Editing Carbon cycle (section)

===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>