raise its renewable energy share." IRENA 2014, p. 24.

[Cowie-2021-1212-a-18] "Biomass-based electricity can provide balancing power needed to maintain power stability and quality as the contribution from solar and wind power increases (Arasto et al., 2017; Lenzen et al., 2016; Li et al., 2020), complementing other balancing options such as battery storage, reservoir hydropower, grid extensions and demand-side management (Göransson & Johnsson, 2018). Beyond its value as a dispatchable resource for electricity generation, biomass is an important option for renewable heating in buildings and industrial processes. In 2019, bioenergy contributed almost 90% of renewable industrial heat consumption and two-thirds of the total modern renewable heating and cooling in buildings and industrial processes (IEA, 2020; IRENA/IEA/REN21, 2020). It is one of the options available to reduce emissions from heavy industries such as iron and steel production (Mandova et al., 2018, 2019) and cement production (IEA, 2018). Furthermore, carbon-based transportation fuels will remain important in the coming decades, as electrification of the transport sector will take time (IEA-AMF/IEA Bioenergy, 2020). Biofuels can contribute to reducing fossil fuel use and associated GHG emissions while there remain vehicles that use carbon-based fuels. In the longer term, biofuels will likely be used in sectors where the substitution of carbon-based fuels is difficult, such as long-distance aviation and marine transportation." Cowie et al. 2021, p. 1212.

[IRENAbaseload-a-19] "There are two distinct categories of renewable power generators: dispatchable and variable. Dispatchable renewable power generators control their output within a specific range, just like conventional fossil power plants. These generators can provide baseload power if needed. Reservoir hydropower plants, biomass (including biogas) power plants, geothermal power plants, and concentrated solar power (CSP) plants with thermal storage (such as in molten salt) all generate dispatchable renewable power. Integrating these renewable sources into power systems does not pose additional challenges. [...] The output of variable renewable energy (VRE) sources, such as wind and solar photovoltaic (PV) is much less controllable." IRENA 2015, p. 7.

[IRENAprojections-k-5-20] According to IRENA, "[...] biomass energy comes from two different sources. One is primary bioenergy, which uses farmland or forests to produce biomass, the other is biomass residue, which is generated as a by-product of food or wood products throughout their supply-consumption chain." IRENA 2014, p. 5.

[eggers-2020-2-b-21] "There are two broad categories for woody biofuels: primary sources, such as logging residues, stumps, and low-quality logs, and secondary sources, i.e., by-products from the forest industries such as bark, saw dust, and black liqueur." Eggers et al. 2020, p. 2.

[shortrotationcoppices-25] "Some fast growing tree species can be cut down to a low stump (or stool) when they are dormant in winter and go on to produce many new stems in the following growing season. This practice is well established in the UK and Europe, having been a traditional method of woodland management over several hundred years for a variety of purposes including charcoal, fencing and shipbuilding." Forest Research 2022c.

[shortrotationforests-26] "While short rotation coppicing (SRC) cuts the tree back to a stool to promote the growth of multiple stems, on a regular cycle of roughly 2-4 years, it is also possible to practice something more closely akin to conventional forestry, though on a shorter timescale. Short rotation forestry (SRF) consists of planting a site and then felling the trees when they have reached a size of typically 10-20 cm diameter at breast height. Depending on tree species this usually takes between 8 and 20 years, and is therefore intermediate in timescale between SRC and conventional forestry. This has the effect of retaining the high productivity of a young plantation, but increasing the wood to bark ratio." Forest Research 2022a.

[iea-2021b-212-a-27] "Woody energy crops: Short‐rotation plantings of woody biomass for bioenergy production, such as coppiced willow and miscanthus." IEA 2021b, p. 212.

[miscanthus-a-33] For instance is the promising crop Miscanthus × giganteus only grown on 30.000 hectares in the EU. See ETIP Bioenergy 2021. 30.000 hectares produces approximately 0.01 EJ annually, given the EU average peak yield of 22 tonnes dry matter per hectare per year (approximately 15 tonnes during spring harvest). See Anderson et al. 2014, p. 79. The energy content in miscanthus biomass is 18 GJ/t. Ghose 2011, p. 263.

[Department_for_Business,_Energy_&_Industrial_Strategy-2021-15-16-34] "We will establish the amount of land that could be used in the UK for perennial energy crop production and for short rotation forestry (SRF). Existing biomass support schemes (Renewables Obligation, Contracts for Difference, RHI & RTFO) already support the use of perennial energy crops such as short rotation coppice and Miscanthus grown specifically for bioenergy purposes and as a material. However, only a small land area (~10,000 hectares) is cultivated with perennial energy crops in the UK at present, and this is mainly used for heat and electricity generation. Currently, there is little to no use of perennial energy crops for low carbon fuels supported under the RTFO due to a lack of commercial-scale processing capacities to convert these resources cost-efficiently into fuel. [...] The CCC's 6th Carbon Budget report highlighted the significant potential for perennial energy crops and SRF to contribute towards our carbon budget targets by increasing soil and biomass carbon stocks while also delivering other ecosystem benefits. In their balanced pathway, the CCC suggests that up to 708,000 hectares of land could be dedicated to energy crop production, which has led to an increased interest in the role of perennial energy crops and SRF as biomass feedstocks to deliver GHG savings in the land use and energy sectors. The Defra land use net zero programme, which is currently building a spatial understanding of the land use trade-offs across a number of policy areas, will help determine the potential scale of future availability of domestically grown biomass and their potential for delivering GHG savings in a landscape where land use change will need to be optimised for multiple benefits. This programme will inform our understanding and evidence on the availability and mix of biomass feedstocks for uses across sectors." Department for Business, Energy & Industrial Strategy 2021, pp. 15–16.

[Brauch-2009-384-a-36] Brauch et al. write that in theory "[...] energy farming on current agricultural (arable and pasture) land could, with projected technological progress, contribute over 800 EJ, without jeopardizing the future world's food supply." The authors also write that "[...] a significant part of the technical potential (around 200 EJ in 2050) for biomass production may be developed at low production costs in the range of US$2/GJ [...] assuming this land is used for perennial crops. Another 100 EJ of biomass could be produced with lower productivity and higher costs at marginal and degraded lands." Brauch et al. 2009, p. 384.

[IRENAprojections-e-38] "According to FAO, the issue is not the volume of available land, which is enough to supply growing demand, but securing the substantial financial investments to actually deploy these potential areas, plus the disparate distribution of land resource by country. For example, 60% of the world's unexploited prime land is held by only thirteen countries. [...] These thirteen countries are Madagascar, Mozambique, Canada, Angola, Kazakhstan, the Democratic Republic of the Congo, China, the Sudan, Australia, Argentina, Russia, the US and Brazil (in ascending order)." IRENA 2014, p. 40.

[IPCC2019cp193-39] "Estimates of marginal/degraded lands currently considered available for bioenergy range from 3.2–14.0 Mkm2, depending on the adopted sustainability criteria, land class definitions, soil conditions, land mapping method and environmental and economic considerations (Campbell et al. 2008; Cai et al. 2011; Lewis and Kelly 2014)." IPCC 2019c, p. 193.

[miscanthus-b-40] Given the EU average peak yield of 22 tonnes dry matter per hectare per year (approximately 15 tonnes during spring harvest). See Anderson et al. 2014, p. 79. The energy content in miscanthus biomass is 18 GJ/t, see Ghose 2011, p. 263. The calculation goes like this: 44990100 hectares times 15 tonnes per hectare times 18 GJ per tonne is 12147327000 GJ, or approximately 12 EJ.

[IRENAprojections-f-45] "Plants convert CO₂ from the atmosphere into biomass. Carbon stored in biomass is called biogenic carbon. Some of this carbon stays above ground and some in the ground. When plants die, decomposition starts. As plant material decays, the stored carbon is released as CO₂ back into the atmosphere." IRENA 2014, p. 45.

[JRC-2014-42-c-47] "Wood from thinnings may, to some extent, be assimilated to harvest residues (especially pre-commercial thinnings). If not collected for bioenergy it would be left in the forest to decay, or combusted at roadside. On the other hand, depending on the wood quality, the use of thinnings wood for bioenergy may compete with other uses, such as pulp and paper or engineered wood. Salvage loggings can also be assimilated to harvest residues. Damaged, dying or dead trees affected by injurious agents, such as wind or ice storms or the spread of invasive epidemic forest pathogens, insects and diseases would remain in the forest and decay or combusted at roadside. Wood removed for prescribed fire hazard control as well can be considered residual wood." JRC 2014, pp. 42–43, table 3.

[pokharel-543-a-48] "This study estimated quantities of logging residues that can physically be recovered from harvest sites and utilized for electricity production in the US South. [...] Although almost all physically available logging residues could be recovered with a relatively short hauling distance, a mail survey indicated that only 4 percent of mills utilized this feedstock." Pokharel et al. 2019, p. 543.

[eggers-2020-2-a-49] "Currently, logging residue extraction, i.e., the harvest of tops and branches left during final felling, occurs on less than 20% of the harvested area in northern Sweden, and about 60% in southern Sweden. Stump harvest occurs to a limited extent today, but is expected to increase to about 5%–10% of the annual clear-felled area in the coming years. Similarly, logging residues constitute the main primary source of woody biofuels in most countries, but in the near future stumps and roundwood may play a more prominent role. Biofuel harvest from early thinnings in dense young forests are currently done to insignificant levels, but will increase, as for stumps, if prize levels rise. Therefore, there is considerable potential for increased extraction rates of primary woody biofuels, especially in northern Sweden, where current extraction rates are relatively low due to longer transport distances and lower harvestable volume per hectare compared to southern Sweden. The situation is similar in other European countries, with large un-used potentials for woody biomass for energy use." Eggers et al. 2020, p. 2.

[dahlberg-2011-a-50] "Logging residues are increasingly being extracted for bioenergy purposes." Dahlberg et al. 2011, p. 1220

[vandenBorn-2014-20-a-51] van den Born et al. distinguish between logging residues in general and dead wood, with the logging residues potential at 14 EJ, and the dead wood potential at 1 EJ annually. For the logging residues potential, see van den Born et al. 2014, p. 20, table 4.2. Regarding the dead wood potential, the authors write: "A biomass pool is dead wood that remains in the forest, either standing or lying, and is transferred to the soil. It is often too costly to harvest dead wood. Besides, it is useful in increasing biodiversity (the proportion of dead wood is a sustainability criteria, (EEA, 2012). The global quantity of dead wood is estimated roughly at 67 Gt of biomass, which is about 11% of the total biomass (FAO, 2010), and about 20 times the annual wood harvest. [...] Dead wood: the global stock of dead wood is estimated at about 1200 EJ of biomass (FAO, 2010). This large pool has built up over a long period of time and in the entire forest area. Assuming an average rotation of 50 to 100 years, this implies a biomass pool of 10 to 20 EJ yr-1 [EJ per year]. When primary forests are excluded because they have not been used (based on FAO, 2010), about 7 to 14 EJ yr-1 of dead biomass remains. Forests with large quantities of dead wood are located in Russia and in parts of Africa. A limitation to the use of salvaged wood is the high costs of access and transport (Niquidet et al., 2012). A conservative estimate of accessible planted forests reduces the pool of available dead wood to about 2 EJ yr-1 biomass (Table 4.2). When an additional assumption is made that half of the dead wood needs to remain in forests to maintain biodiversity (Verkerk et al, 2012), the estimate is about 1 EJ yr-1 biomass available annually for energy production." (p. 15, 19-20)

[Cowie-2021-1215-1216-b-53] "Biomass for bioenergy is usually a by-product of sawlog and pulpwood production for material applications (Dale et al., 2017; Ghaffariyan et al., 2017; Spinelli et al., 2019; Figure 1). Logs that meet quality requirements are used to produce high-value products such as sawnwood and engineered wood products such as cross laminated timber, which can substitute for more carbon-intensive building materials such as concrete, steel and aluminium (Leskinen et al., 2018). Residues from forestry operations (tops, branches, irregular and damaged stem sections, thinnings) and wood processing residues (e.g. sawdust, bark, black liquor) are used for bioenergy (Kittler et al., 2020), including to provide process heat in the forest industry (Hassan et al., 2019). These biomass sources have high likelihood of reducing net GHG emissions when substituting fossil fuels (Hanssen et al., 2017; Matthews et al., 2018), and their use for bioenergy enhances the climate change mitigation value of forests managed for wood production (Cintas, Berndes, Hansson, et al., 2017; Gustavsson et al., 2015, 2021; Schulze et al., 2020; Ximenes et al., 2012). Part of the forest biomass used for bioenergy consists roundwood (also referred to as stemwood), such as small stems from forest thinning. For example, roundwood was estimated to contribute around 20% of the feedstock used for densified wood pellets in the United States in 2018 (US EIA, 2019)." Cowie et al. 2021, pp. 1215–1216.

[internationalforests-55] "The most crucial feedstock for the wood pellet sector is currently sawmill residues (85% of the mix), roundwood (13%), and recovered wood (2%). This mix is likely to change in the coming years with the forecasted expansion of the wood pellet industry. [...] Experience from North America shows that it is possible to use more forest residues as fiber furnish. Although it yields pellets with higher ash content, it is often a lower-cost raw material than, for example, roundwood and wood chips. This practice is increasingly common in both the US South (mainly for pellets exported to Europe) and Canada (mainly exported to Europe and Asia). In Western Canada, the sawmill residue share of the total feedstock has fallen from 97% in 2010 to 72% in 2020, with the balance being forest residues and roundwood." Wood Resources International 2022.

[woodpellets-56] Recalculated from a total production of 43678925 tonnes wood pellets (FAO 2020), with 17 GJ/t energy content.

[woodchips-58] Recalculated from a total production of 265212933 m³ wood chips (FAO 2020), with 3.1 GJ/m³ energy content.

[WBA-2019-3-a-59] "In 2017, 55.6 EJ of biomass was utilized for energy purposes [...]. One of the most promising sectors for growth in bioenergy production is in the form of residues from agriculture sector. Currently, the sector contributes less than 3% to the total bioenergy production." WBA 2019, p. 3.

[vandenBorn-2014-2-21-b-60] The Netherlands Environmental Assessment Agency estimated in 2014 that the total amount of agricultural residues amounts to 78 EJ, with 51 EJ from straw alone (pp. 12-13, table 3.4). "The large production of rice and the relatively low residue flow to the soil makes rice residues the residue with the highest potential for bioenergy, followed by residues from oilcrops, cereals, corn and sugarcane."(p. 19) Because a certain amount should be left in the fields for soil quality purposes, the total amount of agricultural residues that can be sustainably harvested amounts to 24 EJ. van den Born et al. 2014, pp. 2–21.

[bioenergy-statistics-2019-3-b-61] "One of the most promising sectors for growth in bioenergy production is in the form of residues from agriculture sector. Currently, the sector contributes less than 3% to the total bioenergy production. Data shows that utilizing the residues from all major crops for energy can generate approx. 4.3 billion tonnes (low estimate) to 9.4 billion tonnes (high estimate) annually around the world. Utilizing standard energy conversion factors, the theoretical energy potential from residues can be in the range of 17.8 EJ to 82.3 EJ. The major contribution would be from cereals – mainly maize, rice and wheat." WBA 2019, p. 3.

[IRENAprojections-c-62] "In reality, most residues are not utilised for energy because they are difficult to collect or used for specific purposes, such as land conservation, manure and straw incorporation in the field to maintain soil organic matter. This is accounted for in the residue recovery rates. The historical and projected annual crop production growth by region and the residue coefficients are provided in Annex A. About a quarter of the residue generated for each crop is assumed to be recoverable, reflecting an assessment that half the residue could be collected sustainably and half of that amount could be collected economically. After the recoverable fraction of residues is estimated, the amount of residue used for animal feed is calculated separately. This is deducted from the total residue volume." IRENA 2014, p. 9.

[irena2021-63] "At present, traditional methods of space heating and cooking, such as burning firewood, account for 35 EJ, or two-thirds of total biomass use. By 2030, this would give way to modern biomass consumption, including substantially larger shares for power and transport applications. Power and district heating would reach 36 EJ (one-third of total biomass use in 2030) and transport 31EJ (almost 29%), while heat for industry and buildings would reach up to 41 EJ, of which only 6 EJ would be from less sustainable traditional uses. While global biomass potential is sufficient to meet growing demand, different types of biomass resources are distributed unevenly. Global biomass supply potential in 2030 is estimated to range from 97 EJ to 147 EJ per year. Approximately 40% of this total would originate from agricultural residues and waste (37-66 EJ). The remaining supply potential is shared between energy crops (33-39 EJ) and forest products, including forest residues (24-43 EJ). In geographic terms, the largest supply potential — estimated at 43-77 EJ per year — exists in Asia and Europe. North and South America together account for another 45-55 EJ per year." IRENA 2021.

[europeancommission-2018-66] "Advanced (second and third generation) biofuels are biofuels produced from feedstock that do not compete directly with food and feed crops, such as wastes and agricultural residues (i.e. wheat straw, municipal waste), non-food crops (i.e. Miscanthus and short rotation coppice) and algae." European Commission 2018.

[Sm2018p547a-74] "Recent studies by Reza et al. and Smith et al. have reported of the fate of inorganics and heteroatoms during HTC [hydrothermal carbonisation] of Miscanthus and indicate significant removal of the alkali metals, potassium and sodium, along with chlorine. [...] Analysis of ash melting behaviour in Smith et al., showed a significant reduction in the slagging propensity of the resulting fuel, along with the fouling and corrosion risk combined. [...] Consequently HTC offers the potential to upgrade Miscanthus from a reasonably low value fuel into a high grade fuel, with a high calorific value, improved handling properties and favourable ash chemistry. [...] HTC at 250 °C can overcome slagging issues and increase the ash deformation temperature from 1040 °C to 1320 °C for early harvested Miscanthus. The chemistry also suggests a reduction in fouling and corrosion propensity for both 250 °C treated fuels." Smith et al. 2018, pp. 547, 556.

[IRENAprojections-j-79] "The success of large-scale international bioenergy trade will require the transport of high density commodities at low costs. Transport costs can be decreased by introducing pre-treatment into the supply chain. Pre-treatment, including torrefaction, pelletisation and pyrolysis, increases energy density from 2-8 MJ/m³ of raw biomass up to 11-20 MJ/m³ for pre-treated biomass. By optimising the supply chain through incorporating pretreatment, logistics costs could be significantly reduced compared with the raw materials-based supply chain." IRENA 2014, p. 53.

[EASAC-2017-2-a-80] "A critical factor in the use of forest biomass in energy provision is the ‘payback time', during which atmospheric concentrations of carbon dioxide (CO₂) will be increased as a result of using biomass. EASAC concludes that the European Commission should consider the extent to which large-scale forest biomass energy use is compatible with UNFCCC targets (of limiting warming to 1.5 °C above pre-industrial levels), and whether a maximum allowable payback period should be set in its sustainability criteria." EASAC 2017, p. 2.

[Ambrose-2021-81] "The UK's plan to burn more trees to generate “renewable” electricity has come under fire from green groups and sustainable investment campaigners over the controversial claim that biomass energy is carbon-neutral. A letter to the government signed by more than a dozen green groups including Greenpeace and Friends of the Earth warns ministers against relying too heavily on plans to capture carbon emissions to help tackle the climate crisis. The plans are being pioneered by Drax Group, which claims that burning wood pellets is carbon-neutral because trees absorb as much carbon dioxide when they grow as they emit when they are burnt. Capturing the carbon emissions from biomass power plants would then effectively create “negative carbon emissions”, according to Drax. The green groups have disputed these claims and warned that the plans “will be costly” and “will not deliver negative emissions” after accounting for the full carbon footprint of biomass in the power sector." Ambrose 2021.

[Grunwald-2021-82] "By definition, clear-cutting trees and combusting their carbon emits greenhouse gases that heat up the earth. But policymakers in the U.S. Congress and governments around the world have declared that no, burning wood for power isn't a climate threat—it's actually a green climate solution. [...] [T]he [...] basic argument is that the carbon released while trees are burning shouldn't count because it's eventually offset by the carbon absorbed while other trees are growing. That is also currently the official position of the U.S. government, along with many other governments around the world. In documentaries, lawsuits and the teenage activist Greta Thunberg's spirited Twitter feed, critics of the industry have suggested an alternative climate strategy: Let trees grow and absorb carbon, then don't burn them. [...] Cutting down a tree and burning it clearly releases more carbon than leaving the tree alone; replanting the tree can only pay back the carbon debt in the long run, and an even longer run if the replanted tree is eventually reharvested. But biomass defenders say that focusing on one tree or even one clear-cut is far too narrow a way to think about forest carbon, because as long as the carbon absorbed by forests equals the carbon released from forests, the climate doesn't care. [...] The industry's position is that wood pellets actually help expand forests, by making it more lucrative for the private landowners who control most U.S. forest land to stay in the forestry business. The opponents argue that what wood pellets make more lucrative is deforestation. [...] “We can't say, ‘Oh, we can sacrifice forest over here, because it's growing over there. We need to stop sacrificing forest.” Grunwald 2021.

[iea-2020-84] The IEA defines carbon neutrality and carbon negativity like so: "Carbon neutrality, or 'net zero,' means that any CO₂ released into the atmosphere from human activity is balanced by an equivalent amount being removed. Becoming carbon negative requires a company, sector or country to remove more CO₂ from the atmosphere than it emits."IEA 2020.

[Schlamadinger-1996-275-a-85] "Schlamadinger & Marland describe how the atmospheric carbon pool changes depending on what is happening in other carbon pools (living biomass, soils and forest litter, wood and wood products, fossil fuels displaced by biomass fuels, fossil fuels used for forest management activities and for biomass conversion processes, and fossil fuels required to manufacture wood products or their substitutes.)" Schlamadinger & Marland 1996, p. 275. See also Sathre & O'Connor 2010, p. 104

[Cowie-2021-1220-1222-a-86] Some researchers would like to move the counting to the combustion event, but Cowie et al. and others argue against this: "The UNFCCC reporting requirements specify that CO₂ emissions associated with biomass combustion are counted in the land use sector, that is, where the harvest takes place; they are therefore reported as zero in the energy sector to avoid double-counting (Goodwin et al., 2019). This reporting approach is accurate, has no gaps and does not assume that bioenergy is carbon neutral (Haberl at al., 2012; Marland, 2010), although it has sometimes been described as such (e.g. Norton et al., 2019; Searchinger et al., 2009). [...] While the UNFCCC reporting approach is theoretically sound, incomplete coverage of the Kyoto Protocol created a gap in accounting: if an Annex I party (i.e. country with a Kyoto Protocol commitment) imported forest biomass from a country with no Kyoto Protocol commitment, any associated stock change in the forest of the exporting country was not accounted. [...] Several authors (Brack, 2017; Hudiburg et al., 2019; Norton et al., 2019) propose changing the UNFCCC accounting rules by which biomass is treated as having zero emissions at the point of combustion. However, accounting for CO₂ emissions from bioenergy within the energy sector would require revision of the established GHG accounting framework to adjust the land sector values to remove the component related to biomass used for energy, to avoid double-counting of emissions, which would be very difficult to achieve, as explained by Camia et al. (2021). It would create a disincentive for countries to utilize biomass to displace fossil fuels, adversely affecting all types of bioenergy systems irrespective of their potential to provide climate benefits (Pingoud et al., 2010). Rather than changing the accounting convention solely for bioenergy, a flux-based ‘atmospheric flow approach' (Rüter et al., 2019) could potentially be applied to all wood products. However, if carbon fluxes from all wood products were to be reported at the time and place of emission, emissions due to forest harvest for export would not be reported by the country where the harvest takes place, thereby removing incentives for maintaining forest carbon stocks and potentially leading to deforestation because the country where the harvest takes place would report no emissions. Furthermore, reporting only at the time and place of emission would create a disincentive for use and trade in all sustainable wood products, including use for construction and bioenergy (Apps et al., 1997; Cowie et al., 2006; UNFCCC, 2003). [...] With respect to the treatment of bioenergy in UNFCCC reporting and accounting, we disagree with proposals to count emissions at the point of combustion, which could have adverse climate impacts. We recommend that complete and transparent reporting and accounting be applied consistently across the whole land sector, to ensure recognition of the interactions between terrestrial carbon stocks and biomass use for energy and other purposes, and to incentivize land use and management systems that deliver climate benefits." Cowie et al. 2021, pp. 1220–1222.

[EASAC-2017-23-c-87] A graphical explanation of carbon payback and parity times, with carbon debt shown as a curve that moves along a time axis, is available here: EASAC 2017, p. 23.

[Lamers-2013-373-b-88] "The potential carbon debt caused by harvest and the resulting time spans needed to reach pre-harvest carbon levels (payback) or those of a reference case (parity) have become important parameters for climate and bioenergy policy developments." Lamers & Junginger 2013, p. 373.

[Lamers-2013-375-a-90] Lamers & Junginger state that the carbon debt "[...] can be indicated to the site itself (absolute) or against a baseline (relative)." The absolute carbon balance approach (payback time) is chosen to define the time until a site reaches its own pre-harvest carbon level, and the relative carbon balance approach (parity time) is chosen to define the time until an alternative land or biomass use scenario "[...] reaches the same carbon volume as its counterfactual (reference case)." The reference or alternative scenarios can be for example "[...] material use of biomass (e.g. pulp and paper), land protection (no harvest) or conversion to agriculture." According to the authors, "[t]his provides insight whether it is more beneficial from a net carbon perspective to keep biogenic carbon sequestered in plants (subjected to natural disturbances such as insects or wildfire) or use it for energy purposes." Lamers & Junginger 2013, p. 375.

[Camia-2021-83-a-91] EU's Joint Research Centre defines "counterfactual" like so: "The impacts of each bioenergy pathway are evaluated against a counterfactual, i.e. a reference use of the biomass or of the land (thus the results should be interpreted as conditional to the chosen reference)." Camia et al. 2021, p. 83.

[Camia-2018-100-a-92] "It is important to notice that the definition of the reference system (both the energy system and the counterfactual biomass use) is as important as the definition of the bioenergy systems since the stated goal of the study is to assess the mitigation potential of the new systems as compared to the reference one." Camia et al. 2018, p. 100.

[Cowie-2021-1213-1219-1220-a-93] "Critical methodology decisions include the definition of spatial and temporal system boundaries [...] and reference (counterfactual) scenarios [...]. Focus on stack emissions (Option 1) neglects the key differences between fossil and biogenic carbon [...]. Focus on the forest only (Option 2) captures the effects of biomass harvest on forest carbon stocks [...] but omits the climate benefits of displacing fossil fuels. Option 3, the biomass supply chain, overlooks the interactions between biomass and other forest products [...]. Option 4 covers the whole bioeconomy, that is, the forest, the biomass supply chain and all bio-based products from managed forests, and thus provides a more complete assessment of the climate effects of forest bioenergy. In order to quantify the net climate effect of forest bioenergy, assessments should take a whole systems perspective. While this increases the complexity and uncertainty of the assessments, it provides a sound basis for robust decision-making. Biomass for bioenergy should be considered as one component of the bioeconomy (Option 4 [...]). Studies should therefore assess the effects of increasing biomass demand for bioenergy on carbon stocks of the whole forest, and also include the broader indirect impacts on emissions (potentially positive or negative) due to policy- and market-driven influences on land use, use of wood products and GHG-intensive construction materials, and fossil fuel use, outside the bioenergy supply chain. The bioenergy system should be compared with a realistic counterfactual(s) that includes the reference land use and energy systems [...]. This approach is consistent with consequential LCA [...]. The temporal boundary should recognize: forest carbon dynamics, for example, modelling over several rotations; the trajectory for energy system transition; and short- and long-term climate objectives. Matthews et al. (2018) suggest criteria that could be used to identify woody biomass with greater climate benefits when assessed from a full life cycle, whole system perspective." Cowie et al. 2021, pp. 1213, 1219–1220.

[EASAC-2017-23-b-94] A simplified curve, complete with carbon payback and parity times, is available here: EASAC 2017, p. 23.

[JRC-2014-45-a-96] "The GWP is a measure of the effect of the pulse emission of a unit (mass) of a certain gas over its lifetime on the radiative properties of the atmosphere for a certain period of time. In the methodology designed by the IPCC [IPCC 2006], the GWP of CO₂- is taken as the reference value and assigned the value of 1. The reasoning of the authors is that biogenic CO₂- has indeed the same radiative effect of fossil CO₂ on the atmosphere but, while fossil CO₂- can only be reabsorbed by oceans and biosphere (according to the formulation using Bern CC equation, as given by [IPCC 2006]), biogenic-CO₂- has an additional factor which is the reabsorption of the CO₂- via re-growth of vegetation on the same piece of land. By this mathematical formulation, they have been able to assign various values of a so-called GWP_bio- over the typical time horizons of 20, 100 and 500 years and depending on the timing of biomass re-growth. Technically, this factor can then be simply used in a classical LCA and applied as correction factor to the amount of the biogenic-CO₂ emitted by the combustion of biomass." JRC 2014, p. 45.

[EUR-Lex-2018-AnnexVI-b-97] "Annualised emissions from carbon stock changes caused by land-use change, e_l, shall be calculated by dividing total emissions equally over 20 years." European Parliament, Council of the European Union 2018, p. Annex VI.

[EUR-Lex-2018-AnnexVI-c-98] See for instance the European Union's official emission savings percentages for different fuels here: European Parliament, Council of the European Union 2018, p. ANNEX VI. Note that these estimates do not include the average net emissions which results from an eventual land use change prior to planting.

[Camia-2018-89-a-99] "The Renewable Energy Directive (RED), as well as the Fuel Quality Directive (FQD) and the proposal for a RED-Recast (EP 2009, EP 2009b and EC 2016) apply a simplified attributional LCA methodology to assess GHG emissions savings for a series of liquid biofuels pathways used in the transport sector. A similar methodology is also extended to biomass used for power, heat and cooling generation (EC 2016). The RED evaluates the supply-chains GHG emissions of various bioenergy pathways and compares them to each other on a common basis (GHG emission savings with respect to a fossil fuel comparator) to promote the pathways that perform best on this relative scale and to exclude the pathways with the worst technologies and GHG performances." Camia et al. 2018, p. 89.

[Camia-2018-89-91-a-100] "Two main modelling principles are in use in LCA practice: Attributional (A-LCA) and Consequential (C-LCA) modelling, with the former being more widely used for historical and practical reasons. [...] Attributional modelling makes use of historical, fact-based, average, measureable data of known (or at least knowable) uncertainty, and includes all the processes that are identified to relevantly contribute to the system being studied. In attributional modelling, the system is hence modelled “as it is” or “as it was” (or as it is forecasted to be) (EC, 2010). Attributional modelling is also referred to as “accounting”, “book-keeping”, “retrospective”, or “descriptive”. [...] [P]urely attributional LCA studies of bioenergy systems are unable to capture properly all of the complexities linking bioenergy, climate, bioenergy and ecosystem services (e.g. market-mediated effects, biogeophysical, time-dependent effects). [...] The results of these types of assessment are static in time and do not account for biogenic-C flows. It has become established practice in A-LCA to assume that any emission of biogenic CO₂ (release to the atmosphere of the carbon contained in biological resources) is compensated by photosynthesis during the re-growth of the biomass feedstock. This assumption originates from an interpretation of the rules for reporting national GHG inventories to the United Nations Framework Convention on Climate Change (UNFCCC). Biogenic-C flow are accounted for in the land use, land-use change, and forestry (LULUCF) chapter at the time the biomass commodity is harvested and are therefore not accounted for in the energy sector at the time the biomass is burnt (JRC, 2013). It remains valid for system-level analysis, when the changes in biomass carbon stocks are accounted in the land-use sector rather than in the energy sector (EC, 2016c)." Camia et al. 2018, pp. 89–91.

[Cowie-2021-1217-1218-a-103] "Some studies of forest bioenergy consider carbon dynamics at the individual stand level [...]. Stand-level assessments represent the forest system as a strict sequence of events (e.g. site preparation, planting or natural regeneration, thinning and other silvicultural operations, final felling). Results are strongly influenced by the starting point: commencing the assessment at harvest shows upfront emissions, followed by a CO₂ removal phase, giving a delay before forest bioenergy contributes to net reductions in atmospheric CO₂, particularly in long-rotation forests. This delay has been interpreted as diminishing the climate benefit of forest bioenergy [...]. In contrast, commencing at the time of replanting shows the opposite trend: a period of CO₂ removal during forest growth, followed by a pulse emission returning the CO₂ to the atmosphere. Thus, stand-level assessments give inconsistent results and can be misleading as a basis to assess climate impacts of forest systems [...]. Furthermore, when considering only the stand level, it is difficult to identify whether the forest is sustainably managed or subject to unsustainable practices that cause declining productive capacity and decreasing carbon stocks. [...] The alternative to stand level is landscape-scale assessment, that considers the total area of managed forests. Stand- and landscape-level assessments respond to different questions. Stand-level assessment provides detailed information about plant community dynamics, growth patterns and interactions between carbon pools in the forest. But the stand-level perspective overlooks that forests managed for wood production are generally a series of stands of different ages, harvested at different times to produce a continuous supply of wood products. Across the whole forest landscape, that is, at the scale that forests are generally managed, temporal fluctuations observed at stand level are evened out and the forest carbon stock fluctuates around a trend line that can be increasing or decreasing, or roughly stable, depending on the age class distribution and weather patterns (Cowie et al., 2013). Landscape-level assessment provides a more complete representation of the dynamics of forest systems, as it can integrate the effects of all changes in forest management and harvesting taking place in response to—experienced or anticipated—bioenergy demand, and it also incorporates the effects of landscape-scale processes such as fire [...]. In a forest managed such that annual carbon losses due to harvest plus other disturbances and natural turnover equal the annual growth in the forest, there is no change in forest carbon stock when considered at landscape level [...]. To conclude, impacts of bioenergy policy should be assessed at the landscape scale because it is the change in forest carbon stocks at this scale, due to change in management to provide bioenergy along with other forest products, that determines the climate impact. Understanding of stand-level dynamics is critical to forest management and is useful to inform assessments at the landscape scale." Cowie et al. 2021, pp. 1217–1218.

[IPCC2019cp194-104] "Bioenergy from dedicated crops are in some cases held responsible for GHG emissions resulting from indirect land use change (iLUC), that is the bioenergy activity may lead to displacement of agricultural or forest activities into other locations, driven by market-mediated effects. Other mitigation options may also cause iLUC. At a global level of analysis, indirect effects are not relevant because all land-use emissions are direct. iLUC emissions are potentially more significant for crop-based feedstocks such as corn, wheat and soybean, than for advanced biofuels from lignocellulosic materials (Chum et al. 2011; Wicke et al. 2012; Valin et al. 2015; Ahlgren and Di Lucia 2014). Estimates of emissions from iLUC are inherently uncertain, widely debated in the scientific community and are highly dependent on modelling assumptions, such as supply/demand elasticities, productivity estimates, incorporation or exclusion of emission credits for coproducts and scale of biofuel deployment (Rajagopal and Plevin 2013; Finkbeiner 2014; Kim et al. 2014; Zilberman 2017). In some cases, iLUC effects are estimated to result in emission reductions. For example, market-mediated effects of bioenergy in North America showed potential for increased carbon stocks by inducing conversion of pasture or marginal land to forestland (Cintas et al. 2017; Duden et al. 2017; Dale et al. 2017; Baker et al. 2019). There is a wide range of variability in iLUC values for different types of biofuels, from –75–55 gCO₂ MJ–1 (Ahlgren and Di Lucia 2014; Valin et al. 2015; Plevin et al. 2015; Taheripour and Tyner 2013; Bento and Klotz 2014). There is low confidence in attribution of emissions from iLUC to bioenergy." IPCC 2019i, p. 194.

[Bird-2010-5-a-105] One often cited example of indirect land use change is the land use change from forest to agriculture that happened in Brazil after the US started to use some of its harvested corn for ethanol production rather than animal feed. The resulting lower supply of animal feed on the global market was seen as an opportunity by Brazilian farmers, who subsequently cut down forests in order to plant soya beans destined for the animal feed market. See Bird et al. 2010, p. 5, and also Searchinger et al. 2008, pp. 1238–1240 for the original research article.

[Lamers-2013-378-379-a-106] The authors also note that depending on the biome, forest protection implies exposure to natural disturbances, such as wildfires, droughts, or insect infestations. While wildfires have been studied and are often included in the carbon calculations, droughts, insect outbreaks, and other related climate change impact factors on forest are much harder to predict. These natural disturbances "[...] may have severe carbon implications" however. The authors conclude that "[...] forest protection assumptions postulate that the carbon and thus the land will not be used for human economic activities for centuries; an assumption generally questionable in our land-constrained world." Lamers & Junginger 2013, pp. 378–379.

[Cowie-2021-1218-a-107] "Assuming the forest would remain unharvested in the no-bioenergy scenario is not a realistic reference in situations where landholders use the land to generate income, unless landholders can obtain equivalent income from payments for carbon sequestration or other ecosystem services (Srinivasan, 2015). In cases where a no-harvest scenario is a valid reference case, there are challenges in quantifying future carbon stocks: carbon sequestration rate in unharvested forests, especially in the longer term, is uncertain in many cases due to a paucity of relevant data (e.g. Derderian et al., 2016) and uncertain effects of climate change. Furthermore, accumulated carbon is vulnerable to future loss through disturbances such as storm, drought, fire or pest outbreaks. Where more than one alternative is plausible, it is informative to analyse several alternative reference land-use scenarios (Koponen et al., 2018)." Cowie et al. 2021, p. 1218.

[Nabuurs-2017-4-a-112] According to Nabuurs et al., displacement factors takes into consideration the difference in CO₂ emissions per unit of primary energy produced, differences in efficiency of energy conversion (e.g. conversion from primary energy to electricity) and in some cases also the emission differences in the supply chains. See Nabuurs, Arets & Schelhaas 2017, p. 4. See also Cowie et al. 2021, p. 1214.

[Cowie-2021-1214-a-114] "Wood and coal have similar CO₂ emission factors, as the ratio of heating values between the two fuels is similar to the ratio of carbon content [...]. Where biomass is co-fired with coal in large power plants, the conversion efficiency may decrease a few percent, although there is usually no significant efficiency penalty when the co-firing ratio is below 10% [...]. Conversion efficiencies depend on fuel properties including moisture content and grindability in addition to heating value [...]. For low rank coal, biomass co-firing (especially torrefied biomass) can increase the boiler efficiency and net power plant efficiency [...]. Smaller biomass-fired plants can have lower electric conversion efficiency than large coal-fired plants, but as they are typically combined heat and power plants, they also displace heat production from other sources, that could otherwise have generated fossil fuel emissions [...]. Large dedicated biomass units (converted from coal) can operate with roughly the same level of thermal efficiency as delivered historically from coal [...]. Cowie et al. 2021, p. 1214.

[Sathre-2010-104-111-a-115] Sathre & O'Connor found that in general, wood products require less production energy and less use of fossil fuels than what is needed to produce a functionally equivalent amount of metals, concrete, or bricks. They write that a displacement factor of wood product substitution is a measure of the amount of GHG emissions that is avoided when wood is used instead of some other material. In other words, a displacement factor shows the efficiency with which the use of biomass reduces net GHG emissions. The authors also write that a higher displacement factor indicates that more GHG emissions are avoided per unit of wood used. Likewise, a negative displacement factor means that emissions are greater when using the wood product. Sathre & O'Connor 2010, pp. 104–111.

[Myllyviita-2021-2-3-a-116] Myllyviita et al. regrets that most researchers do not include wood storage in their calculations: "If the aim of DFs [displacement factors] is to describe the overall climate effects of wood use, DFs should include all the relevant GHG flows, including changes in forest and HWP [harvested wood products] carbon stock and post-use of HWPs, however, based on this literature review this is not a common practice." Myllyviita et al. 2021, p. 1.

[IEA-net-zero-2050-d-122] "Bioenergy with carbon capture and storage (BECCS) plays a critical role in the NZE Scenario by offsetting emissions from sectors where full decarbonisation is extremely difficult to achieve. In 2050, around 10% of total bioenergy is used in facilities equipped with carbon capture, utilisation and storage, and around 1.3 billion tonnes of CO₂ is captured using BECCS. Around 45% of this CO₂ is captured in biofuels production, 40% in the electricity sector, and the rest in heavy industry, notably cement production." IEA 2021a.

[JRC-2014-23-51-52-a-123] "In case that there is no raw material displacement from other sectors such as food, feed, fibers or changes in land carbon stocks due to direct or indirect land use change, the assumption of carbon neutrality can still be considered valid for annual crops, agriresidues, short-rotation coppices and energy grasses with short rotation cycles. This can also be valid for analysis with time horizons much longer than the feedstock growth cycles. [...] The timeframe of the comparison too plays a relevant role in the performances of the reference system. If the timeframe chosen is short, the current emissions from the reference system can be considered appropriate and constant. In the case of a long-term analysis, though, also the changes in the fossil reference system have to be accounted for. For instance, practically in all of the studies analyzed the reference system (coal or NG) is kept constant and unchanged for the whole duration of the analysis (even centuries), while, according to EU policies, by 2050 the EU should be decarbonized, implying that future savings might be much smaller than current ones. In this case [...] it may happen that the payback time is never reached. [...] On the other hand, if the reference fossil system gets ‘dirtier', as in the case of most of the unconventional fossil energy (shale gas, bituminous coal etc.) the fossil fuel parity may be reached sooner than with a constant reference fossil fuel." JRC 2014, pp. 23, 51–52. Note that the JRC use the term "payback time" in the sense of "parity time" as defined in Carbon accounting principles above. See JRC 2014, p. 16.

[Cowie-2021-1218-b-124] "Studies of real forest landscapes show that the net GHG effects of bioenergy incentives are more variable than suggested by studies that do not consider economic factors and varying conditions in the forest and wood products sector." Cowie et al. 2021, p. 1218.

[JRC-2014-69-a-127] EU's Joint Research Centre describes how the economic boundaries can expand to reach macro-economic size: "Large scale techno-economic modeling: This type of analysis includes a macroeconomic model that estimates the developments of the wood market in terms of imports, quantity of wood used for wood products and for bioenergy etc. as response to a given decision. The market model is coupled with a forest model that can model changes in carbon stocks in all the pools of forests (including living and dead wood, soil-C etc.) and eventually the carbon stocked in wood products. These two models can then be combined with several scenarios for the substitution of wood products in which a typical LCA (biogenic-CO₂ emissions are set to zero) is applied to calculate the GHG savings due to the use of biomass compared to the alternative materials / feedstocks. The combination of these calculations would provide a clear and quantitative forecast of possible carbon savings or emissions due to different policy scenarios and over different time horizons." JRC 2014, p. 69.

[Camia-2021-86-a-128] See for instance Camia et al. 2021, pp. 86, 100.

[Cowie-2021-1213-1221-a-129] "Wide variation in published estimates of payback time for forest bioenergy systems reflects both inherent differences between these systems and different methodology choices [...]. Critical methodology decisions include the definition of spatial and temporal system boundaries [...] and reference (counterfactual) scenarios [...]. Misleading conclusions on the climate effects of forest bioenergy can be produced by studies that focus on emissions at the point of combustion, or consider only carbon balances of individual forest stands, or emphasize short-term mitigation contributions over long-term benefits, or disregard system-level interactions that influence the climate effects of forest bioenergy." Cowie et al. 2021, pp. 1213, 1221.

[Jonker-2013-371-387-a-131] Jonker et al. examined the carbon intensity for southeastern forests in the US, and concluded that due to the large number of possible methodological choices and reference systems, the calculations produce a wide range of payback and parity times, from below 1 year payback time with landscape level carbon accounting to 27 years with stand level accounting, and parity times of 2 –106 years depending on system boundaries and the choice of alternative scenarios. The authors consider landscape-level carbon accounting more appropriate for the examined situation. Under this precondition, the issue of carbon payback time is basically nonexistent. If comparison against a protection scenario is deemed realistic and policy relevant, and assuming that wood pellets directly replace coal in an average coal power plant, the carbon parity time is 12–46 years; i.e. one or two rotations. Switching to intensively managed plantations yields the most drastic reduction in parity time (below 18 years in 9 of 12 cases). The authors conclude that the choice of carbon accounting method has a significant impact on the carbon payback and parity times. Jonker, Junginger & Faaij 2013, pp. 371–387.

[Madsen-2018-1-a-132] "Studies reporting long carbon debt payback times in general assume that the biomass is utilized for electricity production with low conversion efficiencies and that the woody biomass originates from the dedicated harvest of trees for energy from long rotation forestry. Looking at the current use of bioenergy in the EU, there is little evidence that such supply chains dominate." Madsen & Bentsen 2018, p. 1.

[Cowie-2021-1221-1222-a-133] "Misleading conclusions on the climate effects of forest bioenergy can be produced by studies that focus on emissions at the point of combustion, or consider only carbon balances of individual forest stands, or emphasize short-term mitigation contributions over long-term benefits, or disregard system-level interactions that influence the climate effects of forest bioenergy. Payback time calculations are influenced by subjective methodology choices and do not reflect the contribution of bioenergy within a portfolio of mitigation measures, so it is neither possible nor appropriate to declare a generic value for the maximum acceptable payback time for specific forest bioenergy options. To answer the key question ‘what are the climate implications of policies that promote bioenergy?' assessment should be made at the landscape level, and use a full life cycle approach that includes supply chain emissions, changes in land carbon stocks and other variables influenced by the policies studied. Effects on land cover, land management and the wood products and energy sectors need to be considered, including indirect impacts at international level. The bioenergy system should be compared with reference scenarios (counterfactuals) that describe the most likely alternative land use(s) and energy sources that would be displaced by the bioenergy system, and the probable alternative fates for the biomass being utilized. A no-harvest counterfactual is not realistic in most current circumstances, but markets that pay for carbon sequestration and other ecosystem services could change incentives for harvest in the future."Cowie et al. 2021, pp. 1221–1222.

[Bentsen-2017-1211-a-134] Bentsen examined 245 individual studies and found that the carbon payback time of apparently comparable forest bioenergy supply scenarios vary by up to 200 years, which provides ample room for confusion and dispute about the climate benefits of forest bioenergy. He concludes that the outcome of carbon debt studies lie in the assumptions, and that methodological rather than ecosystem and management related assumptions determine the findings. The findings are therefore seen as inadequate for informing and guiding policy development. Bentsen 2017, p. 1211.

[JRC-2014-75-a-135] "There is a large variability in the literature results for fossil fuel parity times. This is due to differences in the characteristics of the forest system considered (growth rate, management), in the carbon pools included, in the system boundaries definition and in the reference baseline used in the analysis." JRC 2014, p. 75.

[Camia-2021-93-a-136] EU's Joint Research Centre recommend that "[...] policymakers and scientists alike recognize that diverging values, worldviews, and ethical perceptions of natural resources and their management are a core part of the debate. These will not be solved by more scientific research, because science is a social endeavour where value-choices and judgements are inevitable. Transparency is key and cooperation with policymakers and co-creation of useful results should be welcomed." Camia et al. 2021, p. 93. In a presentation of this report for IEA Bioenergy, the JRC staff expand on this conclusion. They write that the question “Does forest bioenergy mitigate climate change?” really has no answer, as it is depends on "modelling approaches and the assumptions about hypothetical futures", and that researchers "come to equally valid, but opposite answers depending on assumptions chosen." They also write that "the assumptions chosen will align (consciously or unconsciously) with the worldviews and ethical values of the authors." According to the JRC, supporters of bioenergy usually have a more anthropocentric view of the human-nature relationship, while opposers of bioenergy are more aligned with nature conservation values. These norms lead to different concerns and definitions of what sustainability really is. See Mubareka, Giuntoli & Grassi 2021, pp. 8–9.

[jrc-2014-75-b-137] "Most of the forest feedstocks used for bioenergy, as of today, are industrial residues, waste wood, residual wood (thinnings, harvest residues, salvage loggings, landscape care wood etc.) for which, in the short to medium term, GHG savings may be achieved. On the other hand, in the case of stemwood harvested for bioenergy purposes only, if all the carbon pools and their development with time are considered in both the bioenergy and the reference fossil scenario, there is an actual increase in CO₂ emissions compared to fossil fuels in the short-term (few decades). In the longer term (centuries) also stemwood may reach the fossil fuel parity points and then generate GHG savings if the productivity of the forest is not reduced because of bioenergy production. [...] The results attained are strongly correlated with the following parameters: the fossil fuel replaced, efficiency of the biomass utilization, the future growth rate of the forest, the frequency and intensity of biomass harvests and the initial landscape carbon stock." JRC 2014, p. 75.

[Camia-2021-143-a-140] "Increased removal of FWD [fine woody debris], low stumps, CWD (course woody debris): It depends strongly on the decay rates considered. For instance, (Giuntoli et al., 2015) and (Giuntoli et al., 2016) found that residues with decay rates of 11.5%/year would mitigate climate change compared to natural gas heating and natural gas electricity after about 20 years, but residues with decay rate lower than 2.7%/year would take more than 86 years to payback compared to natural gas heating, or more than a century compared to the current EU power mix. FWD are thus likely to achieve carbon mitigation in a short term. However, decay rates for low stumps have been reported to range between 0.7%/year up to even 11%/year (Persson and Egnell, 2018), depending on climatic conditions and species. Considering a representative decay rate for temperate/boreal forests of between 3 and 6%/year would mean stumps would be unlikely to achieve climate mitigation before 50 years. This is substantiated also by the work of (Laganière et al., 2017). However, we indicate a range of uncertainty across other climate change levels. CWD are very likely to exhibit low decay rates and to have very long payback times." Camia et al. 2021, p. 143. See also JRC 2014, pp. 16–17, 43–44.

[Lamers-2013-379-a-141] Lamers & Junginger examined a number of studies and argue that parity times for residues "[...] mostly vary depending on the respective fossil fuel used in the reference scenario [...]." However, the second most important influencing factor "[...] is the size/diameter of the residue and the forest biome, i.e. conditions affecting the decay rate." The shortest parity times were found for forest residues which would otherwise be burned at the factory or roadside. This immediate carbon release in the alternative scenario causes an immediate carbon benefit and a net zero parity time for the bioenergy scenario. The longest parity times were for stump harvest in the cold boreal forests of northern Finland, when compared to a natural decay scenario for the stumps, and instead production of electricity from natural gas. For stemwood, parity times vary to some degree by forest biome with significantly shorter periods for highly productive regions, such as the temperate moist forests of the South-Eastern USA. In the boreal or sub-boreal forests, parity times against a forest protection scenario are about twice as large, but there are variations between studies. Under specific conditions, for instance where insect infestation has killed a large amount of merchantable timber stock, "[...] bioenergy harvest can reach parity times as low a zero." The high share of fast decaying tree biomass in the protection scenario shortens parity times. Parity times against regular timber harvest (business as usual) vary greatly with the fossil fuel alternative scenario, the shortest being coal and oil compared to natural gas. Afforestation on the other hand has a parity time of zero years if the land area in question would not be sequestering large amounts of carbon otherwise. Lamers & Junginger 2013, p. 379.

[Madsen-2018-1-2-a-142] "Here, we analyze carbon debt and payback time of substituting coal with forest residues for combined heat and power generation (CHP). The analysis is, in contrast to most other studies, based on empirical data from a retrofit of a CHP plant in northern Europe. The results corroborate findings of a carbon debt, here 4.4 kg CO₂eq GJ⁻¹. The carbon debt has a payback time of one year after conversion, and furthermore, the results show that GHG emissions are reduced to 50% relative to continued coal combustion after about 12 years. The findings support the use of residue biomass for energy as an effective means for climate change mitigation. [...] Dehue points out that there is no universally applied definition of ‘carbon debt' and ‘carbon debt payback time', leading authors to apply different definitions in an inconsistent manner. A definition often referred to is by Mitchell et al., where the terms ‘carbon debt', ‘carbon debt repayment', and ‘carbon offset parity point' are introduced. However, this definition only applies to bioenergy scenarios where the source of woody biomass comes from dedicated harvest and forest regrowth is included in the modelling. In contrast, bioenergy sources from wood waste and forest residues are resources that are generated independently of a bioenergy demand. The method that is used here is in line with the typical approach to carbon debt and payback time analyses, allowing for a comparison with other studies." Madsen & Bentsen 2018, pp. 1–2.

[Madsen-2018-2-3-5-7-a-143] "The study covers the direct emissions from the extraction and processing, transportation and the combustion of the fuels. It therefore excludes the embodied emissions in the used fuels or materials, e.g., the emissions related to produce diesel used for transportation of biomass or coal. The system boundary also exclude emissions related to distribution and use of the produced heat and electricity together with emissions that are related to the end of life of the CHP plant. Furthermore, GHG emission related to indirect effects, e.g., indirect land use change or indirect wood use change, of biomass consumption are not considered. The carbon debt concept is adopted from Mitchell et al., but applied to waste and residue resources, as e.g., demonstrated by Sathre et al. (Equation (1)). NE [equals] E_bio − (E_fossil + E_decay) where NE is the annual net GHG emission to the atmosphere, E_bio is the direct GHG emissions from the bioenergy supply chain including emissions from biomass combustion, E_fossil the direct GHG emissions from the counterfactual fossil supply chain, including emissions from fossil fuel combustion, and E_decay the GHG emissions from the counterfactual decay of forest residues. The payback time is determined as the time, where the time integrated NE [equals] 0. (Figure 2). The conceptual carbon emission profile corresponds to modelled profiles for the use of stumps or branches for energy. The payback time is understood as the point in time, where the bioenergy scenario starts to reduce the atmospheric GHG emissions relative to the counterfactual reference scenario. [...] In order to set up a reference scenario, the realistic alternative use of the biomass must be determined. Based on a literature review and interview with some of the biomass suppliers to the plant, the most likely alternative is decomposition on the forest floor, either as logs or as branches. [...] The total emissions per produced GJ are almost identical for both scenarios. Emissions from the biomass scenario are slightly higher by 4.4 kg CO₂eq GJ⁻¹, which represents the carbon debt, equaling 3.2% of the total emissions. [...] Emissions from processing are roughly identical for both scenarios; however, transport emissions are approximately three times higher in the reference scenario than the biomass scenario. This is in line with earlier research and is mainly attributable to longer transport distances for coal. The carbon debt incurred in the transition from coal to biomass is primarily related to the higher carbon intensity of biomass when compared to coal due to a lower carbon to oxygen ratio in biomass. Lower supply chain emissions in the biomass scenario, on the other hand, reduces the carbon debt." Madsen & Bentsen 2018, pp. 2–3, 5, 7. In simpler terms, the calculation starts with the total bioenergy-related emissions, then the coal-related emissions are subtracted (including the emissions from decaying forest residues).

[Madsen-2018-1-2-b-144] "Buchholz et al. conducted a meta-analysis of 59 carbon debt studies, and showed that the majority (47 studies) was based on hypothetical data and only a dozen were based on field data. [...] Data from EUROSTAT show that less than 1% of electricity production in the EU comes from solid biomass fired power plants. Solid biomass is more prevalent in combined heat and power (CHP) and heat production that are plants feeding into district heating systems. 16.3% of heat production to district heating in the EU comes from solid biomass, while the majority comes from natural gas and coal. [...] Contrary to most other studies, which are based on hypothetical scenarios, this analysis benefits from the use of data from an existing power plant retrofit in northern Europe, which is considered to be representative for the use of biomass for CHP in the EU." Madsen & Bentsen 2018, pp. 1–2.

[Holmgren-2021-10-26-a-145] See Holmgren 2021, pp. 10–26. The climate mitigation effect of the established forestry practice was determined by counting the specific annual changes over 10 and 40 years in this scenario's aggregated carbon pools. First, the net annual carbon increase in the national forest carbon pool was calculated: Annual total forest growth minus natural losses minus harvest removals (harvest removals includes both stemwood and residues). Subsequently, carbon used for harvested wood products, and the residues that is left in the forest but not yet decayed, are added to their respective pools (the HWP pool and the dead biomass pool). Then the carbon emitted from the decaying residues (including stumps, roots and branches) is subtracted. Finally, the displaced fossil carbon is added to the displaced fossil carbon pool (the fossil carbon is seen as "held back" in the fossil carbon reservoirs underground since forest biogenic carbon, which is already counted as emissions at harvest, has been used in its place.) As mentioned, and unlike in other studies, the study boundaries here included fossil fuel displacment effects, including from "[...] solid wood products and fibre products, representing normal wood utilization in Sweden where different parts of the tree is used for either of these product categories [...]." A displacement factor of 0.78 tonne fossil carbon displaced per 1 tonne biogenic carbon produced (which "[...] corresponds to displacement effects for the integrated mix of solid wood products, fiber products and bioenergy applied in several studies [...]") (p. 12) is used for both harvested wood products and bioenergy combined, and the forest residues half-life decay rate is set to 10 years (p. 13). In the two forest protection scenarios, the forest increases by 64% and 91% over 40 years respectively, while the actual forestry practice only achieve an increase of 44% (p. 17). However, since the forest in the forest protection scenarios is left to itself, the natural carbon losses (pests, fires etc.) increases by 4% and 6% for the two scenarios, respectively (p. 14). As mentioned above, substantial emissions are caused when the national forest carbon products and energy infrastructure is converted to fossil carbon products and energy infrastructure. The difference compared to the actual forestry practice over these 40 years was calculated by: 1.) Adding the extra amount of carbon that would have been absorbed from the atmospheric carbon pool and stored in the protected forest pool, compared to the amount of carbon stored in the actually managed forest. 2.) Subtracting the projected loss of carbon in the harvested wood products pool during the same time period (since each year some harvested wood products would be cycled out of this carbon pool and into the atmospheric carbon pool because of combustion or rotting when reaching end-of-life, while no new harvested wood products would have entered this pool. 3.) Subtracting the substantial amount of fossil carbon that would have to be moved from the underground fossil reservoirs and into fossil carbon products and energy carriers, in order to a.) replace the forest carbon products and energy carriers, and b.) convert the national forest carbon products and energy infrastructure into a fossil carbon products and energy infrastructure. These subtractions are highest at the beginning of the 40-year period, and (together with the carbon absorption going on elsewhere in the forest) more than compensate for the carbon debt caused by harvest-related carbon emissions (p. 14-17).

[m3-to-dry-tonnes-148] Recalculated from Holmgren's 0.5 tonne CO₂e per m³ (Holmgren 2021, p. 12), 40% spruce and 40% pine in Swedish forests (Swedish Wood, p. Fig. 5), and 320 vs. 390 kg dry mass per m³ for spruce and pine, respectively. Here, an average value of 350 kg/m³ was assumed, together with 50% carbon content, and 3.67 kg CO₂e per kg carbon. The actual forestry practice mitigated in total 3.54 Gt CO₂e, with the displacement-specific mitigation effects estimated at 1.84 Gt CO₂e. If the displacement effects were left out of the calculation, one of the forest protection scenarios (the one with only a 4% increase in natural losses (e.g. wildfires and diseases), had more carbon stored in the forest after 10 and 40 years compared to the actual forestry practice (0.74 vs. 0.55 and 2.41 vs. 1.84 Gt CO₂e, respectively.) The other forest protection scenario (the one with a 6% increase in natural losses) had less carbon absorbed in the forest compared to the actual forestry practice after 40 years (1.56 vs. 1.84 Gt CO₂e) but more after 10 years (0.64 vs. 0.55 Gt CO₂e.) In addition to the forest protection scenarios, the author also included a 10% reduced harvest scenario, which performed similar to the actual forestry scenario.

[Miner-2014-599-a-152] Miner et al. writes that in the eastern parts of the USA, bioenergy from forest residues that otherwise would have been left to decay naturally, typically accomplishes net GHG benefits within a decade when displacing coal-based electricity, and within two decades when displacing natural gas-based electricity. Miner et al. 2014, p. 599.

[hanssen-2017-1407-1410-a-154] See Hanssen et al. 2017, pp. 1407–1410, and table S1 in the supporting information document, link available at the bottom of the article.

[JRC-2014-42-b-1-155] "The studies analyzed report payback times in the range of 0 – 74 years for harvest residues. The main factors affecting these values are mostly similar to the ones described for stemwood. The ratio of fossil carbon displacement is the main parameter. If the residues are used with high efficiency to displace coal (such as in co-firing), the payback times are rather short, if any. In case the residues are heavily processed to produce liquid biofuel the payback time increases dramatically. Also the size of the residue plays a relevant role, as well as the geographic and local conditions that influence the bacterial decomposition rates. Wood from thinnings may, to some extent, be assimilated to harvest residues (especially pre-commercial thinnings). If not collected for bioenergy it would be left in the forest to decay, or combusted at roadside. On the other hand, depending on the wood quality, the use of thinnings wood for bioenergy may compete with other uses, such as pulp and paper or engineered wood. Salvage loggings can also be assimilated to harvest residues. Damaged, dying or dead trees affected by injurious agents, such as wind or ice storms or the spread of invasive epidemic forest pathogens, insects and diseases would remain in the forest and decay or combusted at roadside. Wood removed for prescribed fire hazard control as well can be considered residual wood." JRC 2014, pp. 42–43, table 3. Note that the JRC use the term "payback time" in the sense of "parity time" as defined in Carbon accounting principles above. See JRC 2014, p. 16.

[JRC-2014-42-a-157] "Total GHG emission per energy content from the production of energy from harvest residues. Norway spruce stumps (diameter 30 cm), young stand delimbed thinning wood (diameter 10 cm) and branches (diameter 2 cm). Emissions over a 100 year period after start of the practice in Northern Finland (dotted line) and Southern Finland (solid line) and the entire fuel cycle emissions of some fossil fuels. The total emission estimates of forest bioenergy include emissions resulting from the changes in carbon stocks and the emissions from production chain including collecting, transporting, chipping and combusting the forest residues." JRC 2014, p. 42.

[Camia-2018-100-104-a-161] "Three systems were designed to represent three different power production scales (see Figure 7.15): i) large-scale power plant of 80 MWel. fuelled with wood pellets from forest logging residues (FRel); ii) medium-scale power plant of 15 MWel. fuelled with cereal straw bales (STel); iii) small-scale internal combustion engine of 300 kWel. fuelled with biogas produced from anaerobic digestion of cattle slurry, employing an open or gas-tight tank for digestate storage (Biogas OD/CD). [...] The results are explicit in time, Near-Term Climate Forcers (i.e. ozone precursors and aerosols) are included, an instantaneous, absolute climate metric is used and biogenic-C flows are explicitly accounted for (see Table 7.1 for all methodological details). These results reveal additional details compared to the analysis in section 7.3.1. For instance, they indicate with clarity that power generation from cereal straws and cattle slurry can provide, by 2100, global warming mitigation compared to the current European electricity mix in all of the systems and scenarios considered. Power generation from forest logging residues is an effective mitigation solution only in situations in which the decay rates of the residues on the forest floor were above 5.2% /yr. Even with faster-decomposing feedstocks, bioenergy temporarily causes a climate change worsening compared to the fossil system. Strategies for bioenergy deployment should thus take into account the potential increase in global warming rate and temporary increase in temperature anomaly. Further details on the methodology and on the results of the case studies can be found in Giuntoli et al. (2016)." Camia et al. 2018, pp. 100–104. The supporting documentation show that the "residues" category includes not only small-diameter residues like branches, but also logs and stumps. See JRC 2018 and JRC 2015, p. 92.

[JRC-2014-16-a-162] The JRC here actually use the term "payback time", but define this term in the same way the term "parity time" is defined above: "[...] [A]t the payback time the fossil fuel parity is reached (i.e. the bioenergy system and the fossil counterfactual have emitted the same amount of CO₂ in the atmosphere). After the fossil fuel parity time, the bioenergy system starts to provide CO₂ savings." See JRC 2014, p. 16. Normally, the time it takes for a bioenergy scenario to store as much carbon as a no-bioenergy scenario (i.e. when their net emission level is the same) is known as the carbon parity time: "Eventually carbon levels in the forest return to the level at which they would have been if they had been left unharvested. (Some of the literature employs the term ‘carbon payback period' to describe this longer period, but it is more commonly used to mean the time to parity with fossil fuels; this meaning is used in this paper.)" Chatham House 2017, p. 27.

[mckechnie-2010-163] In the JRC's chart above, landscape level carbon accounting is assumed for this scenario. However, the original research article does not actually say which accounting method is used, only that biomass is sourced "[...] from 5.25 million hectares within the GLSL forest region in Ontario." McKechnie et al. 2010, p. 791.

[Lamers-2013-373-a-166] Lamers & Junginger argue that payback times are mainly determined by plant growth rates, i.e. the forest biome (e.g. climate zone), tree species, site productivity and management. Parity times are primarily influenced by the choice and construction of the reference scenario and fossil carbon displacement efficiencies. The authors write that using "[...] small residual biomass (harvesting/processing), deadwood from highly insect-infected sites, or new plantations on highly productive or marginal land offers (almost) immediate net carbon benefits." The actual climate mitigation potential however is determined by the effectiveness of the fossil fuel displacement. Lamers & Junginger 2013, p. 373.

[JRC-2014-26-a-172] The JRC's example here is from a UK coniferous forest study: "In the case wood is used for bioenergy only the total emissions of the bioenergy system would be −5.5 tCO₂/ha*y (5.1 tCO₂/ha*y from displacement of fossil fuel and 0.4 tCO₂/ha*y due to the sink of the forest system), that, compared to the missed growth of the forest (14 tCO₂/ha*y) [14 tonnes CO₂ per hectare per year] results in net emissions of 8.5 tCO₂/ha*y. This result shows that, in a 40 years timeframe, CO₂ emissions are lower for the suspended management forest than for the forest managed for bioenergy only. The second case is if the wood is used for materials as well as bioenergy (bioenergy from residues). In this case the total emissions of the material and bioenergy system would be −22.8 tCO₂/ha*y, (-6 tCO₂/ha*y in carbon stock of the forest and products and −16.8 tCO₂/ha*y from displacement of products) to which the missed growth of the forest has to be subtracted (14 tCO₂/ha*y) resulting in net GHG savings of 8.8 tCO₂/ha*y. Therefore managing the forest for products determines higher GHG savings than suspending the management." JRC 2014, p. 26.

[JRC-2014-59-61-a-173] "In fact, wood products have multiple climate mitigation benefits: they increase the anthropogenic carbon pools, they are often much less GHG and energy intensive than similar materials of fossil origin (e.g. concrete, metals etc.) and, finally, bioenergy can be obtained from these products at the end of life to replace fossil fuels and guarantee additional substitution. [...] [W]hen wood is used in a cascade utilization, then climate mitigation can be achieved in much shorter times than when wood is used purely for energy. Moreover, with the proper measures (longer storage, substitution of C-intensive materials and fossil fuels), the payback time can be even shortened to zero, as compared to centuries indicated for energy-only use. Studies that fail to consider the wood for material displacement may come to misleading conclusions." JRC 2014, pp. 59, 61. Note that the JRC use the term "payback time" in the sense of "parity time" as defined in Carbon accounting principles above. See JRC 2014, p. 16.

[JRC-2014-16-17-a-174] "The reviewed studies indicate that the use of stemwood from dedicated harvest for bioenergy would cause an actual increase in GHG emissions compared to those from fossil fuels in the short-and medium term (decades), while it may start to generate GHG savings only in the long-term (several decades to centuries), provided that the initial assumptions remain valid. The harvest of stemwood for bioenergy purposes is not common today, however, it is becoming a more common practice that is expected to expand in the future. [...] The GHG saving can be immediate if in the counterfactual scenario the wood would be burnt at roadside. This feedstock is expected to provide most of the additional increment of biomass for bioenergy by 2020. Also in the case of new plantations on agricultural or grazing land the GHG savings can be immediate (in absence of iLUC)." JRC 2014, pp. 16–17.

[JRC-2014-40-41-a-175] "The initial landscape conditions and land-use history are also fundamental in determining the amount of time required for forests to recover the initial additional emissions of the bioenergy system over the fossil one. While Recently Disturbed and Old-Growth landscapes required very long payback times, Post- Agricultural and Rotation Harvest landscapes were capable of recovering the additional emission in relatively short time periods, often within 1 year [Mitchell 2012]. This is a conclusion also of Zanchi et al. [Zanchi 2011]. The reason is that planting a short-rotation forest on unused agricultural land does not start with high carbon stocks so causes an increase in average carbon stocks." JRC 2014, pp. 40–41. Note that the JRC use the term "payback time" in the sense of "parity time" as defined in Carbon accounting principles above. See JRC 2014, p. 16.

[Cowie-2021-1215-1216-a-177] "The source of forest biomass is a key determinant of climate change effects of bioenergy (Matthews et al., 2018). Concerns have been raised that bioenergy demand could lead to widespread harvest of forests solely for bioenergy, causing large GHG emissions and forgone carbon sequestration (Brack, 2017; Norton et al., 2019; Searchinger et al., 2018). However, long-rotation forests are generally not harvested for bioenergy products alone: Biomass for bioenergy is usually a by-product of sawlog and pulpwood production for material applications (Dale et al., 2017; Ghaffariyan et al., 2017; Spinelli et al., 2019; Figure 1). Logs that meet quality requirements are used to produce high-value products such as sawnwood and engineered wood products such as cross laminated timber, which can substitute for more carbon-intensive building materials such as concrete, steel and aluminium (Leskinen et al., 2018). Residues from forestry operations (tops, branches, irregular and damaged stem sections, thinnings) and wood processing residues (e.g. sawdust, bark, black liquor) are used for bioenergy (Kittler et al., 2020), including to provide process heat in the forest industry (Hassan et al., 2019). These biomass sources have high likelihood of reducing net GHG emissions when substituting fossil fuels (Hanssen et al., 2017; Matthews et al., 2018), and their use for bioenergy enhances the climate change mitigation value of forests managed for wood production (Cintas, Berndes, Hansson, et al., 2017; Gustavsson et al., 2015, 2021; Schulze et al., 2020; Ximenes et al., 2012). Part of the forest biomass used for bioenergy consists of roundwood (also referred to as stemwood), such as small stems from forest thinning. For example, roundwood was estimated to contribute around 20% of the feedstock used for densified wood pellets in the United States in 2018 (US EIA, 2019)." Cowie et al. 2021, pp. 1215–1216.

[Hektor-2016-4-a-178] Hektor write that supply chain emissions for biomass are "[...] in most cases less than half of the corresponding emissions from fossil fuels." Hektor, Backéus & Andersson 2016, p. 4.

[JRC-2014-17-a-180] "There is a large variability in the results of forest bioenergy fossil fuel parity times calculations. This large variability depends on the many different characteristics of the systems compared and non-consistent modeling assumptions and approaches. The first, most important assumption is on the fossil fuel displaced. Then, concerning both the bioenergy system and the reference fossil system the following characteristics heavily impact the results: efficiency in the final use, future growth rate of the forest, the frequency and intensity of biomass harvests, the initial forest carbon stock, the forest management practices assumed." JRC 2014, p. 17.

[JRC-2014-34-a-182] "The reviewed studies show payback times ranging from 0 to almost 500 years. This large variability depends on the many different characteristics and assumptions on both the forest/bioenergy system and the reference fossil system. The most straight forward relation is with the fossil fuel used as a reference in the fossil scenario. Obviously, the more carbon intensive the fossil fuel replaced is, the shorter is the payback time. [...] A further correlation exists with the efficiency of the biomass utilization. The less efficient the bioenergy system is, the longer are the payback times. In case of electricity production, in biomass only plants, the electrical efficiency of biomass conversion is lower than the fossil, while thermal conversion energetic efficiency is similar for biomass and fossil fuels. In co-firing plants, biomass generally achieves the same efficiency as coal. An intensive processing, such as for liquid biofuel substitution via lignocellulosic ethanol, causes much longer payback times because of the loss of energy in the biofuels production (about half of the energy content of the biomass is lost in the processing [...]. The slower the forest growth rate is, the longer is the payback time. The forest growth rate depends on the latitude (boreal, temperate, tropical), but also on specific characteristics of the trees species, the microclimate and the soil fertility." JRC 2014, p. 34. Note that the JRC use the term "payback time" in the sense of "parity time" as defined in Carbon accounting principles above. See JRC 2014, p. 16.

[hanssen-2017-3-a-185] See Hanssen et al. 2017: Figure S3 at page 3 in the Supporting Information document, link to document available at the bottom of the article.)

[Camia-2018-95-98-a-188] "The modeller assigns independent inputs to specific parameters, called here decision variables, thus producing a range of results deriving from different configurations of the same supply-chains associated to a single commodity. For instance, the distance at which biomass resources are transported influences significantly the overall impact of bio-based commodities. Conversion efficiencies can be both a source of variability (e.g. range of possible efficiencies for a single engine) and important decision variables (e.g. modelling different conversion technologies).[...] Another source of variation is linked to methodological choices in each assessment (e.g. allocation basis, background processes, etc.); for the benefit of the users, critical methodological choices are reported explicitly in the database. [...] The range of results associated with each pathway is dependent on several factors of variability:1. Transport distances of the feedstock or of the final product, 2. End-use conversion efficiencies, 3. Utilities, 4. Process characteristics, 5. Background data, 6. LCA Methodology. The GHG emissions reported in this work, therefore, should not be interpreted as a universal property associated to the product/commodity, since the changes in methodological choices and background data can largely influence the absolute value of GHG emission. However, the relative benchmarking among similar products and commodities can provide important information. The results [...] show that, most bioenergy pathways emit less GHG along their supply chain than fossil fuel pathways. However, the various pathways can achieve very different GHG emission levels. For instance, using dairy cattle slurry to produce biogas or biomethane can guarantee the highest GHG emissions mitigation due to the emission credits assigned for the avoided methane emissions associated to the use of raw manure as organic fertilizer (Giuntoli et al., 2017). In order for commodities to achieve the highest ambitions in terms of GHG emission savings (>85% savings), generally, high resource efficiency along the supply chain is required, and in particular: 1. Optimized logistics with short or efficient transport options (e.g. biomass feedstock is traded within EU neighbouring countries), 2. High efficiency of final conversion, 3. Use of renewable energy sources to supply process-heat and process-electricity, 4. Optimal process design (e.g. digestate residue from anaerobic digestion is stored in gas-tight tanks), 5. Use of wastes, residual or low-input feedstocks, 6. Assignment of credits to co-products (substitution method). Nonetheless, the results show that even with current technologies, significant optimizations are available to reduce the impacts of each supply chain (lower boundary of floating columns)." Camia et al. 2018, pp. 95, 98.

[IRENAprojections-i-191] "In addition to the EU, the US has also amended its Renewables Fuel Standard 1 (RFS1) to include minimum life-cycle GHG emissions in the RFS2. RFS2 distinguishes between the production of conventional and advanced biofuels, which are defined based on their GHG abatement potential. All biofuels which can save up to 20% GHG in their life cycle compared to the petroleumbased equivalents are categorised as conventional. Conventional biofuel production is limited to 15 billion gallons to 2022. Advanced biofuels production accounts for the remainder 21 billion gallons. A biofuel can be considered advanced if it saves at least 50% GHG. Cellulosic biofuels require a 60% GHG emission reduction compared to the petrochemical equivalent (EPA, 2012). These emissions include ILUC GHG emissions." IRENA 2014, p. 47.

[Eur-Lex-2018-AnnexVI-a-192] The estimates are for the "medium case" considered (case 2a); a pellet mill that uses wood for processing heat, but sources electricity from the grid. Estimates (for forest residue based pellets) reduce to 50–58% when fossil fuels is used for processing heat (case 1), but increase to 84-92% when electricity is sourced from a CHP biomass power plant (case 3a). See European Parliament, Council of the European Union 2018, p. Annex VI.

[Hanssen_et_al.-2017-1415-1416-a-193] "[...] GHG emission reductions of wood-pellet electricity compared to fossil EU grid electricity are 71% (for small roundwood and harvest residues), 69% (for commercial thinnings) or 65% (for mill residues), as shown in more detail in Fig. S3. The GHG reduction percentage of wood-pellet electricity from mill residues was [...] 75% [...]." Hanssen et al. 2017, pp. 1415–1416.

[hanssen-2017-3-4-a-194] See Hanssen et al. 2017: Figure S3 and Table S1 at pages 3–4 in the Supporting Information document, link to document available at the bottom of the article.)

[cowie-2021-1219-a-196] "It is commonly perceived that bioenergy supply chain emissions are substantial, particularly when biomass is transported internationally, and could negate the climate benefits of fossil fuel substitution. However, fossil energy use along domestic forest biomass supply chains, from harvest, processing and transport, is generally small compared to the energy content of the bioenergy product and, with efficient handling and shipping, even when traded internationally [...]. The European Commission's Joint Research Centre determined that shipping pellets between North America and Europe increases supply chain emissions by 3–6 g CO₂/MJ, from around 3–15 g CO₂/MJ for wood chips or pellets dried using bioenergy and transported 500 km by truck (Giuntoli et al., 2017). For context, the EU average emission factors for hard coal are 96 and 16 g CO₂/MJ for combustion and supply respectively (Giuntoli et al., 2017). This underscores the importance of assessing actual supply chains. For example, the international pellet supply chain between the southeast United States and Europe has been intentionally designed to minimize trucking and associated handling costs, with pellet mills and large end users such as power plants located near rail lines, waterways and ports, thereby minimizing transport emissions and increasing net climate benefits (Dwivedi et al., 2014; Favero et al., 2020; Kline et al., 2021)."Cowie et al. 2021, p. 1219.

[Camia-2021-7-42-a-198] "In 2015 the net imports (i.e. imports minus exports) of wood pellets amounted to 3% of the total wood for energy mix (around 16 Mm³). The UK accounted for 97% of EU net imports of wood pellets (JFSQ). The United States was by far the most important source of EU wood pellets imports, with a 77% share (United Nations, 2020)." Camia et al. 2021, pp. 7, 42.

[Camia-2021-33-a-200] "Following the findings of the COST Action EuroCoppice (FP1301)17, coppice forests cover more than 19 Mha in the EU, corresponding to about 12% of the total forest area in 2015. The large majority (17 Mha) are in the EU Mediterranean countries, where about 32% of the forest area is reported as coppice (Unrau et al. 2018)." Camia et al. 2021, p. 33.

[JRC-2014-40-41-b-204] "The initial landscape conditions and land-use history are also fundamental in determining the amount of time required for forests to recover the initial additional emissions of the bioenergy system over the fossil one. While Recently Disturbed and Old-Growth landscapes required very long payback times, Post- Agricultural and Rotation Harvest landscapes were capable of recovering the additional emission in relatively short time periods, often within 1 year [Mitchell 2012]. This is a conclusion also of Zanchi et al. [Zanchi 2011]. The reason is that planting a short-rotation forest on unused agricultural land does not start with high carbon stocks so causes an increase in average carbon stocks." JRC 2014, pp. 40–41. Note that the JRC use the term "payback time" in the sense of "parity time" as defined in Carbon accounting principles above. See JRC 2014, p. 16. Likewise, Liu et al. found an average carbon payback time of 0.97 years for miscanthus planted on marginal, eroded soils in the Loess Plateau in China. Liu et al. 2016, p. 1.

[IRENAprojections-g-205] "If the total biogenic carbon released during biomass decay and/or combustion is sequestered, the system continues to be in balance. As a result, the amount of CO₂ in the atmosphere does not increase. [...] When short-rotation energy crops or agricultural residues are used as fuel, they result in a balanced carbon cycle because they grow/renew themselves annually. In comparison, the rapid expansion of palm oil plantations in Indonesia and Malaysia, for example, has led to major problems associated with bioenergy. Logging rain forests or peat bogs for palm oil plantations has a negative effect. Plantations which were partly built on carbon-rich peat soils in the region resulted in drainage. The subsequent oxidation of peat and natural or anthropogenic fires results in substantial CO₂ emissions. Peat digging also has a negative effect, which results in an increase in CO₂ emissions in the atmosphere." IRENA 2014, p. 45.

[Mc2017p489a-207] "Perennial Miscanthus has energy output/input ratios 10 times higher (47.3 ± 2.2) than annual crops used for energy (4.7 ± 0.2 to 5.5 ± 0.2), and the total carbon cost of energy production (1.12 g CO₂-C eq. MJ⁻¹) is 20–30 times lower than fossil fuels." McCalmont et al. 2017, p. 489.

[Mc2017p500a-208] "The results in Fig. 3c show most of the land in the UK could produce Miscanthus biomass with a carbon index that is substantially lower, at 1.12 g CO₂-C equivalent per MJ energy in the furnace, than coal (33), oil (22), LNG (21), Russian gas (20), and North Sea gas (16) (Bond et al., 2014), thus offering large potential GHG savings over comparable fuels even after accounting for variations in their specific energy contents. Felten et al. (2013) found Miscanthus energy production (from propagation to final conversion) to offer far higher potential GHG savings per unit land area when compared to other bioenergy systems. They found Miscanthus (chips for domestic heating) saved 22.3 ± 0.13 Mg [tonnes] CO₂-eq ha⁻¹ yr⁻¹ [CO₂ equivalents per hectare per year] compared to rapeseed (biodiesel) at 3.2 ± 0.38 and maize (biomass, electricity, and thermal) at 6.3 ± 0.56." McCalmont et al. 2017, p. 500.

[Le2016p2b-209] "The costs and life-cycle assessment of seven miscanthus-based value chains, including small- and large-scale heat and power, ethanol, biogas, and insulation material production, revealed GHG-emission- and fossil-energy-saving potentials of up to 30.6 t CO₂eq C ha⁻¹ y⁻¹ and 429 GJ ha⁻¹ y⁻¹, respectively. Transport distance was identified as an important cost factor. Negative carbon mitigation costs of –78€ t−1 CO₂eq C were recorded for local biomass use. The OPTIMISC results demonstrate the potential of miscanthus as a crop for marginal sites and provide information and technologies for the commercial implementation of miscanthus-based value chains. [...] The overall biomass transport distance was assumed to be 400 km when bales were transported to the bioethanol plant or to the plant producing insulation material as well as in the value chain 'Combined heat and power (CHP) bales.' For the value chains 'CHP pellets' and 'Heat pellets' the bales were transported 100 km to a pelleting plant and from there the pellets were transported 400 km to the power plants. The average farm-to-field distance was assumed to be 2 km. This transport distance is also assumed for the value chain 'heat chips' in which a utilization of the chips as a biomass fuel on the producing farm was assumed. Because of the higher biomass requirements of the biogas plant an average transport distance of 15 km from field to plant was assumed." Lewandowski et al. 2016, pp. 2, 7.

[Mc2017p493b-210] "Any soil disturbance, such as ploughing and cultivation, is likely to result in short-term respiration losses of soil organic carbon, decomposed by stimulated soil microbe populations (Cheng, 2009; Kuzyakov, 2010). Annual disturbance under arable cropping repeats this year after year resulting in reduced SOC levels. Perennial agricultural systems, such as grassland, have time to replace their infrequent disturbance losses which can result in higher steady-state soil carbon contents (Gelfand et al., 2011; Zenone et al., 2013)." McCalmont et al. 2017, p. 493.

[IPCC2019a-211] "Tillage breaks apart soil aggregates which, among other functions, are thought to inhibit soil bacteria, fungi and other microbes from consuming and decomposing SOM (Grandy and Neff 2008). Aggregates reduce microbial access to organic matter by restricting physical access to mineral-stabilised organic compounds as well as reducing oxygen availability (Cotrufo et al. 2015; Lehmann and Kleber 2015). When soil aggregates are broken open with tillage in the conversion of native ecosystems to agriculture, microbial consumption of SOC and subsequent respiration of CO₂ increase dramatically, reducing soil carbon stocks (Grandy and Robertson 2006; Grandy and Neff 2008)." IPCC 2019a, p. 393.

[Ha2015p27a-213] "A systematic review and meta-analysis were used to assess the current state of knowledge and quantify the effects of land use change (LUC) to second generation (2G), non-food bioenergy crops on soil organic carbon (SOC) and greenhouse gas (GHG) emissions of relevance to temperate zone agriculture. Following analysis from 138 original studies, transitions from arable to short rotation coppice (SRC, poplar or willow) or perennial grasses (mostly Miscanthus or switchgrass) resulted in increased SOC (+5.0 ± 7.8% and +25.7 ± 6.7% respectively)." Harris, Spake & Taylor 2015, p. 27.

[Mc2017p493c-214] "[...] it seems likely that arable land converted to Miscanthus will sequester soil carbon; of the 14 comparisons, 11 showed overall increases in SOC [soil organic carbon] over their total sample depths with suggested accumulation rates ranging from 0.42 to 3.8 Mg C ha⁻¹ yr⁻¹. Only three arable comparisons showed lower SOC stocks under Miscanthus, and these suggested insignificant losses between 0.1 and 0.26 Mg ha⁻¹ yr⁻¹." McCalmont et al. 2017, p. 493.

[Mc2017p496c-215] "The correlation between plantation age and SOC can be seen in Fig. 6, [...] the trendline suggests a net accumulation rate of 1.84 Mg C ha⁻¹ yr⁻¹ with similar levels to grassland at equilibrium." McCalmont et al. 2017, p. 496.

[Ka2001p1a-216] Given the EU average peak yield of 22 tonnes dry matter per hectare per year (approximately 15 tonnes during spring harvest). See Anderson et al. 2014, p. 79. 15 tonnes also explicitly quoted as the mean spring yield in Germany, see Felten & Emmerling 2012, p. 662. 48% carbon content; see Kahle et al. 2001, table 3, page 176.

[Wh2018p156c-217] See Whitaker et al. 2018, p. 156, Fig. 3, or Fig. 3 in Appendix S1 (Supplementary Materials)

[Wh2018p151a-218] "The environmental costs and benefits of bioenergy have been the subject of significant debate, particularly for first‐generation biofuels produced from food (e.g. grain and oil seed). Studies have reported life‐cycle GHG savings ranging from an 86% reduction to a 93% increase in GHG emissions compared with fossil fuels (Searchinger et al., 2008; Davis et al., 2009; Liska et al., 2009; Whitaker et al., 2010). In addition, concerns have been raised that N₂O emissions from biofuel feedstock cultivation could have been underestimated (Crutzen et al., 2008; Smith & Searchinger, 2012) and that expansion of feedstock cultivation on agricultural land might displace food production onto land with high carbon stocks or high conservation value (i.e. iLUC) creating a carbon debt which could take decades to repay (Fargione et al., 2008). Other studies have shown that direct nitrogen‐related emissions from annual crop feedstocks can be mitigated through optimized management practices (Davis et al., 2013) or that payback times are less significant than proposed (Mello et al., 2014). However, there are still significant concerns over the impacts of iLUC, despite policy developments aimed at reducing the risk of iLUC occurring (Ahlgren & Di Lucia, 2014; Del Grosso et al., 2014)." Whitaker et al. 2018, p. 151.

[Mi2016p317a-219] "The impact of growing bioenergy and biofuel feedstock crops has been of particular concern, with some suggesting the greenhouse gas (GHG) balance of food crops used for ethanol and biodiesel may be no better or worse than fossil fuels (Fargione et al., 2008; Searchinger et al., 2008). This is controversial, as the allocation of GHG emissions to the management and the use of coproducts can have a large effect on the total carbon footprint of resulting bioenergy products (Whitaker et al., 2010; Davis et al., 2013). The potential consequences of land use change (LUC) to bioenergy on GHG balance through food crop displacement or 'indirect' land use change (iLUC) are also an important consideration (Searchinger et al., 2008)." Milner et al. 2016, pp. 317–318.

[Mc2017p490a-220] "While the initial premise regarding bioenergy was that carbon recently captured from the atmosphere into plants would deliver an immediate reduction in GHG emission from fossil fuel use, the reality proved less straightforward. Studies suggested that GHG emission from energy crop production and land-use change might outweigh any CO₂ mitigation (Searchinger et al., 2008; Lange, 2011). Nitrous oxide (N₂O) production, with its powerful global warming potential (GWP), could be a significant factor in offsetting CO₂ gains (Crutzen et al., 2008) as well as possible acidification and eutrophication of the surrounding environment (Kim & Dale, 2005). However, not all biomass feedstocks are equal, and most studies critical of bioenergy production are concerned with biofuels produced from annual food crops at high fertilizer cost, sometimes using land cleared from natural ecosystems or in direct competition with food production (Naik et al., 2010). Dedicated perennial energy crops, produced on existing, lower grade, agricultural land, offer a sustainable alternative with significant savings in greenhouse gas emissions and soil carbon sequestration when produced with appropriate management (Crutzen et al., 2008; Hastings et al., 2008, 2012; Cherubini et al., 2009; Dondini et al., 2009a; Don et al., 2012; Zatta et al., 2014; Richter et al., 2015)." McCalmont et al. 2017, p. 490.

[Wh2018p156b-221] "Significant reductions in GHG emissions have been demonstrated in many LCA studies across a range of bioenergy technologies and scales (Thornley et al., 2009, 2015). The most significant reductions have been noted for heat and power cases. However, some other studies (particularly on transport fuels) have indicated the opposite, that is that bioenergy systems can increase GHG emissions (Smith & Searchinger, 2012) or fail to achieve increasingly stringent GHG savings thresholds. A number of factors drive this variability in calculated savings, but we know that where significant reductions are not achieved or wide variability is reported there is often associated data uncertainty or variations in the LCA methodology applied (Rowe et al., 2011). For example, data uncertainty in soil carbon stock change following LUC has been shown to significantly influence the GHG intensity of biofuel production pathways (Fig. 3), whilst the shorter term radiative forcing impact of black carbon particles from the combustion of biomass and biofuels also represents significant data uncertainty (Bond et al., 2013)." Whitaker et al. 2018, pp. 156–157.

[Em2017p275a-223] "Miscanthus is one of the very few crops worldwide that reaches true CO₂ neutrality and may function as a CO₂ sink. [...] Related to the combustion of fuel oil, the direct and indirect greenhouse gas emissions can be reduced by a minimum of 96% through the combustion of Miscanthus straw [...]. Due to the C‐sequestration [carbon storage] during Miscanthus growth, this results in a CO₂‐eq mitigation potential of 117%". Emmerling & Pude 2017, pp. 275–276. Emmerling & Pude paraphrase Felten et al. 2013. For yield, carbon sequestration and GHG calculations, see Felten et al. 2013, pp. 160, 166, 168.

[Wh2018p156a-224] "Whilst these values represent the extremes, they demonstrate that site selection for bioenergy crop cultivation can make the difference between large GHG [greenhouse gas] savings or losses, shifting life‐cycle GHG emissions above or below mandated thresholds. Reducing uncertainties in ∆C [carbon increase or decrease] following LUC [land use change] is therefore more important than refining N₂O [nitrous oxide] emission estimates (Berhongaray et al., 2017). Knowledge on initial soil carbon stocks could improve GHG savings achieved through targeted deployment of perennial bioenergy crops on low carbon soils (see section 2). [...] The assumption that annual cropland provides greater potential for soil carbon sequestration than grassland appears to be over‐simplistic, but there is an opportunity to improve predictions of soil carbon sequestration potential using information on the initial soil carbon stock as a stronger predictor of ∆C [change in carbon amount] than prior land use." Whitaker et al. 2018, pp. 156, 160.

[Mi2016p123a-225] "Fig. 3 confirmed either no change or a gain of SOC [soil organic carbon] (positive) through planting Miscanthus on arable land across England and Wales and only a loss of SOC (negative) in parts of Scotland. The total annual SOC change across GB in the transition from arable to Miscanthus if all nonconstrained land was planted with would be 3.3 Tg C yr⁻¹ [3.3 million tonnes carbon per year]. The mean changes for SOC for the different land uses were all positive when histosols were excluded, with improved grasslands yielding the highest Mg C ha⁻¹ yr⁻¹ [tonnes carbon per hectare per year] at 1.49, followed by arable lands at 1.28 and forest at 1. Separating this SOC change by original land use (Fig. 4) reveals that there are large regions of improved grasslands which, if planted with bioenergy crops, are predicted to result in an increase in SOC. A similar result was found when considering the transition from arable land; however for central eastern England, there was a predicted neutral effect on SOC. Scotland, however, is predicted to have a decrease for all land uses, particularly for woodland due mainly to higher SOC and lower Miscanthus yields and hence less input." Milner et al. 2016, p. 123.

[Ha2015p37b-226] "In summary, we have quantified the impacts of LUC [land use change] to bioenergy cropping on SOC [soil organic carbon] and GHG balance. This has identified LUC from arable, in general to lead to increased SOC, with LUC from forests to be associated with reduced SOC and enhanced GHG emissions. Grasslands are highly variable and uncertain in their response to LUC to bioenergy and given their widespread occurrence across the temperate landscape, they remain a cause for concern and one of the main areas where future research efforts should be focussed." Harris, Spake & Taylor 2015, pp. 33, 37 The authors note however that "[t]he average time since transition across all studies was 5.5 years (Xmax 16, Xmin 1) for SOC" and that "[...] the majority of studies considered SOC at the 0–30 cm profile only [...]." Harris, Spake & Taylor 2015, pp. 29–30. Low carbon accumulation rates for young plantations are to be expected, because of accelerated carbon decay at the time of planting (due to soil aeration), and relatively low mean carbon input to the soil during the establishment phase (2-3 years).

[Wh2018p150a-229] "In 2015, a workshop was convened with researchers, policymakers and industry/business representatives from the UK, EU and internationally. Outcomes from global research on bioenergy land‐use change were compared to identify areas of consensus, key uncertainties, and research priorities. [...] Our analysis suggests that the direct impacts of dedicated perennial bioenergy crops on soil carbon and nitrous oxide are increasingly well understood and are often consistent with significant life cycle GHG mitigation from bioenergy relative to conventional energy sources. We conclude that the GHG balance of perennial bioenergy crop cultivation will often be favourable, with maximum GHG savings achieved where crops are grown on soils with low carbon stocks and conservative nutrient application, accruing additional environmental benefits such as improved water quality. The analysis reported here demonstrates there is a mature and increasingly comprehensive evidence base on the environmental benefits and risks of bioenergy cultivation which can support the development of a sustainable bioenergy industry." Whitaker et al. 2018, p. 150.

[JRC-2014-57-58-a-230] "In tropical regions, afforestation may be beneficial since beside sequestering carbon it can lead to cloud formation resulting in a net cooling. In boreal regions, however, low surface albedo of afforested areas might have a warming climatic forcing that 'may exceed the cooling forcing from sequestration' [Thompson 2009]. Bright et al. have defined a possible way of integrating the impact on albedo in the LCA of a forest biofuel [Bright 2012]. [...] In their paper an example for the clear-cut of a boreal forest is reported. In that specific case the increase in albedo that follows the clear-cut harvest may offset about half of the total CO₂ emissions (that include also the biogenic emissions due to carbon stock changes) in a 100 years timeframe. Also Schwaiger and Bird [Schwaiger 2010] have attempted to integrate albedo effects into the bioenergy GHG calculations. They have considered an afforestation project in a south European mountainous area and used average yearly meteorological data. They have concluded that afforestation in the case study area accumulates up to 624 t CO₂ eq./ha, while the change in albedo due to crown cover is equivalent to emissions of roughly 401 t CO₂ eq./ha by the end of the first rotation period (90 years). The net effect, thus, varies around a neutral level with the cumulative result of a slight cooling in the long term. [...] With a similar approach Bright et al. [Bright 2011] have come to the conclusion that for a boreal forest, the albedo effect of the forest management in addition to the fossil fuel replacement leads to a near-neutral climate system. At a global level Bala et al. [Bala 2007] have simulated the climate impacts of deforestation, including the climate forcing of the CO₂ emitted and the albedo changes. They found that global-scale deforestation has a net cooling influence on Earth's climate because the warming carbon-cycle effects of deforestation are overwhelmed by the net cooling associated with changes in albedo and evapotranspiration. Latitude-specific deforestation experiments indicate that afforestation projects in the tropics would be clearly beneficial in mitigating global-scale warming, but would be counterproductive if implemented at high latitudes and would offer only marginal benefits in temperate regions [Betts 2000]." JRC 2014, pp. 57–58.

[JRC-2014-56-58-b-231] "Black carbon, like other aerosol particles, interacts with clouds, changing their reflectivity and lifetime, with effects on local and global climate. In addition, when calculating the climate effect of BC, it is important to realize that it is often mixed with organic carbon (OC) which is also produced during combustion and which reflects sunlight much more strongly than it absorbs it. A low OC-to-BC ratio means a predominantly absorbing aerosol that will contribute to warming. A high OC-to-BC ratio means a predominantly reflecting (or scattering) aerosol that will contribute to cooling. The ratio depends on the emission source: it can be lower than 1 in the case of emissions from diesel engines, but will be much higher in the case of, for example, smoldering wood (Table 9). [...] In other studies [Kulmala 2004] the analysis has been further expanded to include the emissions of organic carbon (mainly terpenes) from boreal forests, that, besides having an intrinsic cooling effect, act as condensation nuclei for cloud formation, thus enhancing the cloud albedo effect and resulting in additional climate cooling to that of the carbon sink. Spraklen et al. [Spraklen 2008] have quantified the relevance of the cooling effect of organic aerosols emissions and compared it to the warming effect of land surface albedo changes. Using a global atmospheric model they have shown that changes in cloud albedo cause a radiative forcing sufficiently large to result in boreal forests having an overall cooling impact on climate. This is the result of emissions of organic vapours and increased cloud formation due to the increased amount of condensation nuclei (doubled). They conclude that the combination of climate forcings related to boreal forests may result in an important global homeostasis [optimization, stableization]. In cold climatic conditions, the snow–vegetation albedo effect dominates and boreal forests warm the climate, whereas in warmer climates they may emit sufficiently large amounts of organic vapour modifying cloud albedo and acting to cool climate." JRC 2014, pp. 56–58.

[JRC-2014-58-a-232] "Georgescu et al. [Georgescu 2011] have shown that the bio-geo-physical effects that result from hypothetical conversion of annual to perennial bioenergy crops across the central United States would have a significant global climate cooling effect, beside the local cooling related mainly to local increases in transpiration, due to higher albedo. They concluded that the reduction in radiative forcing from albedo alone is equivalent to a carbon emission reduction of 78 t C/ha, which is six times larger than the annual biogeochemical effects that arise from offsetting fossil fuel use." JRC 2014, pp. 58.

[JRC-2014-34-b-243] "An intensive processing, such as for liquid biofuel substitution via lignocellulosic ethanol, causes much longer payback times because of the loss of energy in the biofuels production (about half of the energy content of the biomass is lost in the processing [...]." JRC 2014, p. 34. Note that the JRC use the term "payback time" in the sense of "parity time" as defined in Carbon accounting principles above. See JRC 2014, p. 16.

[wattcalc-251] See for instance the estimate of 0.60 W/m² for the 10 t/ha yield above. The calculation is: Yield (t/ha) multiplied with energy content (GJ/t) divided by seconds in a year (31 556 926) multiplied with the number of square metres in one hectare (10 000).

[Hektor-2016-4-b-252] Hektor et al. argue that flue gas condensation devices combined with natural drying of biomass makes it possible to achieve similar or better combustion efficiency than coal: "When burning moist biomass, energy is 'lost' in the evaporation of water. However, modern technology makes it possible to recover a large portion of that energy by flue gas condensation devices." The author also recommends "[...] simpler measures, such as natural drying [...]", and argue that "[...] state-of-the art technologies are nowadays generally included in new applications of biomass energy [...]" and that "[...] taking these factors into consideration, biomass would have about the same gross CO₂ emissions per generated amount of energy as coal [...]." Hektor, Backéus & Andersson 2016, p. 4. See also OECD/IEA 2004, p. 20.

[ISOnw-253] "The raw material for non-woody pellets can be herbaceous biomass, fruit biomass, aquatic biomass or biomass blends and mixtures. These blends and mixtures can also include woody biomass. They are usually manufactured in a die with total moisture content usually less than 15 % of their mass." ISO 2014b.

[TransmissionLoss-254] Transmission loss data from the World Bank, sourced from IEA. The World Bank 2010.

[Smil2015a-256] Additionally, Smil estimates that newly installed photovoltaic solar parks reaches 7–11 W/m² in sunny regions of the world. Smil 2015, p. 191.

[Camia-2021-107-a-261] "Pathways in the first and fourth quadrants are relatively clear situations in which trade-offs are not evident, and should thus clearly be a target for governance measures; in the sense that pathways in quadrant 1 should be incentivised, while pathways in quadrant 4 should be discouraged. Forest bioenergy pathways which fit within the first quadrant are the ones that are very likely to contribute to climate change mitigation in a short-medium term, and at the same time are likely to improve the condition of local ecosystems and biodiversity (or at least do not affect paths of ecosystem restoration). Pathways in the fourth quadrant are the ones that are unlikely to contribute to climate change mitigation in the short-medium term and at the same time are likely to further degrade ecosystems' condition. Conversely, pathways in quadrants 2 and 3 are the ones for which trade-offs between climate mitigation and biodiversity can be identified or assumed. Pathways in quadrant 2 are the ones that even though they are likely to mitigate climate change, they are also likely to negatively impact local biodiversity. For these pathways, safeguards or mitigation strategies should be investigated, and if available, should be considered mandated as contingent to the promotion of bioenergy. This case is also the only case in which the trade-off mentioned above (global climate change mitigation vs. local degradation) could influence the final evaluation of the pathway. Pathways in the third quadrant are likely to improve local ecosystem condition, but might not mitigate climate change in the short term. In these cases, bioenergy production might be seen as a by-product of restoration operations. In both cases in quadrants 2 & 3, trade-offs that cannot be resolved will need to be weighted and discussed during the decision-making process." Camia et al. 2021, p. 107.

[Camia-2021-5-a-265] "In May 2020, the EU Biodiversity Strategy for 2030 (COM/2020/380) was adopted. In the communication, under section 2.2.5 (“Win-win solutions for energy generation”), the Commission committed to publishing this report on the use of forest biomass for energy production in order to inform the EU climate and energy policies that govern the sustainable use of forest biomass for energy production and the accounting of associated carbon impacts, namely the Renewable Energy Directive, the Emissions Trading Scheme (ETS), and the Regulation on land use, land use change and forestry (LULUCF). [...] [T]he study would take stock of the available data related to the use of woody biomass for bioenergy; assess the uses of woody biomass in the EU with a focus on bioenergy; provide suggestions on how to improve the knowledge base on forests in a harmonised way; and expand the evidence basis by highlighting pathways that minimise trade-offs between climate mitigation and biodiversity conservation." Camia et al. 2021, p. 5.

[Camia-2021-6-7-a-266] "In this study, we assess three categories of interventions and their potential impacts: removal of logging residues, afforestation and conversion of natural forests to plantations. These three interventions were chosen because they are considered as practices that aim to supply ‘additional' biomass, i.e. growing biomass that would not be produced in the absence of bioenergy demand, or using biomass, such as residues and wastes, which would otherwise decompose or be burned on site. We acknowledge that, until now, many of these responses have not been triggered as a direct consequence of bioenergy expansion, but they are high on the agenda of potential climate mitigation strategies and could occur, in the EU or outside, as a direct or indirect effect of increased EU demand for forest biomass for wood products and bioenergy." Camia et al. 2021, pp. 6–7.

[Camia-2021-102-103-a-267] "Assessing the impact of forest bioenergy on ecosystems' condition in general, and in particular on biodiversity, is complicated because bioenergy pathways can exert multiple pressures on ecosystems and biodiversity and at the same time alleviate others. This creates an intricate matrix of trade-offs and synergies between forest bioenergy production and biodiversity and the condition of forests. [...] At the local level, intensified forest management to produce additional biomass can increase pressures on forest ecosystems. Similarly, land use change associated to afforestation can drive positive or negative impacts on local biodiversity. Additionally, the supply chain to produce bioenergy commodities is associated to the emission of pollutants which may contribute to acidification, eutrophication, and further climate change. Nevertheless, at the global level, climate change in itself is a major driver of biodiversity loss, therefore the overall benefit to the ecosystems and biodiversity might still be higher from global climate change mitigation if compared with the local level effects mentioned above. The trade-off between potential long-term advantages from climate change mitigation and short term, local ecosystems' degradation is very difficult to quantify. Therefore, under the precautionary principle, we exclude it from this analysis, assuming that we should not evaluate hypothetical long-term benefits versus short-term effects on ecosystems. Instead, we focus our analysis on potential pressures on local biodiversity and ecosystems from land use changes and forest management intensification in order to highlight potential pathways causing negative environmental trade-offs, or “bio-perversities” (Lindenmayer et al., 2012)." Camia et al. 2021, pp. 102–103.

[Camia-2021-148-a-269] "Secondly, concerning the assessment of carbon emissions: the impacts reported here are based on a ‘ceteris paribus' perspective, which is apt to capture only small-scale changes and not suitable to capture the overall impact of large-scale deployment of bioenergy, since it excludes market-mediated effects on other sectors." Camia et al. 2021, p. 148.

[Camia-2021-8-149-a-270] "Win-win management practices that benefit climate change mitigation and have either a neutral or positive effect on biodiversity include removal of slash (fine, woody debris) below thresholds defined according to local conditions, and afforestation of former arable land with mixed forest or naturally regenerating forests. [...] [C]oppice forests are particularly important in Mediterranean countries, they provide many ecosystem services, have relevant socio-economic functions in many rural areas and are mainly utilised for bioenergy. However, in large areas coppices are no longer managed or completely abandoned, resulting in old or overgrown declining stands. In these cases, it is suggested to encourage active forest management, that would enhance the capacity of these ecosystems to store carbon and supply services. Depending on local considerations the preferred option could be active conversion to high forest, or coppice restoration (see Section 5.9.2). [...] [W]e find that collecting slash within the limits of locally recommended thresholds could generate energy without damaging forest ecosystems and at the same time likely contributing to reducing GHG emissions. Similarly, afforesting former agricultural land with mixed species plantations or with naturally regenerating forests would enhance the terrestrial sink even before producing biomass for energy and thus would contribute to climate change mitigation, while at the same time improving ecosystems' conditions. [...] Collecting slash within the limits of locally recommended thresholds could be used to generate energy without damaging forest ecosystems while likely contributing to reducing GHG emissions. Afforesting former agricultural land with mixed species plantations or with naturally regenerating forests would enhance the terrestrial sink even before producing biomass for material and energy uses and thus would contribute to climate change mitigation, while at the same time improving ecosystems' conditions. [...] [C]ollection and use of low stumps within locally established thresholds in climate areas with high decay rates could potentially provide carbon emissions mitigation without damaging local biodiversity; local conditions should be evaluated in these cases." Camia et al. 2021, pp. 8–149.

[Camia-2021-125-147-a-271] "Although not extensively captured in the case studies, there is clear consensus in the literature that afforestation of primary, ancient grassland ecosystems which were never forests, may have very detrimental effects on local biodiversity; some authors compare these effects to the destructive effects of deforestation (Abreu et al., 2017; Bond, 2016; Bond et al., 2019; Feurdean et al., 2018; Veldman et al., 2015a, 2015b). Semi-natural grasslands and anthropogenic heathlands are ecosystems where closed canopy forest did not historically develop because of natural processes such as fire or mega fauna, or because of extensive management by local people. Local biodiversity adapted to open spaces has evolved in those ecosystems, and afforestation or tree planting of closed canopy forests is considered as a significant threat for local biodiversity, as highlighted by IPBES (2018a, b). Bubová et al. (2015) reviewed how abandonment of traditional grassland management followed by natural forest succession or active afforestation, is the main driver for the decline of butterfly diversity in Europe.[...] [P]athways in quadrant 2 may provide a significant contribution to climate change that would benefit global ecosystems and biodiversity even if local ecosystems are damaged in the process. However, this is a very uncertain trade-off and would be contrary to the precautionary principle, as explained in section 5.7. In this quadrant, for instance, we can find afforestation of former agricultural land with monoculture plantations: this intervention is likely to lead to carbon benefits in the short-term, but the impacts on local ecosystem should be evaluated carefully, for instance in the framework of landscape mosaic management and climate change resilience. Afforestation of natural grasslands or anthropogenic heathlands could also produce carbon benefits in the medium term, but the cost for local biodiversity, especially for species adapted to open spaces, could be devastating. Indeed, these practices are already discouraged within the Pan-European Guidelines for Afforestation and Reforestation, but they are still popular around the world (Veldman et al., 2015b, 2015a). Further in this quadrant, operations which should be already discouraged by sustainable management guidelines are classified: removing slash in very high quantities could be detrimental for local biodiversity." Camia et al. 2021, pp. 125–147.

[Camia-2021-125-147-b-272] "The overall carbon impact of afforestation operations needs to be properly calculated including changes in biogenic C-stocks and sinks, the substitution benefits of the newly produced wood, and eventual market-mediated indirect land use change effects. Generally, the overall carbon impact of afforestation is found to be positive, albeit the time scale required might be long (Agostini et al., 2014; Giuntoli et al., 2020b). Nonetheless, not always newly planted forests show a higher C-stock [carbon stock] than existing ecosystems, especially when considering the carbon in soil organic matter. Several studies in our review have tried to provide insights. Bárcena et al. (2014) found increased SOC [soil organic carbon] with afforestation on former cropland and heathland in Northern Europe, however afforestation on former grassland actually decreased SOC levels even for mature forests (>30 years). Laganière et al. (2010) found very similar results from their global meta-analysis, with afforestation on former cropland leading to a significant increase in SOC, but no significant changes in SOC for former pastures and natural grasslands. Furthermore, they also found that the tree species (and thus plantation features) influence the final result, with broadleaves forests generating the highest SOC increase and coniferous forests having the same SOC as the former land use. Li et al. (2012), similarly, found increased SOC for new forests on former cropland and pastureland, but a stable or slightly decreased SOC in former grassland. [...] Pathways in quadrant 3 are probably unlikely to be driven by bioenergy demand, however, they might be definitely valuable for conservation interventions and produce biomass for bioenergy." Camia et al. 2021, pp. 125–147.

[Camia-2021-8-147-a-273] "Lose-lose pathways include removal of coarse woody debris, removal of low stumps, and conversion of primary or natural forests into plantations. [...] Bubová et al. (2015) reviewed how abandonment of traditional grassland management followed by natural forest succession or active afforestation, is the main driver for the decline of butterfly diversity in Europe. [...] Generally, the overall carbon impact of afforestation is found to be positive, albeit the time scale required might be long (Agostini et al., 2014; Giuntoli et al., 2020b). Nonetheless, not always newly planted forests show a higher C-stock than existing ecosystems, especially when considering the carbon in soil organic matter. Several studies in our review have tried to provide insights. Bárcena et al. (2014) found increased SOC with afforestation on former cropland and heathland in Northern Europe, however afforestation on former grassland actually decreased SOC [soil organic carbon] levels even for mature forests (>30 years). Laganière et al. (2010) found very similar results from their global meta-analysis, with afforestation on former cropland leading to a significant increase in SOC, but no significant changes in SOC for former pastures and natural grasslands. Furthermore, they also found that the tree species (and thus plantation features) influence the final result, with broadleaves forests generating the highest SOC increase and coniferous forests having the same SOC as the former land use. Li et al. (2012), similarly, found increased SOC for new forests on former cropland and pastureland, but a stable or slightly decreased SOC in former grassland. [...] [S]everal pathways are categorized in the lose-lose quadrant and should be discouraged. For instance, the removal of CWD [course woody debris] and low stumps can be detrimental to forest ecosystems while at the same time likely not contributing to reducing carbon emissions in the short or even medium term compared to fossil sources. [...] Further, as expected, the conversion of natural and old growth forests to plantations aiming to provide wood for bioenergy would be extremely negative for local biodiversity, and at the same time it would provide no carbon mitigation in the short-medium term and should be thus discouraged. Similar considerations are valid also for the conversion of naturally regenerating forests to high-intensity management plantations: the impact on local biodiversity is highly negative while, even though wood production might increase, the benefits in terms of carbon mitigation are only accrued in the medium to long term. [...] Depending on local conditions, determining the decay rates on the forest floor, the removal of Coarse Woody Debris and low stumps can be detrimental to forest ecosystems while at the same time likely not contribute to reducing carbon emissions in the short or even medium term compared to fossil sources." Camia et al. 2021, pp. 8–147.

[Camia-2021-10-11-a-274] "[W]e are of the opinion that several negative impacts associated with the pathways reviewed in this study could be effectively minimised through swift and robust implementation of the RED II sustainability criteria related to forest biomass, which will be further operationalised through the upcoming EU operational guidance on the evidence for demonstrating compliance with the forest biomass criteria. [...] More specifically, RED II indicates specific no-go areas for agricultural biomass, meaning that biomass for bioenergy cannot be directly produced from land that was, at any time after 2008, classified as highly biodiverse grasslands, primary forest, highly biodiverse forest, or protected areas. However, these criteria do not apply to forest biomass (except for the protected areas criterion). Expanding such land criteria to forest biomass would introduce additional safeguards to ensure that forest biomass for energy is not associated with the afforestation pathways that have the most negative impacts, i.e. those on high-nature value grasslands or anthropogenic heathlands, and it would also forbid the sourcing of wood from plantations established on converted old-growth, primary forest for energy feedstock." Camia et al. 2021, pp. 10–11.

[Camia-2021-93-b-275] "Irrespective from market drivers, a moderate future increase in the production of harvested wood products at EU level may be expected because of forest age dynamics (Grassi et al. 2018 and Korosuo et al. 2020) and, in some circumstance, to reduce risks (or as consequence) of forest fires, pests and windstorms. The residues and the industrial by-products associated with these harvested wood products – along with wood from silvicultural operations specifically aimed at enhancing the quality of trees and the growth of the forest stands - may be meaningfully used for energy production, also contributing to the economic viability of forestry which is an integral element of Sustainable Forest Management." Camia et al. 2021, p. 93.

[Camia-2021-6-83-91-166-a-276] "[...] [A]s scientists, we need to clearly understand our role in this debate: we can gather and synthesise evidence highlighting problems and possible solutions as honest brokers of policy options, but we cannot identify the ‘right' policy tool or the ‘right' policy principle to follow because those issues are within the realm of the political arena and no amount of scientific research will appease ethical disputes. [...] If the question is ‘Is forest bioenergy sustainable?' the answer might be positive or negative depending on who attempts to answer it, and how. [...] [T]o a large extent there are no right or wrong answers, and the definition of ‘good enough' solutions is the role of policymaking, not science. [...] As illustrated schematically in Figure 28, the various tools within the EU legal framework provide incentives towards different management goals for European forests, from incentivising forest bioeconomy to protecting the carbon sink and forest ecosystems. The resulting balance of these different pulling drivers will eventually define both the contribution of forests wood-based products to EU climate mitigation, as well as the resulting state of forests' health (Wolfslehner et al., 2020). As mentioned also in Section 5.2.1, it is natural that different stakeholders with different worldviews, including within the scientific community, have a preference for one driver or another. At the same time, many different equilibrium points are possible and acceptable within the socio-economic context of each Member State. [...] Differences in ethical values on the interaction between humans and nature clearly play a role in defining what ‘sustainable management' means. We think that if we want to de-toxify the debate surrounding the sustainability of forest bioenergy, these divergences in values should be acknowledged and discussed explicitly also within the scientific community." Camia et al. 2021, pp. 6, 83, 91, 166.

[IPCC2019ap375-277] "Traditional biomass (fuelwood, charcoal, agricultural residues, animal dung) used for cooking and heating by some 2.8 billion people (38% of global population) in non-OECD countries accounts for more than half of all bioenergy used worldwide (IEA 2017; REN21 2018) (Cross-Chapter Box 7 in Chapter 6). Cooking with traditional biomass has multiple negative impacts on human health, particularly for women, children and youth (Machisa et al. 2013; Sinha and Ray 2015; Price 2017; Mendum and Njenga 2018; Adefuye et al. 2007) and on household productivity, including high workloads for women and youth (Mendum and Njenga 2018; Brunner et al. 2018; Hou et al. 2018; Njenga et al. 2019). Traditional biomass is land-intensive due to reliance on open fires, inefficient stoves and overharvesting of woodfuel, contributing to land degradation, losses in biodiversity and reduced ecosystem services (IEA 2017; Bailis et al. 2015; Masera et al. 2015; Specht et al. 2015; Fritsche et al. 2017; Fuso Nerini et al. 2017). Traditional woodfuels account for 1.9–2.3% of global GHG emissions, particularly in ‘hotspots' of land degradation and fuelwood depletion in eastern Africa and South Asia, such that one-third of traditional woodfuels globally are harvested unsustainably (Bailis et al. 2015). Scenarios to significantly reduce reliance on traditional biomass in developing countries present multiple co-benefits (high evidence, high agreement), including reduced emissions of black carbon, a short-lived climate forcer that also causes respiratory disease (Shindell et al. 2012). A shift from traditional to modern bioenergy, especially in the African context, contributes to improved livelihoods and can reduce land degradation and impacts on ecosystem services (Smeets et al. 2012; Gasparatos et al. 2018; Mudombi et al. 2018)." IPCC 2019a, p. 375.

[IEA-net-zero-2050-279] "In the NZE Scenario, bioenergy rapidly shifts to 100% sustainable sources of supply, and sustainable use. There is a complete phase-out of the traditional use of solid biomass for cooking, which is inefficient, often linked to deforestation, and whose pollution was responsible for 2.5 million premature deaths in 2020. The traditional use of solid biomass – estimated at around 40% of total bioenergy supply, or around 25 EJ, today – falls to zero by 2030 in the NZE Scenario, in line with achieving UN Sustainable Development Goal 7 on universal access to affordable, reliable, sustainable and modern energy for all. [...] Sustainable use of bioenergy in the NZE Scenario not only avoids negative impacts such as increased deforestation and competition with food production – it also delivers benefits beyond the energy sector. Shifting from traditional use of biomass to modern bioenergy can avoid undue burdens on women often tasked with collecting wood for fuel, bring health benefits from reduced air pollution and proper waste management, and reduce methane emissions from inefficient combustion and waste decomposition. More generally, sustainable bioenergy can provide a valuable source of employment and income for rural communities in emerging economies." IEA 2021a.

[EPA-2020-1-295] See EPA 2020, p. 1. The emission factors are based on the higher heating value (HHV) of the different fuels. The HHV value reflects the actual chemical energy stored in the fuel (mainly carbon and hydrogen molecules), without taking into account 1.) the energy that is lost by producing steam (when the fuel's hydrogen content reacts with oxygen at combustion), and 2.) the energy that is lost in combustion by evaporating the fuel's moisture content. The fuel's lower heating value (LHV) is the energy that remains after the necessary amount of energy has been spent to vaporize all this water. See OECD/IEA 2004, p. 20.

[Cowie-2021-1214-c-298] "Some scientific papers state that burning biomass for energy produces higher emissions of CO₂ per kWh of electricity at the smoke-stack compared with burning coal due to lower energy density of wood and/or less efficient conversion to electricity (e.g. Brack, 2017; Norton et al., 2019; Searchinger et al., 2018; Sterman et al., 2018; Walker et al., 2013), leading to the assertion that ‘biomass is worse for the climate than coal' (Johnston & van Kooten, 2015; McClure, 2014; PFPI, 2011; RSBP, 2012; Tsanova, 2018; Yassa, 2017). However, this interpretation neglects several significant factors. First, stack emissions will not necessarily increase when there is a shift to biomass fuels. The CO₂ emission factor (g CO₂ per GJ of fuel) is solely dependent on the chemical composition of the fuel. Wood and coal have similar CO₂ emission factors, as the ratio of heating values between the two fuels is similar to the ratio of carbon content (ECN, undated; Edwards et al., 2014; US EPA, 2018; van Loo & Koppejan, 2008). Where biomass is co-fired with coal in large power plants, the conversion efficiency may decrease a few percent, although there is usually no significant efficiency penalty when the co-firing ratio is below 10% (van Loo & Koppejan, 2008). Conversion efficiencies depend on fuel properties including moisture content and grindability in addition to heating value (Mun et al., 2016; Shi et al., 2019; Zuwała & Lasek, 2017). For low rank coal, biomass co-firing (especially torrefied biomass) can increase the boiler efficiency and net power plant efficiency (Liu et al., 2019; Thrän et al., 2016). Smaller biomass-fired plants can have lower electric conversion efficiency than large coal-fired plants, but as they are typically combined heat and power plants, they also displace heat production from other sources, that could otherwise have generated fossil fuel emissions (e.g. Madsen & Bentsen, 2018). Large dedicated biomass units (converted from coal) can operate with roughly the same level of thermal efficiency as delivered historically from coal (Koss, 2019)." Cowie et al. 2021, p. 1214.

[buchholz-2017-4-9-a-299] Total stack emissions for wood pellets were 0.897 Mt CO₂/MWh_el vs. 0.877 Mt CO₂/MWh_el for coal. Buchholz & Gunn 2017, pp. 4, 6, 9.

[Drax-2020-1-a-300] "Drax's biomass delivers carbon savings of more than 80% compared to coal – this includes emissions from our supply chain." Drax 2020.

[FutureMetrics-2015-1-2-a-301] See FutureMetrics 2015a, pp. 1–2. Chatham House notes that modern CHP plants (Combined Heat and Power) achieve much higher efficiencies, above 80%, for both fossil fuels and biomass. Chatham House 2017, p. 16.

[IPCC-2006-2.16-2.17-a-304] The individual emission rates are: Wood 112 000 kg CO₂eq per TJ, anthracite 98 300, coking coal 94 600, other bituminous 94 600, sub-bituminous 96 100, lignite 101 000. IPCC 2006a, pp. 2.16–2.17.

[ipcc-2019-a-368-a-305] "Estimating gross emissions only, creates a distorted representation of human impacts on the land sector carbon cycle. While forest harvest for timber and fuelwood and land-use change (deforestation) contribute to gross emissions, to quantify impacts on the atmosphere, it is necessary to estimate net emissions, that is, the balance of gross emissions and gross removals of carbon from the atmosphere through forest regrowth [...]." IPCC 2019a, p. 368.

[ieabioenergy-2019-3-4-a-306] "It is incorrect to determine the climate change effect of using biomass for energy by comparing GHG emissions at the point of combustion [...] the misplaced focus on emissions at the point of combustion blurs the distinction between fossil and biogenic carbon, and it prevents proper evaluation of how displacement of fossil fuels with biomass affects the development of atmospheric GHG concentrations." IEA Bioenergy 2019, pp. 3–4.

[ipcc-2019-a-384-a-308] "Sustainable Forest Management (SFM) is defined as ‘the stewardship and use of forests and forest lands in a way, and at a rate, that maintains their biodiversity, productivity, regeneration capacity, vitality and their potential to fulfill, now and in the future, relevant ecological, economic and social functions, at local, national, and global levels, and that does not cause damage to other ecosystems' [...]. This SFM definition was developed by the Ministerial Conference on the Protection of Forests in Europe and has since been adopted by the Food and Agriculture Organization [of the United Nations (FAO)]." IPCC 2019a, p. 351. Further, IPCC writes: "Sustainable forest management can prevent deforestation, maintain and enhance carbon sinks and can contribute towards GHG emissions-reduction goals. Sustainable forest management generates socio-economic benefits, and provides fibre, timber and biomass to meet society's growing needs. IPCC 2019a, p. 348.

[IPCC-2019-367-a-310] "The trends of productivity shown by several remote-sensing studies (see previous section) are largely consistent with mapping of forest cover and change using a 34-year time series of coarse resolution satellite data (NOAA AVHRR) (Song et al. 2018). This study, based on a thematic classification of satellite data, suggests that (i) global tree canopy cover increased by 2.24 million km² between 1982 and 2016 (corresponding to +7.1%) but with regional differences that contribute a net loss in the tropics and a net gain at higher latitudes, and (ii) the fraction of bare ground decreased by 1.16 million km² (corresponding to –3.1%), mainly in agricultural regions of Asia (Song et al. 2018), see Figure 4.5. Other tree or land cover datasets show opposite global net trends (Li et al. 2018b), but high agreement in terms of net losses in the tropics and large net gains in the temperate and boreal zones (Li et al. 2018b; Song et al. 2018; Hansen et al. 2013)." IPCC 2019a, p. 367.

[Camia-2021-78-a-317] "In the previous version of the Renewable Energy Directive (EU, 2009) in force until 2020, sustainability criteria were defined only for biomass used for the production of biofuels and bioliquids. With the REDII, to be transposed by countries by June 2021, new criteria are defined also cover solid and gaseous biomass fuels used in large installations for the production of power and heating or cooling." Camia et al. 2021, p. 78.

[Stephenson-2014-3-a-319] "Second, our findings are similarly compatible with the well-known age-related decline in productivity at the scale of even-aged forest stands. [...] We highlight the fact that increasing individual tree growth rate does not automatically result in increasing stand productivity because tree mortality can drive orders-of-magnitude reductions in population density. That is, even though the large trees in older, even-aged stands may be growing more rapidly, such stands have fewer trees. Tree population dynamics, especially mortality, can thus be a significant contributor to declining productivity at the scale of the forest stand." Stephenson et al. 2014, p. 3.

[ipcc-2019-f-4.6-323] "Recent studies indicate, that effects of forest management actions on soil C [carbon] stocks can be difficult to quantify and reported effects have been variable and even contradictory (see Box 4.3a)." Because the "current scientific basis is not sufficient", the IPCC will not currently provide soil carbon emission factors for forest management.IPCC 2019f, p. 4.6.

[ipcc-2019-a-351-a-324] "SFM [sustainable forest management] applied at the landscape scale to existing unmanaged forests can first reduce average forest carbon stocks and subsequently increase the rate at which CO₂ is removed from the atmosphere, because net ecosystem production of forest stands is highest in intermediate stand ages (Kurz et al. 2013; Volkova et al. 2018; Tang et al. 2014). The net impact on the atmosphere depends on the magnitude of the reduction in carbon stocks, the fate of the harvested biomass (i.e. use in short – or long-lived products and for bioenergy, and therefore displacement of emissions associated with GHG-intensive building materials and fossil fuels), and the rate of regrowth. Thus, the impacts of SFM on one indicator (e.g., past reduction in carbon stocks in the forested landscape) can be negative, while those on another indicator (e.g., current forest productivity and rate of CO₂ removal from the atmosphere, avoided fossil fuel emissions) can be positive. Sustainably managed forest landscapes can have a lower biomass carbon density than unmanaged forest, but the younger forests can have a higher growth rate, and therefore contribute stronger carbon sinks than older forests (Trofymow et al. 2008; Volkova et al. 2018; Poorter et al. 2016)." IPCC 2019a, p. 351.

[IEA-net-zero-2050-c-325] "Bioenergy provides only 5% of total electricity generation in 2050, but it is an important source of low-emissions flexibility to complement variable generation from solar PV and wind. In the industry sector, where solid bioenergy demand reaches 20 EJ in 2050, it is used to meet high temperature heat needs that cannot be easily electrified such as paper and cement production. In 2050, bioenergy meets 60% of energy demand in the paper sector and 30% of energy demand for cement production." IEA 2021a.

[IEA-net-zero-by-2050-0-94-326] The IEA estimates that replacing short rotation coppice forests with hydrogen production (for heat processing purposes) would cost 4.5 trillion USD: "The additional wind, solar, battery and electrolyser capacity, together with the electricity networks and storage needed to support this higher level of deployment would cost more than USD 5 trillion by 2050. This is USD 4.5 trillion more than would be needed if the use of bioenergy were to be expanded as envisaged in the NZE [Net Zero Emissions scenario], and would increase the total investment needed in the NZE by 3%. While it might therefore be possible still to achieve net‐zero emissions in 2050 without expanding land use for bioenergy, this would make the energy transition significantly more expensive. " IEA 2021b, p. 94.

[NAUFRP-2019-2-a-332] "Research demonstrates that demand for wood helps keep land in forest and incentivizes investments in new and more productive forests, all of which have significant carbon benefits. [...] Failing to consider the effects of markets and investment on carbon impacts can distort the characterization of carbon impacts from forest biomass energy." NAUFRP 2019, p. 2.

[favero-2020-6-a-333] Favero et al. focus on a potential future increase in demand and argues: "Increased bioenergy demand increases forest carbon stocks thanks to afforestation activities and more intensive management relative to a no-bioenergy case [...] higher biomass demand will increase the value of timberland, incentivize additional investment in forest management and afforestation, and result in greater forest carbon stocks over time". Favero, Daigneault & Sohngen 2020, p. 6.

[Cowie-2021-1218-c-334] "Some studies assess unharvested forest as one (and sometimes the only) reference scenario [...] and attribute extra GHG emissions to the bioenergy system based on forgone sequestration in comparison with natural regeneration. Others use a historical baseline reference point, without considering the dynamic nature of carbon stocks under a no-bioenergy scenario [...]. For biomass obtained as a co-product from forests managed for timber production, the relevant reference is commonly management for timber only, with thinning and harvest residues decomposing (or burned) on-site [...]. In some situations, the most likely reference land use could involve land use change. For example, markets for wood products can be an important incentive for private landowners to retain land as managed forest rather than converting to other uses [...]; the reference scenario in this situation may involve: regeneration of natural forest, possibly subject to higher incidence of wildfire; replacement of forest stands with agriculture; or urbanization, each with different impacts on the land carbon stock [...]. Assuming the forest would remain unharvested in the no-bioenergy scenario is not a realistic reference in situations where landholders use the land to generate income, unless landholders can obtain equivalent income from payments for carbon sequestration or other ecosystem services [...]." Cowie et al. 2021, p. 1218.

[FAO-2020-136-a-336] According to FAO, tree cover in Australia is increasing, but carbon stock is only provided for Oceania as a whole. FAO 2020, p. 136.

[FAOSTAT-2020-a-337] Wood chips, mainly used in the paper industry, have similar data; Europe (including Russia) produced 33% and North America 22%, while forest carbon stock increased in both areas. West, Central and East Asia combined produced 18%, and the forest carbon stock in this areas increased from 31.3 to 43.3 Gt. Wood chips production in the areas of the world were carbon stock is decreasing, was 26.9% in 2019. For wood pellet and wood chips production data, see FAOSTAT 2020. For carbon stock data, see FAO 2020, p. 52, table 43.

[EASAC-2017-34-a-339] "The potentially very long payback periods for forest biomass raise important issues given the UNFCCC's aspiration of limiting warming to 1.5 °C above preindustrial levels to ‘significantly reduce the risks and impacts of climate change'. On current trends, this may be exceeded in around a decade. Relying on forest biomass for the EU's renewable energy, with its associated initial increase in atmospheric carbon dioxide levels, increases the risk of overshooting the 1.5°C target if payback periods are longer than this. The European Commission should consider the extent to which large-scale forest biomass energy use is compatible with UNFCCC targets and whether a maximum allowable payback period should be set in its sustainability criteria." EASAC 2017, p. 34.

[Cowie-2021-1213-a-341] "Some authors (e.g. Booth, 2018; Brack, 2017; Norton et al., 2019) propose that forest bioenergy should only receive support under renewable energy policies if it delivers net reduction in atmospheric CO₂ within about a decade, due to the urgent need to reduce GHG emissions. However, besides the subjectivity of payback time analysis raised above, applying a 10-year payback time as a criterion for identifying suitable mitigation options is inconsistent with the long-term temperature goal of the Paris Agreement, which requires that a balance between emission and removals is reached in the second half of this century (Tanaka et al., 2019). Furthermore, it reflects a view on the relationship between net emissions, global warming and climate stabilization that contrasts with the scenarios presented in the SR1.5: The report shows many alternative trajectories towards stabilization temperatures of 1.5 and 2°C warming that reach net zero at different times and require different amounts of CDR (IPCC, 2018). The IPCC report did not determine that individual mitigation measures must meet specific payback times, but rather that a portfolio of mitigation measures is required that together limits the total cumulative global anthropogenic emissions of CO₂." Cowie et al. 2021, p. 1213.

[Cowie-2021-1213-1215-a-342] "Furthermore, applying a payback time criterion when evaluating forest bioenergy, and determining the contribution of bioenergy to meeting the Paris Agreement temperature goal, is complicated by the fact that bioenergy systems operate within the biogenic carbon cycle (see Section 3), which implies a fundamentally different influence on atmospheric CO₂ concentrations over time compared to fossil fuel emissions (Cherubini et al., 2014). [...] [C]omparing GHG emissions from biomass and fossil fuels at the point of combustion ignores the fundamental difference between fossil fuels and biomass fuels. Burning fossil fuels releases carbon that has been locked up in the ground for millions of years. Fossil fuel emissions transfer carbon from the lithosphere to the biosphere–atmosphere system, causing temperature increases that are irreversible on timescales relevant for humans (Archer et al., 2009; Solomon et al., 2009; Ter-Mikaelian, Colombo, & Chen, 2015). In contrast, bioenergy operates within the biosphere–atmosphere system, and burning biomass emits carbon that is part of the continuous exchange of carbon between the biosphere and the atmosphere (Smith et al., 2016)." Cowie et al. 2021, pp. 1213–1215.

[Cowie-2021-1214-d-343] "The IPCC emphasizes the need for transformation of all sectors of society to achieve the ‘well below 2°C' goal of the Paris Agreement (IPCC, 2018). This will entail technology and infrastructure development to generate a portfolio of emissions reduction and CDR strategies. Such investments may include, for example, scaling-up battery manufacturing to support electrification of car fleets, building rail infrastructure and district heating networks and changing the management and harvesting of forests and other lands to provide biomass for biobased products. The mobilization of mitigation options such as these can initially increase net GHG emissions while providing products and services with low, neutral or net negative emissions in the longer term (Cuenot & Hernández, 2016; Hausfather, 2019). The contribution of specific options to mitigation will depend on technology readiness level, costs, resource availability and inertia of existing technologies and systems. Options assessed as having low net GHG emissions per unit energy provided may be restricted by immature development, high cost or dependence on new infrastructure. Other options, including bioenergy, have greater near-term mitigation potential due to being compatible with existing infrastructure and cost competitive in many applications. Strategy development needs to recognize the complementarity of many mitigation options, and balance trade-offs between short- and long-term emissions reduction objectives. Critically, strategies based on assessments of individual technologies in isolation from their broader context, and that apply a strong focus on emissions reduction in the short term, can make long-term climate goals more difficult to achieve (e.g. Berndes at al., 2018; Smyth et al., 2014). Mitigation options available in the near term need to be evaluated beyond the direct effect on GHG emissions, considering also their influence on systems transition and implementation of other mitigation options (see Section 2)." Cowie et al. 2021, p. 1214.

[EASAC-2017-4-a-344] "Some have argued that the length of the carbon payback period does not matter as long as all emissions are eventually absorbed. This ignores the potential impact in the short term on climate tipping points (a concept for which there is some evidence) and on the world's ability to meet the target set in the 2015 Paris Agreement to limit temperature increase to 1.5°C above pre-industrial levels, which requires greenhouse gas emissions to peak in the near term. This suggests that only biomass energy with the shortest carbon payback periods should be eligible for financial and regulatory support." EASAC 2017, p. 4.

[Cowie-2021-1214-b-345] "Risks related to climate tipping points are sometimes raised in relation to the timing of GHG savings: crossing thresholds, for example, associated with forest dieback or thaw of permafrost, could lead to large, irreversible changes in the global climate system (e.g. Grimm et al., 2013). A recent study found a low probability of crossing a tipping point in the global climate system if warming does not exceed 2°C (Fischer et al., 2018). Also, critical threshold values and irreversibility of specific tipping points are uncertain (Collins et al., 2013), and the universal application of critical threshold values is questioned in relation to ecosystem function (Hillebrand et al., 2020). Nevertheless, uncertainties and risks associated with climate tipping points are additional considerations in evaluations of different trajectories towards temperature stabilization. Rather than connecting the timing of GHG savings to specific but uncertain climate tipping points, evaluation of bioenergy options is preferably based on a holistic assessment that considers how bioenergy can contribute to resilience and adaptation to changes in climate along with other environmental stressors." Cowie et al. 2021, p. 1214.

[EASAC-2017-21-a-347] "Harvesting immediately reduces the standing forest carbon stock compared with less (or no) harvesting (Bellassen and Luyssaert, 2014; Sievänen et al., 2014) and it may take from decades to centuries until regrowth restores carbon stocks to their former level—especially if oldgrowth forests are harvested." EASAC 2017, p. 21.

[ChathamHouse-2017-27-a-348] "Following this argument, the carbon dioxide (and other greenhouse gases) released by the burning of woody biomass for energy, along with their associated life-cycle emissions, create what is termed a ‘carbon debt' – i.e. the additional emissions caused by burning biomass instead of the fossil fuels it replaces, plus the emissions absorption foregone from the harvesting of the forests. Over time, regrowth of the harvested forest removes this carbon from the atmosphere, reducing the carbon debt. The period until carbon parity is achieved (i.e. the point at which the net cumulative emissions from biomass use are equivalent to those from a fossil fuel plant generating the same amount of energy) is usually termed the ‘carbon payback period'. After this point, as regrowth continues biomass may begin to yield ‘carbon dividends' in the form of atmospheric greenhouse gas levels lower than would have occurred if fossil fuels had been used. Eventually carbon levels in the forest return to the level at which they would have been if they had been left unharvested. (Some of the literature employs the term ‘carbon payback period' to describe this longer period, but it is more commonly used to mean the time to parity with fossil fuels; this meaning is used in this paper.)" Chatham House 2017, p. 27.

[EASAC-2017-23-a-352] "It has been argued that carbon balances should not be assessed at the stand level since at landscape level depletion of carbon in one stand may be compensated by growth in a stand elsewhere. For scientific analysis of the impact on climate forcing, however, it is necessary to compare the effects of various bioenergy harvest options against a baseline of no bioenergy harvest (or other credible counterfactual scenarios) for the same area of forest. Such studies provide information on the impacts of changes at the stand level, which can then be integrated with other factors (economic, regulatory and social) that may influence effects at landscape level." EASAC 2017, p. 23.

[FutureMetrics-2011b-2-a-353] "It is important to realize that our 3650 ton per year CHP plant does not receive 3650 tons in one delivery and does not release 3650 tons of wood's worth of carbon in one lump either. In fact, the forest products industry can be characterized as a just-in-time manufacturing system. For our CHP plant, 10 tons per day are sustainably harvested and delivered off of our 3650 acre FSC or SFI certified forest. So the carbon released into the atmosphere that day is from 10 tons of wood. The atmosphere “sees” new carbon. But during that same day on our 3650 acre plot, 10 new tons of wood grow and sequester the amount of carbon that was just released." FutureMetrics 2011b, p. 2.

[IEABioenergy-2019-2-a-354] "Forests are generally managed as a series of stands of different ages, harvested at different times, to produce a constant supply of wood products. When considered at plot level, long-rotation forests take many years to regrow after harvest, and the EASAC statement indicates this as a time gap between releasing forest carbon and its reabsorption from the atmosphere. However, across the whole forest estate or landscape, the temporal fluctuations are evened out since other stands continue to grow and sequester carbon, making the time gap as indicated by EASAC less relevant. If annual harvest does not exceed the annual growth in the forest, there is no net reduction in forest carbon." IEA Bioenergy 2019: "The use of forest biomass for climate change mitigation: response to statements of EASAC" IEA Bioenergy 2019, p. 2.

[ipcc-2019j-2.67-a-356] The forest landscape works as a proxy for calculating specifically human GHG emissions: "In the AFOLU [Agriculture, Forestry and Other Land Use] sector, the management of land is used as the best approximation of human influence and thus, estimates of emissions and removals on managed land are used as a proxy for anthropogenic emissions and removals on the basis that the preponderance of anthropogenic effects occurs on managed lands (see Vol. 4 Chapter 1). This allows for consistency, comparability, and transparency in estimation. Referred to as the Managed Land Proxy (MLP), this approach is currently recognised by the IPCC as the only universally applicable approach to estimating anthropogenic emissions and removals in the AFOLU sector (IPCC 2006, IPCC 2010)." IPCC 2019j, p. 2.67.

[IPCC-2019-2.72-a-357] "The natural disturbance component is subtracted from the total estimate of [...] emissions and removals, yielding an estimate of the emissions and removals associated with human activity on managed land." See IPCC 2019j, p. 2.72. "The 2006 IPCC Guidelines are designed to assist in estimating and reporting national inventories of anthropogenic greenhouse gas emissions and removals. For the AFOLU Sector, anthropogenic greenhouse gas emissions and removals by sinks are defined as all those occurring on ‘managed land'. Managed land is land where human interventions and practices have been applied to perform production, ecological or social functions. [...] This approach, i.e., the use of managed land as a proxy for anthropogenic effects, was adopted in the GPG–LULUCF and that use is maintained in the present guidelines. The key rationale for this approach is that the preponderance of anthropogenic effects occurs on managed lands. By definition, all direct human-induced effects on greenhouse gas emissions and removals occur on managed lands only. While it is recognized that no area of the Earth's surface is entirely free of human influence (e.g., CO₂ fertilization), many indirect human influences on greenhouse gases (e.g., increased N deposition, accidental fire) will be manifested predominately on managed lands, where human activities are concentrated. Finally, while local and short-term variability in emissions and removals due to natural causes can be substantial (e.g., emissions from fire, see footnote 1), the natural ‘background' of greenhouse gas emissions and removals by sinks tends to average out over time and space. This leaves the greenhouse gas emissions and removals from managed lands as the dominant result of human activity. Guidance and methods for estimating greenhouse gas emissions and removals for the AFOLU Sector now include: • CO₂ emissions and removals resulting from C stock changes in biomass, dead organic matter and mineral soils, for all managed lands; • CO₂ and non-CO₂ emissions from fire on all managed land; • N₂O emissions from all managed soils; • CO₂ emissions associated with liming and urea application to managed soils; • CH4 emissions from rice cultivation; • CO₂ and N₂O emissions from cultivated organic soils; • CO₂ and N₂O emissions from managed wetlands (with a basis for methodological development for CH4 emissions from flooded land in an Appendix 3); • CH4 emission from livestock (enteric fermentation); • CH4 and N₂O emissions from manure management systems; and • C stock change associated with harvested wood products." See IPCC 2006b, p. 1.5.

[Camia-2021-34-a-358] "Only part of the biomass from felled trees is removed from forests during harvest operations, on average around 80 percent for the EU as a whole during the period 2004 to 2013 (estimate also based on Pilli et al. 2017). The remainder is left as logging (primary) residues." Camia et al. 2021, p. 34.

[Camia-2021-21-68-a-359] "Salvage loggings are any harvesting activity consisting of recovering timber that can still be used, at least in part, from lands affected by natural disturbances (source: EU 2013.); with natural disturbances denominating damages caused by any factor (biotic or abiotic) that adversely affects the vigour and productivity of the forest and that is not a direct result of human activities (FAO 2018). Salvage logging is part of the removals. It includes both the removal of dead trees (belonging to what is reported as natural losses) and living trees (part of the growing stock) to prevent the spread of diseases or pests. Roundwood includes all wood removed with or without bark, including wood removed in its round form, or split, roughly squared or in other form (e.g. branches, roots, stumps and burls (where these are harvested)) and wood that is roughly shaped or pointed. It is a general term referring to wood fuel, including wood for charcoal and industrial roundwood. All roundwood is also referred to as primary wood or primary woody biomass. Fuelwood is roundwood that will be used as fuel for energy purposes such as cooking, heating, or power production. It includes wood harvested from main stems, branches and other parts of trees (where these are harvested for fuel), round or split, and wood that will be used for the production of charcoal (e.g. in pit kilns and portable ovens), wood pellets and other agglomerates. It also includes wood chips to be used for fuel that are made directly (i.e. in the forest) from roundwood. It excludes wood charcoal, pellets, and other agglomerates. Industrial roundwood corresponds to all roundwood except fuelwood. It includes sawlogs and veneer logs; pulpwood, round and split; and other industrial roundwood. As described in Chapter 3, industrial roundwood, although normally intended to be used for manufacturing of woodbased products, can sometimes end up as fuel." Pulpwood is defined like so: "Roundwood that is primarily intended for the production of pulp, particleboard or fibreboard. It includes: roundwood (with or without bark) in its round form or as splitwood or wood chips made directly (i.e. in the forest) from roundwood." Stemwood is defined like so: The wood of the stem(s) of a tree, i.e. the above ground main growing shoot(s). Stemwood includes wood in main axes and in major branches where there is at least X m of ‘straight' length to Y cm top diameter. (Source: Camia et al. 2018). Stemwood, within the context of this study, is the over bark biomass of the stem from 15 cm height (thus excluding the stump) up to a minimum top diameter of 9 cm." Sawnwood is defined like so: "Wood that has been produced from roundwood, either by sawing lengthways or by a profile-chipping process and that exceeds 6 mm in thickness. It includes planks, beams, joists, boards, rafters, scantlings, laths, boxboards and 'lumber', etc., in the following forms: un-planed, planed, end jointed, etc." Camia et al. 2021, pp. 21, 68.

[Camia-2021-92-a-360] "In general, prioritizing residues and a cascade use of wood remains a key overarching principle for maximizing the positive climate impact of bioenergy [...]." Camia et al. 2021, p. 92.

[Camia-2021-7-88-a-361] "Further characterising the primary woody biomass used, we estimate that roughly 20% of the total wood used for energy production is made up of stemwood, while 17% is made up of other wood components (treetops, branches, etc.). Based on available knowledge, at least half of the stemwood used for energy is assumed to be derived from coppice forests, which are particularly important in Mediterranean countries. Coppice forests, for the most part, provide many ecosystem services, and this management system has relevant socio-economic functions in many rural areas. However, in large areas coppices are no longer managed, resulting in old or overgrown declining stands; it is suggested to encourage active coppice restoration or conversion into high forest, depending on local conditions, to enhance the capacity of these ecosystems to store carbon and supply wood and other services. [...] Following what illustrated in section 3.4, for the year 2015 about 20% of this biomass can be broadly estimated as stemwood from primary wood (of which at least half is likely from coppice forests), while a larger part would come from either primary other wood components (tree tops, branches, that would have anyway emitted CO₂ in their decaying processes if left in the forest as residues, about 17%) or from secondary sources (by-products of wood processing industries, bark, post-consumer wood, about 49%); the remaining 14%, being reported as uncategorized, cannot be attributed (see Figure 8). Based on this analysis, it could be preliminary concluded that the large majority of forest bioenergy currently used in the EU is based on residues and the widely recommended “cascade” approach (EU 2015). However, the increase in woody biomass used for energy production from 2005 to 2018 (about 34%, dashed blue line) seems mainly associated to an increase in fuelwood (see Figure 10). More importantly, the large uncertainty in the bioenergy input mix highlighted in chapter 3 prevents to assign a high confidence to the conclusion above." Camia et al. 2021, pp. 7, 88.

[Sathre-2010-111-a-365] Sathre & O'Connor cite research on woody construction materials and write that "[...] over 90% of revenue is gained from the main wood product, with less than 10% gained from other biomass co-products." They also write that "[...] the average economic value added per hectare of forestland is over 40 times greater for main products made from sawlogs than for harvest residues." In other words, "[...] it is unlikely that trees will be harvested solely to produce these low-value products; instead, trees are harvested to produce high-value main products, and by-products are generated simultaneously." Sathre & O'Connor 2010, p. 111.

[Cowie-2021-1216-1217-a-366] The existence of a bioenergy market can improve the financial viability of forest thinning (Cintas et al., 2016), which stimulates production of high-quality timber with the aforementioned climate benefits from product substitution. In addition, extracting (otherwise unutilized) lower quality biomass (e.g. resulting from pest and disease impacts or overstocking) can reduce the frequency and severity of wildfires and associated loss of forest carbon and release of non-CO₂ GHGs, further enhancing the climate benefit (Agee & Skinner, 2005; Evans & Finkral, 2009; Mansuy et al., 2018; Regos et al., 2016; Sun et al., 2018; Verkerk et al., 2018). On the other hand, the mitigation value of forest bioenergy could be diminished if policies supporting bioenergy reduce timber availability for material applications (Favero et al., 2020), thereby reducing the wood products pool and increasing use of GHG-intensive materials; if excessive removal of residues reduces forest productivity (Achat et al., 2015; Helmisaari et al., 2011); or if reforestation displaces food production and results in deforestation elsewhere to provide new cropland. Cowie et al. 2021, pp. 1216–1217.

[FutureMetrics-2017-8-a-368] "In many locations sawmill residuals from structural lumber production are abundant and they supply much of the raw material needed to produce wood pellets. In other locations, there are insufficient sawmill residuals. In those locations, the pellet mills, just like the pulp mills, use the non-sawlog portions of the tree." FutureMetrics 2017, p. 8.

[Lamers-2013-382-a-369] Lamers & Junginger write that the vast majority of wood pellets imported to Europe are based on processing and harvesting residues "[...] with an increasing though still minor share from low-grade roundwood." They state that generally, "[...] the higher economic value for timber and cellulose products makes large-scale use of whole-trees for energy purposes highly unlikely wherever there is regional competition for the fiber." The technical potential for additional use of harvest residues is high in areas with large forests and harvest residues is therefore "[...] more likely to be used as feedstock when process-based wood waste streams become scarce." Lamers & Junginger 2013, p. 382.

[Camia-2021-57-58-a-370] "Synergies as well as competition within the wood-based economy are evident. Similar to the energy sector, the wood-based panel and pulp industries are likewise largely based on forest industry by-products. Therefore, the energy sector, wood-based panel, and pulp industries are all dependant on the demand for sawnwood, and they compete for the same feedstocks." Camia et al. 2021, pp. 57–58. In Germany, "[...] two thirds of the distributed sawmill by-products are delivered to panel and pulp industry; only 12% are distributed directly to energy producers (11% pellets, 1% directly to power plants)." Hoefnagels et al. 2017, p. 50.

[IPCC-2019-638-a-373] "Limitations on bioenergy and BECCS can result in increases in the cost of mitigation (Kriegler et al. 2014; Edmonds et al. 2013). Studies have also examined limiting CDR, including reforestation, afforestation, and bioenergy and BECCS (Kriegler et al. 2018a,b). These studies find that limiting CDR can increase mitigation costs, increase food prices, and even preclude limiting warming to less than 1.5°C above pre-industrial levels (Kriegler et al. 2018a,b; Muratori et al. 2016)." IPCC 2019e, p. 638.

[iea-bioenergy-2019-4-a-374] "Concern about near-term emissions is not a strong argument for stopping investments that contribute to net emissions reduction beyond 2030, be it the scaling-up of battery manufacturing to support electrification of car fleets, the development of rail infrastructure, or the development of biomass supply systems and innovation to provide biobased products displacing fossil fuels, cement and other GHG-intensive products. We assert that it is critical to focus on the global emissions trajectory required to achieve climate stabilization, acknowledging possible trade-offs between short- and long-term emissions reduction objectives. A strong focus on short-term carbon balances may result in decisions that make long-term climate objectives more difficult to meet."IEA Bioenergy 2019, p. 4.

[naufrp-2019-1-2-a-375] "Comparisons between forest biomass emissions and fossil fuel emissions at the time of combustion and for short periods thereafter do not account for long term carbon accumulation in the atmosphere and can significantly distort or ignore comparative carbon impacts over time. [...] The most common timeframe for measuring the impacts of greenhouse gases is 100 years, as illustrated by the widespread use of 100-year global warming potentials. This timeframe provides a more accurate accounting of cumulative emissions than shorter intervals." NAUFRP 2019, pp. 1–2.

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[ao]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[ap]

[33]

[34]

[35]

[36]

[aq]

[ar]

[as]

[at]

[37]

[au]

[av]

[aw]

[ax]

[ay]

[38]

[az]

[ba]

[bb]

[bc]

[bd]

[39]

[be]

[bf]

[bg]

[bh]

[bi]

Categories

Biomass harvested directly for energy

Biomass in the form of residues and waste