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Carbon cycle

January 31, 2023

For the thermonuclear reaction involving carbon that powers some stars, see CNO cycle. For organic chemical ring-shaped structures, see Cyclic compound. For the geochemical cycle, see Carbonate–silicate cycle.

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of Earth. Carbon is the main component of biological compounds as well as a major component of many minerals such as limestone. Along with the nitrogen cycle and the water cycle, the carbon cycle comprises a sequence of events that are key to make Earth capable of sustaining life. It describes the movement of carbon as it is recycled and reused throughout the biosphere, as well as long-term processes of carbon sequestration to and release from carbon sinks. Carbon sinks in the land and the ocean each currently take up about one-quarter of anthropogenic carbon emissions each year.

Fast carbon cycle showing the movement of carbon between land, atmosphere, and oceans in billions of tons (gigatons) per year. Yellow numbers are natural fluxes, red are human contributions, white are stored carbon. The effects of the slow carbon cycle, such as volcanic and tectonic activity are not included.[1]

Humans have disturbed the biological carbon cycle for many centuries by modifying land use, and moreover with the recent industrial-scale mining of fossil carbon (coal, petroleum, and gas extraction, and cement manufacture) from the geosphere.[1][2] Carbon dioxide in the atmosphere had increased nearly 52% over pre-industrial levels by 2020, forcing greater atmospheric and Earth surface heating by the Sun.[3][4] The increased carbon dioxide has also increased the acidity of the ocean surface by about 30% due to dissolved carbon dioxide, carbonic acid and other compounds, and is fundamentally altering marine chemistry.[5][6] The majority of fossil carbon has been extracted over just the past half century, and rates continue to rise rapidly, contributing to human-caused climate change.[7][8] The largest consequences to the carbon cycle, and to the biosphere which critically enables human civilization, are still set to unfold due to the vast yet limited inertia of the Earth system.[1][9][10] Restoring balance to this natural system is an international priority, described in both the Paris Climate Agreement and Sustainable Development Goal 13.

Main components

 
Detail of anthropogenic carbon flows, showing cumulative mass in gigatons during years 1850–2018 (left) and the annual mass average during 2009–2018 (right).[2]

The carbon cycle was first described by Antoine Lavoisier and Joseph Priestley, and popularised by Humphry Davy.[11] The global carbon cycle is now usually divided into the following major reservoirs of carbon interconnected by pathways of exchange:[12]: 5–6 

The carbon exchanges between reservoirs occur as the result of various chemical, physical, geological, and biological processes. The ocean contains the largest active pool of carbon near the surface of the Earth.[13] The natural flows of carbon between the atmosphere, ocean, terrestrial ecosystems, and sediments are fairly balanced; so carbon levels would be roughly stable without human influence.[3][14]

Atmosphere

Main article: Atmospheric carbon cycle
Computer model showing a year in the life of atmospheric carbon dioxide and how it travels around the globe [15]

Carbon in the Earth's atmosphere exists in two main forms: carbon dioxide and methane. Both of these gases absorb and retain heat in the atmosphere and are partially responsible for the greenhouse effect.[13] Methane produces a larger greenhouse effect per volume as compared to carbon dioxide, but it exists in much lower concentrations and is more short-lived than carbon dioxide, making carbon dioxide the more important greenhouse gas of the two.[16]

Carbon dioxide is removed from the atmosphere primarily through photosynthesis and enters the terrestrial and oceanic biospheres. Carbon dioxide also dissolves directly from the atmosphere into bodies of water (ocean, lakes, etc.), as well as dissolving in precipitation as raindrops fall through the atmosphere. When dissolved in water, carbon dioxide reacts with water molecules and forms carbonic acid, which contributes to ocean acidity. It can then be absorbed by rocks through weathering. It also can acidify other surfaces it touches or be washed into the ocean.[17]

 
CO2 concentrations over the last 800,000 years as measured from ice cores (blue/green) and directly (black)

Human activities over the past two centuries have increased the amount of carbon in the atmosphere by nearly 50% as of year 2020, mainly in the form of carbon dioxide, both by modifying ecosystems' ability to extract carbon dioxide from the atmosphere and by emitting it directly, e.g., by burning fossil fuels and manufacturing concrete.[4][13]

In the far future (2 to 3 billion years), the rate at which carbon dioxide is absorbed into the soil via the carbonate–silicate cycle will likely increase due to expected changes in the sun as it ages. The expected increased luminosity of the Sun will likely speed up the rate of surface weathering.[18] This will eventually cause most of the carbon dioxide in the atmosphere to be squelched into the Earth's crust as carbonate.[19][20] Once the concentration of carbon dioxide in the atmosphere falls below approximately 50 parts per million (tolerances vary among species), C3 photosynthesis will no longer be possible.[20] This has been predicted to occur 600 million years from the present, though models vary.[21]

Once the oceans on the Earth evaporate in about 1.1 billion years from now,[18] plate tectonics will very likely stop due to the lack of water to lubricate them. The lack of volcanoes pumping out carbon dioxide will cause the carbon cycle to end between 1 billion and 2 billion years into the future.[22]

Terrestrial biosphere

 
Amount of carbon stored in Earth's various terrestrial ecosystems, in gigatonnes.[23]
Main article: Terrestrial biological carbon cycle

The terrestrial biosphere includes the organic carbon in all land-living organisms, both alive and dead, as well as carbon stored in soils. About 500 gigatons of carbon are stored above ground in plants and other living organisms,[3] while soil holds approximately 1,500 gigatons of carbon.[24] Most carbon in the terrestrial biosphere is organic carbon,[25] while about a third of soil carbon is stored in inorganic forms, such as calcium carbonate.[26] Organic carbon is a major component of all organisms living on earth. Autotrophs extract it from the air in the form of carbon dioxide, converting it into organic carbon, while heterotrophs receive carbon by consuming other organisms.

Because carbon uptake in the terrestrial biosphere is dependent on biotic factors, it follows a diurnal and seasonal cycle. In CO2 measurements, this feature is apparent in the Keeling curve. It is strongest in the northern hemisphere because this hemisphere has more land mass than the southern hemisphere and thus more room for ecosystems to absorb and emit carbon.

 
A portable soil respiration system measuring soil CO2 flux.

Carbon leaves the terrestrial biosphere in several ways and on different time scales. The combustion or respiration of organic carbon releases it rapidly into the atmosphere. It can also be exported into the ocean through rivers or remain sequestered in soils in the form of inert carbon.[27] Carbon stored in soil can remain there for up to thousands of years before being washed into rivers by erosion or released into the atmosphere through soil respiration. Between 1989 and 2008 soil respiration increased by about 0.1% per year.[28] In 2008, the global total of CO2 released by soil respiration was roughly 98 billion tonnes, about 3 times more carbon than humans are now putting into the atmosphere each year by burning fossil fuel (this does not represent a net transfer of carbon from soil to atmosphere, as the respiration is largely offset by inputs to soil carbon). There are a few plausible explanations for this trend, but the most likely explanation is that increasing temperatures have increased rates of decomposition of soil organic matter, which has increased the flow of CO2. The length of carbon sequestering in soil is dependent on local climatic conditions and thus changes in the course of climate change.[29]

Size of major carbon pools on the Earth (year 2000 estimates)[13]
Pool Quantity
(gigatons)
Atmosphere 720
Ocean (total) 38,400
Total inorganic 37,400
Total organic 1,000
Surface layer 670
Deep layer 36,730
Lithosphere
Sedimentary carbonates > 60,000,000
Kerogens 15,000,000
Terrestrial biosphere (total) 2,000
Living biomass 600 – 1,000
Dead biomass 1,200
Aquatic biosphere 1 – 2
Fossil fuels (total) 4,130
Coal 3,510
Oil 230
Gas 140
Other (peat) 250

Ocean

Main article: Oceanic carbon cycle

The ocean can be conceptually divided into a surface layer within which water makes frequent (daily to annual) contact with the atmosphere, and a deep layer below the typical mixed layer depth of a few hundred meters or less, within which the time between consecutive contacts may be centuries. The dissolved inorganic carbon (DIC) in the surface layer is exchanged rapidly with the atmosphere, maintaining equilibrium. Partly because its concentration of DIC is about 15% higher[30] but mainly due to its larger volume, the deep ocean contains far more carbon—it is the largest pool of actively cycled carbon in the world, containing 50 times more than the atmosphere[13]—but the timescale to reach equilibrium with the atmosphere is hundreds of years: the exchange of carbon between the two layers, driven by thermohaline circulation, is slow.[13]

Carbon enters the ocean mainly through the dissolution of atmospheric carbon dioxide, a small fraction of which is converted into carbonate. It can also enter the ocean through rivers as dissolved organic carbon. It is converted by organisms into organic carbon through photosynthesis and can either be exchanged throughout the food chain or precipitated into the oceans' deeper, more carbon-rich layers as dead soft tissue or in shells as calcium carbonate. It circulates in this layer for long periods of time before either being deposited as sediment or, eventually, returned to the surface waters through thermohaline circulation.[3] Oceans are basic (~pH 8.2), hence CO2 acidification shifts the pH of the ocean towards neutral.

Oceanic absorption of CO2 is one of the most important forms of carbon sequestering which limit the human-caused rise of carbon dioxide in the atmosphere. However, this process is limited by a number of factors. CO2 absorption makes water more acidic, which affects ocean biosystems. The projected rate of increasing oceanic acidity could slow the biological precipitation of calcium carbonates, thus decreasing the ocean's capacity to absorb CO2.[31][32]

Geosphere

Main article: Carbonate–silicate cycle
 
Diagram showing relative sizes (in gigatonnes) of the main storage pools of carbon on Earth. Cumulative changes (thru year 2014) from land use and emissions of fossil carbon are included for comparison.[23]

The geologic component of the carbon cycle operates slowly in comparison to the other parts of the global carbon cycle. It is one of the most important determinants of the amount of carbon in the atmosphere, and thus of global temperatures.[33]

Most of the earth's carbon is stored inertly in the earth's lithosphere.[13] Much of the carbon stored in the earth's mantle was stored there when the earth formed.[34] Some of it was deposited in the form of organic carbon from the biosphere.[35] Of the carbon stored in the geosphere, about 80% is limestone and its derivatives, which form from the sedimentation of calcium carbonate stored in the shells of marine organisms. The remaining 20% is stored as kerogens formed through the sedimentation and burial of terrestrial organisms under high heat and pressure. Organic carbon stored in the geosphere can remain there for millions of years.[33]

Carbon can leave the geosphere in several ways. Carbon dioxide is released during the metamorphism of carbonate rocks when they are subducted into the earth's mantle. This carbon dioxide can be released into the atmosphere and ocean through volcanoes and hotspots.[34] It can also be removed by humans through the direct extraction of kerogens in the form of fossil fuels. After extraction, fossil fuels are burned to release energy and emit the carbon they store into the atmosphere.

Terrestrial carbon in the water cycle

 
Where terrestrial carbon goes when water flows [36]

In the diagram on the right: [36]

  1. Atmospheric particles act as cloud condensation nuclei, promoting cloud formation.[37][38]
  2. Raindrops absorb organic and inorganic carbon through particle scavenging and adsorption of organic vapors while falling toward Earth.[39][40]
  3. Burning and volcanic eruptions produce highly condensed polycyclic aromatic molecules (i.e. black carbon) that is returned to the atmosphere along with greenhouse gases such as CO2.[41][42]
  4. Terrestrial plants fix atmospheric CO2 through photosynthesis, returning a fraction back to the atmosphere through respiration.[43] Lignin and celluloses represent as much as 80% of the organic carbon in forests and 60% in pastures.[44][45]
  5. Litterfall and root organic carbon mix with sedimentary material to form organic soils where plant-derived and petrogenic organic carbon is both stored and transformed by microbial and fungal activity.[46][47][48]
  6. Water absorbs plant and settled aerosol-derived dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) as it passes over forest canopies (i.e. throughfall) and along plant trunks/stems (i.e. stemflow).[49] Biogeochemical transformations take place as water soaks into soil solution and groundwater reservoirs[50][51] and overland flow occurs when soils are completely saturated,[52] or rainfall occurs more rapidly than saturation into soils.[53]
  7. Organic carbon derived from the terrestrial biosphere and in situ primary production is decomposed by microbial communities in rivers and streams along with physical decomposition (i.e. photo-oxidation), resulting in a flux of CO2 from rivers to the atmosphere that are the same order of magnitude as the amount of carbon sequestered annually by the terrestrial biosphere.[54][55][56] Terrestrially-derived macromolecules such as lignin [57] and black carbon [58] are decomposed into smaller components and monomers, ultimately being converted to CO2, metabolic intermediates, or biomass.
  8. Lakes, reservoirs, and floodplains typically store large amounts of organic carbon and sediments, but also experience net heterotrophy in the water column, resulting in a net flux of CO2 to the atmosphere that is roughly one order of magnitude less than rivers.[59][56] Methane production is also typically high in the anoxic sediments of floodplains, lakes, and reservoirs.[60]
  9. Primary production is typically enhanced in river plumes due to the export of fluvial nutrients.[61][62] Nevertheless, estuarine waters are a source of CO2 to the atmosphere, globally.[63]
  10. Coastal marshes both store and export blue carbon.[64][65][66] Marshes and wetlands are suggested to have an equivalent flux of CO2 to the atmosphere as rivers, globally.[67]
  11. Continental shelves and the open ocean typically absorb CO2 from the atmosphere.[63]
  12. The marine biological pump sequesters a small but significant fraction of the absorbed CO2 as organic carbon in marine sediments (see next section).[68][36]

The marine biological pump

 
Flow of carbon through the open ocean
Main article: Biological pump

The marine biological pump is the ocean's biologically driven sequestration of carbon from the atmosphere and land runoff to the deep ocean interior and seafloor sediments.[69] The biological pump is not so much the result of a single process, but rather the sum of a number of processes each of which can influence biological pumping. The pump transfers about 11 billion tonnes of carbon every year into the ocean's interior. An ocean without the biological pump would result in atmospheric CO2 levels about 400 ppm higher than the present day.[70][71][72]

Most carbon incorporated in organic and inorganic biological matter is formed at the sea surface where it can then start sinking to the ocean floor. The deep ocean gets most of its nutrients from the higher water column when they sink down in the form of marine snow. This is made up of dead or dying animals and microbes, fecal matter, sand and other inorganic material.[73]

The biological pump is responsible for transforming dissolved inorganic carbon (DIC) into organic biomass and pumping it in particulate or dissolved form into the deep ocean. Inorganic nutrients and carbon dioxide are fixed during photosynthesis by phytoplankton, which both release dissolved organic matter (DOM) and are consumed by herbivorous zooplankton. Larger zooplankton - such as copepods, egest fecal pellets - which can be reingested, and sink or collect with other organic detritus into larger, more-rapidly-sinking aggregates. DOM is partially consumed by bacteria and respired; the remaining refractory DOM is advected and mixed into the deep sea. DOM and aggregates exported into the deep water are consumed and respired, thus returning organic carbon into the enormous deep ocean reservoir of DIC.[74]

A single phytoplankton cell has a sinking rate around one metre per day. Given that the average depth of the ocean is about four kilometres, it can take over ten years for these cells to reach the ocean floor. However, through processes such as coagulation and expulsion in predator fecal pellets, these cells form aggregates. These aggregates have sinking rates orders of magnitude greater than individual cells and complete their journey to the deep in a matter of days.[75]

About 1% of the particles leaving the surface ocean reach the seabed and are consumed, respired, or buried in the sediments. The net effect of these processes is to remove carbon in organic form from the surface and return it to DIC at greater depths, maintaining a surface-to-deep ocean gradient of DIC. Thermohaline circulation returns deep-ocean DIC to the atmosphere on millennial timescales. The carbon buried in the sediments can be subducted into the earth's mantle and stored for millions of years as part of the slow carbon cycle (see next section).[74]

Fast and slow cycles

 
The slow carbon cycle operates through rocks
The fast carbon cycle operates through the biosphere, see diagram at start of article ↑

There is a fast and a slow carbon cycle. The fast cycle operates in the biosphere and the slow cycle operates in rocks. The fast or biological cycle can complete within years, moving carbon from atmosphere to biosphere, then back to the atmosphere. The slow or geological cycle can take millions of years to complete, moving carbon through the Earth's crust between rocks, soil, ocean and atmosphere.[76]

The fast carbon cycle involves relatively short-term biogeochemical processes between the environment and living organisms in the biosphere (see diagram at start of article). It includes movements of carbon between the atmosphere and terrestrial and marine ecosystems, as well as soils and seafloor sediments. The fast cycle includes annual cycles involving photosynthesis and decadal cycles involving vegetative growth and decomposition. The reactions of the fast carbon cycle to human activities will determine many of the more immediate impacts of climate change.[77][78][79][80]

The slow carbon cycle involves medium to long-term geochemical processes belonging to the rock cycle (see diagram on the right). The exchange between the ocean and atmosphere can take centuries, and the weathering of rocks can take millions of years. Carbon in the ocean precipitates to the ocean floor where it can form sedimentary rock and be subducted into the earth's mantle. Mountain building processes result in the return of this geologic carbon to the Earth's surface. There the rocks are weathered and carbon is returned to the atmosphere by degassing and to the ocean by rivers. Other geologic carbon returns to the ocean through the hydrothermal emission of calcium ions. In a given year between 10 and 100 million tonnes of carbon moves around this slow cycle. This includes volcanoes returning geologic carbon directly to the atmosphere in the form of carbon dioxide. However, this is less than one percent of the carbon dioxide put into the atmosphere by burning fossil fuels.[76][77]

Deep carbon cycle

 
Movement of oceanic plates—which carry carbon compounds—through the mantle
Main article: Deep carbon cycle

Although deep carbon cycling is not as well-understood as carbon movement through the atmosphere, terrestrial biosphere, ocean, and geosphere, it is nonetheless an important process.[81] The deep carbon cycle is intimately connected to the movement of carbon in the Earth's surface and atmosphere. If the process did not exist, carbon would remain in the atmosphere, where it would accumulate to extremely high levels over long periods of time.[82] Therefore, by allowing carbon to return to the Earth, the deep carbon cycle plays a critical role in maintaining the terrestrial conditions necessary for life to exist.

Furthermore, the process is also significant simply due to the massive quantities of carbon it transports through the planet. In fact, studying the composition of basaltic magma and measuring carbon dioxide flux out of volcanoes reveals that the amount of carbon in the mantle is actually greater than that on the Earth's surface by a factor of one thousand.[83] Drilling down and physically observing deep-Earth carbon processes is evidently extremely difficult, as the lower mantle and core extend from 660 to 2,891 km and 2,891 to 6,371  km deep into the Earth respectively. Accordingly, not much is conclusively known regarding the role of carbon in the deep Earth. Nonetheless, several pieces of evidence—many of which come from laboratory simulations of deep Earth conditions—have indicated mechanisms for the element's movement down into the lower mantle, as well as the forms that carbon takes at the extreme temperatures and pressures of said layer. Furthermore, techniques like seismology have led to a greater understanding of the potential presence of carbon in the Earth's core.

Carbon in the lower mantle

 
Carbon outgassing through various processes [84]

Carbon principally enters the mantle in the form of carbonate-rich sediments on tectonic plates of ocean crust, which pull the carbon into the mantle upon undergoing subduction. Not much is known about carbon circulation in the mantle, especially in the deep Earth, but many studies have attempted to augment our understanding of the element's movement and forms within the region. For instance, a 2011 study demonstrated that carbon cycling extends all the way to the lower mantle. The study analyzed rare, super-deep diamonds at a site in Juina, Brazil, determining that the bulk composition of some of the diamonds' inclusions matched the expected result of basalt melting and crytallisation under lower mantle temperatures and pressures.[85] Thus, the investigation's findings indicate that pieces of basaltic oceanic lithosphere act as the principle transport mechanism for carbon to Earth's deep interior. These subducted carbonates can interact with lower mantle silicates, eventually forming super-deep diamonds like the one found.[86]

However, carbonates descending to the lower mantle encounter other fates in addition to forming diamonds. In 2011, carbonates were subjected to an environment similar to that of 1800 km deep into the Earth, well within the lower mantle. Doing so resulted in the formations of magnesite, siderite, and numerous varieties of graphite.[87] Other experiments—as well as petrologic observations—support this claim, indicating that magnesite is actually the most stable carbonate phase in most part of the mantle. This is largely a result of its higher melting temperature.[88] Consequently, scientists have concluded that carbonates undergo reduction as they descend into the mantle before being stabilised at depth by low oxygen fugacity environments. Magnesium, iron, and other metallic compounds act as buffers throughout the process.[89] The presence of reduced, elemental forms of carbon like graphite would indicate that carbon compounds are reduced as they descend into the mantle.

 
Carbon is tetrahedrally bonded to oxygen

Polymorphism alters carbonate compounds' stability at different depths within the Earth. To illustrate, laboratory simulations and density functional theory calculations suggest that tetrahedrally coordinated carbonates are most stable at depths approaching the core–mantle boundary.[90][87] A 2015 study indicates that the lower mantle's high pressure causes carbon bonds to transition from sp2 to sp3 hybridised orbitals, resulting in carbon tetrahedrally bonding to oxygen.[91] CO3 trigonal groups cannot form polymerisable networks, while tetrahedral CO4 can, signifying an increase in carbon's coordination number, and therefore drastic changes in carbonate compounds' properties in the lower mantle. As an example, preliminary theoretical studies suggest that high pressure causes carbonate melt viscosity to increase; the melts' lower mobility as a result of its increased viscosity causes large deposits of carbon deep into the mantle.[92]

Accordingly, carbon can remain in the lower mantle for long periods of time, but large concentrations of carbon frequently find their way back to the lithosphere. This process, called carbon outgassing, is the result of carbonated mantle undergoing decompression melting, as well as mantle plumes carrying carbon compounds up towards the crust.[93] Carbon is oxidised upon its ascent towards volcanic hotspots, where it is then released as CO2. This occurs so that the carbon atom matches the oxidation state of the basalts erupting in such areas.[94]

 
Knowledge about carbon in the core can be gained by analysing shear wave velocities

Carbon in the core

Although the presence of carbon in the Earth's core is well-constrained, recent studies suggest large inventories of carbon could be stored in this region.[clarification needed] Shear (S) waves moving through the inner core travel at about fifty percent of the velocity expected for most iron-rich alloys.[95] Because the core's composition is believed to be an alloy of crystalline iron and a small amount of nickel, this seismic anomaly indicates the presence of light elements, including carbon, in the core. In fact, studies using diamond anvil cells to replicate the conditions in the Earth's core indicate that iron carbide (Fe7C3) matches the inner core's wave speed and density. Therefore, the iron carbide model could serve as an evidence that the core holds as much as 67% of the Earth's carbon.[96] Furthermore, another study found that in the pressure and temperature condition of the Earth's inner core, carbon dissolved in iron and formed a stable phase with the same Fe7C3 composition—albeit with a different structure from the one previously mentioned.[97] In summary, although the amount of carbon potentially stored in the Earth's core is not known, recent studies indicate that the presence of iron carbides can explain some of the geophysical observations.

Human influence on carbon cycle

Carbon dioxide emissions and partitioning
 
Emissions of CO2 have been caused by different sources ramping up one after the other (Global Carbon Project)
 
Partitioning of CO2 emissions show that most emissions are being absorbed by carbon sinks, including plant growth, soil uptake, and ocean uptake (Global Carbon Project)
 
Schematic representation of the overall perturbation of the global carbon cycle caused by anthropogenic activities, averaged from 2010 to 2019.
Main articles: Climate change and Ocean acidification

Since the industrial revolution, and especially since the end of WWII, human activity has substantially disturbed the global carbon cycle by redistributing massive amounts of carbon from the geosphere.[1] Humans have also continued to shift the natural component functions of the terrestrial biosphere with changes to vegetation and other land use.[13] Man-made (synthetic) carbon compounds have been designed and mass-manufactured that will persist for decades to millennia in air, water, and sediments as pollutants.[98][99] Climate change is amplifying and forcing further indirect human changes to the carbon cycle as a consequence various positive and negative feedbacks.[29]

Land use changes

Main articles: Agriculture and Deforestation

Since the invention of agriculture, humans have directly and gradually influenced the carbon cycle over century-long timescales by modifying the mixture of vegetation in the terrestrial biosphere.[100] Over the past several centuries, direct and indirect human-caused land use and land cover change (LUCC) has led to the loss of biodiversity, which lowers ecosystems' resilience to environmental stresses and decreases their ability to remove carbon from the atmosphere. More directly, it often leads to the release of carbon from terrestrial ecosystems into the atmosphere.

Deforestation for agricultural purposes removes forests, which hold large amounts of carbon, and replaces them, generally with agricultural or urban areas. Both of these replacement land cover types store comparatively small amounts of carbon so that the net result of the transition is that more carbon stays in the atmosphere. However, the effects on the atmosphere and overall carbon cycle can be intentionally and/or naturally reversed with reforestation.

Herbivore impacts

Increased herbivore populations can alter the amount of carbon dioxide produced from a ecosystem, overall impacting the carbon cycle. Large mobile herbivores have the ability to alter both the above and below ground composition of a ecosystem, through selective feeding, trampling, and waste, all of which decrease plant production.[101] Selectively feeding on high quality plants decreases the aboveground plant composition, on the other hand trampling results in soil compaction resulting in higher soil bulk density and less soil oxygen.[101] The amount of carbon dioxide released back into the atmosphere is increased due to large herbivores waste.[102] The impact large herbivores have on the ecosystem suggest their importance to the carbon cycle, as with the help of natural disturbances, increased herbivore populations can shift a carbon sink to a source. The boreal forest is a prime example of how increased herbivore populations can negatively impact a ecosystem. Increased herbivore populations significant negative impact on the ecosystem, suggests that they can even be identified as a invasive species.[103]

Fossil carbon extraction

See also: Coal mining and Extraction of petroleum

The largest and one of the fastest growing human impacts on the carbon cycle and biosphere is the extraction and burning of fossil fuels, which directly transfer carbon from the geosphere into the atmosphere. Carbon dioxide is also produced and released during the calcination of limestone for clinker production.[104] Clinker is an industrial precursor of cement.

As of 2020, about 450 gigatons of fossil carbon have been extracted in total; an amount approaching the carbon contained in all of Earth's living terrestrial biomass.[2] Recent rates of global emissions directly into the atmosphere have exceeded the uptake by vegetation and the oceans.[105][106][107][108] These sinks have been expected and observed to remove about half of the added atmospheric carbon within about a century.[2][100][109] Nevertheless, sinks like the ocean have evolving saturation properties, and a substantial fraction (20–35%, based on coupled models) of the added carbon is projected remain in the atmosphere for centuries to millennia.[110][111] Fossil carbon extraction that increases atmospheric greenhouse gases is thus described by the IPCC, atmospheric and oceanic scientists as a long-term commitment by society to living in a changing climate and, ultimately, a warmer world.[4][112]

Man-made chemicals

Smaller amounts of man-made petrochemicals, containing fossil carbon, can have unexpected and outsized effects on the biological carbon cycle. This occurs in part because they have been purposely created by humans to decompose slowly, which enables their unnatural persistence and buildup throughout the biosphere. In many cases their pathways through the broader carbon cycle are also not yet well-characterized or understood.

 
The pathway by which plastics enter the world's oceans.

Plastics

See also: Plastic pollution and Microplastics

Close to 400 million tons of plastic were manufactured globally during year 2018 with annual growth rates approaching 10%, and over 6 gigatons produced in total since 1950.[99] Plastics eventually undergo fragmentation as a typical first step in their decay, and this enables their widespread distribution by air and water currents. Animals easily internalize microplastics and nanoplastics through ingestion and inhalation, accompanied by risks of bioaccumulation. Biodegradable plastics placed into landfills generate methane and carbon dioxide which cycles through the atmosphere unless captured.[113] A major review of the scientific evidence as of year 2019 did not identify major consequences for human society at current levels, but does foresee substantial risks emerging within the next century.[114] A 2019 study indicated that degradation of plastics through sun exposure, releases both carbon dioxide and other greenhouse gases.[115] Bioplastics with a more natural and rapid carbon cycle have been developed as an alternative to other petroleum-based single-use plastics.[116]

Halocarbons

See also: Halocarbons and Fluorinated gases

Halocarbons are less prolific compounds developed for diverse uses throughout industry; for example as solvents and refrigerants. Nevertheless, the buildup of relatively small concentrations (parts per trillion) of chlorofluorocarbon, hydrofluorocarbon, and perfluorocarbon gases in the atmosphere is responsible for about 10% of the total direct radiative forcing from all long-lived greenhouse gases (year 2019); which includes forcing from the much larger concentrations of carbon dioxide and methane.[117] Chlorofluorocarbons also cause stratospheric ozone depletion. International efforts are ongoing under the Montreal Protocol and Kyoto Protocol to control rapid growth in the industrial manufacturing and use of these environmentally potent gases. For some applications more benign alternatives such as hydrofluoroolefins have been developed and are being gradually introduced.[118]

 
Climate–carbon cycle feedbacks and state variables
as represented in a stylised model
Carbon stored on land in vegetation and soils is aggregated into a single stock ct. Ocean mixed layer carbon, cm, is the only explicitly modelled ocean stock of carbon; though to estimate carbon cycle feedbacks the total ocean carbon is also calculated.[119]

Climate change feedbacks

Main article: Climate change feedback

Current trends in climate change lead to higher ocean temperatures and acidity, thus modifying marine ecosystems.[120] Also, acid rain and polluted runoff from agriculture and industry change the ocean's chemical composition. Such changes can have dramatic effects on highly sensitive ecosystems such as coral reefs,[121] thus limiting the ocean's ability to absorb carbon from the atmosphere on a regional scale and reducing oceanic biodiversity globally.

The exchanges of carbon between the atmosphere and other components of the Earth system, collectively known as the carbon cycle, currently constitute important negative (dampening) feedbacks on the effect of anthropogenic carbon emissions on climate change. Carbon sinks in the land and the ocean each currently take up about one-quarter of anthropogenic carbon emissions each year.[122][119]

These feedbacks are expected to weaken in the future, amplifying the effect of anthropogenic carbon emissions on climate change.[123] The degree to which they will weaken, however, is highly uncertain, with Earth system models predicting a wide range of land and ocean carbon uptakes even under identical atmospheric concentration or emission scenarios.[124][119][125] Arctic methane emissions indirectly caused by anthropogenic global warming also affect the carbon cycle and contribute to further warming.

See also

References

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Further reading

  • Appenzeller, Tim (February 2004). "The case of the missing carbon". National Geographic Magazine. (Article about the missing carbon sink.)
  • Houghton, R. A. (2005). "The contemporary carbon cycle". In William H Schlesinger (ed.). Biogeochemistry. Amsterdam: Elsevier Science. pp. 473–513. ISBN 978-0-08-044642-4.
  • Reichle, David E. (2020). The global carbon cycle and climate change : scaling ecological energetics from organism to the biosphere. Amsterdam, Netherlands. ISBN 978-0-12-821767-2. OCLC 1128830826.
  • Janzen, H. H. (2004). "Carbon cycling in earth systems—a soil science perspective". Agriculture, Ecosystems & Environment. 104 (3): 399–417. CiteSeerX 10.1.1.466.622. doi:10.1016/j.agee.2004.01.040.
  • Millero, Frank J. (2005). Chemical Oceanography (3 ed.). CRC Press. ISBN 978-0-8493-2280-8.

External links

 
Wikimedia Commons has media related to carbon cycle.
  • – an interagency partnership.
  • NOAA's Carbon Cycle Greenhouse Gases Group
  • Global Carbon Project – initiative of the Earth System Science Partnership
  • UNEP – The present carbon cycle – Climate Change 15 September 2008 at the Wayback Machine carbon levels and flows
  • NASA's Orbiting Carbon Observatory 9 September 2018 at the Wayback Machine
  • CarboSchools, a European website with many resources to study carbon cycle in secondary schools.
  • Carbon and Climate, an educational website with a carbon cycle applet for modeling your own projection.

January 31, 2023
carbon, cycle, thermonuclear, reaction, involving, carbon, that, powers, some, stars, cycle, organic, chemical, ring, shaped, structures, cyclic, compound, geochemical, cycle, carbonate, silicate, cycle, carbon, cycle, biogeochemical, cycle, which, carbon, exc. For the thermonuclear reaction involving carbon that powers some stars see CNO cycle For organic chemical ring shaped structures see Cyclic compound For the geochemical cycle see Carbonate silicate cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere pedosphere geosphere hydrosphere and atmosphere of Earth Carbon is the main component of biological compounds as well as a major component of many minerals such as limestone Along with the nitrogen cycle and the water cycle the carbon cycle comprises a sequence of events that are key to make Earth capable of sustaining life It describes the movement of carbon as it is recycled and reused throughout the biosphere as well as long term processes of carbon sequestration to and release from carbon sinks Carbon sinks in the land and the ocean each currently take up about one quarter of anthropogenic carbon emissions each year Fast carbon cycle showing the movement of carbon between land atmosphere and oceans in billions of tons gigatons per year Yellow numbers are natural fluxes red are human contributions white are stored carbon The effects of the slow carbon cycle such as volcanic and tectonic activity are not included 1 Humans have disturbed the biological carbon cycle for many centuries by modifying land use and moreover with the recent industrial scale mining of fossil carbon coal petroleum and gas extraction and cement manufacture from the geosphere 1 2 Carbon dioxide in the atmosphere had increased nearly 52 over pre industrial levels by 2020 forcing greater atmospheric and Earth surface heating by the Sun 3 4 The increased carbon dioxide has also increased the acidity of the ocean surface by about 30 due to dissolved carbon dioxide carbonic acid and other compounds and is fundamentally altering marine chemistry 5 6 The majority of fossil carbon has been extracted over just the past half century and rates continue to rise rapidly contributing to human caused climate change 7 8 The largest consequences to the carbon cycle and to the biosphere which critically enables human civilization are still set to unfold due to the vast yet limited inertia of the Earth system 1 9 10 Restoring balance to this natural system is an international priority described in both the Paris Climate Agreement and Sustainable Development Goal 13 Contents 1 Main components 1 1 Atmosphere 1 2 Terrestrial biosphere 1 3 Ocean 1 4 Geosphere 2 Terrestrial carbon in the water cycle 3 The marine biological pump 4 Fast and slow cycles 5 Deep carbon cycle 5 1 Carbon in the lower mantle 5 2 Carbon in the core 6 Human influence on carbon cycle 6 1 Land use changes 6 2 Herbivore impacts 6 3 Fossil carbon extraction 6 4 Man made chemicals 6 4 1 Plastics 6 4 2 Halocarbons 6 5 Climate change feedbacks 7 See also 8 References 9 Further reading 10 External linksMain components Edit Detail of anthropogenic carbon flows showing cumulative mass in gigatons during years 1850 2018 left and the annual mass average during 2009 2018 right 2 The carbon cycle was first described by Antoine Lavoisier and Joseph Priestley and popularised by Humphry Davy 11 The global carbon cycle is now usually divided into the following major reservoirs of carbon interconnected by pathways of exchange 12 5 6 The atmosphere The terrestrial biosphere The ocean including dissolved inorganic carbon and living and non living marine biota The sediments including fossil fuels freshwater systems and non living organic material The Earth s interior mantle and crust These carbon stores interact with the other components through geological processes The carbon exchanges between reservoirs occur as the result of various chemical physical geological and biological processes The ocean contains the largest active pool of carbon near the surface of the Earth 13 The natural flows of carbon between the atmosphere ocean terrestrial ecosystems and sediments are fairly balanced so carbon levels would be roughly stable without human influence 3 14 Atmosphere Edit Main article Atmospheric carbon cycle source source source source source source source source source source source source source source Computer model showing a year in the life of atmospheric carbon dioxide and how it travels around the globe 15 Carbon in the Earth s atmosphere exists in two main forms carbon dioxide and methane Both of these gases absorb and retain heat in the atmosphere and are partially responsible for the greenhouse effect 13 Methane produces a larger greenhouse effect per volume as compared to carbon dioxide but it exists in much lower concentrations and is more short lived than carbon dioxide making carbon dioxide the more important greenhouse gas of the two 16 Carbon dioxide is removed from the atmosphere primarily through photosynthesis and enters the terrestrial and oceanic biospheres Carbon dioxide also dissolves directly from the atmosphere into bodies of water ocean lakes etc as well as dissolving in precipitation as raindrops fall through the atmosphere When dissolved in water carbon dioxide reacts with water molecules and forms carbonic acid which contributes to ocean acidity It can then be absorbed by rocks through weathering It also can acidify other surfaces it touches or be washed into the ocean 17 CO2 concentrations over the last 800 000 years as measured from ice cores blue green and directly black Human activities over the past two centuries have increased the amount of carbon in the atmosphere by nearly 50 as of year 2020 mainly in the form of carbon dioxide both by modifying ecosystems ability to extract carbon dioxide from the atmosphere and by emitting it directly e g by burning fossil fuels and manufacturing concrete 4 13 In the far future 2 to 3 billion years the rate at which carbon dioxide is absorbed into the soil via the carbonate silicate cycle will likely increase due to expected changes in the sun as it ages The expected increased luminosity of the Sun will likely speed up the rate of surface weathering 18 This will eventually cause most of the carbon dioxide in the atmosphere to be squelched into the Earth s crust as carbonate 19 20 Once the concentration of carbon dioxide in the atmosphere falls below approximately 50 parts per million tolerances vary among species C3 photosynthesis will no longer be possible 20 This has been predicted to occur 600 million years from the present though models vary 21 Once the oceans on the Earth evaporate in about 1 1 billion years from now 18 plate tectonics will very likely stop due to the lack of water to lubricate them The lack of volcanoes pumping out carbon dioxide will cause the carbon cycle to end between 1 billion and 2 billion years into the future 22 Terrestrial biosphere Edit Amount of carbon stored in Earth s various terrestrial ecosystems in gigatonnes 23 Main article Terrestrial biological carbon cycle The terrestrial biosphere includes the organic carbon in all land living organisms both alive and dead as well as carbon stored in soils About 500 gigatons of carbon are stored above ground in plants and other living organisms 3 while soil holds approximately 1 500 gigatons of carbon 24 Most carbon in the terrestrial biosphere is organic carbon 25 while about a third of soil carbon is stored in inorganic forms such as calcium carbonate 26 Organic carbon is a major component of all organisms living on earth Autotrophs extract it from the air in the form of carbon dioxide converting it into organic carbon while heterotrophs receive carbon by consuming other organisms Because carbon uptake in the terrestrial biosphere is dependent on biotic factors it follows a diurnal and seasonal cycle In CO2 measurements this feature is apparent in the Keeling curve It is strongest in the northern hemisphere because this hemisphere has more land mass than the southern hemisphere and thus more room for ecosystems to absorb and emit carbon A portable soil respiration system measuring soil CO2 flux Carbon leaves the terrestrial biosphere in several ways and on different time scales The combustion or respiration of organic carbon releases it rapidly into the atmosphere It can also be exported into the ocean through rivers or remain sequestered in soils in the form of inert carbon 27 Carbon stored in soil can remain there for up to thousands of years before being washed into rivers by erosion or released into the atmosphere through soil respiration Between 1989 and 2008 soil respiration increased by about 0 1 per year 28 In 2008 the global total of CO2 released by soil respiration was roughly 98 billion tonnes about 3 times more carbon than humans are now putting into the atmosphere each year by burning fossil fuel this does not represent a net transfer of carbon from soil to atmosphere as the respiration is largely offset by inputs to soil carbon There are a few plausible explanations for this trend but the most likely explanation is that increasing temperatures have increased rates of decomposition of soil organic matter which has increased the flow of CO2 The length of carbon sequestering in soil is dependent on local climatic conditions and thus changes in the course of climate change 29 Size of major carbon pools on the Earth year 2000 estimates 13 Pool Quantity gigatons Atmosphere 720Ocean total 38 400Total inorganic 37 400Total organic 1 000Surface layer 670Deep layer 36 730LithosphereSedimentary carbonates gt 60 000 000Kerogens 15 000 000Terrestrial biosphere total 2 000Living biomass 600 1 000Dead biomass 1 200Aquatic biosphere 1 2Fossil fuels total 4 130Coal 3 510Oil 230Gas 140Other peat 250Ocean Edit Main article Oceanic carbon cycle The ocean can be conceptually divided into a surface layer within which water makes frequent daily to annual contact with the atmosphere and a deep layer below the typical mixed layer depth of a few hundred meters or less within which the time between consecutive contacts may be centuries The dissolved inorganic carbon DIC in the surface layer is exchanged rapidly with the atmosphere maintaining equilibrium Partly because its concentration of DIC is about 15 higher 30 but mainly due to its larger volume the deep ocean contains far more carbon it is the largest pool of actively cycled carbon in the world containing 50 times more than the atmosphere 13 but the timescale to reach equilibrium with the atmosphere is hundreds of years the exchange of carbon between the two layers driven by thermohaline circulation is slow 13 Carbon enters the ocean mainly through the dissolution of atmospheric carbon dioxide a small fraction of which is converted into carbonate It can also enter the ocean through rivers as dissolved organic carbon It is converted by organisms into organic carbon through photosynthesis and can either be exchanged throughout the food chain or precipitated into the oceans deeper more carbon rich layers as dead soft tissue or in shells as calcium carbonate It circulates in this layer for long periods of time before either being deposited as sediment or eventually returned to the surface waters through thermohaline circulation 3 Oceans are basic pH 8 2 hence CO2 acidification shifts the pH of the ocean towards neutral Oceanic absorption of CO2 is one of the most important forms of carbon sequestering which limit the human caused rise of carbon dioxide in the atmosphere However this process is limited by a number of factors CO2 absorption makes water more acidic which affects ocean biosystems The projected rate of increasing oceanic acidity could slow the biological precipitation of calcium carbonates thus decreasing the ocean s capacity to absorb CO2 31 32 Geosphere Edit Main article Carbonate silicate cycle Diagram showing relative sizes in gigatonnes of the main storage pools of carbon on Earth Cumulative changes thru year 2014 from land use and emissions of fossil carbon are included for comparison 23 The geologic component of the carbon cycle operates slowly in comparison to the other parts of the global carbon cycle It is one of the most important determinants of the amount of carbon in the atmosphere and thus of global temperatures 33 Most of the earth s carbon is stored inertly in the earth s lithosphere 13 Much of the carbon stored in the earth s mantle was stored there when the earth formed 34 Some of it was deposited in the form of organic carbon from the biosphere 35 Of the carbon stored in the geosphere about 80 is limestone and its derivatives which form from the sedimentation of calcium carbonate stored in the shells of marine organisms The remaining 20 is stored as kerogens formed through the sedimentation and burial of terrestrial organisms under high heat and pressure Organic carbon stored in the geosphere can remain there for millions of years 33 Carbon can leave the geosphere in several ways Carbon dioxide is released during the metamorphism of carbonate rocks when they are subducted into the earth s mantle This carbon dioxide can be released into the atmosphere and ocean through volcanoes and hotspots 34 It can also be removed by humans through the direct extraction of kerogens in the form of fossil fuels After extraction fossil fuels are burned to release energy and emit the carbon they store into the atmosphere Terrestrial carbon in the water cycle Edit Where terrestrial carbon goes when water flows 36 In the diagram on the right 36 Atmospheric particles act as cloud condensation nuclei promoting cloud formation 37 38 Raindrops absorb organic and inorganic carbon through particle scavenging and adsorption of organic vapors while falling toward Earth 39 40 Burning and volcanic eruptions produce highly condensed polycyclic aromatic molecules i e black carbon that is returned to the atmosphere along with greenhouse gases such as CO2 41 42 Terrestrial plants fix atmospheric CO2 through photosynthesis returning a fraction back to the atmosphere through respiration 43 Lignin and celluloses represent as much as 80 of the organic carbon in forests and 60 in pastures 44 45 Litterfall and root organic carbon mix with sedimentary material to form organic soils where plant derived and petrogenic organic carbon is both stored and transformed by microbial and fungal activity 46 47 48 Water absorbs plant and settled aerosol derived dissolved organic carbon DOC and dissolved inorganic carbon DIC as it passes over forest canopies i e throughfall and along plant trunks stems i e stemflow 49 Biogeochemical transformations take place as water soaks into soil solution and groundwater reservoirs 50 51 and overland flow occurs when soils are completely saturated 52 or rainfall occurs more rapidly than saturation into soils 53 Organic carbon derived from the terrestrial biosphere and in situ primary production is decomposed by microbial communities in rivers and streams along with physical decomposition i e photo oxidation resulting in a flux of CO2 from rivers to the atmosphere that are the same order of magnitude as the amount of carbon sequestered annually by the terrestrial biosphere 54 55 56 Terrestrially derived macromolecules such as lignin 57 and black carbon 58 are decomposed into smaller components and monomers ultimately being converted to CO2 metabolic intermediates or biomass Lakes reservoirs and floodplains typically store large amounts of organic carbon and sediments but also experience net heterotrophy in the water column resulting in a net flux of CO2 to the atmosphere that is roughly one order of magnitude less than rivers 59 56 Methane production is also typically high in the anoxic sediments of floodplains lakes and reservoirs 60 Primary production is typically enhanced in river plumes due to the export of fluvial nutrients 61 62 Nevertheless estuarine waters are a source of CO2 to the atmosphere globally 63 Coastal marshes both store and export blue carbon 64 65 66 Marshes and wetlands are suggested to have an equivalent flux of CO2 to the atmosphere as rivers globally 67 Continental shelves and the open ocean typically absorb CO2 from the atmosphere 63 The marine biological pump sequesters a small but significant fraction of the absorbed CO2 as organic carbon in marine sediments see next section 68 36 The marine biological pump Edit Flow of carbon through the open ocean Main article Biological pump The marine biological pump is the ocean s biologically driven sequestration of carbon from the atmosphere and land runoff to the deep ocean interior and seafloor sediments 69 The biological pump is not so much the result of a single process but rather the sum of a number of processes each of which can influence biological pumping The pump transfers about 11 billion tonnes of carbon every year into the ocean s interior An ocean without the biological pump would result in atmospheric CO2 levels about 400 ppm higher than the present day 70 71 72 Most carbon incorporated in organic and inorganic biological matter is formed at the sea surface where it can then start sinking to the ocean floor The deep ocean gets most of its nutrients from the higher water column when they sink down in the form of marine snow This is made up of dead or dying animals and microbes fecal matter sand and other inorganic material 73 The biological pump is responsible for transforming dissolved inorganic carbon DIC into organic biomass and pumping it in particulate or dissolved form into the deep ocean Inorganic nutrients and carbon dioxide are fixed during photosynthesis by phytoplankton which both release dissolved organic matter DOM and are consumed by herbivorous zooplankton Larger zooplankton such as copepods egest fecal pellets which can be reingested and sink or collect with other organic detritus into larger more rapidly sinking aggregates DOM is partially consumed by bacteria and respired the remaining refractory DOM is advected and mixed into the deep sea DOM and aggregates exported into the deep water are consumed and respired thus returning organic carbon into the enormous deep ocean reservoir of DIC 74 A single phytoplankton cell has a sinking rate around one metre per day Given that the average depth of the ocean is about four kilometres it can take over ten years for these cells to reach the ocean floor However through processes such as coagulation and expulsion in predator fecal pellets these cells form aggregates These aggregates have sinking rates orders of magnitude greater than individual cells and complete their journey to the deep in a matter of days 75 About 1 of the particles leaving the surface ocean reach the seabed and are consumed respired or buried in the sediments The net effect of these processes is to remove carbon in organic form from the surface and return it to DIC at greater depths maintaining a surface to deep ocean gradient of DIC Thermohaline circulation returns deep ocean DIC to the atmosphere on millennial timescales The carbon buried in the sediments can be subducted into the earth s mantle and stored for millions of years as part of the slow carbon cycle see next section 74 Fast and slow cycles Edit The slow carbon cycle operates through rocksThe fast carbon cycle operates through the biosphere see diagram at start of article There is a fast and a slow carbon cycle The fast cycle operates in the biosphere and the slow cycle operates in rocks The fast or biological cycle can complete within years moving carbon from atmosphere to biosphere then back to the atmosphere The slow or geological cycle can take millions of years to complete moving carbon through the Earth s crust between rocks soil ocean and atmosphere 76 The fast carbon cycle involves relatively short term biogeochemical processes between the environment and living organisms in the biosphere see diagram at start of article It includes movements of carbon between the atmosphere and terrestrial and marine ecosystems as well as soils and seafloor sediments The fast cycle includes annual cycles involving photosynthesis and decadal cycles involving vegetative growth and decomposition The reactions of the fast carbon cycle to human activities will determine many of the more immediate impacts of climate change 77 78 79 80 The slow carbon cycle involves medium to long term geochemical processes belonging to the rock cycle see diagram on the right The exchange between the ocean and atmosphere can take centuries and the weathering of rocks can take millions of years Carbon in the ocean precipitates to the ocean floor where it can form sedimentary rock and be subducted into the earth s mantle Mountain building processes result in the return of this geologic carbon to the Earth s surface There the rocks are weathered and carbon is returned to the atmosphere by degassing and to the ocean by rivers Other geologic carbon returns to the ocean through the hydrothermal emission of calcium ions In a given year between 10 and 100 million tonnes of carbon moves around this slow cycle This includes volcanoes returning geologic carbon directly to the atmosphere in the form of carbon dioxide However this is less than one percent of the carbon dioxide put into the atmosphere by burning fossil fuels 76 77 Deep carbon cycle Edit Movement of oceanic plates which carry carbon compounds through the mantle Main article Deep carbon cycle Although deep carbon cycling is not as well understood as carbon movement through the atmosphere terrestrial biosphere ocean and geosphere it is nonetheless an important process 81 The deep carbon cycle is intimately connected to the movement of carbon in the Earth s surface and atmosphere If the process did not exist carbon would remain in the atmosphere where it would accumulate to extremely high levels over long periods of time 82 Therefore by allowing carbon to return to the Earth the deep carbon cycle plays a critical role in maintaining the terrestrial conditions necessary for life to exist Furthermore the process is also significant simply due to the massive quantities of carbon it transports through the planet In fact studying the composition of basaltic magma and measuring carbon dioxide flux out of volcanoes reveals that the amount of carbon in the mantle is actually greater than that on the Earth s surface by a factor of one thousand 83 Drilling down and physically observing deep Earth carbon processes is evidently extremely difficult as the lower mantle and core extend from 660 to 2 891 km and 2 891 to 6 371 km deep into the Earth respectively Accordingly not much is conclusively known regarding the role of carbon in the deep Earth Nonetheless several pieces of evidence many of which come from laboratory simulations of deep Earth conditions have indicated mechanisms for the element s movement down into the lower mantle as well as the forms that carbon takes at the extreme temperatures and pressures of said layer Furthermore techniques like seismology have led to a greater understanding of the potential presence of carbon in the Earth s core Carbon in the lower mantle Edit Carbon outgassing through various processes 84 Carbon principally enters the mantle in the form of carbonate rich sediments on tectonic plates of ocean crust which pull the carbon into the mantle upon undergoing subduction Not much is known about carbon circulation in the mantle especially in the deep Earth but many studies have attempted to augment our understanding of the element s movement and forms within the region For instance a 2011 study demonstrated that carbon cycling extends all the way to the lower mantle The study analyzed rare super deep diamonds at a site in Juina Brazil determining that the bulk composition of some of the diamonds inclusions matched the expected result of basalt melting and crytallisation under lower mantle temperatures and pressures 85 Thus the investigation s findings indicate that pieces of basaltic oceanic lithosphere act as the principle transport mechanism for carbon to Earth s deep interior These subducted carbonates can interact with lower mantle silicates eventually forming super deep diamonds like the one found 86 However carbonates descending to the lower mantle encounter other fates in addition to forming diamonds In 2011 carbonates were subjected to an environment similar to that of 1800 km deep into the Earth well within the lower mantle Doing so resulted in the formations of magnesite siderite and numerous varieties of graphite 87 Other experiments as well as petrologic observations support this claim indicating that magnesite is actually the most stable carbonate phase in most part of the mantle This is largely a result of its higher melting temperature 88 Consequently scientists have concluded that carbonates undergo reduction as they descend into the mantle before being stabilised at depth by low oxygen fugacity environments Magnesium iron and other metallic compounds act as buffers throughout the process 89 The presence of reduced elemental forms of carbon like graphite would indicate that carbon compounds are reduced as they descend into the mantle Carbon is tetrahedrally bonded to oxygen Polymorphism alters carbonate compounds stability at different depths within the Earth To illustrate laboratory simulations and density functional theory calculations suggest that tetrahedrally coordinated carbonates are most stable at depths approaching the core mantle boundary 90 87 A 2015 study indicates that the lower mantle s high pressure causes carbon bonds to transition from sp2 to sp3 hybridised orbitals resulting in carbon tetrahedrally bonding to oxygen 91 CO3 trigonal groups cannot form polymerisable networks while tetrahedral CO4 can signifying an increase in carbon s coordination number and therefore drastic changes in carbonate compounds properties in the lower mantle As an example preliminary theoretical studies suggest that high pressure causes carbonate melt viscosity to increase the melts lower mobility as a result of its increased viscosity causes large deposits of carbon deep into the mantle 92 Accordingly carbon can remain in the lower mantle for long periods of time but large concentrations of carbon frequently find their way back to the lithosphere This process called carbon outgassing is the result of carbonated mantle undergoing decompression melting as well as mantle plumes carrying carbon compounds up towards the crust 93 Carbon is oxidised upon its ascent towards volcanic hotspots where it is then released as CO2 This occurs so that the carbon atom matches the oxidation state of the basalts erupting in such areas 94 Knowledge about carbon in the core can be gained by analysing shear wave velocities Carbon in the core Edit Although the presence of carbon in the Earth s core is well constrained recent studies suggest large inventories of carbon could be stored in this region clarification needed Shear S waves moving through the inner core travel at about fifty percent of the velocity expected for most iron rich alloys 95 Because the core s composition is believed to be an alloy of crystalline iron and a small amount of nickel this seismic anomaly indicates the presence of light elements including carbon in the core In fact studies using diamond anvil cells to replicate the conditions in the Earth s core indicate that iron carbide Fe7C3 matches the inner core s wave speed and density Therefore the iron carbide model could serve as an evidence that the core holds as much as 67 of the Earth s carbon 96 Furthermore another study found that in the pressure and temperature condition of the Earth s inner core carbon dissolved in iron and formed a stable phase with the same Fe7C3 composition albeit with a different structure from the one previously mentioned 97 In summary although the amount of carbon potentially stored in the Earth s core is not known recent studies indicate that the presence of iron carbides can explain some of the geophysical observations Human influence on carbon cycle EditCarbon dioxide emissions and partitioning Emissions of CO2 have been caused by different sources ramping up one after the other Global Carbon Project Partitioning of CO2 emissions show that most emissions are being absorbed by carbon sinks including plant growth soil uptake and ocean uptake Global Carbon Project Schematic representation of the overall perturbation of the global carbon cycle caused by anthropogenic activities averaged from 2010 to 2019 Main articles Climate change and Ocean acidification Since the industrial revolution and especially since the end of WWII human activity has substantially disturbed the global carbon cycle by redistributing massive amounts of carbon from the geosphere 1 Humans have also continued to shift the natural component functions of the terrestrial biosphere with changes to vegetation and other land use 13 Man made synthetic carbon compounds have been designed and mass manufactured that will persist for decades to millennia in air water and sediments as pollutants 98 99 Climate change is amplifying and forcing further indirect human changes to the carbon cycle as a consequence various positive and negative feedbacks 29 Land use changes Edit Main articles Agriculture and Deforestation Since the invention of agriculture humans have directly and gradually influenced the carbon cycle over century long timescales by modifying the mixture of vegetation in the terrestrial biosphere 100 Over the past several centuries direct and indirect human caused land use and land cover change LUCC has led to the loss of biodiversity which lowers ecosystems resilience to environmental stresses and decreases their ability to remove carbon from the atmosphere More directly it often leads to the release of carbon from terrestrial ecosystems into the atmosphere Deforestation for agricultural purposes removes forests which hold large amounts of carbon and replaces them generally with agricultural or urban areas Both of these replacement land cover types store comparatively small amounts of carbon so that the net result of the transition is that more carbon stays in the atmosphere However the effects on the atmosphere and overall carbon cycle can be intentionally and or naturally reversed with reforestation Herbivore impacts Edit Increased herbivore populations can alter the amount of carbon dioxide produced from a ecosystem overall impacting the carbon cycle Large mobile herbivores have the ability to alter both the above and below ground composition of a ecosystem through selective feeding trampling and waste all of which decrease plant production 101 Selectively feeding on high quality plants decreases the aboveground plant composition on the other hand trampling results in soil compaction resulting in higher soil bulk density and less soil oxygen 101 The amount of carbon dioxide released back into the atmosphere is increased due to large herbivores waste 102 The impact large herbivores have on the ecosystem suggest their importance to the carbon cycle as with the help of natural disturbances increased herbivore populations can shift a carbon sink to a source The boreal forest is a prime example of how increased herbivore populations can negatively impact a ecosystem Increased herbivore populations significant negative impact on the ecosystem suggests that they can even be identified as a invasive species 103 Fossil carbon extraction Edit See also Coal mining and Extraction of petroleum The largest and one of the fastest growing human impacts on the carbon cycle and biosphere is the extraction and burning of fossil fuels which directly transfer carbon from the geosphere into the atmosphere Carbon dioxide is also produced and released during the calcination of limestone for clinker production 104 Clinker is an industrial precursor of cement As of 2020 update about 450 gigatons of fossil carbon have been extracted in total an amount approaching the carbon contained in all of Earth s living terrestrial biomass 2 Recent rates of global emissions directly into the atmosphere have exceeded the uptake by vegetation and the oceans 105 106 107 108 These sinks have been expected and observed to remove about half of the added atmospheric carbon within about a century 2 100 109 Nevertheless sinks like the ocean have evolving saturation properties and a substantial fraction 20 35 based on coupled models of the added carbon is projected remain in the atmosphere for centuries to millennia 110 111 Fossil carbon extraction that increases atmospheric greenhouse gases is thus described by the IPCC atmospheric and oceanic scientists as a long term commitment by society to living in a changing climate and ultimately a warmer world 4 112 Man made chemicals Edit Smaller amounts of man made petrochemicals containing fossil carbon can have unexpected and outsized effects on the biological carbon cycle This occurs in part because they have been purposely created by humans to decompose slowly which enables their unnatural persistence and buildup throughout the biosphere In many cases their pathways through the broader carbon cycle are also not yet well characterized or understood The pathway by which plastics enter the world s oceans Plastics Edit See also Plastic pollution and Microplastics Close to 400 million tons of plastic were manufactured globally during year 2018 with annual growth rates approaching 10 and over 6 gigatons produced in total since 1950 99 Plastics eventually undergo fragmentation as a typical first step in their decay and this enables their widespread distribution by air and water currents Animals easily internalize microplastics and nanoplastics through ingestion and inhalation accompanied by risks of bioaccumulation Biodegradable plastics placed into landfills generate methane and carbon dioxide which cycles through the atmosphere unless captured 113 A major review of the scientific evidence as of year 2019 did not identify major consequences for human society at current levels but does foresee substantial risks emerging within the next century 114 A 2019 study indicated that degradation of plastics through sun exposure releases both carbon dioxide and other greenhouse gases 115 Bioplastics with a more natural and rapid carbon cycle have been developed as an alternative to other petroleum based single use plastics 116 Halocarbons Edit See also Halocarbons and Fluorinated gases Halocarbons are less prolific compounds developed for diverse uses throughout industry for example as solvents and refrigerants Nevertheless the buildup of relatively small concentrations parts per trillion of chlorofluorocarbon hydrofluorocarbon and perfluorocarbon gases in the atmosphere is responsible for about 10 of the total direct radiative forcing from all long lived greenhouse gases year 2019 which includes forcing from the much larger concentrations of carbon dioxide and methane 117 Chlorofluorocarbons also cause stratospheric ozone depletion International efforts are ongoing under the Montreal Protocol and Kyoto Protocol to control rapid growth in the industrial manufacturing and use of these environmentally potent gases For some applications more benign alternatives such as hydrofluoroolefins have been developed and are being gradually introduced 118 Climate carbon cycle feedbacks and state variablesas represented in a stylised model Carbon stored on land in vegetation and soils is aggregated into a single stock ct Ocean mixed layer carbon cm is the only explicitly modelled ocean stock of carbon though to estimate carbon cycle feedbacks the total ocean carbon is also calculated 119 Climate change feedbacks Edit Main article Climate change feedback Current trends in climate change lead to higher ocean temperatures and acidity thus modifying marine ecosystems 120 Also acid rain and polluted runoff from agriculture and industry change the ocean s chemical composition Such changes can have dramatic effects on highly sensitive ecosystems such as coral reefs 121 thus limiting the ocean s ability to absorb carbon from the atmosphere on a regional scale and reducing oceanic biodiversity globally The exchanges of carbon between the atmosphere and other components of the Earth system collectively known as the carbon cycle currently constitute important negative dampening feedbacks on the effect of anthropogenic carbon emissions on climate change Carbon sinks in the land and the ocean each currently take up about one quarter of anthropogenic carbon emissions each year 122 119 These feedbacks are expected to weaken in the future amplifying the effect of anthropogenic carbon emissions on climate change 123 The degree to which they will weaken however is highly uncertain with Earth system models predicting a wide range of land and ocean carbon uptakes even under identical atmospheric concentration or emission scenarios 124 119 125 Arctic methane emissions indirectly caused by anthropogenic global warming also affect 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Analysis How carbon cycle feedbacks could make global warming worse Carbon Brief Archived from the original on 16 April 2020 Retrieved 4 January 2022 Further reading EditAppenzeller Tim February 2004 The case of the missing carbon National Geographic Magazine Article about the missing carbon sink Houghton R A 2005 The contemporary carbon cycle In William H Schlesinger ed Biogeochemistry Amsterdam Elsevier Science pp 473 513 ISBN 978 0 08 044642 4 Reichle David E 2020 The global carbon cycle and climate change scaling ecological energetics from organism to the biosphere Amsterdam Netherlands ISBN 978 0 12 821767 2 OCLC 1128830826 Janzen H H 2004 Carbon cycling in earth systems a soil science perspective Agriculture Ecosystems amp Environment 104 3 399 417 CiteSeerX 10 1 1 466 622 doi 10 1016 j agee 2004 01 040 Millero Frank J 2005 Chemical Oceanography 3 ed CRC Press ISBN 978 0 8493 2280 8 External links Edit Wikimedia Commons has media related to carbon cycle Carbon Cycle Science Program an interagency partnership NOAA s Carbon Cycle Greenhouse Gases Group Global Carbon Project initiative of the Earth System Science Partnership UNEP The present carbon cycle Climate Change Archived 15 September 2008 at the Wayback Machine carbon levels and flows NASA s Orbiting Carbon Observatory Archived 9 September 2018 at the Wayback Machine CarboSchools a European website with many resources to study carbon cycle in secondary schools Carbon and Climate an educational website with a carbon cycle applet for modeling your own projection Retrieved from https en wikipedia org w index php title Carbon cycle amp oldid 1135356632, wikipedia, wiki, book, books, library,

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