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{{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=1.2|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 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 several greenhouse gases in the atmosphere of Earth. The current global average concentration of carbon dioxide (CO2) in the atmosphere is 421 ppm as of May 2022 (0.04%).WEB, Carbon dioxide now more than 50% higher than pre-industrial levels {{!, National Oceanic and Atmospheric Administration |url=https://www.noaa.gov/news-release/carbon-dioxide-now-more-than-50-higher-than-pre-industrial-levels |access-date=2022-06-14 |website=www.noaa.gov |date=3 June 2022 |archive-date=5 June 2022 |archive-url=https://web.archive.org/web/20220605004925weblink |url-status=live }} 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.BOOK, Eggleton, Tony,weblink A Short Introduction to Climate Change, 2013, Cambridge University Press, 9781107618763, 52, 14 March 2023, 14 March 2023,weblink live, WEB, The NOAA Annual Greenhouse Gas Index (AGGI) – An Introduction,weblink live,weblink 27 November 2020, 2020-12-18, NOAA Global Monitoring Laboratory/Earth System Research Laboratories, The increase is due to human activity.JOURNAL, Etheridge, D.M., L.P. Steele, R.L. Langenfelds, R.J. Francey, J.-M. Barnola, V.I. Morgan, 1996, Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn, Journal of Geophysical Research, 101, D2, 4115–28, 1996JGR...101.4115E, 10.1029/95JD03410, 19674607, 0148-0227, As of March 2024, the monthly average concentration of CO2 reached a new record high of 425.22 parts per million (ppm), marking an increase of 4.7 ppm over March 2023. By the latest measurement, levels had further escalated to 427.48 ppm.WEB, Pierre-Louis, Kendra, May 10, 2024, Carbon Dioxide Just Took an Ominous, Record-Breaking Jump,weblink 2024-05-13, www.bloomberg.com, This continuous increase in CO2 concentrations is a clear indicator of ongoing global environmental stress, primarily driven by the burning of fossil fuels, which is the principal cause of this rise and also a major contributor to climate change.IPCC (2022) Summary for policy makers {{Webarchive|url=https://web.archive.org/web/20230312040126weblink|date=12 March 2023}} in 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/20220802125242weblink|date=2 August 2022}}, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA Other significant human activities that emit CO2 include cement production, deforestation, and biomass burning.Carbon dioxide is a greenhouse gas. It absorbs and emits infrared radiation at its two infrared-active vibrational frequencies. The two wavelengths are 4.26 Î¼m (2,347 cm−1) (asymmetric stretching vibrational mode) and 14.99 Î¼m (667 cm−1) (bending vibrational mode). CO2 plays a significant role in influencing Earth's surface temperature through the greenhouse effect.JOURNAL, Petty, G.W., 2004, A First Course in Atmospheric Radiation, Eos Transactions, 85, 36, 229–51, 2004EOSTr..85..341P, 10.1029/2004EO360007, free, Light emission from the Earth's surface is most intense in the infrared region between 200 and 2500 cm−1,BOOK,weblink Atkins' Physical Chemistry, Atkins P, de Paula J, 2006, W. H. Freeman, 978-0-7167-8759-4, 8th, 462, Peter Atkins, 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.WEB, 2012, Carbon Dioxide Absorbs and Re-emits Infrared Radiation,weblink live,weblink 21 September 2017, 9 September 2017, UCAR Center for Science Education, 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 rise in average global temperature and ocean acidification. Another direct effect is the 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. The present atmospheric concentration of {{CO2}} is the highest for 14 million years.WEB, AHMED, Issam, Current carbon dioxide levels last seen 14 million years ago,weblink 2024-02-08, phys.org, en, Concentrations of {{CO2}} in the atmosphere were as high as 4,000 ppm during the Cambrian period about 500 million years ago, and as low as 180 ppm during the Quaternary glaciation of the last two million years. 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).WEB, Climate and CO2 in the Atmosphere,weblink live,weblink" title="web.archive.org/web/20181006151450weblink">weblink 6 October 2018, 10 October 2007, {{TOC limit|3}}

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 ProjectGlobal Carbon Project

Current situation

Since the start of the Industrial Revolution, atmospheric {{co2}} concentration have been increasing, causing global warming and ocean acidification.JOURNAL, Friedlingstein, Pierre, O'Sullivan, Michael, Jones, Matthew W., Andrew, Robbie M., Gregor, Luke, Hauck, Judith, Le Quéré, Corinne, Luijkx, Ingrid T., Olsen, Are, Peters, Glen P., Peters, Wouter, Pongratz, Julia, Schwingshackl, Clemens, Sitch, Stephen, Canadell, Josep G., 2022-11-11, Global Carbon Budget 2022, Earth System Science Data, 14, 11, 4811–4900, 2022ESSD...14.4811F, 10.5194/essd-14-4811-2022, free, 20.500.11850/594889, free, {{Creative Commons text attribution notice|cc=by4|from this source=yes}} 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)."Parts per million" refers to the number of carbon dioxide molecules per million molecules of dry air. WEB, Carbon Dioxide LATEST MEASUREMENT, NASA Global Climate Change,weblink Climate Change: Vital Signs of the Planet, 17 April 2022,weblink live, Updated monthly. Figures are published monthly by the National Oceanic & Atmospheric Administration (NOAA).WEB, Global Monitoring Laboratory - Trends in Atmospheric Carbon Dioxide, National Oceanic & Atmospheric Administration,weblink Latest figure, and graphs of trend; frequently updatedWEB, Table of atmospheric CO₂ since 1958, updated monthly, National Oceanic & Atmospheric Administration,weblink 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., The value had been about 280 ppm during the 10,000 years up to the mid-18th century.Each part per million of {{co2}} in the atmosphere represents approximately 2.13 gigatonnes of carbon, or 7.82 gigatonnes of {{CO2}}.WEB, 18 July 2020, Conversion Tables,weblink live,weblink" title="web.archive.org/web/20170927015828weblink">weblink 27 September 2017, 18 July 2020, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Alt URL {{Webarchive|url=https://web.archive.org/web/20160223232340weblink|date=23 February 2016}}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".{{rp|515}}It has been estimated that 2,400 gigatons of CO₂ have been emitted by human activity since 1850, with some absorbed by oceans and land, and about 950 gigatons remaining in the atmosphere. Around 2020 the emission rate was over 40 gigatons per year.WEB, The World Counts, The World Counts,weblink 4 December 2023, 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.JOURNAL, Archer D, 2009, Atmospheric lifetime of fossil fuel carbon dioxide,weblink live, Annual Review of Earth and Planetary Sciences, 37, 1, 117–34, 2009AREPS..37..117A, 10.1146/annurev.earth.031208.100206,weblink 24 February 2021, 7 March 2021, 2268/12933, JOURNAL, 6, Joos F, Roth R, Fuglestvedt JS, Peters GP, Enting IG, Von Bloh W, Brovkin V, Burke EJ, Eby M, Edwards NR, Friedrich T, 2013, Carbon dioxide and climate impulse response functions for the computation of greenhouse gas metrics: A multi-model analysis,weblink live, Atmospheric Chemistry and Physics, 13, 5, 2793–2825, 10.5194/acpd-12-19799-2012,weblink 22 July 2020, 7 March 2021, free, free, 20.500.11850/58316,

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 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 plant biomass 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.WEB, Rasmussen, Carl Edward, Atmospheric Carbon Dioxide Growth Rate,weblink 14 March 2023, 14 March 2023,weblink live, WEB, Frequently Asked Questions,weblink dead,weblink" title="web.archive.org/web/20110817044713weblink">weblink 17 August 2011, 13 June 2007, Carbon Dioxide Information Analysis Center (CDIAC), Concentrations also vary on a regional basis, most strongly near the ground with much smaller variations aloft. In urban areas concentrations are generally higherJOURNAL, George K, Ziska LH, Bunce JA, Quebedeaux B, 2007, Elevated atmospheric CO2 concentration and temperature across an urban–rural transect,weblink live, Atmospheric Environment, 41, 35, 7654–7665, 2007AtmEn..41.7654G, 10.1016/j.atmosenv.2007.08.018,weblink 15 October 2019, 12 September 2019, 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.WEB, Tans, Pieter, Trends in Carbon Dioxide,weblink 2009-12-11, National Oceanic and Atmospheric Administration, NOAA/Earth System Research Laboratories, ESRL, 25 January 2013,weblink" title="web.archive.org/web/20130125014026weblink">weblink live, WEB, Carbon Budget 2009 Highlights,weblink dead,weblink" title="web.archive.org/web/20111216001323weblink">weblink 16 December 2011, 2012-11-02, globalcarbonproject.org,
• The daily average concentration of atmospheric {{co2}} at Mauna Loa Observatory first exceeded 400 ppm on 10 May 2013NEWS,weblink BBC, Carbon dioxide passes symbolic mark, 10 May 2013, 10 May 2013, 23 May 2019,weblink live, WEB,weblink Up-to-date weekly average CO2 at Mauna Loa, 2019-06-01, NOAA, 24 May 2019,weblink live, although this concentration had already been reached in the Arctic in June 2012.NEWS,weblink The Guardian, 11 May 2013, 1 June 2012, Greenhouse gas levels pass symbolic 400ppm CO2 milestone, Associated Press, 22 January 2014,weblink" title="web.archive.org/web/20140122000841weblink">weblink live, 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."WEB, Kunzig, Robert, 2013-05-09, Climate Milestone: Earth's CO2 Level Passes 400 ppm,weblink 2013-05-12, National Geographic (magazine), National Geographic, 15 December 2013,weblink" title="web.archive.org/web/20131215081625weblink">weblink dead,
• As of 2018, {{CO2}} concentrations were measured to be 410 ppm.WEB, Trends in Atmospheric Carbon Dioxide,weblink Earth System Research Laboratories, NOAA, 14 March 2023, 25 January 2013,weblink" title="web.archive.org/web/20130125014026weblink">weblink live,

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 just ppm). To convert from the usual ppmv units to ppm mass, multiply by the ratio of the molar weight of CO2 to that of air, i.e. times 1.52 (44.01 divided by 28.96).The first reproducibly accurate measurements of atmospheric CO2 were from flask sample measurements made by Dave Keeling at Caltech in the 1950s.WEB, 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/20221008043137weblink |url-status=live }} 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 NOAA/ERSL,WEB,weblink NOAA CCGG page Retrieved 2 March 2016, 14 March 2023, 11 August 2011,weblink" title="web.archive.org/web/20110811160744weblink">weblink live, WDCGG,WDCGG webpage {{Webarchive|url=https://web.archive.org/web/20160406090043weblink|date=6 April 2016}} Retrieved 2 March 2016 and RAMCES.RAMCES webpage {{dead link|date=November 2016|bot=InternetArchiveBot|fix-attempted=yes}} Retrieved 2 March 2016 The NOAA/ESRL Baseline Observatory Network, and the Scripps Institution of Oceanography NetworkWEB,weblink CDIAC CO2 page Retrieved 9 February 2016, 14 March 2023, 13 August 2011,weblink" title="web.archive.org/web/20110813142008weblink">weblink live, data are hosted at the CDIAC at ORNL. The World Data Centre for Greenhouse Gases (WDCGG), part of GAW, data are hosted by the JMA. The Reseau Atmospherique de Mesure des Composes an Effet de Serre database (RAMCES) is part of 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.WEB,weblink GLOBALVIEW-CO2 information page. Retrieved 9 February 2016, 14 March 2023, 31 January 2020,weblink live, Ongoing ground-based total column measurements began more recently. Column measurements typically refer to an averaged column amount denoted X{{CO2}}, rather than a surface only measurement. These measurements are made by the TCCON. These data are also hosted on the CDIAC, and made publicly available according to the data use policy.WEB,weblink TCCON data use policy webpage Retrieved 9 February 2016, 14 March 2023, 17 October 2020,weblink live,

Satellite measurements

Space-based measurements of carbon dioxide are also a recent addition to atmospheric X{{CO2}} measurements. SCIAMACHY aboard ESA's ENVISAT made global column X{{CO2}} measurements from 2002 to 2012. AIRS aboard NASA's Aqua satellite makes global X{{CO2}} measurements and was launched shortly after ENVISAT in 2012. More recent satellites have significantly improved the data density and precision of global measurements. Newer missions have higher spectral and spatial resolutions. JAXA's GOSAT was the first dedicated GHG monitoring satellite to successfully achieve orbit in 2009. NASA's OCO-2 launched in 2014 was the second. Various other satellites missions to measure atmospheric X{{CO2}} are planned.

Analytical methods to investigate sources of CO2

• The burning of long-buried fossil fuels releases {{CO2}} containing carbon of different isotopic ratios to those of living plants, enabling distinction between natural and human-caused contributions to {{CO2}} concentration.e.g. JOURNAL, Gosh, Prosenjit, Brand, Willi A., 2003, Stable isotope ratio mass spectrometry in global climate change research,weblink International Journal of Mass Spectrometry, 228, 1, 1–33, 2003IJMSp.228....1G, 10.1.1.173.2083, 10.1016/S1387-3806(03)00289-6, 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/20170811075226weblink |url-status=live }}
• 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.JOURNAL, Keeling, Charles D., Piper, Stephen C., Whorf, Timothy P., Keeling, Ralph F., 2011, Evolution of natural and anthropogenic fluxes of atmospheric CO2 from 1957 to 2003, Tellus B, 63, 1, 1–22, 2011TellB..63....1K, 10.1111/j.1600-0889.2010.00507.x, 0280-6509, free,
• Atmospheric O{{sub|2}} levels are decreasing in Earth's atmosphere as it reacts with the carbon in fossil fuels to form {{CO2}}.JOURNAL, Bender, Michael L., Ho, David T., Hendricks, Melissa B., Mika, Robert, Battle, Mark O., Tans, Pieter P., Conway, Thomas J., Sturtevant, Blake, Cassar, Nicolas, 2005, Atmospheric O2/N2changes, 1993–2002: Implications for the partitioning of fossil fuel CO2sequestration, Global Biogeochemical Cycles, 19, 4, n/a, 2005GBioC..19.4017B, 10.1029/2004GB002410, 0886-6236, free,

Causes of the current increase

{{anchor|Anthropogenic CO2 increase}} Anthropogenic CO2 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.WEB, Evans, Simon, 5 October 2021, Analysis: Which countries are historically responsible for climate change? / Historical responsibility for climate change is at the heart of debates over climate justice.,weblink live,weblink 26 October 2021, CarbonBrief.org, Carbon Brief, 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)., )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.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: Chapter 3: Human Influence on the Climate System {{Webarchive|url=https://web.archive.org/web/20230307162843weblink |date=7 March 2023 }}. In 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/20210809131444weblink |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}} Anthropogenic carbon emissions exceed the amount that can be taken up or balanced out by natural sinks.JOURNAL, Ballantyne, A.P., Alden, C.B., Miller, J.B., Tans, P.P., White, J.W.C., 2012, Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years, Nature, 488, 7409, 70–72, 2012Natur.488...70B, 10.1038/nature11299, 0028-0836, 22859203, 4335259, Thus carbon dioxide has gradually accumulated in the atmosphere and, as of May 2022, its concentration is 50% above pre-industrial levels.The extraction and burning of fossil fuels, releasing carbon that has been underground for many millions of years, has increased the atmospheric concentration of {{CO2}}. As of year 2019 the extraction and burning of geologic fossil carbon by humans releases over 30 gigatonnes of {{CO2}} (9 billion tonnes carbon) each year.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). This larger disruption to the natural balance is responsible for recent growth in the atmospheric {{CO2}} concentration.WEB, Dlugokencky, E., 5 February 2016, Annual Mean Carbon Dioxide Data,weblink 12 February 2016, Earth System Research Laboratories, NOAA, 14 March 2023,weblink" title="web.archive.org/web/20230314104205weblink">weblink live, Currently about half of the carbon dioxide released from the burning of fossil fuels is not absorbed by vegetation and the oceans and remains in the atmosphere.JOURNAL, A.P. Ballantyne, C.B. Alden, J.B. Miller, P.P. Tans, J.W. C. White, 2012, Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years, Nature (journal), Nature, 488, 7409, 70–72, 2012Natur.488...70B, 10.1038/nature11299, 22859203, 4335259, Burning fossil fuels such as coal, petroleum, and natural gas is the leading cause of increased anthropogenic {{CO2}}; deforestation is the second major cause. In 2010, 9.14 gigatonnes of carbon (GtC, equivalent to 33.5 gigatonnes 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.WEB, Global carbon budget 2010 (summary),weblink dead,weblink" title="web.archive.org/web/20120723220134weblink">weblink 23 July 2012, Tyndall Centre for Climate Change Research, In addition, land use change contributed 0.87 GtC in 2010, compared to 1.45 GtC in 1990. 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.Calculated from file global.1751_2013.csv in weblink {{Webarchive|url=https://web.archive.org/web/20111022125534weblink|date=22 October 2011}} from the Carbon Dioxide Information Analysis Center.The International Energy Agency estimates that the top 1% of emitters globally each had 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.IEA (2023), The world’s top 1% of emitters produce over 1000 times more {{CO2}} than the bottom 1%, IEA, Parisweblink , License: CC BY 4.0

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|CO2 reduces the flux of thermal radiation emitted to space (causing the large dip near 667 cm−1), thereby contributing to the greenhouse effect.)File:Longwave Absorption Coefficients of H2O and CO2.svg|thumb|Longwave-infrared absorption coefficientabsorption coefficientEarth's natural greenhouse effect makes life as we know it possible and carbon dioxide 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.WEB,weblink Annex II Glossary, Intergovernmental Panel on Climate Change, 15 October 2010, 3 November 2018,weblink" title="web.archive.org/web/20181103000935weblink">weblink dead, A concise description of the greenhouse effect is given in the Intergovernmental Panel on Climate Change Fourth Assessment Report, "What is the Greenhouse Effect?" FAQ 1.3 – AR4 WGI Chapter 1: Historical Overview of Climate Change Science {{Webarchive|url=https://web.archive.org/web/20181130152334weblink |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."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.E. Claussen, V.A. Cochran, and D.P. Davis, Climate Change: Science, Strategies, & Solutions, University of Michigan, 2001. p. 373.A. Allaby and M. Allaby, A Dictionary of Earth Sciences, Oxford University Press, 1999, {{ISBN|0-19-280079-5}}, p. 244.BOOK Water is responsible for most (about 36–70%) of the total greenhouse effect, and the role of water vapor as a greenhouse gas depends on temperature. On Earth, carbon dioxide is the most relevant, direct anthropologically influenced greenhouse gas. 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 CO2 was estimated to be responsible for 1.82 W m−2 of the 2.63 W m−2 change in radiative forcing on Earth (about 70%).WEB,weblink IPCC Fifth Assessment Report – Chapter 8: Anthropogenic and Natural Radiative Forcing., 14 March 2023, 22 October 2018,weblink live, The concept of atmospheric CO2 increasing ground temperature was first published by Svante Arrhenius in 1896.JOURNAL, Svante, Arrhenius, On the influence of carbonic acid in the air upon the temperature of the ground, Philosophical Magazine and Journal of Science, 1896, 237–76,weblink 14 March 2023, 18 November 2020,weblink live, The increased radiative forcing due to increased CO2 in the Earth's atmosphere is based on the physical properties of CO2 and the non-saturated absorption windows where CO2 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.{{carbon cycle|Carbon dioxide}}

Carbon cycle

(File:Carbon cycle.jpg|thumb|left|This diagram of the fast 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.WEB, Riebeek, Holli, The Carbon Cycle,weblink Earth Observatory, NASA, 5 April 2018, 16 June 2011,weblink" title="web.archive.org/web/20160305010126weblink">weblink 5 March 2016, live, dmy-all, )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 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 matter, wildfires and the respiration processes of living aerobic organisms. Man-made sources of {{CO2}} include the burning of fossil fuels for heating, power generation and transport, as well as some industrial processes such as cement making. It is also produced by various microorganisms from fermentation and cellular respiration. Plants, algae and cyanobacteria convert carbon dioxide to carbohydrates by a process called photosynthesis. They gain the energy needed for this reaction from absorption of sunlight by chlorophyll and other pigments. Oxygen, produced as a by-product of photosynthesis, is released into the atmosphere and subsequently used for respiration by heterotrophic organisms and other plants, forming a cycle with carbon.(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.)Most sources of {{CO2}} emissions are natural, and are balanced to various degrees by similar {{CO2}} sinks. For example, the decay of organic material in forests, grasslands, and other land vegetation - including forest fires - results in the release of about 436 gigatonnes of {{CO2}} (containing 119 gigatonnes carbon) every year, while {{CO2}} uptake by new growth on land counteracts these releases, absorbing 451 Gt (123 Gt C).BOOK,weblink Considering Forest and Grassland Carbon in Land Management, Kayler, Z., Janowiak, M., Swanston, C., General Technical Report WTO-GTR-95, United States Department of Agriculture, Forest Service, The Global Carbon Cycle, 3–9, 2017, 14 March 2023, 7 July 2022,weblink live, Although much {{CO2}} in the early atmosphere of the young Earth was produced by volcanic activity, modern volcanic activity releases only 130 to 230 megatonnes of {{CO2}} each year.JOURNAL, Gerlach, T.M., Present-day CO2 emissions from volcanoes, Eos (journal), Eos, Transactions, American Geophysical Union, 72, 23, 249, 254–55, 4 June 1991, 10.1029/90EO10192, 1991EOSTr..72..249., Natural sources are more or less balanced by natural sinks, in the form of chemical and biological processes which remove {{CO2}} from the atmosphere.Overall, there is a large natural flux of atmospheric {{CO2}} into and out of the biosphere, both on land and in the oceans.BOOK, Cappelluti, G., Bösch, H., Monks, P.S., Use of remote sensing techniques for the detection and monitoring of GHG emissions from the Scottish land use sector, Scottish Government, 2009, 978-0-7559-7738-3,weblink 28 January 2011, 8 June 2011,weblink" title="web.archive.org/web/20110608110514weblink">weblink live, In the pre-industrial era, each of these fluxes were in balance to such a degree that little net {{CO2}} flowed between the land and ocean reservoirs of carbon, and little change resulted in the atmospheric concentration. From the human pre-industrial era to 1940, the terrestrial biosphere represented a net source of atmospheric {{CO2}} (driven largely by land-use changes), but subsequently switched to a net sink with growing fossil carbon emissions. In 2012, about 57% of human-emitted {{CO2}}, mostly from the burning of fossil carbon, was taken up by land and ocean sinks.JOURNAL, Canadell JG, Le Quéré C, Raupach MR, Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks, Proc. Natl. Acad. Sci. U.S.A., 104, 47, 18866–70, November 2007, 17962418, 2141868, 10.1073/pnas.0702737104, 2007PNAS..10418866C, etal, free, JOURNAL, Junling Huang, Michael B. McElroy,weblink The Contemporary and Historical Budget of Atmospheric CO2, Canadian Journal of Physics, 90, 8, 707–16, 2012, 10.1139/p2012-033, 2012CaJPh..90..707H, 14 March 2023, 3 August 2017,weblink live, The ratio of the increase in atmospheric {{CO2}} to emitted {{CO2}} is known as the airborne fraction. This ratio varies in the short-term and is typically about 45% over longer (5-year) periods. Estimated carbon in global terrestrial vegetation increased from approximately 740 gigatonnes in 1910 to 780 gigatonnes in 1990.JOURNAL, Post WM, King AW, Wullschleger SD, Hoffman FM, Historical Variations in Terrestrial Biospheric Carbon Storage, DOE Research Summary, 34, 1, June 1997,weblink 1997GBioC..11...99P, 99–109, 10.1029/96GB03942, free, 28 May 2011, 28 July 2011,weblink" title="web.archive.org/web/20110728063520weblink">weblink live,

Photosynthesis

{{See also|photosynthesis|Carbon fixation|C3 carbon fixation|l3={{C3}} carbon fixation|C4 carbon fixation|l4={{C4}} carbon fixation}}{{stack|(File:Simple photosynthesis overview.svg|thumb|upright|Photosynthesis changes sunlight into chemical energy, splits water to liberate O2, and fixes CO2 into sugar.)}}Carbon dioxide in the Earth's atmosphere is essential to life and to most of the planetary biosphere. The average rate of energy capture by photosynthesis globally is approximately 130 terawatts,JOURNAL, Nealson KH, Conrad PG, Life: past, present and future, Philos. Trans. R. Soc. Lond. B Biol. Sci., 354, 1392, 1923–39, December 1999, 10670014, 1692713, 10.1098/rstb.1999.0532, BOOK, Singhal GS, Renger G, Sopory SK, Irrgang KD, Govindjee, Whitmarsh J, Govindjee, Concepts in photobiology: photosynthesis and photomorphogenesis, The photosynthetic process, Kluwer Academic Publishers, Boston, 1999, 11–51, 978-0-7923-5519-9,weblink 100 x 1015 grams of carbon/year fixed by photosynthetic organisms which is equivalent to 4 x 1018 kJ/yr = 4 x 1021J/yr of free energy stored as reduced carbon; (4 x 1018 kJ/yr) / (31,556,900 sec/yr) = 1.27 x 1014 J/yr; (1.27 x 1014 J/yr) / (1012 J/sec / TW) = 127 TW., 20 March 2014, 14 August 2010,weblink" title="web.archive.org/web/20100814191216weblink">weblink live, BOOK, Steger U, Achterberg W, Blok K, Bode H, Frenz W, Gather C, Hanekamp G, Imboden D, Jahnke M, Kost M, Kurz R, Nutzinger HG, Ziesemer T, Sustainable development and innovation in the energy sector, Springer, Berlin, 2005, 32, 978-3-540-23103-5,weblink The average global rate of photosynthesis is 130 TW (1 TW = 1 terawatt = 1012 watt)., 2023-03-14, 2023-03-14,weblink live, which is about six times larger than the current power consumption of human civilization.WEB, Energy Information Administration,weblink World Consumption of Primary Energy by Energy Type and Selected Country Groups, 1980–2004, XLS, 31 July 2006, 2007-01-20, dead,weblink" title="web.archive.org/web/20061109125803weblink">weblink 9 November 2006, dmy, Photosynthetic organisms also convert around 100–115 billion metric tonnes of carbon into biomass per year.JOURNAL, Field CB, Behrenfeld MJ, Randerson JT, Falkowski P, Primary production of the biosphere: integrating terrestrial and oceanic components, Science, 281, 5374, 237–40, July 1998, 9657713, 10.1126/science.281.5374.237, 1998Sci...281..237F,weblink 2023-03-14, 2018-09-25,weblink live, BOOK, McGraw-Hill Encyclopedia of Science & Technology, McGraw-Hill, New York, 2007, 978-0-07-144143-8, 13, Photosynthesis, Photosynthetic organisms are photoautotrophs, which means that they are able to synthesize food directly from {{CO2}} and water using energy from light. However, not all organisms that use light as a source of energy carry out photosynthesis, since photoheterotrophs use organic compounds, rather than {{CO2}}, as a source of carbon.JOURNAL, Bryant DA, Frigaard NU, Prokaryotic photosynthesis and phototrophy illuminated, Trends Microbiol., 14, 11, 488–96, November 2006, 16997562, 10.1016/j.tim.2006.09.001, In plants, algae and cyanobacteria, photosynthesis releases oxygen. This is called oxygenic photosynthesis. Although there are some differences between oxygenic photosynthesis in plants, algae, and cyanobacteria, the overall process is quite similar in these organisms. Some types of bacteria, however, carry out anoxygenic photosynthesis, which consumes {{CO2}} but does not release oxygen.{{citation needed|date=September 2023}}Carbon dioxide is converted into sugars in a process called carbon fixation. Carbon fixation is an endothermic redox reaction, so photosynthesis needs to supply both the source of energy to drive this process and the electrons needed to convert {{CO2}} into a carbohydrate. This addition of the electrons is a reduction reaction. In general outline and in effect, photosynthesis is the opposite of cellular respiration, in which glucose and other compounds are oxidized to produce {{CO2}} and water, and to release exothermic chemical energy to drive the organism's metabolism. The two processes take place through a different sequence of chemical reactions, however, and in different cellular compartments.{{citation needed|date=September 2023}}

Oceanic carbon cycle

{{See also|Biological pump|Solubility pump|Ocean acidification}}(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. One example is the dissolution of calcium carbonate: Reactions like this tend to buffer changes in atmospheric {{CO2}}. Since the right side of the reaction produces an acidic compound, adding {{CO2}} on the left side decreases the pH of seawater, a process which has been termed ocean acidification (pH of the ocean becomes more acidic although the pH value remains in the alkaline range). Reactions between {{CO2}} and non-carbonate rocks also add bicarbonate to the seas. This can later undergo the reverse of the above reaction to form carbonate rocks, releasing half of the bicarbonate as {{CO2}}. Over hundreds of millions of years, this has produced huge quantities of carbonate rocks.From 1850 until 2022, the ocean has absorbed 26% of total anthropogenic emissions. 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.JOURNAL, Susan Solomon, Gian-Kasper Plattner, Reto Knutti, Pierre Friedlingstein, February 2009, Irreversible climate change due to carbon dioxide emissions, Proc. Natl. Acad. Sci. USA, 106, 6, 1704–09, 2009PNAS..106.1704S, 10.1073/pnas.0812721106, 2632717, 19179281, free, JOURNAL, Archer, David, Eby, Michael, Brovkin, Victor, Ridgwell, Andy, Cao, Long, Mikolajewicz, Uwe, Caldeira, Ken, Matsumoto, Katsumi, Munhoven, Guy, Montenegro, Alvaro, Tokos, Kathy, 2009, Atmospheric Lifetime of Fossil Fuel Carbon Dioxide,weblink Annual Review of Earth and Planetary Sciences, 37, 1, 117–34, 2009AREPS..37..117A, 10.1146/annurev.earth.031208.100206, 0084-6597, 2268/12933, 14 March 2023, 14 March 2023,weblink live, free, Carbon moves between the atmosphere, vegetation (dead and alive), the soil, the surface layer of the ocean, and the deep ocean.

Effects of current increase

Direct effects

File:Physical Drivers of climate change.svg|thumb|Radiative forcingRadiative forcingDirect effects of increasing CO2 concentrations in the atmosphere include increasing global temperatures, ocean acidification and a CO2 fertilization effect on plants and crops.JOURNAL, Keeling, Charles D., 1997-08-05, Climate change and carbon dioxide: An introduction, Proceedings of the National Academy of Sciences, en, 94, 16, 8273–8274, 10.1073/pnas.94.16.8273, 0027-8424, 33714, 11607732, 1997PNAS...94.8273K, free,

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|file=no}}

CO2 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.JOURNAL, Pisoft, Petr, May 25, 2021, Stratospheric contraction caused by increasing greenhouse gases, Environmental Research Letters, 16, 6, 064038, 2021ERL....16f4038P, 10.1088/1748-9326/abfe2b, free,

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

Wildfire caused by heat and dryness, Coral bleaching>bleached coral caused by ocean acidification and heating, coastal flooding caused by storms 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 CO2 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.)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, CO2 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. 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 deglaciationdeglaciationEstimates 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. However the IPCC Sixth Assessment Report estimated similar levels 3 to 3.3 million years ago in the mid-Pliocene warm period. This period can be a proxy for likely climate outcomes with current levels of {{CO2}}.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: Chapter 2: Changing State of the Climate System. In 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.{{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 reduce the gas to carbon compounds and oxygen. Methane, a very active greenhouse gas, may have been more prevalent as well.JOURNAL, Walker, James C.G., June 1985, Carbon dioxide on the early earth,weblink Origins of Life and Evolution of the Biosphere, 16, 2, 117–27, 1985OrLi...16..117W, 10.1007/BF01809466, 11542014, 2010-01-30, free, 2027.42/43349, 206804461, 14 September 2012,weblink" title="web.archive.org/web/20120914033408weblink">weblink live, JOURNAL, Pavlov, Alexander A., Kasting, James F., Brown, Lisa L., Rages, Kathy A., Freedman, Richard, May 2000, Greenhouse warming by CH4 in the atmosphere of early Earth, Journal of Geophysical Research, 105, E5, 11981–90, 2000JGR...10511981P, 10.1029/1999JE001134, 11543544, free, 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.JOURNAL, Zahnle, K., Schaefer, L., Laura K. Schaefer, Fegley, B., 2010, Earth's Earliest Atmospheres, Cold Spring Harbor Perspectives in Biology, 2, 10, a004895, 10.1101/cshperspect.a004895, 2944365, 20573713, A major part of carbon dioxide emissions were soon dissolved in water and incorporated in carbonate sediments.The production of free oxygen by cyanobacterial photosynthesis eventually led to the 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 period about 500 million years ago to as low as 180 parts per million 20,000 years ago . Drivers of ancient-Earth CO2 concentration {{See also|Biogeochemical cycle}}On long timescales, atmospheric {{CO2}} concentration is determined by the balance among geochemical processes including organic carbon burial in sediments, silicate rock weathering, and 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 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 evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide as sources of electrons, rather than water.JOURNAL, Olson JM, May 2006, Photosynthesis in the Archean era, Photosynth. Res., 88, 2, 109–17, 10.1007/s11120-006-9040-5, 16453059, 2006PhoRe..88..109O, 20364747, Cyanobacteria appeared later, and the excess oxygen they produced contributed to the oxygen catastrophe,JOURNAL, Buick R, August 2008, When did oxygenic photosynthesis evolve?, Philos. Trans. R. Soc. Lond. B Biol. Sci., 363, 1504, 2731–43, 10.1098/rstb.2008.0041, 2606769, 18468984, which rendered the 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}} plants which increased greatly in abundance between 7 and 5 million years ago over plants that use the less efficient {{C3}} metabolic pathway.JOURNAL, Osborne, C.P., Beerling, D.J., David Beerling, 2006, 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}} 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.JOURNAL, Lovelock, J. E., 1972, Gaia as seen through the atmosphere,weblink dead, Atmospheric Environment, 6, 8, 579–580, 1972AtmEn...6..579L, 10.1016/0004-6981(72)90076-5,weblink" title="web.archive.org/web/20111103150707weblink">weblink 2011-11-03, 2014-03-22, JOURNAL, Li, K.-F., 2009-05-30, Atmospheric pressure as a natural climate regulator for a terrestrial planet with a biosphere,weblink Proceedings of the National Academy of Sciences, 106, 24, 9576–9579, 2009PNAS..106.9576L, 10.1073/pnas.0809436106, 2701016, 19487662, 2014-03-22, free, 12 February 2013,weblink" title="web.archive.org/web/20130212184727weblink">weblink live, Measuring ancient-Earth CO2 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 CO2 (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)The most direct method for measuring atmospheric carbon dioxide concentrations for periods before instrumental sampling is to measure bubbles of air (fluid or gas inclusions) trapped in the 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 years.WEB, Etheridge, D.M., Steele, L.P., Langenfelds, R.L., Francey, R.J., Barnola, JM, Morgan, VI, June 1998, Historical CO2 record derived from a spline fit (20-year cutoff) of the Law Dome DE08 and DE08-2 ice cores,weblink dead,weblink" title="web.archive.org/web/20120305110732weblink">weblink 5 March 2012, 2007-06-12, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, JOURNAL, Flückiger, Jacqueline, 2002, High-resolution Holocene {{chem, N, 2, O, ice core record and its relationship with {{chem|CH|4}} and CO2 |journal=Global Biogeochemical Cycles |volume=16 |issue=1 |page=1010 |bibcode=2002GBioC..16.1010F |doi=10.1029/2001GB001417 |doi-access=free}} The longest ice core record comes from East Antarctica, where ice has been sampled to an age of 800,000 years.NEWS, Amos, J., 4 September 2006, Deep ice tells long climate story, BBC News,weblink 28 April 2010, 23 January 2013,weblink" title="web.archive.org/web/20130123202651weblink">weblink live, During this time, the atmospheric carbon dioxide concentration has varied between 180 and 210 ppm during ice ages, increasing to 280–300 ppm during warmer interglacials.JOURNAL, Hileman B., November 2005, Ice Core Record Extended: Analyses of trapped air show current CO2 at highest level in 650,000 years,weblink Chemical & Engineering News, 83, 48, 7, 10.1021/cen-v083n048.p007, 0009-2347, 28 January 2010, 15 May 2019,weblink" title="web.archive.org/web/20190515033556weblink">weblink live, Vostok Ice Core Data {{Webarchive|url=https://web.archive.org/web/20150227064302weblink |date=27 February 2015 }}, ncdc.noaa.gov {{Webarchive|url=https://web.archive.org/web/20210422034738weblink |date=22 April 2021 }}{{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 ppm during the period ten to seven thousand years ago,JOURNAL, Friederike Wagner, Bent Aaby, Henk Visscher, 2002, Rapid atmospheric CO2 changes associated with the 8,200-years-B.P. cooling event, Proc. Natl. Acad. Sci. USA, 99, 19, 12011–14, 2002PNAS...9912011W, 10.1073/pnas.182420699, 129389, 12202744, free, though others have argued that these findings more likely reflect calibration or contamination problems rather than actual CO2 variability.JOURNAL, Andreas Indermühle, Bernhard Stauffer, Thomas F. Stocker, 1999, Early Holocene Atmospheric CO2 Concentrations, Science, 286, 5446, 1815, 10.1126/science.286.5446.1815a, free, JOURNAL, IndermÃœhle, A, 1999, Early Holocene atmospheric CO2concentrations, Science, 286, 5446, 1815a–15, 10.1126/science.286.5446.1815a, free, JOURNAL, H. J. Smith, M. Wahlen, D. Mastroianni, 1997, The CO2 concentration of air trapped in GISP2 ice from the Last Glacial Maximum-Holocene transition, Geophysical Research Letters, 24, 1, 1–4, 1997GeoRL..24....1S, 10.1029/96GL03700, 129667062, 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 CO2 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 CO2 mole fractions stayed within a range of 180 ppm to 280 ppm throughout the last 800,000 years, until the increase of the last 250 years. However, various proxy measurements and models suggest larger variations in past epochs: 500 million years ago CO2 levels were likely 10 times higher than now.(:File:Phanerozoic Carbon Dioxide.png) Various proxy measurements have been used to try to determine atmospheric CO2 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.Phytane is a type of diterpenoid alkane. It is a breakdown product of chlorophyll, and is now used to estimate ancient {{CO2}} levels.JOURNAL, Witkowski, Caitlyn, 28 November 2018, Molecular fossils from phytoplankton reveal secular pCO2 trend over the Phanerozoic, Science Advances, 2, 11, eaat4556, 2018SciA....4.4556W, 10.1126/sciadv.aat4556, 6261654, 30498776, 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. 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.WEB, IPCC: Climate Change 2001: The Scientific Basis,weblink live,weblink 29 August 2022, 14 March 2023, {{Failed verification|date=May 2024}}Indeed, higher CO2 concentrations are thought to have prevailed throughout most of the Phanerozoic 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.JOURNAL, Berner, Robert A., January 1994, GEOCARB II: a revised model of atmospheric CO2 over Phanerozoic time, American Journal of Science, 294, 1, 56–91, 1994AmJS..294...56B, 10.2475/ajs.294.1.56, free, JOURNAL, Royer, D.L., R.A. Berner, D.J. Beerling, David Beerling, 2001, Phanerozoic atmospheric CO2 change: evaluating geochemical and paleobiological approaches, Earth-Science Reviews, 54, 4, 349–92, 2001ESRv...54..349R, 10.1016/S0012-8252(00)00042-8, JOURNAL, Berner, Robert A., Kothavala, Zavareth, 2001, GEOCARB III: a revised model of atmospheric CO2 over Phanerozoic time,weblink live, American Journal of Science, 301, 2, 182–204, 2001AmJS..301..182B, 10.1.1.393.582, 10.2475/ajs.301.2.182,weblink 25 April 2006, The spread of land plants is thought to have reduced CO2 concentrations during the late Devonian, and plant activities as both sources and sinks of CO2 have since been important in providing stabilizing feedbacks.JOURNAL, Beerling, D.J., David Beerling, Berner, R.A., 2005, Feedbacks and the co-evolution of plants and atmospheric CO2, Proc. Natl. Acad. Sci. USA, 102, 5, 1302–05, 2005PNAS..102.1302B, 10.1073/pnas.0408724102, 547859, 15668402, free, 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 mm per day.JOURNAL, Hoffmann, PF, AJ Kaufman, GP Halverson, DP Schrag, 1998, A neoproterozoic snowball earth, Science, 281, 5381, 1342–46, 1998Sci...281.1342H, 10.1126/science.281.5381.1342, 9721097, 13046760, 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 CO2 emission of comparable scale has occurred since. In the modern era, emissions to the atmosphere from volcanoes are approximately 0.645 billion tons of {{CO2}} per year, whereas humans contribute 29 billion tons of {{CO2}} each year.NEWS, Siegel, Ethan, How Much CO2 Does A Single Volcano Emit?, en, Forbes,weblink live, 2018-09-06,weblink 6 June 2017, JOURNAL, Gerlach, TM, 1991, Present-day CO2 emissions from volcanoes, Transactions of the American Geophysical Union, 72, 23, 249–55, 1991EOSTr..72..249., 10.1029/90EO10192, See also: WEB, 14 June 2011, U.S. Geological Survey,weblink live,weblink" title="web.archive.org/web/20120925170105weblink">weblink 25 September 2012, 15 October 2012, 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,WEB, 13 September 2009, New CO2 data helps unlock the secrets of Antarctic formation,weblink live,weblink" title="web.archive.org/web/20110715082838weblink">weblink 15 July 2011, 28 January 2010, Physorg.com, 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.JOURNAL, Pagani, Mark, Huber, Matthew, Liu, Zhonghui, Bohaty, Steven M., Henderiks, Jorijntje, Sijp, Willem, Krishnan, Srinath, Deconto, Robert M., 2 December 2011, Drop in carbon dioxide levels led to polar ice sheet, study finds,weblink live, Science, 334, 6060, 1261–4, 2011Sci...334.1261P, 10.1126/science.1203909, 22144622, 206533232,weblink" title="web.archive.org/web/20130522065758weblink">weblink 22 May 2013, 14 May 2013, Low {{CO2}} concentrations may have been the stimulus that favored the evolution of {{C4}} plants, which increased greatly in abundance between 7 and 5 million years ago.{{anchor|Greenhouse gas#water vapor feedback|Greenhouse gas#water-vapor feedback}} See also Carbon budget Global temperature record Notes {{Notelist}} References {{Reflist}} External links Current global map of carbon dioxide concentrations. Global Carbon Dioxide Circulation (NASA; 13 December 2016) Video (03:10) – A Year in the Life of Earth's {{CO2}} (NASA; 17 November 2014) {{Climate change}}{{Authority control}}