Climate change feedbacks

"Climate feedback" redirects here. For the fact-checking website, see Climate Feedback.

Climate change feedbacks are processes in the climate system which amplify or diminish the effect of forces that initially cause the warming. Positive feedbacks enhance global warming while negative feedbacks weaken it.^[3]: 2233 Feedbacks are important in the understanding of climate change because they play an important part in determining the sensitivity of the climate to warming forces. Climate forcings and feedbacks together determine how much and how fast the climate changes.^[4]

Examples of some effects of global warming that can amplify (positive feedbacks) or reduce (negative feedbacks) global warming^[1][2]: 96

The main positive feedback is that warming increases the amount of atmospheric water vapor, which is a powerful greenhouse gas.^[5] Another positive feedback is the loss of reflective snow and ice cover. Positive carbon cycle feedbacks occur when organic matter burns or decays, releasing CO₂ back into the atmosphere. Loss of organic matter can happen through rainforest drying, forest fires, and desertification. Methane can also be released into the atmosphere by thawing permafrost.

The main cooling effect is called the Planck response, which comes from the Stefan–Boltzmann law. It states that the total energy radiated per unit surface area per unit time is directly proportional to the fourth power of the black body's temperature. The carbon cycle acts a negative feedback as it absorbs more than half of CO₂ emissions every year. Atmospheric CO₂ gets absorbed into rocks and into plants. It also gets dissolved in the ocean where it leads to ocean acidification.

There are several types feedbacks: physical feedbacks, biological feedbacks and carbon cycle feedbacks. Calculations can give different results depending on the time frame and location that is used. Carbon cycle feedbacks are negative, which means that as atmospheric concentrations increase, carbon uptake also increases. However, higher temperatures and saturation of carbon sinks decrease that negative feedback effect. Overall feedbacks are expected to trend in a positive direction for the near future, though the Planck response will become increasingly negative as the planet warms.^{[6]: 94–95} There is no threat of a runaway greenhouse effect from current climate change.

Definition and terminology

See also: Climate system, positive feedback, and negative feedback

In climate science, a feedback that amplifies an initial warming is called a positive feedback.^[1] On the other hand, a feedback that reduces an initial warming is called a negative feedback.^[1] Naming a feedback positive or negative does not imply that the feedback is good or bad.^[7]

A 2021 IPCC glossary defines a positive feedback as one in which an initial perturbation is enhanced, and a negative feedback as one in which the initial perturbation is weakened by the changes it causes.^[8]: 2222 The glossary explains that the initial perturbation may be externally forced, or may arise through the climate system's internal variability.^[8]: 2222

Here, external forcing refers to "a forcing agent outside the climate system causing a change in the climate system"^[8]: 2229 that may push the climate system in the direction of warming or cooling.^[9] External forcings may be human-caused (for example, greenhouse gas emissions or land use change) or natural (for example, volcanic eruptions).^[8]: 2229

Physical feedbacks

Planck response (negative)

Planck response is "the most fundamental feedback in the climate system".^[10]: 19 As the temperature of a black body increases, the emission of infrared radiation increases with the fourth power of its absolute temperature according to the Stefan–Boltzmann law. This increases the amount of outgoing radiation back into space as the Earth warms.^[11] It is a strong stabilizing response and has sometimes been called the "no-feedback response" because it is an intensive property of a thermodynamic system when considered to be purely a function of temperature.^[12] Although Earth has an effective emissivity less than unity, the ideal black body radiation emerges as a separable quantity when investigating perturbations to the planet's outgoing radiation.

The Planck "feedback" or Planck response is the comparable radiative response obtained from analysis of practical observations or global climate models (GCMs). Its expected strength has been most simply estimated from the derivative of the Stefan-Boltzmann equation as -4σT³ = -3.8 W/m²/K (watts per square meter per degree of warming).^[11][12] Accounting from GCM applications has sometimes yielded a reduced strength, as caused by extensive properties of the stratosphere and similar residual artifacts subsequently identified as being absent from such models.^[12] Most extensive "grey body" properties of Earth that influence the outgoing radiation are usually postulated to be encompassed by the other GCM feedback components, and to be distributed in accordance with a particular forcing-feedback formulation of the climate system.^[13] Ideally the Planck response strength obtained from GCMs, indirect measurements, and black body estimates will further converge as analysis methods continue to mature.

Water vapor feedback (positive)

 

Atmospheric gases only absorb some wavelengths of energy but are transparent to others. The absorption patterns of water vapor (blue peaks) and carbon dioxide (pink peaks) overlap in some wavelengths.^[14]

According to Clausius–Clapeyron relation, saturation vapor pressure is higher in a warmer atmosphere, and so the absolute amount of water vapor will increase as the atmosphere warms. It is sometimes also called the specific humidity feedback,^[15]: 969 because the relative humidity stays nearly constant or even decreases slightly due to the warmer air.^[16] Since water vapor is a greenhouse gas, the increase in water vapor content makes the atmosphere warm further, which allows the atmosphere to hold still more water vapor. Thus, a positive feedback loop is formed, which continues until the negative feedbacks bring the system to equilibrium.^[15]: 969

Increases in atmospheric water vapor have been detected from satellites, and calculations based on these observations place this feedback strength at 1.85 ± 0.32 m²/K. This is very similar to model estimates, which are at 1.77 ± 0.20 m²/K^[15]: 969 Either value effectively doubles the warming that would otherwise occur from CO₂ increases alone.^[17] Like with the other physical feedbacks, this is already accounted for in the warming projections under climate change scenarios.

Lapse rate (negative)

Main article: Lapse rate

The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude.^[18][19] It is therefore a quantification of temperature, related to radiation, as a function of altitude, and is not a separate phenomenon in this context. The lapse rate feedback is generally a negative feedback. However, it is in fact a positive feedback in polar regions where it strongly contributed to polar amplified warming, one of the biggest consequences of climate change.^[20] This is because in regions with strong inversions, such as the polar regions, the lapse rate feedback can be positive because the surface warms faster than higher altitudes, resulting in inefficient longwave cooling.^[21][22][23]

The atmosphere's temperature decreases with height in the troposphere. Since emission of infrared radiation varies with temperature, longwave radiation escaping to space from the relatively cold upper atmosphere is less than that emitted toward the ground from the lower atmosphere. Thus, the strength of the greenhouse effect depends on the atmosphere's rate of temperature decrease with height. Both theory and climate models indicate that global warming will reduce the rate of temperature decrease with height, producing a negative lapse rate feedback that weakens the greenhouse effect.^[21] Measurements of the rate of temperature change with height are very sensitive to small errors in observations, making it difficult to establish whether the models agree with observations.^{[10]: 25 [24]}

Surface albedo feedback (positive)

Main articles: Arctic sea ice decline and Ice–albedo feedback

 

Average decadal extent and area of the Arctic Ocean sea ice since the start of satellite observations.

 

Annual trend in the Arctic sea ice extent and area for the 2011-2022 time period.

Albedo is the measure of how strongly the planetary surface can reflect solar radiation, which prevents its absorption and thus has a cooling effect. Brighter and more reflective surfaces have a high albedo and darker surfaces have a low albedo, so they heat up more. The most reflective surfaces are ice and snow, so surface albedo changes are overwhelmingly associated with what is known as the ice-albedo feedback. A minority of the effect is also associated with changes in physical oceanography, soil moisture and vegetation cover.^[15]: 970

The presence of ice cover and sea ice makes the North Pole and the South Pole colder than they would have been without it.^[25] During glacial periods, additional ice increases the reflectivity, which reduces the absorption of solar radiation, resulting in more cooling through a continuing cycle.^[26] But when warming occurs and the ice melts, darker land or open water takes its place and this causes more warming, which in turn causes more melting, and this cycle continues until an equilibrium is found.^[27][28] Consequently, recent Arctic sea ice decline is one of the primary factors behind the Arctic warming nearly four times faster than the global average since 1979 (the year when continuous satellite readings of the Arctic sea ice began), in a phenomenon known as Arctic amplification.^[29][30] Conversely, the high stability of ice cover in Antarctica, where the East Antarctic ice sheet rises nearly 4 km above the sea level, means that it has experienced very little net warming over the past seven decades.^{[31][32][33][34]}

 

Aerial photograph showing a section of sea ice. The lighter blue areas are melt ponds and the darkest areas are open water; both have a lower albedo than the white sea ice, so their presence increases local and global temperatures, which helps to spur more melting

As of 2021, the total surface feedback strength is estimated at 0.35 [0.10 to 0.60] W m²/K.^[2]: 95 On its own, Arctic sea ice decline between 1979 and 2011 is estimated to have been responsible for 0.21 (W/m²) of radiative forcing, which is equivalent to a quarter of radiative forcing from CO₂ increases over the same period.^[28] The combined change in all sea ice cover between 1992 and 2018 is equivalent to 10% of all the anthropogenic greenhouse gas emissions.^[35] When CMIP5 models estimate a total loss of Arctic sea ice cover from June to September (a plausible outcome under higher levels of warming), it increases the global temperatures by 0.19 °C (0.34 °F), with a range of 0.16–0.21 °C, while the regional temperatures would increase by over 1.5 °C (2.7 °F). This estimate includes the second-order effects such the impact from ice loss on lapse rate feedback, the changes in water vapor concentrations and regional cloud feedbacks.^[36] All of these calculations are already part of every CMIP5 and CMIP6 model, so they would not represent a source of "additional" warming on top of their existing projections.^[37]

Cloud feedback (positive)

 

Details of how clouds interact with shortwave and longwave radiation at different atmospheric heights^[38]

Main article: Cloud feedback

Seen from below, clouds emit infrared radiation back to the surface, which has a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, leading to a cooling effect. Low clouds are bright and very reflective, so they lead to strong cooling, while high clouds are too thin and transparent to effectively reflect sunlight, so they cause overall warming.^[39] As a whole, clouds have a substantial cooling effect.^[15]: 1022 However, climate change is expected to alter the distribution of cloud types in a way that reduces their cooling and thus accelerates overall warming, and so changes to clouds represent a positive feedback.^[2]: 95

As of 2021, cloud feedback strength is estimated at 0.42 [–0.10 to 0.94] W m²/K.^[2]: 95 This is the largest confidence interval of any climate feedback, and it occurs because some cloud types (most of which are present over the oceans) have been very difficult to observe, so climate models don't have as much data to go on with when they attempt to simulate their behaviour.^[15]: 975 Additionally, clouds have been strongly affected by aerosol particles, mainly from the unfiltered burning of sulfur-rich fossil fuels such as coal and bunker fuel. Any estimate of cloud feedback needs to disentangle the effects of so-called global dimming caused by these particles as well.^[40][41]

Thus, estimates of cloud feedback differ sharply between climate models. Models with the strongest cloud feedback have the highest climate sensitivity, which means that they simulate much stronger warming in response to a doubling of CO₂ (or equivalent greenhouse gas) concentrations than the rest.^[42][43] Around 2020, a small fraction of models was found to simulate so much warming as the result that they had contradicted paleoclimate evidence from fossils,^[44][45] and their output was effectively excluded from the climate sensitivity estimate of the IPCC Sixth Assessment Report.^{[2]: 93 [46]}

Biogeophysical and biogeochemical feedbacks

CO₂ feedbacks (mostly negative)

See also: Carbon cycle and Soil carbon feedback

 

The impulse response following a 100 GtC injection of CO₂ into Earth's atmosphere.^[47] The majority of excess carbon is removed by ocean and land sinks in less than a few centuries, while a substantial portion persists.

There are positive and negative climate feedbacks from Earth's carbon cycle. Negative feedbacks are large, and play a great role in the studies of climate inertia or of dynamic (time-dependent) climate change. Because they are considered relatively insensitive to temperature changes, they are sometimes considered separately or disregarded in studies which aim to quantify climate sensitivity.^{[13] [48]}Global warming projections have included carbon cycle feedbacks since the IPCC Fourth Assessment Report (AR4) in 2007.^[49] While the scientific understanding of these feedbacks was limited at the time, it had improved since then.^[50] These positive feedbacks include an increase in wildfire frequency and severity, substantial losses from tropical rainforests due to fires and drying and tree losses elsewhere.^{[51][52][53][54] [55][56][57][58]}

Altogether, carbon sinkss in the land and ocean absorb around half of the current emissions. Their future absorption is dynamic. In the future, if the emissions decrease, the fraction they absorb will increase, and they will absorb up to three-quarters of the remaining emissions - yet, the raw amount absorbed will decrease from the present. On the contrary, if the emissions will increase, then the raw amount absorbed will increase from now, yet the fraction could decline to one-third by the end of the 21st century.^[59]: 22 If the emissions remain very high after the 21st century, carbon sinks would eventually be completely overwhelmed, with the ocean sink diminished further and land ecosystems outright becoming a net source.^[60]: 677 Hypothetically, very strong carbon dioxide removal could also result in land and ocean carbon sinks becoming net sources for several decades.^[60]: 677

Role of oceans

Following Le Chatelier's principle, the chemical equilibrium of the Earth's carbon cycle will shift in response to anthropogenic CO₂ emissions. The primary driver of this is the ocean, which absorbs anthropogenic CO₂ via the so-called solubility pump. At present this accounts for only about one third of the current emissions, but ultimately most (~75%) of the CO₂ emitted by human activities will dissolve in the ocean over a period of centuries: "A better approximation of the lifetime of fossil fuel CO₂ for public discussion might be 300 years, plus 25% that lasts forever".^[61] However, the rate at which the ocean will take it up in the future is less certain, and will be affected by stratification induced by warming and, potentially, changes in the ocean's thermohaline circulation. It is believed that the single largest factor in determining the total strength of the global carbon sink is the state of the Southern Ocean - particularly of the Southern Ocean overturning circulation.^[62]

Chemical weathering

Chemical weathering over the geological long term acts to remove CO₂ from the atmosphere. With current global warming, weathering is increasing, demonstrating significant feedbacks between climate and Earth surface.^[63] Biosequestration also captures and stores CO₂ by biological processes. The formation of shells by organisms in the ocean, over a very long time, removes CO₂ from the oceans.^[64] The complete conversion of CO₂ to limestone takes thousands to hundreds of thousands of years.^[65]

Primary production through photosynthesis

Net primary productivity of plants' and phytoplankton grows as the increased CO₂ fuels their photosynthesis in what is known as the CO2 fertilization effect. Additionally, plants require less water as the atmospheric CO₂ concentrations increase, because they lose less moisture to evapotranspiration through open stomata (the pores in leaves through which CO₂ is absorbed). However, increased droughts in certain regions can still limit plant growth, and the warming beyond optimum conditions has a consistently negative impact. Thus, estimates for the 21st century show that plants would become more a lot more abundant at high latitudes near the poles but grow much less near the tropics - there is only medium confidence that tropical ecosystems would gain more carbon relative to now. However, there is high condidence that the total land cabrbon sink will remain positive.^[60]: 677

Non-CO₂ (unclear)

 

Methane climate feedbacks in natural ecosystems.

Release of gases of biological origin would be affected by global warming, and this includes climate-relevant gases such as methane, nitrous oxide or dimethyl sulfide.^[66][67] Others, such as dimethyl sulfide released from oceans, have indirect effects.^[68] Emissions of methane from land (particularly from wetlands) and of nitrous oxide from land and oceans are a known positive feedback.^[69] I.e. long-term warming changes the balance in the methane-related microbial community within freshwater ecosystems so they produce more methane while proportionately less is oxidised to carbon dioxide.^[70] There would also be biogeophysical changes which affect the albedo. For instance, larch in some sub-arctic forests are being replaced by spruce trees. This has a limited contribution to warming, because larch trees shed their needles in winter and so they end up more extensively covered in snow than the spruce trees which retain their dark needles all year.^[71]

On the other hand, changes in emissions of compounds such sea salt, dimethyl sulphide, dust, ozone and a range of biogenic volatile organic compounds are expected to be negative overall. As of 2021, all of these non-CO₂ feedbacks are believed to practically cancel each other out, but there is only low confidence, and the combined feedbacks could be up to 0.25 W m²/K in either direction.^[15]: 967

Permafrost (positive)

Permafrost is not included in the estimates above, as it is difficult to model, and the estimates of its role is strongly time-dependent as its carbon pools are depleted at different rates under different warming levels.^[15]: 967 Instead, it is treated as a separate process that will contribute to near-term warming, with the best estimates shown below.

This section is an excerpt from Permafrost § Impact on global temperatures.[edit]

 

Nine probable scenarios of greenhouse gas emissions from permafrost thaw during the 21st century, which show a limited, moderate and intense CO₂ and CH₄ emission response to low, medium and high-emission Representative Concentration Pathways. The vertical bar uses emissions of selected large countries as a comparison: the right-hand side of the scale shows their cumulative emissions since the start of the Industrial Revolution, while the left-hand side shows each country's cumulative emissions for the rest of the 21st century if they remained unchanged from their 2019 levels.^[72]

Altogether, it is expected that cumulative greenhouse gas emissions from permafrost thaw will be smaller than the cumulative anthropogenic emissions, yet still substantial on a global scale, with some experts comparing them to emissions caused by deforestation.^[72] The IPCC Sixth Assessment Report estimates that carbon dioxide and methane released from permafrost could amount to the equivalent of 14–175 billion tonnes of carbon dioxide per 1 °C (1.8 °F) of warming.^[73]: 1237 For comparison, by 2019, annual anthropogenic emissions of carbon dioxide alone stood around 40 billion tonnes.^[73]: 1237 A major review published in the year 2022 concluded that if the goal of preventing 2 °C (3.6 °F) of warming was realized, then the average annual permafrost emissions throughout the 21st century would be equivalent to the year 2019 annual emissions of Russia. Under RCP4.5, a scenario considered close to the current trajectory and where the warming stays slightly below 3 °C (5.4 °F), annual permafrost emissions would be comparable to year 2019 emissions of Western Europe or the United States, while under the scenario of high global warming and worst-case permafrost feedback response, they would approach year 2019 emissions of China.^[72]

Fewer studies have attempted to describe the impact directly in terms of warming. A 2018 paper estimated that if global warming was limited to 2 °C (3.6 °F), gradual permafrost thaw would add around 0.09 °C (0.16 °F) to global temperatures by 2100,^[74] while a 2022 review concluded that every 1 °C (1.8 °F) of global warming would cause 0.04 °C (0.072 °F) and 0.11 °C (0.20 °F) from abrupt thaw by the year 2100 and 2300. Around 4 °C (7.2 °F) of global warming, abrupt (around 50 years) and widespread collapse of permafrost areas could occur, resulting in an additional warming of 0.2–0.4 °C (0.36–0.72 °F).^[75][76]

Long-term feedbacks

Ice sheets

 

Global warming caused by the potential disappearance of the four notable ice masses and their albedo, assuming an average warming level of 1.5 °C (2.7 °F) throughout.^[36] While mountain glaciers and summer Arctic sea ice can be lost in a century or two, total ice sheet loss requires multiple millennia.^[77][78]

The Earth's two remaining ice sheets, the Greenland ice sheet and the Antarctic ice sheet, cover the world's largest island and an entire continent, and both of them are also around 2 km (1 mi) thick on average.^[79][80] Due to this immense size, their response to warming is measured in thousands of years and is believed to occur in two stages.^[15]: 977

The first stage would be the effect from ice melt on thermohaline circulation. Because meltwater is completely fresh, it makes it harder for the surface layer of water to sink beneath the lower layers, and this disrupts the exchange of oxygen, nutrients and heat between the layers. This would act as a negative feedback - sometimes estimated as a cooling effect of 0.2 °C (0.36 °F) over a 1000-year average, though the research on these timescales has been limited.^[15]: 977 An even longer-term effect is the ice-albedo feedback from ice sheets reaching their ultimate state in response to whatever the long-term temperature change would be. Unless the warming is reversed entirely, this feedback would be positive.^[15]: 977 The total loss of the Greenland Ice Sheet is estimated to add0.13 °C (0.23 °F) to global warming (with a range of 0.04–0.06 °C), while the loss of the West Antarctic Ice Sheet adds 0.05 °C (0.090 °F) (0.04–0.06 °C), and East Antarctic ice sheet 0.6 °C (1.1 °F)^[36] Total loss of the Greenland ice sheet would also increase regional temperatures in the Arctic by between 0.5 °C (0.90 °F) and 3 °C (5.4 °F), while the regional temperature in Antarctica is likely to go up by 1 °C (1.8 °F) after the loss of the West Antarctic ice sheet and 2 °C (3.6 °F) after the loss of the East Antarctic ice sheet.^[77][78]

These estimates assume that global warming stays at an average of 1.5 °C (2.7 °F). Because of the logarithmic growth of the greenhouse effect,^[2]: 80 the impact from ice loss would be larger at the slightly lower warming level of 2020s, but it would become lower if the warming proceeds towards higher levels.^[36] While Greenland and the West Antartic ice sheet are likely committed to melting entirely if the long-term warming is around 1.5 °C (2.7 °F), the East Antarctic ice sheet would not be at risk of complete disappearance until the very high global warming of 5–10 °C (9.0–18.0 °F)^[77][78]

Methane hydrates

See also: Clathrate gun hypothesis

Methane hydrates or methane clathrates] are frozen compounds where a large amount of methane is trapped within a crystal structure of water, forming a solid similar to ice.^[81] On Earth, they generally lie beneath sediments on the ocean floors, (approximately 1,100 m (3,600 ft) below the sea level).^[82] Around 2008, there was a serious concern that a large amount of hydrates from relatively shallow deposits in the Arctic, particularly around the East Siberian Arctic Shelf, could quickly break down and release large amounts of methane, potentially leading to 6 °C (11 °F) within 80 years.^[83][84] Current research shows that hydrates react very slowly to warming, and that it's very difficult for methane to reach the atmosphere after dissociation on the seafloor.^[85][86] Thus, no "detectable" impact on the global temperatures is expected to occur in this century due to methane hydrates.^[60]: 677 Some research suggests hydrate dissociation can still cause a warming of 0.4–0.5 °C (0.72–0.90 °F) over several millennia.^[87]

Mathematical formulation of global energy imbalance

Earth is a thermodynamic system for which long-term temperature changes follow the global energy imbalance (EEI stands for Earth's energy imbalance):

${\displaystyle EEI\equiv ASR-OLR}$ 

where ASR is the absorbed solar radiation and OLR is the outgoing longwave radiation at top of atmosphere. When EEI is positive the system is warming, when it is negative they system is cooling, and when it is approximately zero then there is neither warming or cooling. The ASR and OLR terms in this expression encompass many temperature-dependent properties and complex interactions that govern system behavior.^[88]

In order to diagnose that behavior around a relatively stable equilibrium state, one may consider a perturbation to EEI as indicated by the symbol Δ. Such a perturbation is induced by a radiative forcing (ΔF) which can be natural or man-made. Responses within the system to either return back towards the stable state, or to move further away from the stable state are called feedbacks λΔT:

${\displaystyle \Delta EEI=\Delta F+\lambda \Delta T}$ .

Collectively the feedbacks are approximated by the linearized parameter λ and the perturbed temperature ΔT because all components of λ (assumed to be first-order to act independently and additively) are also functions of temperature, albeit to varying extents, by definition for a thermodynamic system:

${\displaystyle \lambda =\sum _{i}\lambda _{i}=(\lambda _{wv}+\lambda _{c}+\lambda _{a}+\lambda _{cc}+\lambda _{p}+\lambda _{lr}+...)}$ .

Some feedback components having significant influence on EEI are: ${\displaystyle wv}$ = water vapor, ${\displaystyle c}$ = clouds, ${\displaystyle a}$ = surface albedo, ${\displaystyle cc}$ = carbon cycle, ${\displaystyle p}$ = Planck response, and ${\displaystyle lr}$ = lapse rate. All quantities are understood to be global averages, while T is usually translated to temperature at the surface because of its direct relevance to humans and much other life.^[13]

The negative Planck response, being an especially strong function of temperature, is sometimes factored out to give an expression in terms of the relative feedback gains g_i from other components:

${\displaystyle \lambda =\neg \lambda _{p}\times (1-\sum _{i}g_{i})}$ .

For example ${\displaystyle g_{wv}\approx 0.5}$  for the water vapor feedback.

Within the context of modern numerical climate modelling and analysis, the linearized formulation has limited use. One such use is to diagnose the relative strengths of different feedback mechanisms. An estimate of climate sensitivity to a forcing is then obtained for the case where the net feedback remains negative and the system reaches a new equilibrium state (ΔEEI=0) after some time has passed:^{[10]: 19–20}

${\displaystyle \Delta T={\frac {\Delta F}{\lambda _{p}\times (1-\sum _{i}g_{i})}}}$ .

Implications for climate policy

See also: Climate sensitivity

Uncertainty over climate change feedbacks has implications for climate policy. For instance, uncertainty over carbon cycle feedbacks may affect targets for reducing greenhouse gas emissions (climate change mitigation).^[89] Emissions targets are often based on a target stabilization level of atmospheric greenhouse gas concentrations, or on a target for limiting global warming to a particular magnitude. Both of these targets (concentrations or temperatures) require an understanding of future changes in the carbon cycle. If models incorrectly project future changes in the carbon cycle, then concentration or temperature targets could be missed. For example, if models underestimate the amount of carbon released into the atmosphere due to positive feedbacks (e.g., due to thawing permafrost), then they may also underestimate the extent of emissions reductions necessary to meet a concentration or temperature target.^{[citation needed]}

See also

• Climate variability and change
• Climate inertia
• Complex system
• Effects of climate change
• Parametrization (climate)
• Tipping points in the climate system

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