Editing Carbon cycle (section)

==Main compartments==
The carbon cycle was first described by [[Antoine Lavoisier]] and [[Joseph Priestley]], and popularised by [[Humphry Davy]].<ref name="AOW">{{cite book |last1=Holmes |first1=Richard |title=The Age of Wonder: How the Romantic Generation Discovered the Beauty and Terror of Science |date=2008 |publisher=Pantheon Books |isbn=978-0-375-42222-5 }}{{pn|date=July 2024}}</ref>  The global carbon cycle is now usually divided into the following major ''reservoirs of carbon'' (also called [[Carbon pool|carbon pools]]) interconnected by pathways of exchange:<ref>{{cite book |last1=Archer |first1=David |title=The Global Carbon Cycle |date=2010 |publisher=Princeton University Press |isbn=978-1-4008-3707-6 |pages=5–6 }}</ref>
* [[Atmosphere]]
* Terrestrial [[biosphere]]
* [[Ocean]], including [[total inorganic carbon|dissolved inorganic carbon]] and living and non-living marine biota
* [[Sediment]]s, including [[fossil fuel]]s, freshwater systems, and non-living organic material.
* Earth's interior ([[mantle (geology)|mantle]] and [[crust (geology)|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.<ref name=GlobalCarbonCycle/>
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.<ref name=Prentice_etal_2001>{{cite book |last=Prentice |first=I.C. |hdl=10067/381670151162165141 |chapter=The carbon cycle and atmospheric carbon dioxide |title=Climate change 2001: the scientific basis: contribution of Working Group I to the Third Assessment Report of the Intergouvernmental Panel on Climate Change |editor1-last=Houghton |editor1-first=J.T. |year=2001 }}</ref><ref name=U>{{cite web|title=An Introduction to the Global Carbon Cycle|publisher=University of New Hampshire|url=http://globecarboncycle.unh.edu/CarbonCycleBackground.pdf|year=2009|access-date=6 February 2016|archive-url=https://web.archive.org/web/20161008110835/http://globecarboncycle.unh.edu/CarbonCycleBackground.pdf|archive-date=8 October 2016|url-status=live|df=dmy-all}}</ref>

===Atmosphere===
{{Main|Atmospheric carbon cycle}}
[[File:NASA - A Year in the Life of Earth's CO2 x1SgmFa0r04.webm|thumb|upright=1.2|left|{{center|Computer model showing a year in the life of atmospheric carbon dioxide and how it travels around the globe{{hsp}}<ref>{{cite press release |title=A Year In The Life Of Earth's CO2 |url=https://svs.gsfc.nasa.gov/11719 |publisher=NASA's Goddard Space Flight Center |date=17 November 2014 }}</ref>}}]]
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]].<ref name=GlobalCarbonCycle>{{Cite journal |last1=Falkowski |first1=P. |last2=Scholes |first2=R. J. |last3=Boyle |first3=E. |last4=Canadell |first4=J. |last5=Canfield |first5=D. |last6=Elser |first6=J. |last7=Gruber |first7=N. |last8=Hibbard |first8=K. |last9=Högberg |first9=P. | last10 = Linder | first10 = S. |last11=MacKenzie |first11=F. T. |last12=Moore, III |first12=B. |last13=Pedersen |first13=T. |last14=Rosenthal |first14=Y. |last15=Seitzinger |first15=S. |last16=Smetacek |first16=V. |last17=Steffen |first17=W. |title=The Global Carbon Cycle: A Test of Our Knowledge of Earth as a System |doi=10.1126/science.290.5490.291 |journal=Science |volume=290 |issue=5490 |pages=291–296 |year=2000 |pmid=11030643 |bibcode=2000Sci...290..291F}}</ref> 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. Thus, carbon dioxide contributes more to the global greenhouse effect than methane.<ref name=Forster2007>{{Cite journal |title=Changes in atmospheric constituents and in radiative forcing |year=2007 |journal=Climate Change 2007: The Physical Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change |last1=Forster |first1=P. |last2=Ramawamy |first2=V. |last3=Artaxo |first3=P. |last4=Berntsen |first4=T. |last5=Betts |first5=R. |last6=Fahey |first6=D.W. |last7=Haywood |first7=J. |last8=Lean |first8=J. |author8-link=Judith Lean |last9=Lowe |first9=D.C. | last10 =  Myhre  | first10 =  G. |last11=Nganga |first11=J. |last12=Prinn |first12=R. |last13=Raga |first13=G. |last14=Schulz |first14=M. |last15=Van Dorland |first15=R. }}</ref>

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.<ref name=Planet>{{Cite journal |title=Many Planets, One Earth // Section 4: Carbon Cycling and Earth's Climate |url=http://www.learner.org/courses/envsci/unit/text.php?unit=1&secNum=4 |journal=Many Planets, One Earth |volume=4 |access-date=2012-06-24 |archive-url=https://web.archive.org/web/20120417175417/http://www.learner.org/courses/envsci/unit/text.php?unit=1&secNum=4 |archive-date=17 April 2012 |url-status=live |df=dmy-all }}</ref>
[[File:Carbon Dioxide 800kyr.svg|thumb|upright=1.35|CO<sub>2</sub> 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.<ref name="noaagi"/><ref name="GlobalCarbonCycle"/>

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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 [[Formation and evolution of the Solar System#Future|expected changes in the sun]] as it ages.  The expected increased luminosity of the Sun will likely speed up the rate of surface weathering.<ref name=swansong>{{cite journal |last1=O'Malley-James |first1=Jack T. |last2=Greaves |first2=Jane S. |last3=Raven |first3=John A. |last4=Cockell |first4=Charles S. |title=Swansong Biospheres: Refuges for life and novel microbial biospheres on terrestrial planets near the end of their habitable lifetimes |journal=[[International Journal of Astrobiology]] |date=2012 |volume=12 |issue=2 |pages=99–112  |arxiv=1210.5721 |bibcode=2013IJAsB..12...99O |doi=10.1017/S147355041200047X |s2cid=73722450 }}</ref> This will eventually cause most of the carbon dioxide in the atmosphere to be squelched into the Earth's crust as carbonate.<ref>{{cite journal |last1=Walker |first1=James C. G. |last2=Hays |first2=P. B. |last3=Kasting |first3=J. F. |title=A negative feedback mechanism for the long-term stabilization of Earth's surface temperature |journal=Journal of Geophysical Research: Oceans |date=20 October 1981 |volume=86 |issue=C10 |pages=9776–9782 |doi=10.1029/JC086iC10p09776 |bibcode=1981JGR....86.9776W }}</ref><ref name=":1">{{cite report |type=Preprint |last1=Heath |first1=Martin J. |last2=Doyle |first2=Laurance R. |title=Circumstellar Habitable Zones to Ecodynamic Domains: A Preliminary Review and Suggested Future Directions |date=2009 |arxiv=0912.2482 }}</ref><ref>{{cite journal |last1=Crockford |first1=Peter W. |last2=Bar On |first2=Yinon M. |last3=Ward |first3=Luce M. |last4=Milo |first4=Ron |last5=Halevy |first5=Itay |title=The geologic history of primary productivity |journal=Current Biology |date=November 2023 |volume=33 |issue=21 |pages=4741–4750.e5 |doi=10.1016/j.cub.2023.09.040 |pmid=37827153 |bibcode=2023CBio...33E4741C }}</ref> Once the concentration of carbon dioxide in the atmosphere falls below approximately 50 parts per million (tolerances vary among species), [[C3 carbon fixation|C<sub>3</sub> photosynthesis]] will no longer be possible.<ref name=":1" /> This has been predicted to occur 600 million years from the present, though models vary.<ref>{{cite journal |last1=Lenton |first1=Timothy M. |last2=von Bloh |first2=Werner |title=Biotic feedback extends the life span of the biosphere |journal=Geophysical Research Letters |date=May 2001 |volume=28 |issue=9 |pages=1715–1718 |doi=10.1029/2000GL012198 |bibcode=2001GeoRL..28.1715L |doi-access=free }}</ref>

Once the oceans on the Earth evaporate in about 1.1 billion years from now,<ref name="swansong"/> 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.<ref>{{cite book | last1=Brownlee | first1=Donald E. | date=2010 | chapter=Planetary habitability on astronomical time scales | title=Heliophysics: Evolving Solar Activity and the Climates of Space and Earth | editor1-first=Carolus J. | editor1-last=Schrijver | editor2-first=George L. | editor2-last=Siscoe | editor2-link=George Siscoe | chapter-url=https://books.google.com/books?id=M8NwTYEl0ngC&pg=PA94 | publisher=Cambridge University Press | isbn=978-0-521-11294-9 | page=94 |doi=10.1017/CBO9780511760358 }}</ref>

===Terrestrial biosphere===
[[File:Carbon stored in ecosystems.png|thumb|right|upright=1.35|Amount of carbon stored in Earth's various terrestrial ecosystems, in gigatonnes.<ref name="janow">{{cite report |doi=10.2737/WO-GTR-95 |doi-access=free |title=Considering Forest and Grassland Carbon in Land Management |date=2017 |last1=Janowiak |first1=M. |last2=Connelly |first2=W.J. |last3=Dante-Wood |first3=K. |last4=Domke |first4=G.M. |last5=Giardina |first5=C. |last6=Kayler |first6=Z. |last7=Marcinkowski |first7=K. |last8=Ontl |first8=T. |last9=Rodriguez-Franco |first9=C. |last10=Swanston |first10=C. |last11=Woodall |first11=C.W. |last12=Buford |first12=M. |publisher=United States Department of Agriculture, Forest Service }}</ref>]]
{{Main|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 [[soil]]s. About 500 gigatons of carbon are stored above ground in plants and other living organisms,<ref name=Prentice_etal_2001/> while soil holds approximately 1,500 gigatons of carbon.<ref>{{cite journal|last1=Rice|first1=Charles W.|title=Storing carbon in soil: Why and how?|journal=Geotimes|date=January 2002|volume=47|issue=1|pages=14&ndash;17|url=http://www.geotimes.org/jan02/feature_carbon.html|access-date=5 April 2018|archive-url=https://web.archive.org/web/20180405153123/http://www.geotimes.org/jan02/feature_carbon.html|archive-date=5 April 2018|url-status=live|df=dmy-all}}</ref> Most carbon in the terrestrial biosphere is organic carbon,<ref>{{cite journal|doi=10.1111/gcbb.12401|title=Investigating the biochar effects on C-mineralization and sequestration of carbon in soil compared with conventional amendments using the stable isotope (δ<sup>13</sup>C) approach|journal=GCB Bioenergy|volume=9|issue=6|pages=1085–1099|year=2016|last1=Yousaf|first1=Balal|last2=Liu|first2=Guijian|last3=Wang|first3=Ruwei|last4=Abbas|first4=Qumber|last5=Imtiaz|first5=Muhammad|last6=Liu|first6=Ruijia|doi-access=free}}</ref>  while about a third of [[soil carbon]] is stored in inorganic forms, such as [[calcium carbonate]].<ref name=Lal-2008>{{Cite journal |doi=10.1039/b809492f |title=Sequestration of atmospheric CO<sub>2</sub> in global carbon pools |last=Lal |first=Rattan |journal=Energy and Environmental Science |volume=1 |pages=86–100 |year=2008|issue=1 |bibcode=2008EnEnS...1...86L }}</ref> 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 to 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 CO<sub>2</sub> measurements, this feature is apparent in the [[Keeling curve]]. It is strongest in the northern [[Hemisphere of the Earth|hemisphere]] because this hemisphere has more land mass than the southern hemisphere and thus more room for ecosystems to absorb and emit carbon.

[[File:SRS1000 being used to measure soil respiration in the field..jpg|thumb|upright=1.2|left|A portable soil respiration system measuring soil CO<sub>2</sub> flux.]]

Carbon leaves the terrestrial biosphere in several ways and on different time scales. The [[combustion]] or [[Cellular respiration|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.<ref>{{cite journal |doi=10.1016/j.ecolind.2017.04.049 |title=The carbon flux of global rivers: A re-evaluation of amount and spatial patterns |journal=Ecological Indicators |volume=80 |pages=40–51 |year=2017 |last1=Li |first1=Mingxu |last2=Peng |first2=Changhui |last3=Wang |first3=Meng |last4=Xue |first4=Wei |last5=Zhang |first5=Kerou |last6=Wang |first6=Kefeng |last7=Shi |first7=Guohua |last8=Zhu |first8=Qiuan |bibcode=2017EcInd..80...40L }}</ref> 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.<ref>{{cite journal |doi=10.1038/nature08930 |pmid=20336143 |title=Temperature-associated increases in the global soil respiration record |journal=Nature |volume=464 |issue=7288 |pages=579–582 |year=2010 |last1=Bond-Lamberty |first1=Ben |last2=Thomson |first2=Allison |bibcode=2010Natur.464..579B |s2cid=4412623 }}</ref> In 2008, the global total of CO<sub>2</sub> released by soil respiration was roughly 98 billion tonnes{{citation needed|date=April 2024}}, 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).{{citation needed|date=April 2024}} 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 CO<sub>2</sub>. The length of carbon sequestering in soil is dependent on local climatic conditions and thus changes in the course of [[climate change]].<ref name="Varney">{{cite journal |last1=Varney |first1=Rebecca M. |last2=Chadburn |first2=Sarah E. |last3=Friedlingstein |first3=Pierre |last4=Burke |first4=Eleanor J. |last5=Koven |first5=Charles D. |last6=Hugelius |first6=Gustaf |last7=Cox |first7=Peter M. |title=A spatial emergent constraint on the sensitivity of soil carbon turnover to global warming |journal=Nature Communications |date=2 November 2020 |volume=11 |issue=1 |page=5544 |doi=10.1038/s41467-020-19208-8 |pmid=33139706 |pmc=7608627 |bibcode=2020NatCo..11.5544V }}</ref> <!-- From pre-industrial era to 2010, the terrestrial biosphere represented a net source of atmospheric CO<sub>2</sub> prior to 1940, switching subsequently to a net sink.<ref>{{cite journal |last1=Huang |first1=Junling |last2=McElroy |first2=Michael B. |title=The contemporary and historical budget of atmospheric CO 2 1 This article is part of a Special Issue that honours the work of Dr. Donald M. Hunten FRSC who passed away in December 2010 after a very illustrious career. |journal=Canadian Journal of Physics |date=August 2012 |volume=90 |issue=8 |pages=707–716 |doi=10.1139/p2012-033 }}</ref> -->

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{| class=wikitable align="right" style="text-align:left; font-size:0.9em"
|+ Size of major carbon pools on the Earth (year 2000 estimates)<ref name=GlobalCarbonCycle/>
! Pool !! Quantity<br>(gigatons)
|-
| Atmosphere || 720
|-
| Ocean (total) || 38,400
|-
| style="padding-left: 2em" | Total inorganic || 37,400
|-
| style="padding-left: 2em" | Total organic || 1,000
|-
| style="padding-left: 2em" | Surface layer || 670
|-
| style="padding-left: 2em" | Deep layer || 36,730
|-
| [[Lithosphere]] || 
|-
| style="padding-left: 2em" | Sedimentary carbonates || > 60,000,000
|-
| style="padding-left: 2em" | [[Kerogen]]s || 15,000,000
|-
| Terrestrial biosphere (total) || 2,000
|-
| style="padding-left: 2em" | Living biomass || 600 – 1,000
|-
| style="padding-left: 2em" | Dead biomass || 1,200
|-
| Aquatic biosphere || 1 – 2
|-
| Fossil fuels (total) || 4,130
|-
| style="padding-left: 2em" | Coal || 3,510
|-
| style="padding-left: 2em" | Oil || 230
|-
| style="padding-left: 2em" | Gas || 140
|-
| style="padding-left: 2em" | Other ([[peat]]) || 250
|}

===Ocean===
{{Main|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<ref name=Sarmiento_and_Gruber_2006>{{cite book |last1=Sarmiento |first1=Jorge L. |last2=Gruber |first2=Nicolas |title=Ocean Biogeochemical Dynamics |date=2006 |publisher=Princeton University Press |isbn=978-0-691-01707-5 }}{{pn|date=October 2024}}</ref> 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<ref name=GlobalCarbonCycle/>—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.<ref name=GlobalCarbonCycle/>

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.<ref name=Prentice_etal_2001/> 

Oceans are basic (with a current [[Ocean#pH|pH value]] of 8.1 to 8.2). The increase in atmospheric CO<sub>2</sub> shifts the pH of the ocean towards neutral in a process called [[ocean acidification]]. Oceanic absorption of CO<sub>2</sub> is one of the most important forms of [[carbon sequestering]]. The projected rate of pH reduction could slow the biological precipitation of [[calcium carbonate]]s, thus decreasing the ocean's capacity to absorb CO<sub>2</sub>.<ref name=Klyepas1999>{{Cite journal |last1=Kleypas |first1=J. A. |last2=Buddemeier |first2=R. W. |last3=Archer |first3=D. |last4=Gattuso |first4=J. P. |last5=Langdon |first5=C. |last6=Opdyke |first6=B. N. |title=Geochemical Consequences of Increased Atmospheric Carbon Dioxide on Coral Reefs |doi=10.1126/science.284.5411.118 |journal=Science |volume=284 |issue=5411 |pages=118–120 |year=1999 |pmid=10102806 |bibcode=1999Sci...284..118K}}</ref><ref name=Langdon2000>{{Cite journal |last1=Langdon |first1=C. |last2=Takahashi |first2=T. |last3=Sweeney |first3=C. |last4=Chipman |first4=D. |last5=Goddard |first5=J. |last6=Marubini |first6=F. |last7=Aceves |first7=H. |last8=Barnett |first8=H. |last9=Atkinson |first9=M. J. |doi=10.1029/1999GB001195 |title=Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef |journal=Global Biogeochemical Cycles |volume=14 |issue=2 |pages=639 |year=2000 |bibcode=2000GBioC..14..639L|s2cid=128987509 |doi-access=free }}</ref>

===Geosphere===
{{Main|Carbonate–silicate cycle}}
[[File:Global carbon stocks.png|thumb|left|upright=1.8|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.<ref name="janow"/>]]
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.<ref name=NASA>{{Cite web |title=The Slow Carbon Cycle |url=http://earthobservatory.nasa.gov/Features/CarbonCycle/page2.php |publisher=NASA |access-date=2012-06-24 |archive-url=https://web.archive.org/web/20120616151904/http://earthobservatory.nasa.gov/Features/CarbonCycle/page2.php |archive-date=16 June 2012 |url-status=live |df=dmy-all |date=2011-06-16 }}</ref>

Most of the Earth's carbon is stored inertly in the Earth's [[lithosphere]].<ref name=GlobalCarbonCycle/> Much of the carbon stored in the Earth's mantle was stored there when the Earth formed.<ref name=DiVenere2012>[http://www.columbia.edu/~vjd1/carbon.htm The Carbon Cycle and Earth's Climate] {{Webarchive|url=https://web.archive.org/web/20030623195122/http://www.columbia.edu/~vjd1/carbon.htm |date=23 June 2003 }} Information sheet for Columbia University Summer Session 2012 Earth and Environmental Sciences Introduction to Earth Sciences I</ref> Some of it was deposited in the form of organic carbon from the biosphere.<ref name=Berner1999>{{cite journal |last1=Berner |first1=Robert A. |title=A New Look at the Long-term Carbon Cycle|journal=GSA Today |date=November 1999 |volume=9 |issue=11 |pages=1&ndash;6 |url=https://www.geosociety.org/gsatoday/archive/9/11/pdf/gt9911.pdf |archive-url=https://web.archive.org/web/20190213183546/https://www.geosociety.org/gsatoday/archive/9/11/pdf/gt9911.pdf |archive-date=2019-02-13 |url-status=live }}</ref> 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 [[kerogen]]s 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.<ref name=NASA/>

Carbon can leave the geosphere in several ways. Carbon dioxide is released during the [[metamorphism]] of carbonate rocks when they are [[Subduction|subducted]] into the Earth's mantle. This carbon dioxide can be released into the atmosphere and ocean through [[Volcanism|volcanoes]] and [[Hotspot (geology)|hotspots]].<ref name=DiVenere2012/> 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.

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