View source for Carbon dioxide in Earth's atmosphere

{{Short description|Atmospheric constituent and greenhouse gas}}
{{Use dmy dates|date=October 2020}}
{{Use American English|date=January 2019}}
[[File:Mauna Loa CO2 monthly mean concentration.svg|thumb|upright=1.5|Atmospheric {{CO2}} concentration measured at [[Mauna Loa Observatory]] in Hawaii from 1958 to 2023 (also called the [[Keeling Curve]]). The rise in {{CO2}} over that time period is clearly visible. The concentration is expressed as μmole per mole, or [[Parts-per notation|ppm]]. ]]

In [[Earth's atmosphere]], [[carbon dioxide]] is a [[trace gas]] that plays an integral part in the [[greenhouse effect]], [[carbon cycle]], [[photosynthesis]] and [[oceanic carbon cycle]]. It is one of three main [[Greenhouse gas|greenhouse gases]] in the atmosphere of [[Earth]]. The concentration of carbon dioxide ({{CO2}}) in the atmosphere reached 427 [[Parts per million|ppm]] (0.0427%) on a [[mole fraction|molar basis]] in 2024, representing 3341 gigatonnes of {{CO2}}.<!--If we change the ppms, we should multiply by the mass of the atmosphere divided by the average molecular weight of air and multiplied by the molecular weight of CO2 to get the new value for the mass of CO2 in the atmosphere. That is, pps times (5148)(44.01/28.96)=7.823 gigatonnes/ppm.--><ref>{{Cite web |last=Change |first=NASA Global Climate |title=Carbon Dioxide Concentration {{!}} NASA Global Climate Change |url=https://climate.nasa.gov/vital-signs/carbon-dioxide/?intent=121 |access-date=2024-11-03 |website=Climate Change: Vital Signs of the Planet |language=en}}</ref> This is an increase of 50% since the start of the [[Industrial Revolution]], up from 280 ppm during the 10,000 years prior to the mid-18th century.<ref name="Eggleton-2013">{{cite book |last1=Eggleton |first1=Tony |url=https://books.google.com/books?id=jeSwRly2M_cC&q=280&pg=PA52 |title=A Short Introduction to Climate Change |date=2013 |publisher=Cambridge University Press |isbn=9781107618763 |page=52 |access-date=14 March 2023 |archive-date=14 March 2023 |archive-url=https://web.archive.org/web/20230314104202/https://books.google.com/books?id=jeSwRly2M_cC&q=280&pg=PA52 |url-status=live }}</ref><ref name="NOAA-June2022">{{Cite web |date=3 June 2022 |title=Carbon dioxide now more than 50% higher than pre-industrial levels |url=https://www.noaa.gov/news-release/carbon-dioxide-now-more-than-50-higher-than-pre-industrial-levels |url-status=live |archive-url=https://web.archive.org/web/20220605004925/https://www.noaa.gov/news-release/carbon-dioxide-now-more-than-50-higher-than-pre-industrial-levels |archive-date=5 June 2022 |access-date=2022-06-14 |publisher=[[National Oceanic and Atmospheric Administration]]}}</ref><ref name="NOAA-2020">{{Cite web |title=The NOAA Annual Greenhouse Gas Index (AGGI) – An Introduction |url=https://www.esrl.noaa.gov/gmd/aggi/ |url-status=live |archive-url=https://web.archive.org/web/20201127013113/https://www.esrl.noaa.gov/gmd/aggi/ |archive-date=27 November 2020 |access-date=2020-12-18 |publisher=[[NOAA]] Global Monitoring Laboratory/Earth System Research Laboratories}}</ref> The increase [[Climate change attribution|is due to human activity]].<ref name="Etheridge-1996">{{cite journal |last=Etheridge |first=D.M. |author2=L.P. Steele |author3=R.L. Langenfelds |author4=R.J. Francey |author5=J.-M. Barnola |author6=V.I. Morgan |year=1996 |title=Natural and anthropogenic changes in atmospheric {{CO2}} over the last 1000 years from air in Antarctic ice and firn |journal=Journal of Geophysical Research |volume=101 |issue=D2 |pages=4115–28 |bibcode=1996JGR...101.4115E |doi=10.1029/95JD03410 |s2cid=19674607 |issn=0148-0227}}</ref> 

The current increase in {{CO2}} concentrations is primarily driven by the burning of [[Fossil fuel|fossil fuels]].<ref name="IPCC-AR6">IPCC (2022) [https://www.ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_SPM.pdf Summary for policy makers] {{Webarchive|url=https://web.archive.org/web/20230312040126/https://www.ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_SPM.pdf|date=12 March 2023}} in [https://www.ipcc.ch/report/ar6/wg3/ Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] {{Webarchive|url=https://web.archive.org/web/20220802125242/https://www.ipcc.ch/report/ar6/wg3/|date=2 August 2022}}, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA</ref> Other significant human activities that emit {{CO2}} include [[cement]] production, [[deforestation]], and [[biomass]] burning. The increase in atmospheric concentrations of {{CO2}} and other long-lived greenhouse gases such as [[methane]] increase the absorption and emission of infrared radiation by the atmosphere. This has led to a [[Instrumental temperature record|rise in average global temperature]] and [[ocean acidification]]. Another direct effect is the [[CO2 fertilization effect|{{CO2}} fertilization effect]]. The increase in atmospheric concentrations of {{CO2}} causes a range of further [[effects of climate change]] on the environment and human living conditions.

Carbon dioxide is a greenhouse gas. It absorbs and emits [[infrared radiation]] at its two infrared-active vibrational frequencies. The two [[wavelength]]s are 4.26&nbsp;[[μm]] (2,347&nbsp;cm<sup>−1</sup>) (asymmetric stretching [[infrared spectroscopy|vibrational mode]]) and 14.99&nbsp;μm (667&nbsp;cm<sup>−1</sup>) (bending vibrational mode). {{CO2}} plays a significant role in influencing [[Earth]]'s surface temperature through the greenhouse effect.<ref>{{cite journal |last1=Petty |first1=G.W. |year=2004 |title=A First Course in Atmospheric Radiation |journal=Eos Transactions |volume=85 |issue=36 |pages=229–51 |bibcode=2004EOSTr..85..341P |doi=10.1029/2004EO360007 |doi-access=free}}</ref> Light emission from the Earth's surface is most intense in the infrared region between 200 and 2500&nbsp;cm<sup>−1</sup>,<ref>{{cite book |last1=Atkins |first1=P. |author-link=Peter Atkins |url=https://archive.org/details/atkinsphysicalch00pwat/page/462 |title=Atkins' Physical Chemistry |last2=de Paula |first2=J. |date=2006 |publisher=W.H. Freeman |isbn=978-0-7167-8759-4 |edition=8th |page=[https://archive.org/details/atkinsphysicalch00pwat/page/462 462]}}</ref> as opposed to light emission from the much hotter [[Sun]] which is most intense in the visible region. Absorption of infrared light at the vibrational frequencies of atmospheric {{CO2}} traps energy near the surface, warming the surface of Earth and its lower atmosphere. Less energy reaches the upper atmosphere, which is therefore cooler because of this absorption.<ref name="UCAR-2012">{{cite web |date=2012 |title=Carbon Dioxide Absorbs and Re-emits Infrared Radiation |url=https://scied.ucar.edu/carbon-dioxide-absorbs-and-re-emits-infrared-radiation |url-status=live |archive-url=https://web.archive.org/web/20170921012448/https://scied.ucar.edu/carbon-dioxide-absorbs-and-re-emits-infrared-radiation |archive-date=21 September 2017 |access-date=9 September 2017 |publisher=UCAR Center for Science Education}}</ref>

The present atmospheric concentration of {{CO2}} is the highest for 14 million years.<ref name="Ahmed-2023">{{Cite web |last=Ahmed |first=Issam |title=Current carbon dioxide levels last seen 14 million years ago |url=https://phys.org/news/2023-12-current-carbon-dioxide-million-years.html |access-date=2024-02-08 |website=phys.org |language=en}}</ref> Concentrations of {{CO2}} in the atmosphere were as high as 4,000 ppm during the [[Cambrian|Cambrian period]] about 500 million years ago, and as low as 180 ppm during the [[Quaternary glaciation]] of the last two million years.<ref name="Eggleton-2013" /> Reconstructed temperature records for the last 420 million years indicate that atmospheric {{CO2}} concentrations peaked at approximately 2,000 ppm. This peak happened during the [[Devonian]] period (400 million years ago). Another peak occurred in the [[Triassic]] period (220–200 million years ago).<ref>{{cite web |title=Climate and {{CO2}} in the Atmosphere |url=http://earthguide.ucsd.edu/virtualmuseum/climatechange2/07_1.shtml |url-status=live |archive-url=https://web.archive.org/web/20181006151450/http://earthguide.ucsd.edu/virtualmuseum/climatechange2/07_1.shtml |archive-date=6 October 2018 |access-date=10 October 2007}}</ref>

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==Current concentration and future trends==
{{See also|Climate change|Climate variability and change|Atmospheric methane|Holocene#Climate|Quaternary#Climate|label 4=Holocene climate|label 5=Quaternary climate}}
[[File:Carbon Sources and Sinks.svg|thumb|upright=1.35|Between 1850 and 2019 the [[Global Carbon Project]] estimates that about 2/3rds of excess carbon dioxide emissions have been caused by burning fossil fuels, and a little less than half of that has stayed in the atmosphere.]]

=== Current situation ===
Since the start of the [[Industrial Revolution]], atmospheric {{co2}} concentration have been increasing, causing [[global warming]] and [[ocean acidification]].<ref name="carbon budget 2022">{{Cite journal |last1=Friedlingstein |first1=Pierre |last2=O'Sullivan |first2=Michael |last3=Jones |first3=Matthew W. |last4=Andrew |first4=Robbie M. |last5=Gregor |first5=Luke |last6=Hauck |first6=Judith |last7=Le Quéré |first7=Corinne |last8=Luijkx |first8=Ingrid T. |last9=Olsen |first9=Are |last10=Peters |first10=Glen P. |last11=Peters |first11=Wouter |last12=Pongratz |first12=Julia |last13=Schwingshackl |first13=Clemens |last14=Sitch |first14=Stephen |last15=Canadell |first15=Josep G. |date=2022-11-11 |title=Global Carbon Budget 2022 |journal=Earth System Science Data |volume=14 |issue=11 |pages=4811–4900 |bibcode=2022ESSD...14.4811F |doi=10.5194/essd-14-4811-2022 |doi-access=free|hdl=20.500.11850/594889 |hdl-access=free }} {{Creative Commons text attribution notice|cc=by4|from this source=yes}}</ref> In October 2023 the average level of {{CO2}} in Earth's atmosphere, adjusted for seasonal variation, was 422.17 [[parts per million]] by volume (ppm).<ref>"Parts per million" refers to the number of carbon dioxide molecules per million molecules of dry air. {{Cite web |title=Carbon Dioxide LATEST MEASUREMENT|publisher=NASA Global Climate Change |url=https://climate.nasa.gov/vital-signs/carbon-dioxide|website=Climate Change: Vital Signs of the Planet |archive-date=17 April 2022 |archive-url=https://web.archive.org/web/20220417141013/https://climate.nasa.gov/vital-signs/carbon-dioxide/ |url-status=live }} Updated monthly.</ref> Figures are published monthly by the [[National Oceanic and Atmospheric Administration|National Oceanic & Atmospheric Administration]] (NOAA).<ref>{{Cite web
| title = Global Monitoring Laboratory - Trends in Atmospheric Carbon Dioxide
| publisher = National Oceanic & Atmospheric Administration  
| url = https://gml.noaa.gov/ccgg/trends/
}} Latest figure, and graphs of trend; frequently updated</ref><ref>{{cite web | title=Table of atmospheric CO₂ since 1958, updated monthly |publisher = National Oceanic & Atmospheric Administration|url=https://gml.noaa.gov/webdata/ccgg/trends/co2/co2_mm_mlo.txt|quote=The actual figures fluctuate month-by-month throughout the year, so figures for the same month of different years should be compared, or a seasonally corrected figure used.}}</ref> The value had been about 280 ppm during the 10,000 years up to the mid-18th century.<ref name="Eggleton-2013" /><ref name="NOAA-June2022" /><ref name="NOAA-2020" />

Each part per million of {{co2}} in the atmosphere represents approximately 2.13 [[gigatonne]]s of carbon, or 7.82 gigatonnes of {{CO2}}.<ref>{{cite web |date=18 July 2020 |title=Conversion Tables |url=https://cdiac.ess-dive.lbl.gov/pns/convert.html |url-status=live |archive-url=https://web.archive.org/web/20170927015828/http://cdiac.ess-dive.lbl.gov/pns/convert.html |archive-date=27 September 2017 |access-date=18 July 2020 |website=Carbon Dioxide Information Analysis Center |publisher=Oak Ridge National Laboratory}} [http://cdiac.ornl.gov/pns/convert.html Alt URL] {{Webarchive|url=https://web.archive.org/web/20160223232340/http://cdiac.ornl.gov/pns/convert.html|date=23 February 2016}}</ref>

It was pointed out in 2021 that "the current rates of increase of the concentration of the major greenhouse gases (carbon dioxide, methane and nitrous oxide) are unprecedented over at least the last 800,000 years".<ref name="Eyring-2021" />{{rp|515}}

{{As of|2024|post=,}} it is estimated that 2,650 gigatonnes of {{CO2}} have been emitted by human activity since 1850, with annual emissions of 42 gigatonnes per year. About 1,050 gigatonnes remain in the atmosphere following absorption by oceans and land.<ref name=GlobalCarbonBudget2024>{{cite |last=Friedlingstein |first=Pierre |last2=O'Sullivan |first2=Michael |last3=Jones |first3=Matthew W. |last4=Andrew |first4=Robbie M. |last5=Hauck |first5=Judith |last6=Landschützer |first6=Peter |last7=Le Quéré |first7=Corinne |last8=Li |first8=Hongmei |last9=Luijkx |first9=Ingrid T. |last10=Olsen |first10=Are |last11=Peters |first11=Glen P. |last12=Peters |first12=Wouter |last13=Pongratz |first13=Julia |last14=Schwingshackl |first14=Clemens |last15=Sitch |first15=Stephen |last16=Canadell |first16=Josep G. |last17=Ciais |first17=Philippe |last18=Jackson |first18=Robert B. |last19=Alin |first19=Simone R. |last20=Arneth |first20=Almut |last21=Arora |first21=Vivek |last22=Bates |first22=Nicholas R. |last23=Becker |first23=Meike |last24=Bellouin |first24=Nicolas |last25=Berghoff |first25=Carla F. |last26=Bittig |first26=Henry C. |last27=Bopp |first27=Laurent |last28=Cadule |first28=Patricia |last29=Campbell |first29=Katie |last30=Chamberlain |first30=Matthew A. |last31=Chandra |first31=Naveen |last32=Chevallier |first32=Frédéric |last33=Chini |first33=Louise P. |last34=Colligan |first34=Thomas |last35=Decayeux |first35=Jeanne |last36=Djeutchouang |first36=Laique |last37=Dou |first37=Xinyu |last38=Duran Rojas |first38=Carolina |last39=Enyo |first39=Kazutaka |last40=Evans |first40=Wiley |last41=Fay |first41=Amanda |last42=Feely |first42=Richard A. |last43=Ford |first43=Daniel J. |last44=Foster |first44=Adrianna |last45=Gasser |first45=Thomas |last46=Gehlen |first46=Marion |last47=Gkritzalis |first47=Thanos |last48=Grassi |first48=Giacomo |last49=Gregor |first49=Luke |last50=Gruber |first50=Nicolas |last51=Gürses |first51=Özgür |last52=Harris |first52=Ian |last53=Hefner |first53=Matthew |last54=Heinke |first54=Jens |last55=Hurtt |first55=George C. |last56=Iida |first56=Yosuke |last57=Ilyina |first57=Tatiana |last58=Jacobson |first58=Andrew R. |last59=Jain |first59=Atul |last60=Jarníková |first60=Tereza |last61=Jersild |first61=Annika |last62=Jiang |first62=Fei |last63=Jin |first63=Zhe |last64=Kato |first64=Etsushi |last65=Keeling |first65=Ralph F. |last66=Klein Goldewijk |first66=Kees |last67=Knauer |first67=Jürgen |last68=Korsbakken |first68=Jan Ivar |last69=Lauvset |first69=Siv K. |last70=Lefèvre |first70=Nathalie |last71=Liu |first71=Zhu |last72=Liu |first72=Junjie |last73=Ma |first73=Lei |last74=Maksyutov |first74=Shamil |last75=Marland |first75=Gregg |last76=Mayot |first76=Nicolas |last77=McGuire |first77=Patrick |last78=Metzl |first78=Nicolas |last79=Monacci |first79=Natalie M. |last80=Morgan |first80=Eric J. |last81=Nakaoka |first81=Shin-Ichiro |last82=Neill |first82=Craig |last83=Niwa |first83=Yosuke |last84=Nützel |first84=Tobias |last85=Olivier |first85=Lea |last86=Ono |first86=Tsuneo |last87=Palmer |first87=Paul I. |last88=Pierrot |first88=Denis |last89=Qin |first89=Zhangcai |last90=Resplandy |first90=Laure |last91=Roobaert |first91=Alizée |last92=Rosan |first92=Thais M. |last93=Rödenbeck |first93=Christian |last94=Schwinger |first94=Jörg |last95=Smallman |first95=T. Luke |last96=Smith |first96=Stephen |last97=Sospedra-Alfonso |first97=Reinel |last98=Steinhoff |first98=Tobias |last99=Sun |first99=Qing |last100=Sutton |first100=Adrienne J. |last101=Séférian |first101=Roland |last102=Takao |first102=Shintaro |last103=Tatebe |first103=Hiroaki |last104=Tian |first104=Hanqin |last105=Tilbrook |first105=Bronte |last106=Torres |first106=Olivier |last107=Tourigny |first107=Etienne |last108=Tsujino |first108=Hiroyuki |last109=Tubiello |first109=Francesco |last110=Van Der Werf |first110=Guido |last111=Wanninkhof |first111=Rik |last112=Wang |first112=Xuhui |last113=Yang |first113=Dongxu |last114=Yang |first114=Xiaojuan |last115=Yu |first115=Zhen |last116=Yuan |first116=Wenping |last117=Yue |first117=Xu |last118=Zaehle |first118=Sönke |last119=Zeng |first119=Ning |last120=Zeng |first120=Jiye |title=Global Carbon Budget 2024 |date=13 November 2024 |doi=10.5194/essd-2024-519 |doi-access=free}}</ref><!--Source (Table 8) provides emissions as GtC: Total emissions = 655 GtC in 2019, 725 GtC in 2024; G_ATM = 260 GtC in 2019, 290 GtC in 2024. As per Table 1 of the source, multiply by 44.01/12.011 = 3.664 to convert to GtCO2. Yearly anthropogenic CO2 emissions were predicted to be 43.1±3.2 GtCO2 for 2019 and 41.6 GtCO2 for 2024.-->

Some fraction (a projected 20–35%) of the fossil carbon transferred thus far will persist in the atmosphere as elevated {{CO2}} levels for many thousands of years after these carbon transfer activities begin to subside.<ref>{{cite journal |vauthors=Archer D |year=2009 |title=Atmospheric lifetime of fossil fuel carbon dioxide |url=https://orbi.uliege.be/handle/2268/12933 |url-status=live |journal=Annual Review of Earth and Planetary Sciences |volume=37 |issue=1 |pages=117–34 |bibcode=2009AREPS..37..117A |doi=10.1146/annurev.earth.031208.100206 |archive-url=https://web.archive.org/web/20210224064427/https://orbi.uliege.be/handle/2268/12933 |archive-date=24 February 2021 |access-date=7 March 2021 |hdl=2268/12933}}</ref><ref>{{Cite journal |display-authors=6 |vauthors=Joos F, Roth R, Fuglestvedt JS, Peters GP, Enting IG, Von Bloh W, Brovkin V, Burke EJ, Eby M, Edwards NR, Friedrich T |year=2013 |title=Carbon dioxide and climate impulse response functions for the computation of greenhouse gas metrics: A multi-model analysis |url=https://www.atmos-chem-phys.net/13/2793/2013/ |url-status=live |journal=Atmospheric Chemistry and Physics |volume=13 |issue=5 |pages=2793–2825 |doi=10.5194/acpd-12-19799-2012 |archive-url=https://web.archive.org/web/20200722130540/https://www.atmos-chem-phys.net/13/2793/2013/ |archive-date=22 July 2020 |access-date=7 March 2021 |doi-access=free |hdl-access=free |hdl=20.500.11850/58316}}</ref>

=== Annual and regional fluctuations ===
Atmospheric {{CO2}} concentrations fluctuate slightly with the seasons, falling during the [[Northern Hemisphere]] spring and summer as plants consume the gas and rising during northern autumn and winter as plants go dormant or die and decay. The level drops by about 6 or 7 ppm (about 50&nbsp;Gt) from May to September during the Northern Hemisphere's growing season, and then goes up by about 8 or 9 ppm. The [[Northern Hemisphere]] dominates the annual cycle of {{CO2}} concentration because it has much greater land area and [[Biomass (ecology)|plant biomass]] in mid-latitudes (30-60 degrees) than the [[Southern Hemisphere]]. Concentrations reach a peak in May as the Northern Hemisphere spring greenup begins, and decline to a minimum in October, near the end of the growing season.<ref name="Atmospheric Carbon Dioxide Growth Rate">{{cite web |author=Rasmussen, Carl Edward |title=Atmospheric Carbon Dioxide Growth Rate |url=http://mlg.eng.cam.ac.uk/carl/words/carbon.html |access-date=14 March 2023 |archive-date=14 March 2023 |archive-url=https://web.archive.org/web/20230314010417/https://mlg.eng.cam.ac.uk/carl/words/carbon.html |url-status=live }}</ref><ref>{{cite web |title=Frequently Asked Questions |url=http://cdiac.ornl.gov/pns/faq.html |url-status=dead |archive-url=https://web.archive.org/web/20110817044713/http://cdiac.ornl.gov/pns/faq.html |archive-date=17 August 2011 |access-date=13 June 2007 |publisher=Carbon Dioxide Information Analysis Center (CDIAC)}}</ref>

Concentrations also vary on a regional basis, most strongly [[Planetary boundary layer|near the ground]] with much smaller variations aloft. In urban areas concentrations are generally higher<ref>{{Cite journal |vauthors=George K, Ziska LH, Bunce JA, Quebedeaux B |year=2007 |title=Elevated atmospheric CO<sub>2</sub> concentration and temperature across an urban–rural transect |url=https://zenodo.org/record/1258774 |url-status=live |journal=Atmospheric Environment |volume=41 |issue=35 |pages=7654–7665 |bibcode=2007AtmEn..41.7654G |doi=10.1016/j.atmosenv.2007.08.018 |archive-url=https://web.archive.org/web/20191015185617/https://zenodo.org/record/1258774 |archive-date=15 October 2019 |access-date=12 September 2019}}</ref> and indoors they can reach 10 times background levels.

=== Measurements and predictions made in the recent past ===

* Data from 2009 found that the global mean {{CO2}} concentration was rising at a rate of approximately 2 ppm/year and accelerating.<ref name="Carbon Trends">{{cite web |author=Tans, Pieter |title=Trends in Carbon Dioxide |url=http://www.esrl.noaa.gov/gmd/ccgg/trends/ |access-date=2009-12-11 |publisher=[[National Oceanic and Atmospheric Administration|NOAA]]/[[Earth System Research Laboratories|ESRL]] |archive-date=25 January 2013 |archive-url=https://web.archive.org/web/20130125014026/http://www.esrl.noaa.gov/gmd/ccgg/trends/ |url-status=live }}</ref><ref name="carbon budget">{{cite web |title=Carbon Budget 2009 Highlights |url=http://www.globalcarbonproject.org/carbonbudget/09/hl-full.htm |url-status=dead |archive-url=https://web.archive.org/web/20111216001323/http://www.globalcarbonproject.org/carbonbudget/09/hl-full.htm |archive-date=16 December 2011 |access-date=2012-11-02 |publisher=globalcarbonproject.org}}</ref>
* The daily average concentration of atmospheric {{co2}} at [[Mauna Loa Observatory]] first exceeded 400 ppm on 10 May 2013<ref name="bbc.co.uk">{{cite news|url=https://www.bbc.co.uk/news/science-environment-22486153|work=BBC|title=Carbon dioxide passes symbolic mark|date=10 May 2013|access-date=10 May 2013|archive-date=23 May 2019|archive-url=https://web.archive.org/web/20190523133252/https://www.bbc.co.uk/news/science-environment-22486153|url-status=live}}</ref><ref name="NOAA-2019">{{cite web | url=http://www.esrl.noaa.gov/gmd/ccgg/trends/weekly.html | title=Up-to-date weekly average CO<sub>2</sub> at Mauna Loa | access-date=2019-06-01 | publisher=[[NOAA]] | archive-date=24 May 2019 | archive-url=https://web.archive.org/web/20190524180525/https://www.esrl.noaa.gov/gmd/ccgg/trends/weekly.html | url-status=live }}</ref> although this concentration had already been reached in the Arctic in June 2012.<ref>{{cite news | url = https://www.theguardian.com/environment/2012/jun/01/record-greenhouse-gas-trouble-scientists | newspaper = [[The Guardian]] | access-date = 11 May 2013 | date = 1 June 2012 | title = Greenhouse gas levels pass symbolic 400ppm CO<sub>2</sub> milestone | agency = Associated Press | archive-date = 22 January 2014 | archive-url = https://web.archive.org/web/20140122000841/http://www.theguardian.com/environment/2012/jun/01/record-greenhouse-gas-trouble-scientists | url-status = live }}</ref> Data from 2013 showed that the concentration of carbon dioxide in the atmosphere is this high "for the first time in 55 years of measurement—and probably more than 3 million years of Earth history."<ref>{{cite web |last=Kunzig |first=Robert |date=2013-05-09 |title=Climate Milestone: Earth's CO<sub>2</sub> Level Passes 400 ppm |url=http://news.nationalgeographic.com/news/energy/2013/05/130510-earth-co2-milestone-400-ppm/ |access-date=2013-05-12 |work=[[National Geographic (magazine)|National Geographic]] |archive-date=15 December 2013 |archive-url=https://web.archive.org/web/20131215081625/http://news.nationalgeographic.com/news/energy/2013/05/130510-earth-co2-milestone-400-ppm/ |url-status=dead }}</ref>
* As of 2018, {{CO2}} concentrations were measured to be 410 ppm.<ref name="Carbon Trends" /><ref name="NOAA-2013">{{cite web|title=Trends in Atmospheric Carbon Dioxide|url=https://www.esrl.noaa.gov/gmd/ccgg/trends/|website=Earth System Research Laboratories|publisher=[[NOAA]]|access-date=14 March 2023|archive-date=25 January 2013|archive-url=https://web.archive.org/web/20130125014026/http://www.esrl.noaa.gov/gmd/ccgg/trends/|url-status=live}}</ref>

== Measurement techniques ==
{{See also|Total Carbon Column Observing Network|Space-based measurements of carbon dioxide}}
[[File:Global distribution of Carbon Dioxide.jpg|thumb|Carbon dioxide observations from 2008 to 2017 showing the seasonal variations and the difference between northern and southern hemispheres]]

The concentrations of carbon dioxide in the atmosphere are expressed as parts per million by volume (abbreviated as ppmv, or ppm(v), or just ppm). To convert from the usual ppmv units to ppm mass (abbreviated as ppmm, or ppm(m)), multiply by the ratio of the [[molar mass]] of CO<sub>2</sub> to that of air, i.e. times 1.52 (44.01 divided by 28.96).

The first reproducibly accurate measurements of atmospheric CO<sub>2</sub> were from flask sample measurements made by [[Charles David Keeling|Dave Keeling]] at [[California Institute of Technology|Caltech]] in the 1950s.<ref>{{Cite web |title=The Early Keeling Curve {{pipe}} Scripps {{CO2}} Program |url=https://scrippsco2.ucsd.edu/history_legacy/early_keeling_curve.html |website=scrippsco2.ucsd.edu |access-date=14 March 2023 |archive-date=8 October 2022 |archive-url=https://web.archive.org/web/20221008043137/https://scrippsco2.ucsd.edu/history_legacy/early_keeling_curve.html |url-status=live }}</ref> Measurements at Mauna Loa have been ongoing since 1958. Additionally, measurements are also made at many other sites around the world. Many measurement sites are part of larger global networks. Global network data are often made publicly available.

=== Data networks ===
There are several surface measurement (including flasks and continuous in situ) networks including [[National Oceanic and Atmospheric Administration|NOAA]]/[[Earth System Research Laboratories|ERSL]],<ref>{{Cite web |url=http://www.esrl.noaa.gov/gmd/ccgg/index.html |title=NOAA CCGG page Retrieved 2 March 2016 |access-date=14 March 2023 |archive-date=11 August 2011 |archive-url=https://web.archive.org/web/20110811160744/http://www.esrl.noaa.gov/gmd/ccgg/index.html |url-status=live }}</ref> WDCGG,<ref>[http://ds.data.jma.go.jp/gmd/wdcgg/ WDCGG webpage] {{Webarchive|url=https://web.archive.org/web/20160406090043/http://ds.data.jma.go.jp/gmd/wdcgg/|date=6 April 2016}} Retrieved 2 March 2016</ref> and RAMCES.<ref>[https://www.lsce.ipsl.fr/en/cycles-transferts/icos-ramces/ RAMCES webpage] </ref> The NOAA/ESRL Baseline Observatory Network, and the [[Scripps Institution of Oceanography]] Network<ref>{{Cite web |url=http://cdiac.ornl.gov/trends/co2/ |title=CDIAC CO2 page Retrieved 9 February 2016 |access-date=14 March 2023 |archive-date=13 August 2011 |archive-url=https://web.archive.org/web/20110813142008/http://cdiac.ornl.gov/trends/co2/ |url-status=live }}</ref> data are hosted at the [[Carbon Dioxide Information Analysis Center|CDIAC]] at [[Oak Ridge National Laboratory|ORNL]]. The World Data Centre for Greenhouse Gases (WDCGG), part of [[Global Atmospheric Watch|GAW]], data are hosted by the [[Japan Meteorological Agency|JMA]]. The Reseau Atmospherique de Mesure des Composes an Effet de Serre database (RAMCES) is part of [[Institute Pierre Simon Laplace|IPSL]].

From these measurements, further products are made which integrate data from the various sources. These products also address issues such as data discontinuity and sparseness. GLOBALVIEW-{{CO2}} is one of these products.<ref>{{Cite web |url=http://www.esrl.noaa.gov/gmd/ccgg/globalview/co2/co2_intro.html |title=GLOBALVIEW-CO2 information page. Retrieved 9 February 2016 |access-date=14 March 2023 |archive-date=31 January 2020 |archive-url=https://web.archive.org/web/20200131032546/https://www.esrl.noaa.gov/gmd/ccgg/globalview/co2/co2_intro.html |url-status=live }}</ref>

=== Analytical methods to investigate sources of CO<sub>2</sub> ===

* The burning of long-buried fossil fuels releases {{CO2}} containing carbon of different [[isotopic ratio]]s to those of living plants, enabling distinction between natural and human-caused contributions to {{CO2}} concentration.<ref name="Gosh-2003">e.g. {{cite journal |last1=Gosh |first1=Prosenjit |last2=Brand |first2=Willi A. |year=2003 |title=Stable isotope ratio mass spectrometry in global climate change research |url=http://www.bgc.mpg.de/service/iso_gas_lab/publications/PG_WB_IJMS.pdf |journal=[[International Journal of Mass Spectrometry]] |volume=228 |issue=1 |pages=1–33 |bibcode=2003IJMSp.228....1G |citeseerx=10.1.1.173.2083 |doi=10.1016/S1387-3806(03)00289-6 |quote=Global change issues have become significant due to the sustained rise in atmospheric trace gas concentrations ({{CO2}}, {{chem|N|2|O}}, {{chem|CH|4}}) over recent years, attributable to the increased per capita energy consumption of a growing global population. |access-date=2 July 2012 |archive-date=11 August 2017 |archive-url=https://web.archive.org/web/20170811075226/http://www.bgc.mpg.de/service/iso_gas_lab/publications/PG_WB_IJMS.pdf |url-status=live }}</ref>
* There are higher atmospheric {{CO2}} concentrations in the Northern Hemisphere, where most of the world's population lives (and emissions originate from), compared to the southern hemisphere. This difference has increased as anthropogenic emissions have increased.<ref name="Keeling-2011">{{cite journal |last1=Keeling |first1=Charles D. |last2=Piper |first2=Stephen C. |last3=Whorf |first3=Timothy P. |last4=Keeling |first4=Ralph F. |year=2011 |title=Evolution of natural and anthropogenic fluxes of atmospheric CO<sub>2</sub> from 1957 to 2003 |journal=Tellus B |volume=63 |issue=1 |pages=1–22 |bibcode=2011TellB..63....1K |doi=10.1111/j.1600-0889.2010.00507.x |issn=0280-6509 |doi-access=free}}</ref>
* Atmospheric O{{sub|2}} levels are decreasing in Earth's atmosphere as it reacts with the carbon in fossil fuels to form {{CO2}}.<ref name="Bender-2005">{{cite journal |last1=Bender |first1=Michael L. |last2=Ho |first2=David T. |last3=Hendricks |first3=Melissa B. |last4=Mika |first4=Robert |last5=Battle |first5=Mark O. |last6=Tans |first6=Pieter P. |last7=Conway |first7=Thomas J. |last8=Sturtevant |first8=Blake |last9=Cassar |first9=Nicolas |year=2005 |title=Atmospheric O2/N2changes, 1993–2002: Implications for the partitioning of fossil fuel CO2sequestration |journal=Global Biogeochemical Cycles |volume=19 |issue=4 |pages=n/a |bibcode=2005GBioC..19.4017B |doi=10.1029/2004GB002410 |issn=0886-6236 |doi-access=free}}</ref>

== Causes of the current increase ==

=== {{anchor|Anthropogenic CO2 increase}} Anthropogenic CO<sub>2</sub> emissions ===
{{See also|Greenhouse gas emissions|Causes of climate change|Radiative forcing}}
[[File:20211026 Cumulative carbon dioxide CO2 emissions by country - bar chart.svg|thumb|upright=1.5| The US, China and Russia have cumulatively contributed the greatest amounts of {{CO2}} since 1850.<ref name="Evans-2021">{{cite web |last1=Evans |first1=Simon |date=5 October 2021 |title=Analysis: Which countries are historically responsible for climate change? / Historical responsibility for climate change is at the heart of debates over climate justice. |url=https://www.carbonbrief.org/analysis-which-countries-are-historically-responsible-for-climate-change |url-status=live |archive-url=https://web.archive.org/web/20211026094104/https://www.carbonbrief.org/analysis-which-countries-are-historically-responsible-for-climate-change |archive-date=26 October 2021 |website=CarbonBrief.org |publisher=Carbon Brief |quote=Source: Carbon Brief analysis of figures from the Global Carbon Project, CDIAC, Our World in Data, Carbon Monitor, Houghton and Nassikas (2017) and Hansis et al (2015).}}</ref>]]While {{CO2}} absorption and release is always happening as a result of natural processes, the recent rise in {{CO2}} levels in the atmosphere is known to be mainly due to human (anthropogenic) activity.<ref name="Eyring-2021">Eyring, V., N.P. Gillett, K.M. Achuta Rao, R. Barimalala, M. Barreiro Parrillo, N. Bellouin, C. Cassou, P.J. Durack, Y. Kosaka, S.  McGregor, S. Min, O. Morgenstern, and Y. Sun, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter03.pdf Chapter 3: Human Influence on the Climate System] {{Webarchive|url=https://web.archive.org/web/20230307162843/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter03.pdf |date=7 March 2023 }}. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] {{Webarchive|url=https://web.archive.org/web/20210809131444/https://www.ipcc.ch/report/ar6/wg1/ |date=9 August 2021 }} [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L.  Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 423–552, {{doi|10.1017/9781009157896.005}}</ref> Anthropogenic carbon emissions exceed the amount that can be taken up or balanced out by natural sinks.<ref name="Ballantyne-2012">{{cite journal |last1=Ballantyne |first1=A.P. |last2=Alden |first2=C.B. |last3=Miller |first3=J.B. |last4=Tans |first4=P.P. |last5=White |first5=J.W.C. |year=2012 |title=Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years |journal=Nature |volume=488 |issue=7409 |pages=70–72 |bibcode=2012Natur.488...70B |doi=10.1038/nature11299 |issn=0028-0836 |pmid=22859203 |s2cid=4335259}}</ref> Thus carbon dioxide has gradually accumulated in the atmosphere and, as of May 2022, its concentration is 50% above pre-industrial levels.<ref name="NOAA-June2022" />

The extraction and burning of fossil fuels, releasing carbon that has been [[Earth's crust|underground]] for many millions of years, has increased the atmospheric concentration of {{CO2}}.<ref name="NOAA-2020" /><ref name="carbon budget 2022" /> As of year 2019 the extraction and burning of geologic fossil carbon by humans releases over 30&nbsp;gigatonnes of {{CO2}} (9&nbsp;billion tonnes carbon) each year.<ref name="Friedlingstein-2019">Friedlingstein, P., Jones, M., O'Sullivan, M., Andrew, R., Hauck, J., Peters, G., Peters, W., Pongratz, J., Sitch, S., Le Quéré, C. and 66 others (2019) "Global carbon budget 2019". ''Earth System Science Data'', '''11'''(4): 1783–1838. {{doi|10.5194/essd-11-1783-2019}}. [[File:CC-BY_icon.svg|50x50px]] Material was copied from this source, which is available under a [[creativecommons:by/4.0/|Creative Commons Attribution 4.0 International License]].</ref> This larger disruption to the natural balance is responsible for recent growth in the atmospheric {{CO2}} concentration.<ref name="NOAA-2013" /><ref name="Dlugokencky-2016">{{cite web |last1=Dlugokencky |first1=E. |date=5 February 2016 |title=Annual Mean Carbon Dioxide Data |url=ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_annmean_gl.txt |access-date=12 February 2016 |website=Earth System Research Laboratories |publisher=[[NOAA]] |archive-date=14 March 2023 |archive-url=https://web.archive.org/web/20230314104205/ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_annmean_gl.txt |url-status=live }}</ref>  Currently about half of the carbon dioxide released from the [[Global warming|burning of fossil fuels]] is not absorbed by vegetation and the oceans and remains in the [[atmosphere]].<ref>{{Cite journal |author1=A.P. Ballantyne |author2=C.B. Alden |author3=J.B. Miller |author4=P.P. Tans |author5=J.W. C. White |year=2012 |title=Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years |journal=[[Nature (journal)|Nature]] |volume=488 |issue=7409 |pages=70–72 |bibcode=2012Natur.488...70B |doi=10.1038/nature11299 |pmid=22859203 |s2cid=4335259}}</ref>

Burning fossil fuels such as [[coal]], [[petroleum]], and [[natural gas]] is the leading cause of increased [[human impact on the environment|anthropogenic]] {{CO2}}; [[deforestation]] is the second major cause. In 2010, 9.14 gigatonnes of carbon (GtC, equivalent to 33.5 [[gigatonne]]s of {{CO2}} or about 4.3 ppm in Earth's atmosphere) were released from fossil fuels and cement production worldwide, compared to 6.15 GtC in 1990.<ref name="TyndallCtr">{{cite web |title=Global carbon budget 2010 (summary) |url=http://www.tyndall.ac.uk/global-carbon-budget-2010 |url-status=dead |archive-url=https://web.archive.org/web/20120723220134/http://www.tyndall.ac.uk/global-carbon-budget-2010 |archive-date=23 July 2012 |publisher=[[Tyndall Centre for Climate Change Research]]}}</ref> In addition, [[land use change]] contributed 0.87 GtC in 2010, compared to 1.45 GtC in 1990.<ref name="TyndallCtr" />  In the period 1751 to 1900, about 12 GtC were released as {{CO2}} to the atmosphere from burning of fossil fuels, whereas from 1901 to 2013 the figure was about 380 GtC.<ref>Calculated from file global.1751_2013.csv in [http://cdiac.ornl.gov/ftp/ndp030/CSV-FILES] {{Webarchive|url=https://web.archive.org/web/20111022125534/http://cdiac.ornl.gov/ftp/ndp030/CSV-FILES/|date=22 October 2011}} from the [[Carbon Dioxide Information Analysis Center]].</ref>

The [[International Energy Agency]] estimates that the top 1% of emitters globally each had [[Carbon footprint|carbon footprints]] of over 50 tonnes of {{CO2}} in 2021, more than 1,000 times greater than those of the bottom 1% of emitters. The global average energy-related carbon footprint is around 4.7 tonnes of {{CO2}} per person.<ref>IEA (2023), The world’s top 1% of emitters produce over 1000 times more {{CO2}} than the bottom 1%, IEA, Paris https://www.iea.org/commentaries/the-world-s-top-1-of-emitters-produce-over-1000-times-more-co2-than-the-bottom-1 , License: CC BY 4.0</ref>

== Roles in natural processes on Earth ==

=== Greenhouse effect ===
[[File:Climate Change Schematic.svg|thumb|left|Greenhouse gases allow sunlight to pass through the atmosphere, heating the planet, but then absorb and redirect the infrared radiation (heat) the planet emits]]
[[File:Spectral Greenhouse Effect.png|thumb|CO<sub>2</sub> reduces the flux of thermal radiation emitted to space (causing the large dip near 667 cm<sup>−1</sup>), thereby contributing to the greenhouse effect.]]
[[File:Longwave Absorption Coefficients of H2O and CO2.svg|thumb|[[Outgoing longwave radiation|Longwave-infrared]] [[absorption coefficient]]s of water vapor and carbon dioxide. For wavelengths near 15-microns, CO<sub>2</sub> is a much stronger absorber than water vapor.]]

{{Main|Greenhouse effect|3 = Radiative forcing}}

On Earth, carbon dioxide is the most relevant, direct [[greenhouse gas]] that is influenced by human activities. Water is responsible for most (about 36–70%) of the total greenhouse effect, and the [[Greenhouse gas#Role of water vapor|role of water vapor]] as a greenhouse gas depends on temperature. Carbon dioxide is often mentioned in the context of its increased influence as a greenhouse gas since the pre-industrial (1750) era. In 2013, the increase in CO<sub>2</sub> was estimated to be responsible for 1.82 W m<sup>−2</sup> of the 2.63 W m<sup>−2</sup> change in [[radiative forcing]] on Earth (about 70%).<ref name="IPCC-AR5">{{Cite web |url=https://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter08_FINAL.pdf |title=IPCC Fifth Assessment Report – Chapter 8: Anthropogenic and Natural Radiative Forcing. |access-date=14 March 2023 |archive-date=22 October 2018 |archive-url=https://web.archive.org/web/20181022073632/https://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter08_FINAL.pdf |url-status=live }}</ref>

Earth's natural [[greenhouse effect]] makes life as we know it possible, and carbon dioxide in the atmosphere plays a significant role in providing for the relatively high temperature on Earth. The greenhouse effect is a process by which thermal radiation from a planetary atmosphere warms the planet's surface beyond the temperature it would have in the absence of its atmosphere.<ref name="IPCC-2018">{{cite web |title=Annex II Glossary |url=http://www.ipcc.ch/publications_and_data/ar4/syr/en/annexessglossary-e-i.html |url-status=dead |archive-url=https://web.archive.org/web/20181103000935/http://www.ipcc.ch/publications_and_data/ar4/syr/en/annexessglossary-e-i.html |archive-date=3 November 2018 |access-date=15 October 2010 |publisher=Intergovernmental Panel on Climate Change}}</ref><ref name="ipcc-AR4WG1">A concise description of the greenhouse effect is given in the ''Intergovernmental Panel on Climate Change Fourth Assessment Report,'' "What is the Greenhouse Effect?" [http://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-1-3.html FAQ 1.3 – AR4 WGI Chapter 1: Historical Overview of Climate Change Science] {{Webarchive|url=https://web.archive.org/web/20181130152334/https://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-1-3.html|date=30 November 2018}}, IPCC Fourth Assessment Report, Chapter 1, p. 115: "To balance the absorbed incoming [solar] energy, the Earth must, on average, radiate the same amount of energy back to space. Because the Earth is much colder than the Sun, it radiates at much longer wavelengths, primarily in the infrared part of the spectrum (see Figure 1). Much of this thermal radiation emitted by the land and ocean is absorbed by the atmosphere, including clouds, and reradiated back to Earth. This is called the greenhouse effect."<br />Stephen H. Schneider, in ''Geosphere-biosphere Interactions and Climate,'' Lennart O. Bengtsson and Claus U. Hammer, eds., Cambridge University Press, 2001, {{ISBN|0-521-78238-4}}, pp. 90–91.<br />E. Claussen, V.A. Cochran, and D.P. Davis, ''Climate Change: Science, Strategies, & Solutions,'' University of Michigan, 2001. p. 373.<br />A. Allaby and M. Allaby, ''A Dictionary of Earth Sciences,'' Oxford University Press, 1999, {{ISBN|0-19-280079-5}}, p. 244.</ref><ref>{{cite book |first=Vaclav |last=Smil |author-link=Vaclav Smil  |url=https://books.google.com/books?id=8ntHWPMUgpMC&pg=PA107 |title=The Earth's Biosphere: Evolution, Dynamics, and Change |publisher=MIT Press |year=2003 |isbn=978-0-262-69298-4 |page=107 |access-date=14 March 2023 |archive-url=https://web.archive.org/web/20230314102307/https://books.google.com/books?id=8ntHWPMUgpMC&pg=PA107 |archive-date=14 March 2023 |url-status=live}}</ref> 

The concept of more atmospheric CO<sub>2</sub> increasing ground temperature was first published by [[Svante Arrhenius#Greenhouse effect|Svante Arrhenius]] in 1896.<ref name="Arrhenius-1896">{{cite journal |first=Svante |last=Arrhenius |title=On the influence of carbonic acid in the air upon the temperature of the ground |journal=Philosophical Magazine and Journal of Science |date=1896 |pages=237–76 |url=http://www.rsc.org/images/Arrhenius1896_tcm18-173546.pdf |access-date=14 March 2023 |archive-date=18 November 2020 |archive-url=https://web.archive.org/web/20201118065555/https://www.rsc.org/images/Arrhenius1896_tcm18-173546.pdf |url-status=live }}</ref> The increased radiative forcing due to increased CO<sub>2</sub> in the Earth's atmosphere is based on the physical properties of CO<sub>2</sub> and the non-saturated absorption windows where CO<sub>2</sub> absorbs outgoing long-wave energy.  The increased forcing drives further changes in [[Earth's energy balance]] and, over the longer term, in Earth's climate.<ref name="Eyring-2021" />

=== Carbon cycle ===
{{Main | Carbon cycle| Atmospheric carbon cycle}}
[[File:Carbon cycle.jpg|thumb|left|This diagram of the carbon cycle shows the movement of carbon between land, atmosphere, and oceans in billions of metric tons of carbon per year. Yellow numbers are natural fluxes, red are human contributions, white are stored carbon.<ref name="Riebeek-2011">{{cite web|last1=Riebeek|first1=Holli|title=The Carbon Cycle|url=http://earthobservatory.nasa.gov/Features/CarbonCycle/?src=eoa-features|website=Earth Observatory|publisher=NASA|access-date=5 April 2018|date=16 June 2011|archive-url=https://web.archive.org/web/20160305010126/http://earthobservatory.nasa.gov/Features/CarbonCycle/?src=eoa-features|archive-date=5 March 2016|url-status=live|df=dmy-all}}</ref>]]
Atmospheric carbon dioxide plays an integral role in the Earth's [[carbon cycle]] whereby {{CO2}} is removed from the atmosphere by some natural processes such as [[photosynthesis]] and deposition of [[carbonates]], to form limestones for example, and added back to the atmosphere by other natural processes such as [[Cellular respiration|respiration]] and the acid dissolution of carbonate deposits. There are two broad carbon cycles on Earth: the fast carbon cycle and the slow carbon cycle. The fast carbon cycle refers to movements of carbon between the environment and living things in the [[biosphere]] whereas the slow carbon cycle involves the movement of carbon between the atmosphere, oceans, soil, rocks, and volcanism. Both cycles are intrinsically interconnected and atmospheric {{CO2}} facilitates the linkage.

Natural sources of atmospheric {{CO2}} include [[volcanic]] [[outgassing]], the [[combustion]] of [[organic compound|organic matter]], [[wildfires]] and the [[Respiration (physiology)|respiration]] processes of living [[aerobic organism]]s. Man-made sources of {{CO2}} include the burning of [[fossil fuels]], as well as some industrial processes such as cement making. 

[[File:Global carbon budget components.png|thumb|Annual {{CO2}} flows from anthropogenic sources (left) into Earth's atmosphere, land, and ocean sinks (right) since year 1960. Units in equivalent gigatonnes carbon per year.<ref name="Friedlingstein-2019"/>]]
Natural sources of  {{CO2}} are more or less balanced by natural [[Carbon sink|carbon sinks]], in the form of chemical and biological processes which remove {{CO2}} from the atmosphere. For example, the decay of organic material in forests, grasslands, and other land vegetation - including forest fires - results in the release of about 436&nbsp;[[tonne|gigatonnes]] of {{CO2}} (containing 119&nbsp;gigatonnes carbon) every year, while {{CO2}} uptake by new growth on land counteracts these releases, absorbing 451 Gt (123 Gt C).<ref>{{Cite book |url=https://www.fs.fed.us/research/publications/gtr/gtr_wo95.pdf |title=Considering Forest and Grassland Carbon in Land Management |last1=Kayler |first1=Z. |last2=Janowiak |first2=M. |last3=Swanston |first3=C. |journal=General Technical Report WTO-GTR-95 |publisher=United States Department of Agriculture, Forest Service |chapter=The Global Carbon Cycle |pages=3–9 |year=2017 |access-date=14 March 2023 |archive-date=7 July 2022 |archive-url=https://web.archive.org/web/20220707152049/https://www.fs.fed.us/research/publications/gtr/gtr_wo95.pdf |url-status=live }}</ref>  Although much {{CO2}} in the early atmosphere of the young Earth was produced by [[volcano|volcanic activity]], modern volcanic activity releases only 130 to 230&nbsp;[[tonne|megatonnes]] of {{CO2}} each year.<ref>{{cite journal |author=Gerlach, T.M. |title=Present-day CO<sub>2</sub> emissions from volcanoes |journal=[[Eos (journal)|Eos, Transactions, American Geophysical Union]] |volume=72 |issue=23 |pages=249, 254–55 |date=4 June 1991 |doi=10.1029/90EO10192 |bibcode=1991EOSTr..72..249.}}</ref> 

From the human pre-industrial era to 1940, the terrestrial biosphere represented a net source of atmospheric {{CO2}} (driven largely by [[land-use change]]s), but subsequently switched to a net sink with growing fossil carbon emissions.<ref>{{cite journal |author1=Junling Huang |author2=Michael B. McElroy |year=2012 |title=The Contemporary and Historical Budget of Atmospheric CO<sub>2</sub> |url=http://dash.harvard.edu/bitstream/handle/1/10981610/The%20contemporary%20and%20historical%20budget%20of%20atmospheric%20CO2_1%202.pdf?sequence=9 |url-status=live |journal=Canadian Journal of Physics |volume=90 |issue=8 |pages=707–16 |bibcode=2012CaJPh..90..707H |doi=10.1139/p2012-033 |archive-url=https://web.archive.org/web/20170803143812/https://dash.harvard.edu/bitstream/handle/1/10981610/The%20contemporary%20and%20historical%20budget%20of%20atmospheric%20CO2_1%202.pdf?sequence=9 |archive-date=3 August 2017 |access-date=14 March 2023}}</ref>

Carbon moves between the atmosphere, vegetation (dead and alive), the soil, the surface layer of the ocean, and the deep ocean. A detailed model has been developed by [[Fortunat Joos]] in [[Bern]] and colleagues, called the Bern model.<ref>{{cite journal|display-authors=etal|last1=Fortunat Joos |title=Global warming feedbacks on terrestrial carbon uptake under the Intergovernmental Panel on Climate Change (IPCC) Emission Scenarios |journal=Global Biogeochemical Cycles |date=Dec 2001 |volume=15 |issue=4 |pages=891–907 |doi=10.1029/2000GB001375 |bibcode=2001GBioC..15..891J |url=https://www.researchgate.net/publication/228956254}}</ref>
A simpler model based on it gives the fraction of {{co2}} remaining in the atmosphere as a function of the number of years after it is emitted into the atmosphere:<ref>{{cite journal |last1=Morgan Edwards & Jessika Trancik |title=Supplementary Information |journal=Nature Climate Change |date=Apr 25, 2014 |doi=10.1038/nclimate2204 |hdl=1721.1/96138 |url=https://static-content.springer.com/esm/art%3A10.1038%2Fnclimate2204/MediaObjects/41558_2014_BFnclimate2204_MOESM362_ESM.pdf}}, supplement to [https://web.archive.org/web/20150116044523/https://www.see.ed.ac.uk/~shs/Methane/Methane%20and%20time.pdf Climate impacts of energy technologies depend on emissions timing]</ref>

:<math>f(t)=0.217+0.259\exp(-t/172.9)+0.338\exp(-t/18,51)+0.186\exp(-t/1.186)</math>

According to this model, 21.7% of the carbon dioxide released into the air stays there forever, but of course this is not true if carbon-containing material is removed from the cycle (and stored) in ways that are not operative at present ([[Carbon sink#Artificial sequestration|artificial sequestration]]).

==== Oceanic carbon cycle ====
{{Main|Oceanic carbon cycle|Biological pump}}
[[File:CO2 pump hg.svg|thumb|Air-sea exchange of {{CO2}}]]

The Earth's oceans contain a large amount of {{CO2}} in the form of bicarbonate and carbonate ions—much more than the amount in the atmosphere. The bicarbonate is produced in reactions between rock, water, and carbon dioxide.  

From 1850 until 2022, the ocean has absorbed 26% of total anthropogenic emissions.<ref name="carbon budget 2022" /> However, the rate at which the ocean will take it up in the future is less certain. Even if equilibrium is reached, including dissolution of carbonate minerals, the increased concentration of bicarbonate and decreased or unchanged concentration of carbonate ion will give rise to a higher concentration of un-ionized carbonic acid and dissolved {{CO2}}. This higher concentration in the seas, along with higher temperatures, would mean a higher equilibrium concentration of {{CO2}} in the air.<ref name="Friedlingstein-2009">{{cite journal |author=[[Susan Solomon]] |author2=Gian-Kasper Plattner |author3=Reto Knutti |author4=Pierre Friedlingstein |date=February 2009 |title=Irreversible climate change due to carbon dioxide emissions |journal=Proc. Natl. Acad. Sci. USA |volume=106 |issue=6 |pages=1704–09 |bibcode=2009PNAS..106.1704S |doi=10.1073/pnas.0812721106 |pmc=2632717 |pmid=19179281 |doi-access=free}}</ref><ref name="ArcherEby2009">{{cite journal |last1=Archer |first1=David |last2=Eby |first2=Michael |last3=Brovkin |first3=Victor |last4=Ridgwell |first4=Andy |last5=Cao |first5=Long |last6=Mikolajewicz |first6=Uwe |last7=Caldeira |first7=Ken |last8=Matsumoto |first8=Katsumi |last9=Munhoven |first9=Guy |last10=Montenegro |first10=Alvaro |last11=Tokos |first11=Kathy |year=2009 |title=Atmospheric Lifetime of Fossil Fuel Carbon Dioxide |url=http://orbi.ulg.ac.be/handle/2268/12933 |journal=Annual Review of Earth and Planetary Sciences |volume=37 |issue=1 |pages=117–34 |bibcode=2009AREPS..37..117A |doi=10.1146/annurev.earth.031208.100206 |issn=0084-6597 |hdl=2268/12933 |access-date=14 March 2023 |archive-date=14 March 2023 |archive-url=https://web.archive.org/web/20230314104229/https://orbi.uliege.be/handle/2268/12933 |url-status=live |hdl-access=free }}</ref>

A study published in ''[[Science Advances]]'' in 2025 concluded that faster flow of the [[Antarctic Circumpolar Current]] (ACC) at higher latitudes causes upwelling of isotopically light deep waters around Antarctica, likely increasing atmospheric carbon dioxide levels and thereby potentially constituting a critical [[positive feedback]] for future warming.<ref name=SciAdvances_20250101>{{cite journal |last1=Starr |first1=Aidan |last2=Hall |first2=Ian R. |last3=Barker |first3=Stephen |last4=Nederbragt |first4=Alexandra |last5=Owen |first5=Lindsey |last6=Hemming |first6=Sidney R. |title=Shifting Antarctic Circumpolar Current south of Africa over the past 1.9 million years |journal=Science Advances |date=1 January 2025 |volume=11 |issue=1 |doi=10.1126/sciadv.adp1692|pmc=11691690 }}</ref>

== Effects of current increase ==

=== Direct effects ===
[[File:Physical Drivers of climate change.svg|thumb|upright=1.35|[[Radiative forcing|Physical drivers]] of global warming that has happened so far. Future [[global warming potential]] for long lived drivers like carbon dioxide emissions is not represented. Whiskers on each bar show the possible [[Observational error|error range]].]]

Direct effects of increasing CO<sub>2</sub> concentrations in the atmosphere include [[Instrumental temperature record|increasing global temperatures]], [[ocean acidification]] and a [[CO2 fertilization effect|CO<sub>2</sub> fertilization effect]] on plants and crops.<ref>{{Cite journal |last=Keeling |first=Charles D. |date=1997-08-05 |title=Climate change and carbon dioxide: An introduction |journal=Proceedings of the National Academy of Sciences |language=en |volume=94 |issue=16 |pages=8273–8274 |doi=10.1073/pnas.94.16.8273 |issn=0027-8424 |pmc=33714 |pmid=11607732|bibcode=1997PNAS...94.8273K |doi-access=free }}</ref>

====Temperature rise on land====
{{excerpt|Instrumental temperature record#Total warming and trends|paragraphs=1-2|file=no}}

====Temperature rise in oceans====
{{See also|Ocean heat content}}
{{excerpt|Effects of climate change on oceans#Rising ocean temperature|paragraphs=1-2|file=no}}

==== Ocean acidification ====
{{excerpt|ocean acidification|paragraphs=1-2}}

==== CO<sub>2</sub> fertilization effect ====
{{excerpt|CO2 fertilization effect|displaytitle={{CO2}} fertilization effect|paragraphs=1-3|file=no}}

====Other direct effects====
{{CO2}} emissions have also led to the stratosphere contracting by 400 meters since 1980, which could affect satellite operations, GPS systems and radio communications.<ref>{{cite journal |last1=Pisoft |first1=Petr |date=May 25, 2021 |title=Stratospheric contraction caused by increasing greenhouse gases |journal=Environmental Research Letters |volume=16 |issue=6 |page=064038 |bibcode=2021ERL....16f4038P |doi=10.1088/1748-9326/abfe2b |doi-access=free}}</ref>

=== Indirect effects and impacts ===
{{multiple image
| perrow            = 2
| total_width       = 400
| image1            = 062821Yreka Fire CalFire -2wiki.jpg
| alt1              = Thick orange-brown smoke blocks half a blue sky, with conifers in the foreground
| image2            = Bleachedcoral.jpg
| alt2              = A few grey fish swim over grey coral with white spikes
| image3            = Village Telly in Mali.jpg
| alt3              = Desert sand half covers a village of small flat-roofed houses with scattered green trees
| image4            = US Navy 071120-M-8966H-005 An aerial view over southern Bangladesh reveals extensive flooding as a result of Cyclone Sidr.jpg
| alt4              = large areas of still water behind riverside buildings
| footer            = Some climate change effects, clockwise from top left: [[Wildfire]] caused by heat and dryness, [[Coral bleaching|bleached coral]] caused by ocean acidification and heating, [[coastal flooding]] caused by [[storm]]s and sea level rise, and [[environmental migration]] caused by [[desertification]]
}}

{{excerpt|effects of climate change|paragraphs=1}}
{{excerpt|effects of climate change on oceans|paragraphs=1}}

== Approaches for reducing CO<sub>2</sub> concentrations ==
[[File:Following Carbon Dioxide Through the Atmosphere.webm|thumb|A model of the behavior of carbon in the atmosphere from 1 September 2014 to 31 August 2015. The height of Earth's atmosphere and topography have been vertically exaggerated and appear approximately 40 times higher than normal to show the complexity of the atmospheric flow.]]
{{Main|Climate change mitigation|Carbon sequestration|Carbon dioxide removal|Carbon capture and storage}}
Carbon dioxide has unique long-term effects on climate change that are nearly "irreversible" for a thousand years after emissions stop (zero further emissions). The greenhouse gases [[methane]] and [[nitrous oxide]] do not persist over time in the same way as carbon dioxide. Even if human carbon dioxide emissions were to completely cease, atmospheric temperatures are not expected to decrease significantly in the short term. This is because the air temperature is determined by a balance between heating, due to greenhouse gases, and cooling due to heat transfer to the ocean. If emissions were to stop, CO<sub>2</sub> levels and the heating effect would slowly decrease, but simultaneously the cooling due to heat transfer would diminish (because sea temperatures would get closer to the air temperature), with the result that the air temperature would decrease only slowly. Sea temperatures would continue to rise, causing thermal expansion and some sea level rise.<ref name="Friedlingstein-2009" /> Lowering global temperatures more rapidly would require [[carbon sequestration]] or [[geoengineering]].

Various techniques have been proposed for removing excess carbon dioxide from the atmosphere.
{{excerpt|carbon dioxide removal|paragraphs=1|file=no}}

==Concentrations in the geologic past==
{{See also|Paleoclimatology|Great Oxidation Event|Faint young Sun paradox}}
[[File:Phanerozoic Carbon Dioxide.png|thumb|{{CO2}} concentrations over the last 500 Million years|315x315px]]
[[File:CO2 40k.png|thumb|Concentration of atmospheric {{CO2}} over the last 40,000 years, from the [[Last Glacial Maximum]] to the present day. The current rate of increase is much higher than at any point during the last [[deglaciation]].]]
Estimates in 2023 found that the current carbon dioxide concentration in the atmosphere may be the highest it has been in the last 14 million years.<ref name="Ahmed-2023" /> However the [[IPCC Sixth Assessment Report]] estimated similar levels 3 to 3.3 million years ago in the [[Pliocene climate#Mid-Pliocene and future climate|mid-Pliocene warm period]]. This period can be a [[Proxy (climate)|proxy]] for likely climate outcomes with current levels of {{CO2}}.<ref>Gulev, S.K., P.W. Thorne, J. Ahn, F.J. Dentener, C.M. Domingues, S. Gerland, D. Gong, D.S. Kaufman, H.C. Nnamchi, J.  Quaas, J.A. Rivera, S. Sathyendranath, S.L. Smith, B. Trewin, K. von Schuckmann, and R.S. Vose, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter02.pdf Chapter 2: Changing State of the Climate System]. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R.  Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 287–422, doi:10.1017/9781009157896.004.</ref>{{rp|Figure 2.34}}

Carbon dioxide is believed to have played an important effect in regulating Earth's temperature throughout its 4.54 billion year history. Early in the Earth's life, scientists have found evidence of liquid water indicating a warm world even though the Sun's output is believed to have only been 70% of what it is today. Higher carbon dioxide concentrations in the early Earth's atmosphere might help explain this [[faint young sun paradox]]. When Earth first formed, [[Earth's atmosphere]] may have contained more greenhouse gases and {{CO2}} concentrations may have been higher, with estimated [[partial pressure]] as large as {{convert|1000|kPa|bar|abbr=on|lk=on}}, because there was no bacterial [[photosynthesis]] to [[redox|reduce]] the gas to carbon compounds and oxygen. [[Methane]], a very active greenhouse gas, may have been more prevalent as well.<ref name="Walker-1985">{{cite journal |last=Walker |first=James C.G. |date=June 1985 |title=Carbon dioxide on the early earth |url=http://deepblue.lib.umich.edu/bitstream/2027.42/43349/1/11084_2005_Article_BF01809466.pdf |journal=Origins of Life and Evolution of the Biosphere |volume=16 |issue=2 |pages=117–27 |bibcode=1985OrLi...16..117W |doi=10.1007/BF01809466 |pmid=11542014 |access-date=2010-01-30 |hdl-access=free |hdl=2027.42/43349 |s2cid=206804461 |archive-date=14 September 2012 |archive-url=https://web.archive.org/web/20120914033408/http://deepblue.lib.umich.edu/bitstream/2027.42/43349/1/11084_2005_Article_BF01809466.pdf |url-status=live }}</ref><ref>{{cite journal |author1=Pavlov, Alexander A. |author2=Kasting, James F. |author3=Brown, Lisa L. |author4=Rages, Kathy A. |author5=Freedman, Richard |date=May 2000 |title=Greenhouse warming by CH<sub>4</sub> in the atmosphere of early Earth |journal=Journal of Geophysical Research |volume=105 |issue=E5 |pages=11981–90 |bibcode=2000JGR...10511981P |doi=10.1029/1999JE001134 |pmid=11543544 |doi-access=free}}</ref>

Carbon dioxide concentrations have shown several cycles of variation from about 180 parts per million during the deep glaciations of the [[Holocene]] and [[Pleistocene]] to 280 parts per million during the interglacial periods. Carbon dioxide concentrations have varied widely over the Earth's history. It is believed to have been present in Earth's first atmosphere, shortly after Earth's formation. The second atmosphere, consisting largely of [[nitrogen]] and {{Chem|C|O|2}} was produced by outgassing from [[volcanism]], supplemented by gases produced during the [[late heavy bombardment]] of Earth by huge [[asteroids]].<ref name="Zahnle-2010">{{cite journal |last1=Zahnle |first1=K. |last2=Schaefer |first2=L. |author2-link=Laura K. Schaefer |last3=Fegley |first3=B. |year=2010 |title=Earth's Earliest Atmospheres |journal=Cold Spring Harbor Perspectives in Biology |volume=2 |issue=10 |pages=a004895 |doi=10.1101/cshperspect.a004895 |pmc=2944365 |pmid=20573713}}</ref> A major part of carbon dioxide emissions were soon dissolved in water and incorporated in carbonate sediments.

The production of free oxygen by [[cyanobacteria]]l photosynthesis eventually led to the [[Great Oxygenation Event|oxygen catastrophe]] that ended Earth's second atmosphere and brought about the Earth's third atmosphere (the modern atmosphere) 2.4 billion years ago. Carbon dioxide concentrations dropped from 4,000 parts per million during the [[Cambrian|Cambrian period]] about 500 million years ago to as low as 180 parts per million 20,000 years ago .<ref name="Eggleton-2013" /><!-- It is unclear if this figure is based on modeled data, or reconstructions. -->

=== Drivers of ancient-Earth CO<sub>2</sub> concentration ===
{{See also|Biogeochemical cycle}}

On long timescales, atmospheric {{CO2}} concentration is determined by the balance among [[Geochemical cycle|geochemical processes]] including organic carbon burial in sediments, silicate rock [[weathering]], and [[Volcanic gas|volcanic degassing]]. The net effect of slight imbalances in the [[carbon cycle]] over tens to hundreds of millions of years has been to reduce atmospheric {{CO2}}. On a timescale of billions of years, such downward trend appears bound to continue indefinitely as occasional massive historical releases of buried carbon due to volcanism will become less frequent (as earth mantle cooling and progressive exhaustion of [[Geothermal gradient|internal radioactive heat]] proceed further). The rates of these processes are extremely slow; hence they are of no relevance to the atmospheric {{CO2}} concentration over the next hundreds or thousands of years.

=== Photosynthesis in the geologic past ===
Over the course of Earth's geologic history {{CO2}} concentrations have played a role in biological evolution. The first photosynthetic organisms probably [[evolution|evolved]] early in the [[evolutionary history of life]] and most likely used [[reducing agent]]s such as [[hydrogen]] or [[hydrogen sulfide]] as sources of electrons, rather than water.<ref>{{cite journal |author=Olson JM |date=May 2006 |title=Photosynthesis in the Archean era |journal=Photosynth. Res. |volume=88 |issue=2 |pages=109–17 |doi=10.1007/s11120-006-9040-5 |pmid=16453059 |bibcode=2006PhoRe..88..109O |s2cid=20364747}}</ref> Cyanobacteria appeared later, and the excess oxygen they produced contributed to the [[oxygen catastrophe]],<ref name="Buick R-2008">{{cite journal |author=Buick R |date=August 2008 |title=When did oxygenic photosynthesis evolve? |journal=Philos. Trans. R. Soc. Lond. B Biol. Sci. |volume=363 |issue=1504 |pages=2731–43 |doi=10.1098/rstb.2008.0041 |pmc=2606769 |pmid=18468984|bibcode=2008RSPTB.363.2731B }}</ref> which rendered the [[Evolution of multicellularity|evolution of complex life]] possible. In recent geologic times, low {{CO2}} concentrations below 600 parts per million might have been the stimulus that favored the evolution of [[C4 carbon fixation|{{C4}}]] plants which increased greatly in abundance between 7 and 5 million years ago over plants that use the less efficient [[C3 carbon fixation|{{C3}}]] metabolic pathway.<ref name="Beerling-2006">{{cite journal |author=Osborne, C.P. |author2=Beerling, D.J. |author-link2=David Beerling |year=2006 |title=Nature's green revolution: the remarkable evolutionary rise of {{C4}} plants |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=361 |issue=1465 |pages=173–94 |doi=10.1098/rstb.2005.1737 |pmc=1626541 |pmid=16553316}}</ref> At current atmospheric pressures photosynthesis shuts down when atmospheric {{CO2}} concentrations fall below 150 ppm and 200 ppm although some microbes can extract carbon from the air at much lower concentrations.<ref>{{cite journal |last1=Lovelock |first1=J. E. |year=1972 |title=Gaia as seen through the atmosphere |url=http://www.jameslovelock.org/page33.html/ |url-status=dead |journal=Atmospheric Environment |volume=6 |issue=8 |pages=579–580 |bibcode=1972AtmEn...6..579L |doi=10.1016/0004-6981(72)90076-5 |archive-url=https://web.archive.org/web/20111103150707/http://www.jameslovelock.org/page33.html |archive-date=2011-11-03 |access-date=2014-03-22}}</ref><ref>{{cite journal |last1=Li |first1=K.-F. |date=2009-05-30 |title=Atmospheric pressure as a natural climate regulator for a terrestrial planet with a biosphere |url=http://www.pnas.org/content/early/2009/06/01/0809436106.full.pdf+html |journal=Proceedings of the National Academy of Sciences |volume=106 |issue=24 |pages=9576–9579 |bibcode=2009PNAS..106.9576L |doi=10.1073/pnas.0809436106 |pmc=2701016 |pmid=19487662 |access-date=2014-03-22 |doi-access=free |archive-date=12 February 2013 |archive-url=https://web.archive.org/web/20130212184727/http://www.pnas.org/content/early/2009/06/01/0809436106.full.pdf+html |url-status=live }}</ref>

=== Measuring ancient-Earth CO<sub>2</sub> concentration ===
{{See also|Proxy (climate)|label 1=Climate reconstruction proxies}}
[[File:Vostok Petit data.svg|thumb|Over 400,000 years of ice core data: Graph of CO<sub>2</sub> (green), reconstructed temperature (blue) and dust (red) from the Vostok ice core]]
[[File:Temperature-change-and-carbon-dioxide-change-measured-from-the-EPICA-Dome-C-ice-core-in-Antarctica-v2.jpg|thumb|Correspondence between temperature and atmospheric {{CO2}} during the last 800,000 years]]<!--alternative: File:Co2-temperature-records.svg -->

The most direct method for measuring atmospheric carbon dioxide concentrations for periods before instrumental sampling is to measure bubbles of air ([[fluid inclusions|fluid or gas inclusions]]) trapped in the [[Antarctica|Antarctic]] or [[Greenland]] ice sheets. The most widely accepted of such studies come from a variety of Antarctic cores and indicate that atmospheric {{CO2}} concentrations were about 260–280 ppm immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 [[annum|years]].<ref>{{cite web |last1=Etheridge |first1=D.M. |last2=Steele |first2=L.P. |last3=Langenfelds |first3=R.L. |last4=Francey |first4=R.J. |last5=Barnola |first5=JM |last6=Morgan |first6=VI |date=June 1998 |title=Historical CO<sub>2</sub> record derived from a spline fit (20-year cutoff) of the Law Dome DE08 and DE08-2 ice cores |url=http://cdiac.ornl.gov/ftp/trends/co2/lawdome.smoothed.yr20 |url-status=dead |archive-url=https://web.archive.org/web/20120305110732/http://cdiac.ornl.gov/ftp/trends/co2/lawdome.smoothed.yr20 |archive-date=5 March 2012 |access-date=2007-06-12 |website=Carbon Dioxide Information Analysis Center |publisher=[[Oak Ridge National Laboratory]]}}</ref><ref>{{cite journal |last1=Flückiger |first1=Jacqueline |year=2002 |title=High-resolution Holocene {{chem|N|2|O}} ice core record and its relationship with {{chem|CH|4}} and CO<sub>2</sub> |journal=Global Biogeochemical Cycles |volume=16 |issue=1 |page=1010 |bibcode=2002GBioC..16.1010F |doi=10.1029/2001GB001417 |doi-access=free}}</ref> The longest [[ice core]] record comes from East Antarctica, where ice has been sampled to an age of 800,000 years.<ref name="Amos-2006">{{cite news |last1=Amos |first1=J. |date=4 September 2006 |title=Deep ice tells long climate story |work=BBC News |url=http://news.bbc.co.uk/2/hi/science/nature/5314592.stm |access-date=28 April 2010 |archive-date=23 January 2013 |archive-url=https://web.archive.org/web/20130123202651/http://news.bbc.co.uk/2/hi/science/nature/5314592.stm |url-status=live }}</ref> During this time, the atmospheric carbon dioxide concentration has varied between 180 and 210 ppm during [[ice age]]s, increasing to 280–300 ppm during warmer [[interglacial]]s.<ref>{{cite journal |author=Hileman B. |date=November 2005 |title=Ice Core Record Extended: Analyses of trapped air show current CO<sub>2</sub> at highest level in 650,000 years |url=http://pubs.acs.org/cen/news/83/i48/8348notw1.html |journal=[[Chemical & Engineering News]] |volume=83 |issue=48 |pages=7 |doi=10.1021/cen-v083n048.p007 |issn=0009-2347 |access-date=28 January 2010 |archive-date=15 May 2019 |archive-url=https://web.archive.org/web/20190515033556/http://pubs.acs.org/cen/news/83/i48/8348notw1.html |url-status=live }}</ref><ref>[http://www.ncdc.noaa.gov/paleo/icecore/antarctica/vostok/vostok_data.html Vostok Ice Core Data] {{Webarchive|url=https://web.archive.org/web/20150227064302/http://www.ncdc.noaa.gov/paleo/icecore/antarctica/vostok/vostok_data.html |date=27 February 2015 }}, [http://www.ncdc.noaa.gov ncdc.noaa.gov] {{Webarchive|url=https://web.archive.org/web/20210422034738/https://www.ncdc.noaa.gov/ |date=22 April 2021 }}</ref>

{{CO2}} mole fractions in the atmosphere have gone up by around 35 percent since the 1900s, rising from 280 parts per million by volume to 387 parts per million in 2009. One study using evidence from [[stomata]] of fossilized leaves suggests greater variability, with {{CO2}} mole fractions above 300&nbsp;ppm during the period ten to seven thousand years ago,<ref>{{cite journal |author1=Friederike Wagner |author2=Bent Aaby |author3=Henk Visscher |year=2002 |title=Rapid atmospheric CO<sub>2</sub> changes associated with the 8,200-years-B.P. cooling event |journal=Proc. Natl. Acad. Sci. USA |volume=99 |issue=19 |pages=12011–14 |bibcode=2002PNAS...9912011W |doi=10.1073/pnas.182420699 |pmc=129389 |pmid=12202744 |doi-access=free}}</ref> though others have argued that these findings more likely reflect calibration or contamination problems rather than actual CO<sub>2</sub> variability.<ref>{{cite journal |author1=Andreas Indermühle |author2=Bernhard Stauffer |author3=Thomas F. Stocker |year=1999 |title=Early Holocene Atmospheric CO<sub>2</sub> Concentrations |journal=Science |volume=286 |issue=5446 |page=1815 |doi=10.1126/science.286.5446.1815a |doi-access=free}} {{cite journal |last1=IndermÜhle |first1=A |year=1999 |title=Early Holocene atmospheric CO<sub>2</sub>concentrations |journal=Science |volume=286 |issue=5446 |pages=1815a–15 |doi=10.1126/science.286.5446.1815a |doi-access=free}}</ref><ref>{{cite journal |author1=H. J. Smith |author2=M. Wahlen |author3=D. Mastroianni |year=1997 |title=The CO<sub>2</sub> concentration of air trapped in GISP2 ice from the Last Glacial Maximum-Holocene transition |journal=Geophysical Research Letters |volume=24 |issue=1 |pages=1–4 |bibcode=1997GeoRL..24....1S |doi=10.1029/96GL03700 |s2cid=129667062}}</ref> Because of the way air is trapped in ice (pores in the ice close off slowly to form bubbles deep within the [[firn]]) and the time period represented in each ice sample analyzed, these figures represent averages of atmospheric concentrations of up to a few centuries rather than annual or decadal levels.

Ice cores provide evidence for greenhouse gas concentration variations over the past 800,000 years. Both CO<sub>2</sub> and {{chem|CH|4}} concentrations vary between glacial and interglacial phases, and these variations correlate strongly with temperature. Direct data does not exist for periods earlier than those represented in the ice core record, a record that indicates that CO<sub>2</sub> mole fractions stayed within a range of 180&nbsp;ppm to 280&nbsp;ppm throughout the last 800,000 years, until the increase of the last 250 years. However, various [[Proxy (climate)|proxy measurements]] and models suggest larger variations in past epochs: 500&nbsp;million years ago CO<sub>2</sub> levels were likely 10 times higher than now.<ref>[[:File:Phanerozoic Carbon Dioxide.png]]</ref>  

Various proxy measurements have been used to try to determine atmospheric CO<sub>2</sub> concentrations millions of years in the past. These include [[boron]] and [[carbon]] [[isotope]] ratios in certain types of marine sediments, and the numbers of [[stomata]] observed on fossil plant leaves.<ref name="Beerling-2006" />

[[Phytane]] is a type of [[diterpenoid]] [[alkane]]. It is a breakdown product of chlorophyll, and is now used to estimate ancient {{CO2}} levels.<ref name="Witkowski-2018">{{cite journal |last1=Witkowski |first1=Caitlyn |date=28 November 2018 |title=Molecular fossils from phytoplankton reveal secular pCO<sub>2</sub> trend over the Phanerozoic |journal=Science Advances |volume=2 |issue=11 |pages=eaat4556 |bibcode=2018SciA....4.4556W |doi=10.1126/sciadv.aat4556 |pmc=6261654 |pmid=30498776}}</ref> Phytane gives both a continuous record of {{CO2}} concentrations but it also can overlap a break in the {{CO2}} record of over 500 million years.<ref name="Witkowski-2018" />

==== 600 to 400 million years ago ====
There is evidence for high {{CO2}} concentrations of over 6,000 ppm between 600 and 400 million years ago, and of over 3,000 ppm between 200 and 150 million years ago.<ref name="IPCC-2001">{{Cite web |title=IPCC: Climate Change 2001: The Scientific Basis |url=https://www.ipcc.ch/site/assets/uploads/2018/07/WG1_TAR_FM.pdf |url-status=live |archive-url=https://web.archive.org/web/20220829124925/https://www.ipcc.ch/site/assets/uploads/2018/07/WG1_TAR_FM.pdf |archive-date=29 August 2022 |access-date=14 March 2023}}</ref>{{Failed verification|date=May 2024}}

Indeed, higher CO<sub>2</sub> concentrations are thought to have prevailed throughout most of the [[Phanerozoic]] [[Eon (geology)|Eon]], with concentrations four to six times current concentrations during the Mesozoic era, and ten to fifteen times current concentrations during the early Palaeozoic era until the middle of the [[Devonian]] period, about 400 million years ago.<ref name="Berner-1994">{{cite journal |last=Berner |first=Robert A. |date=January 1994 |title=GEOCARB II: a revised model of atmospheric CO<sub>2</sub> over Phanerozoic time |journal=American Journal of Science |volume=294 |issue=1 |pages=56–91 |bibcode=1994AmJS..294...56B |doi=10.2475/ajs.294.1.56 |doi-access=free}}</ref><ref>{{cite journal |last=Royer |first=D.L. |author2=R.A. Berner |author3=D.J. Beerling |author-link3=David Beerling |year=2001 |title=Phanerozoic atmospheric CO<sub>2</sub> change: evaluating geochemical and paleobiological approaches |journal=Earth-Science Reviews |volume=54 |issue=4 |pages=349–92 |bibcode=2001ESRv...54..349R |doi=10.1016/S0012-8252(00)00042-8}}</ref><ref name="Berner-2001">{{cite journal |last=Berner |first=Robert A. |author2=Kothavala, Zavareth |year=2001 |title=GEOCARB III: a revised model of atmospheric CO<sub>2</sub> over Phanerozoic time |url=https://earth.geology.yale.edu/~ajs/2001/Feb/qn020100182.pdf |url-status=live |journal=American Journal of Science |volume=301 |issue=2 |pages=182–204 |bibcode=2001AmJS..301..182B |citeseerx=10.1.1.393.582 |doi=10.2475/ajs.301.2.182 |archive-url=https://web.archive.org/web/20060425205109/https://earth.geology.yale.edu/~ajs/2001/Feb/qn020100182.pdf |archive-date=25 April 2006}}</ref> The spread of land plants is thought to have reduced CO<sub>2</sub> concentrations during the late Devonian, and plant activities as both sources and sinks of CO<sub>2</sub> have since been important in providing stabilizing feedbacks.<ref name="Beerling-2005">{{cite journal |last=Beerling |first=D.J. |author-link=David Beerling |author2=Berner, R.A. |year=2005 |title=Feedbacks and the co-evolution of plants and atmospheric CO<sub>2</sub> |journal=Proc. Natl. Acad. Sci. USA |volume=102 |issue=5 |pages=1302–05 |bibcode=2005PNAS..102.1302B |doi=10.1073/pnas.0408724102 |pmc=547859 |pmid=15668402 |doi-access=free}}</ref> 

Earlier still, a 200-million year period of intermittent, widespread glaciation extending close to the equator ([[Snowball Earth]]) appears to have been ended suddenly, about 550 Ma, by a colossal volcanic outgassing that raised the {{CO2}} concentration of the atmosphere abruptly to 12%, about 350 times modern levels, causing extreme greenhouse conditions and carbonate deposition as [[limestone]] at the rate of about 1&nbsp;mm per day.<ref name="Hoffmann-1998">{{cite journal |last=Hoffmann |first=PF |author2=AJ Kaufman |author3=GP Halverson |author4=DP Schrag |year=1998 |title=A neoproterozoic snowball earth |journal=Science |volume=281 |issue=5381 |pages=1342–46 |bibcode=1998Sci...281.1342H |doi=10.1126/science.281.5381.1342 |pmid=9721097 |s2cid=13046760}}</ref> This episode marked the close of the [[Precambrian]] Eon, and was succeeded by the generally warmer conditions of the Phanerozoic, during which multicellular animal and plant life evolved. No volcanic CO<sub>2</sub> emission of comparable scale has occurred since. In the modern era, emissions to the atmosphere from volcanoes are approximately 0.645&nbsp;billion tons of {{CO2}} per year, whereas humans contribute 29&nbsp;billion tons of {{CO2}} each year.<ref>{{Cite news |last=Siegel |first=Ethan |title=How Much CO2 Does A Single Volcano Emit? |language=en |work=Forbes |url=https://www.forbes.com/sites/startswithabang/2017/06/06/how-much-co2-does-a-single-volcano-emit/ |url-status=live |access-date=2018-09-06 |archive-url=https://web.archive.org/web/20170606122158/https://www.forbes.com/sites/startswithabang/2017/06/06/how-much-co2-does-a-single-volcano-emit/ |archive-date=6 June 2017}}</ref><ref name="Hoffmann-1998" /><ref name="Gerlach-1991">{{cite journal |last=Gerlach |first=TM |year=1991 |title=Present-day CO<sub>2</sub> emissions from volcanoes |journal=Transactions of the American Geophysical Union |volume=72 |issue=23 |pages=249–55 |bibcode=1991EOSTr..72..249. |doi=10.1029/90EO10192}}</ref><ref>See also: {{cite web |date=14 June 2011 |title=U.S. Geological Survey |url=http://www.usgs.gov/newsroom/article.asp?ID=2827&from=rss_home |url-status=live |archive-url=https://web.archive.org/web/20120925170105/http://www.usgs.gov/newsroom/article.asp?ID=2827&from=rss_home |archive-date=25 September 2012 |access-date=15 October 2012}}</ref>

==== 60 to 5 million years ago ====
Atmospheric {{CO2}} concentration continued to fall after about 60 million years ago. About 34 million years ago, the time of the [[Eocene–Oligocene extinction event]] and when the [[Antarctic ice sheet]] started to take its current form, {{CO2}} was about 760 ppm,<ref>{{cite web |date=13 September 2009 |title=New CO<sub>2</sub> data helps unlock the secrets of Antarctic formation |url=http://www.physorg.com/news172072921.html |url-status=live |archive-url=https://web.archive.org/web/20110715082838/http://www.physorg.com/news172072921.html |archive-date=15 July 2011 |access-date=28 January 2010 |publisher=Physorg.com}}</ref> and there is geochemical evidence that concentrations were less than 300 ppm by about 20 million years ago. Decreasing {{CO2}} concentration, with a tipping point of 600 ppm, was the primary agent forcing Antarctic glaciation.<ref>{{cite journal |last1=Pagani |first1=Mark |last2=Huber |first2=Matthew |last3=Liu |first3=Zhonghui |last4=Bohaty |first4=Steven M. |last5=Henderiks |first5=Jorijntje |last6=Sijp |first6=Willem |last7=Krishnan |first7=Srinath |last8=Deconto |first8=Robert M. |date=2 December 2011 |title=Drop in carbon dioxide levels led to polar ice sheet, study finds |url=https://www.sciencedaily.com/releases/2011/12/111201174225.htm |url-status=live |journal=Science |volume=334 |issue=6060 |pages=1261–4 |bibcode=2011Sci...334.1261P |doi=10.1126/science.1203909 |pmid=22144622 |s2cid=206533232 |archive-url=https://web.archive.org/web/20130522065758/http://www.sciencedaily.com/releases/2011/12/111201174225.htm |archive-date=22 May 2013 |access-date=14 May 2013}}</ref> Low {{CO2}} concentrations may have been the stimulus that favored the evolution of [[C4 carbon fixation|{{C4}}]] plants, which increased greatly in abundance between 7 and 5 million years ago.<ref name="Beerling-2006" />

{{anchor|Greenhouse gas#water vapor feedback|Greenhouse gas#water-vapor feedback}}

==See also==
* [[Carbon budget]]
* [[Global temperature record]]

==References==
{{Reflist}}

==External links==
*[https://earth.nullschool.net/#current/chem/surface/level/overlay=co2sc/winkel3 Current global map of carbon dioxide concentrations.]
*[https://www.jpl.nasa.gov/news/nasa-releases-new-eye-popping-view-of-carbon-dioxide Global Carbon Dioxide Circulation] ([[NASA]]; 13 December 2016)
*[https://www.youtube.com/watch?v=x1SgmFa0r04 Video (03:10) – A Year in the Life of Earth's {{CO2}}] ([[NASA]]; 17 November 2014)

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[[Category:Atmosphere of Earth]]
[[Category:Carbon dioxide]]
[[Category:Greenhouse gases]]

[[fr:Dioxyde de carbone#CO2 dans l'atmosphère terrestre]]