Atmospheric CO2 concentration measured atMauna Loa Observatory in Hawaii from 1958 to 2023 (also called theKeeling Curve). The rise in CO2 over that time period is clearly visible. The concentration is expressed as μmole per mole, orppm.
The current increase in CO2 concentrations is primarily driven by the burning offossil fuels.[6] Other significant human activities that emit CO2 includecement production,deforestation, andbiomass burning. The increase in atmospheric concentrations of CO2 and other long-lived greenhouse gases such asmethane increase the absorption and emission of infrared radiation by the atmosphere. This has led to arise in average global temperature andocean acidification. Another direct effect is theCO2 fertilization effect. The increase in atmospheric concentrations of CO2 causes a range of furthereffects of climate change on the environment and human living conditions.
Carbon dioxide is a greenhouse gas. It absorbs and emitsinfrared radiation at its two infrared-active vibrational frequencies. The twowavelengths are 4.26 μm (2,347 cm−1) (antisymmetric stretchingvibrational mode) and 14.99 μm (667 cm−1) (bending vibrational mode). CO2 plays a significant role in influencingEarth's surface temperature through the greenhouse effect.[7] Light emission from the Earth's surface is most intense in the infrared region between 200 and 2500 cm−1,[8] as opposed to light emission from the much hotterSun 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.[9]
The present atmospheric concentration of CO2 is the highest for 14 million years.[10] Concentrations of CO2 in the atmosphere were as high as 4,000 ppm during theCambrian period about 500 million years ago, and as low as 180 ppm during theQuaternary glaciation of the last two million years.[2] Reconstructed temperature records for the last 420 million years indicate that atmospheric CO2 concentrations peaked at approximately 2,000 ppm. This peak happened during theDevonian period (400 million years ago). Another peak occurred in theTriassic period (220–200 million years ago).[11]
Between 1850 and 2019 theGlobal 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.
Each part per million of CO2 in the atmosphere represents approximately 2.13gigatonnes of carbon, or 7.82 gigatonnes of CO2.[16]
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".[17]: 515
As of 2024,[update] 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.[18]
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.[19][20]
Atmospheric CO2 concentrations fluctuate slightly with the seasons, falling during theNorthern 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. TheNorthern Hemisphere dominates the annual cycle of CO2 concentration because it has much greater land area andplant biomass in mid-latitudes (30-60 degrees) than theSouthern 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.[21][22]
Concentrations also vary on a regional basis, most stronglynear the ground with much smaller variations aloft. In urban areas concentrations are generally higher[23] and indoors they can reach 10 times background levels.
Measurements and predictions made in the recent past
The concentration of atmospheric carbon dioxide has been increasing in progressively greater amounts.[24]
Data from 2009 found that the global mean CO2 concentration was rising at a rate of approximately 2 ppm/year and accelerating.[25][26]
The daily average concentration of atmospheric CO2 atMauna Loa Observatory first exceeded 400 ppm on 10 May 2013[27][28] although this concentration had already been reached in the Arctic in June 2012.[29] 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."[30]
As of 2018, CO2 concentrations were measured to be 410 ppm.[25][31]
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 themolar mass 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 byDave Keeling atCaltech in the 1950s.[32] 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.
There are several surface measurement (including flasks and continuous in situ) networks includingNOAA/ERSL,[33] WDCGG,[34] and RAMCES.[35] The NOAA/ESRL Baseline Observatory Network, and theScripps Institution of Oceanography Network[36] data are hosted at theCDIAC atORNL. The World Data Centre for Greenhouse Gases (WDCGG), part ofGAW, data are hosted by theJMA. The Reseau Atmospherique de Mesure des Composes an Effet de Serre database (RAMCES) is part ofIPSL.
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.[37]
The burning of long-buried fossil fuels releases CO2 containing carbon of differentisotopic ratios to those of living plants, enabling distinction between natural and human-caused contributions to CO2 concentration.[38]
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.[39]
Atmospheric O2 levels are decreasing in Earth's atmosphere as it reacts with the carbon in fossil fuels to form CO2.[40]
The US, China and Russia have cumulatively contributed the greatest amounts of CO2 since 1850.[41]
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.[17] Anthropogenic carbon emissions exceed the amount that can be taken up or balanced out by natural sinks.[42] Thus carbon dioxide has gradually accumulated in the atmosphere and, as of May 2022, its concentration is 50% above pre-industrial levels.[3]
The extraction and burning of fossil fuels, releasing carbon that has beenunderground for many millions of years, has increased the atmospheric concentration of CO2.[4][12] 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.[43] This larger disruption to the natural balance is responsible for recent growth in the atmospheric CO2 concentration.[31][44] Currently about half of the carbon dioxide released from theburning of fossil fuels is not absorbed by vegetation and the oceans and remains in theatmosphere.[42]
Burning fossil fuels such ascoal,petroleum, andnatural gas is the leading cause of increasedanthropogenic CO2;deforestation is the second major cause. In 2010, 9.14 gigatonnes of carbon (GtC, equivalent to 33.5gigatonnes 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.[45] In addition,land use change contributed 0.87 GtC in 2010, compared to 1.45 GtC in 1990.[45] 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.[46]
TheInternational Energy Agency estimates that the top 1% of emitters globally each hadcarbon 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.[47]
Greenhouse gases allow sunlight to pass through the atmosphere, heating the planet, but then absorb and redirect the infrared radiation (heat) the planet emits.CO2 reduces the flux of thermal radiation emitted to space (causing the large dip near 667 cm−1), thereby contributing to the greenhouse effect.Longwave-infraredabsorption coefficients of water vapor and carbon dioxide. For wavelengths near 15-microns, CO2 is a much stronger absorber than water vapor.
On Earth, carbon dioxide is the most relevant, directgreenhouse gas that is influenced by human activities. Water is responsible for most (about 36–70%) of the total greenhouse effect, and therole 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 CO2 was estimated to be responsible for 1.82 W m−2 of the 2.63 W m−2 change inradiative forcing on Earth (about 70%).[48]
Earth's naturalgreenhouse 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.[49][50][51]
The concept of more atmospheric CO2 increasing ground temperature was first published bySvante Arrhenius in 1896.[52] 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 inEarth's energy balance and, over the longer term, in Earth's climate.[17]
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.[53]
Atmospheric carbon dioxide plays an integral role in the Earth'scarbon cycle whereby CO2 is removed from the atmosphere by some natural processes such asphotosynthesis and deposition ofcarbonates, to form limestones for example, and added back to the atmosphere by other natural processes such asrespiration 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 thebiosphere 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.
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.[43]
Natural sources of CO2 are more or less balanced by naturalcarbon 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 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).[54] Although much CO2 in the early atmosphere of the young Earth was produced byvolcanic activity, modern volcanic activity releases only 130 to 230 megatonnes of CO2 each year.[55]
From the human pre-industrial era to 1940, the terrestrial biosphere represented a net source of atmospheric CO2 (driven largely byland-use changes), but subsequently switched to a net sink with growing fossil carbon emissions.[56]
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 byFortunat Joos inBern and colleagues, called the Bern model.[57]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:[58]
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 (artificial sequestration).
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.[12] 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.[59][60]
A study published inScience Advances in 2025 concluded that faster flow of theAntarctic 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 criticalpositive feedback for future warming.[61]
Physical drivers of global warming that has happened so far. Futureglobal warming potential for long lived drivers like carbon dioxide emissions is not represented. Whiskers on each bar show the possibleerror range.
The global average and combined land andocean surface temperature, show a warming of 1.09 °C (range: 0.95 to 1.20 °C) from 1850–1900 to 2011–2020, based on multiple independently produced datasets.[63]: 5 The trend is faster since the 1970s than in any other 50-year period over at least the last 2000 years.[63]: 8
It is clear that the ocean is warming as a result of climate change, and this rate of warming is increasing.[64]: 9 The global ocean was the warmest it had ever been recorded by humans in 2022.[65] This is determined by theocean heat content, which exceeded the previous 2021 maximum in 2022.[65] The steady rise in ocean temperatures is an unavoidable result of theEarth's energy imbalance, which is primarily caused by rising levels of greenhouse gases.[65] Between pre-industrial times and the 2011–2020 decade, the ocean's surface has heated between 0.68 and 1.01 °C.[66]: 1214
The majority of ocean heat gain occurs in theSouthern Ocean. For example, between the 1950s and the 1980s, the temperature of the Antarctic Southern Ocean rose by 0.17 °C (0.31 °F), nearly twice the rate of the global ocean.[67]
A change in pH by 0.1 represents a 26% increase in hydrogen ion concentration in the world's oceans (the pH scale is logarithmic, so a change of one in pH units is equivalent to a tenfold change in hydrogen ion concentration). Sea-surface pH and carbonate saturation states vary depending on ocean depth and location. Colder and higher latitude waters are capable of absorbing more CO2. This can cause acidity to rise, lowering the pH and carbonate saturation levels in these areas. There are several other factors that influence the atmosphere-ocean CO2 exchange, and thus local ocean acidification. These includeocean currents andupwelling zones, proximity to large continental rivers,sea ice coverage, and atmospheric exchange withnitrogen andsulfur fromfossil fuel burning andagriculture.[72][73][74]
TheCO2 fertilization effect or carbon fertilization effect causes an increased rate ofphotosynthesis while limiting leaf transpiration in plants. Both processes result from increased levels ofatmospheric carbon dioxide (CO2).[75][76] The carbon fertilization effect varies depending on plant species, air and soil temperature, and availability of water and nutrients.[77][78] Netprimary productivity (NPP) might positively respond to the carbon fertilization effect,[79] although evidence shows that enhanced rates of photosynthesis in plants due to CO2 fertilization do not directly enhance all plant growth, and thus carbon storage.[77] The carbon fertilization effect has been reported to be the cause of 44% ofgross primary productivity (GPP) increase since the 2000s.[80]Earth System Models, Land System Models andDynamic Global Vegetation Models are used to investigate and interpret vegetation trends related to increasing levels of atmospheric CO2.[77][81] However, theecosystem processes associated with the CO2 fertilization effect remain uncertain and therefore are challenging to model.[82][83]
Terrestrial ecosystems have reduced atmospheric CO2 concentrations and have partially mitigatedclimate change effects.[84] The response by plants to the carbon fertilization effect is unlikely to significantly reduce atmospheric CO2 concentration over the next century due to the increasing anthropogenic influences on atmospheric CO2.[76][77][85][86] Earth's vegetated lands have shown significant greening since the early 1980s[87] largely due to rising levels of atmospheric CO2.[88][89][90][91]
Theory predicts thetropics to have the largest uptake due to the carbon fertilization effect, but this has not been observed. The amount of CO2 uptake from CO2 fertilization also depends on how forests respond to climate change, and if they are protected fromdeforestation.[92]
CO2 emissions have also led to the stratosphere contracting by 400 meters since 1980, which could affect satellite operations, GPS systems and radio communications.[93]
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 gasesmethane andnitrous 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.[59] Lowering global temperatures more rapidly would requirecarbon sequestration orgeoengineering.
Various techniques have been proposed for removing excess carbon dioxide from the atmosphere.
Carbon dioxide removal (CDR) is a process in which carbon dioxide (CO2) is removed from the atmosphere by deliberate human activities and durably stored in geological, terrestrial, or ocean reservoirs, or in products.[98]: 2221 This process is also known as carbon removal, greenhouse gas removal or negative emissions. CDR is more and more often integrated intoclimate policy, as an element ofclimate change mitigation strategies.[99][100] Achievingnet zero emissions will require first and foremost deep and sustained cuts in emissions, and then—in addition—the use of CDR ("CDR is what puts thenet intonet zero emissions"[101]). In the future, CDR may be able to counterbalance emissions that are technically difficult to eliminate, such as some agricultural and industrial emissions.[102]: 114
CO2 concentrations over the last 500 million yearsConcentration of atmospheric CO2 over the last 40,000 years, from theLast Glacial Maximum to the present day. The current rate of increase is much higher than at any point during the lastdeglaciation.
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.[10] However theIPCC Sixth Assessment Report estimated similar levels 3 to 3.3 million years ago in themid-Pliocene warm period. This period can be aproxy for likely climate outcomes with current levels of CO2.[103]: 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 thisfaint young sun paradox. When Earth first formed,Earth's atmosphere may have contained more greenhouse gases and CO2 concentrations may have been higher, with estimatedpartial pressure as large as 1,000 kPa (10 bar), because there was no bacterialphotosynthesis toreduce the gas to carbon compounds and oxygen.Methane, a very active greenhouse gas, may have been more prevalent as well.[104][105]
Carbon dioxide concentrations have shown several cycles of variation from about 180 parts per million during the deep glaciations of theHolocene andPleistocene 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 ofnitrogen andCO 2 was produced by outgassing fromvolcanism, supplemented by gases produced during thelate heavy bombardment of Earth by hugeasteroids.[106] A major part of carbon dioxide emissions were soon dissolved in water and incorporated in carbonate sediments.
The production of free oxygen bycyanobacterial photosynthesis eventually led to theoxygen 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 theCambrian period about 500 million years ago to as low as 180 parts per million 20,000 years ago .[2]
On long timescales, atmospheric CO2 concentration is determined by the balance amonggeochemical processes including organic carbon burial in sediments, silicate rockweathering, andvolcanic degassing. The net effect of slight imbalances in thecarbon 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 ofinternal 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.
Over the course of Earth's geologic history CO2 concentrations have played a role in biological evolution. The first photosynthetic organisms probablyevolved early in theevolutionary history of life and most likely usedreducing agents such ashydrogen orhydrogen sulfide as sources of electrons, rather than water.[107] Cyanobacteria appeared later, and the excess oxygen they produced contributed to theoxygen catastrophe,[108] which rendered theevolution 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 ofC4 plants which increased greatly in abundance between 7 and 5 million years ago over plants that use the less efficientC3 metabolic pathway.[109] 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.[110][111]
Over 400,000 years of ice core data: Graph of CO2 (green), reconstructed temperature (blue) and dust (red) from the Vostok ice coreCorrespondence 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 theAntarctic orGreenland 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 10,000years.[112][113] The longestice core record comes from East Antarctica, where ice has been sampled to an age of 800,000 years.[114] During this time, the atmospheric carbon dioxide concentration has varied between 180 and 210 ppm duringice ages, increasing to 280–300 ppm during warmerinterglacials.[115][116]
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 fromstomata of fossilized leaves suggests greater variability, with CO2 mole fractions above 300 ppm during the period ten to seven thousand years ago,[117] though others have argued that these findings more likely reflect calibration or contamination problems rather than actual CO2 variability.[118][119] Because of the way air is trapped in ice (pores in the ice close off slowly to form bubbles deep within thefirn) 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 andCH 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, variousproxy measurements and models suggest larger variations in past epochs: 500 million years ago CO2 levels were likely 10 times higher than now.[120]
Various proxy measurements have been used to try to determine atmospheric CO2 concentrations millions of years in the past. These includeboron andcarbonisotope ratios in certain types of marine sediments, and the numbers ofstomata observed on fossil plant leaves.[109]
Phytane is a type ofditerpenoidalkane. It is a breakdown product of chlorophyll, and is now used to estimate ancient CO2 levels.[121] 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.[121]
Geochemical modelling suggests that prior to the mid-Ordovician (450 million years ago) atmospheric CO2 reached 1000s of ppm, but proxy evidence of this time remains unreliable. Some Phytane estimates of the Ordovician suggest concentrations of ~300-700ppm.[122]
Indeed, higher CO2 concentrations are thought to have prevailed throughout most of thePhanerozoicEon, 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 theDevonian period, about 400 million years ago.[123][124][125] 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.[126]
Earlier in Earth's history, in theNeoproterozoic Era, an 82-million year period of intermittent, widespread glaciation extending to the equator (Snowball Earth) ended suddenly at 635 Ma.[127] after CO2 released during volcanic outgassing built up to ~12% (~120,000 ppm). This caused extreme greenhouse conditions, rapid deglaciation, and carbonate deposition aslimestone at rates which may have been as fast as 40 cm per year. The end of the Snowball Earth glaciations marks the transition between theCryogenian andEdiacaran Periods, and may have contributed to the radiation of metazoan life in thePhanerozoic.[128]
Atmospheric CO2 concentration continued to fall after about 60 million years ago. About 34 million years ago, the time of theEocene–Oligocene extinction event and when theAntarctic ice sheet started to take its current form, CO2 was about 760 ppm,[129] 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.[130] Low CO2 concentrations may have been the stimulus that favored the evolution ofC4 plants, which increased greatly in abundance between 7 and 5 million years ago.[109]
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