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Carbon cycle

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For the thermonuclear reaction involving carbon that powers some stars, see CNO cycle. For organic chemical ring-shaped structures, see Cyclic compounds.

Movement of carbon between land, atmosphere, and ocean in billions of tons per year. Yellow numbers are natural fluxes, red are human contributions, white are stored carbon. The effects of volcanic and tectonic activity are not included.

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the 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.

The carbon cycle was discovered by Joseph Priestley and Antoine Lavoisier, and popularized by Humphry Davy.[1]

Main components

Carbon pools in the major reservoirs on earth.[2]
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
The ocean and land have continued to, over time, absorb about half of all carbon dioxide emissions, even as those emissions have risen dramatically in recent decades. It remains unclear if carbon absorption will continue at this rate.

The global carbon cycle is now usually divided into the following major reservoirs of carbon interconnected by pathways of exchange:[3]: 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.[2] The natural flows of carbon between the atmosphere, ocean, terrestrial ecosystems, and sediments is fairly balanced, so that carbon levels would be roughly stable without human influence.[4][5]

Atmosphere

Main article: Atmospheric carbon cycle
Epiphytes on electric wires. This kind of plant takes both CO2 and water from the atmosphere for living and growing.

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.[2] 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.[6]

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.[7]

Human activities over the past two centuries have significantly increased the amount of carbon in the atmosphere, 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.[2]

Terrestrial biosphere

Main article: Terrestrial biological carbon cycle
A portable soil respiration system measuring soil CO2 flux

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,[4] while soil holds approximately 1,500 gigatons of carbon.[8] Most carbon in the terrestrial biosphere is organic carbon,[9] while about a third of soil carbon is stored in inorganic forms, such as calcium carbonate.[10] 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.

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.[11] 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.[12] In 2008, the global total of CO2 released by soil respiration was roughly 98 billion tonnes, about 10 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.

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[13] but mainly due to its larger volume, the deep ocean contains far more carbon—it's the largest pool of actively cycled carbon in the world, containing 50 times more than the atmosphere[2]—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.[2]

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 ocean's 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.[4] 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 limiting 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 carbon dioxide.[14][15]

Earth's interior

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.[16]

Most of the earth's carbon is stored inertly in the earth's lithosphere.[2] Much of the carbon stored in the earth's mantle was stored there when the earth formed.[17] Some of it was deposited in the form of organic carbon from the biosphere.[18] 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.[16]

Carbon can leave the geosphere in several ways. Carbon dioxide is released during the metamorphosis 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.[17] 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, thus emitting the carbon they store into the atmosphere

Human influence

Main article: Global warming
Human perturbation of the carbon cycle
Human activity since the industrial era has changed the balance in the natural carbon cycle. Units are in gigatons.[4]
CO2 in Earth's atmosphere if half of global-warming emissions are not absorbed.[19][20][21][22]
(NASA computer simulation).

Since the industrial revolution, human activity has modified the carbon cycle by changing its components' functions and directly adding carbon to the atmosphere.[2]

The largest human impact on the carbon cycle is through direct emissions from burning fossil fuels, which transfers carbon from the geosphere into the atmosphere. The rest of this increase is caused mostly by changes in land-use, particularly deforestation.

Another direct human impact on the carbon cycle is the chemical process of calcination of limestone for clinker production, which releases CO2.[23] Clinker is an industrial precursor of cement.

Humans also influence the carbon cycle indirectly by changing the terrestrial and oceanic biosphere.[24] 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 product of the process is that more carbon stays in the atmosphere.

Other human-caused changes to the environment change ecosystems' productivity and their ability to remove carbon from the atmosphere. Air pollution, for example, damages plants and soils, while many agricultural and land use practices lead to higher erosion rates, washing carbon out of soils and decreasing plant productivity.

Humans also affect the oceanic carbon cycle.[24] Current trends in climate change lead to higher ocean temperatures, thus modifying ecosystems.[25][26][27] 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,[28][29][30] thus limiting the ocean's ability to absorb carbon from the atmosphere on a regional scale and reducing oceanic biodiversity globally.

Arctic methane emissions indirectly caused by anthropogenic global warming also affect the carbon cycle, and contribute to further warming in what is known as climate change feedback.

On 12 November 2015, NASA scientists reported that human-made carbon dioxide (CO2) continues to increase, reaching levels not seen in hundreds of thousands of years: currently, the rate carbon dioxide release by the burning of fossil fuels is about double the net uptake by vegetation and the ocean.[19][20][21][22]

See also

References

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  2. ^ a b c d e f g h Falkowski, P.; Scholes, R. J.; Boyle, E.; Canadell, J.; Canfield, D.; Elser, J.; Gruber, N.; Hibbard, K.; Högberg, P.; Linder, S.; MacKenzie, F. T.; Moore b, 3.; Pedersen, T.; Rosenthal, Y.; Seitzinger, S.; Smetacek, V.; Steffen, W. (2000). "The Global Carbon Cycle: A Test of Our Knowledge of Earth as a System". Science. 290 (5490): 291–296. Bibcode:2000Sci...290..291F. doi:10.1126/science.290.5490.291. PMID 11030643. {{cite journal}}: |first12= has numeric name (help)
  3. ^ Archer, David (2010). The global carbon cycle. Princeton: Princeton University Press. ISBN 9781400837076.
  4. ^ a b c d Prentice, I.C. (2001). "The carbon cycle and atmospheric carbon dioxide". Climate change 2001: the scientific basis: contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change / Houghton, J.T. [edit.] Retrieved 31 May 2012.
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  8. ^ Rice, Charles W. (January 2002). "Storing carbon in soil: Why and how?". Geotimes. 47 (1): 14–17. Retrieved 5 April 2018.
  9. ^ Yousaf, Balal; Liu, Guijian; Wang, Ruwei; Abbas, Qumber; Imtiaz, Muhammad; Liu, Ruijia (2016). "Investigating the biochar effects on C-mineralization and sequestration of carbon in soil compared with conventional amendments using the stable isotope (δ13C) approach". GCB Bioenergy. 9 (6): 1085–1099. doi:10.1111/gcbb.12401.
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  19. ^ a b Buis, Alan; Ramsayer, Kate; Rasmussen, Carol (12 November 2015). "A Breathing Planet, Off Balance". NASA. Retrieved 13 November 2015.
  20. ^ a b Staff (12 November 2015). "Audio (66:01) - NASA News Conference - Carbon & Climate Telecon". NASA. Retrieved 12 November 2015.
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Further reading

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