Carbon dioxide in Earth's atmosphere: Difference between revisions

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Carbon dioxide in Earth's atmosphere is a trace gas, having a global average concentration of 421 ppm as at May 2022.^[3] The concentration is expressed as "parts per million" (ppm, on a molar basis). This increase of CO₂ in Earth's atmosphere has resulted in global warming and ocean acidification. Carbon dioxide (CO₂) in Earth's atmosphere is an integral part of the carbon cycle, a biogeochemical cycle in which carbon is exchanged between the Earth's oceans, soil, rocks and the biosphere. Plants and other photoautotrophs use solar energy to produce carbohydrate from atmospheric carbon dioxide and water by photosynthesis. Almost all other organisms depend on carbohydrate derived from photosynthesis as their primary source of energy and carbon compounds. CO₂ absorbs and emits infrared radiation at wavelengths of 4.26 μm (2,347 cm⁻¹) (asymmetric stretching vibrational mode) and 14.99 μm (667 cm⁻¹) (bending vibrational mode) and consequently is a greenhouse gas that plays a significant role in influencing Earth's surface temperature through the greenhouse effect.^[4]

Global annual mean CO₂ concentration has increased by 50% since the start of the Industrial Revolution, from 280 ppm during the 10,000 years up to the mid-18th century^[5] to 421 ppm as of May 2022.^[3]^[6] The increase has been attributed to human activity, particularly deforestation and the burning of fossil fuels.^[7] The extraction and burning of fossil fuels, using carbon that has been sequestered for many millions of years in the lithosphere, has caused the atmospheric concentration of CO₂ to increase.^[6]^[8] Most CO₂ from human activities is released from burning coal, petroleum, and natural gas. Other large anthropogenic sources include cement production, deforestation, and biomass burning.

While transparent to visible light, carbon dioxide is a greenhouse gas, absorbing and emitting infrared radiation at its two infrared-active vibrational frequencies. Light emission from the Earth's surface is most intense in the infrared region between 200 and 2500 cm⁻¹,^[9] 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 and the lower atmosphere. Less energy reaches the upper atmosphere, which is therefore cooler because of this absorption.^[10]

Increases in atmospheric concentrations of CO₂ and other long-lived greenhouse gases such as methane, nitrous oxide and ozone have strengthened their absorption and emission of infrared radiation, causing the rise in average global temperature since the mid-20th century. Carbon dioxide is of greatest concern because it exerts a larger overall warming influence than all of these other gases combined.^[6] It furthermore has an atmospheric lifetime that increases with the cumulative amount of fossil carbon extracted and burned, due to the imbalance that this activity has imposed on Earth's fast carbon cycle.^[11] This means that 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.^[12]^[13]^[14]

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 present concentration is the highest for 14 million years.^[15] Concentrations of CO₂ in the atmosphere were as high as 4,000 ppm during the Cambrian period about 500 million years ago to as low as 180 ppm during the Quaternary glaciation of the last two million years.^[5] Reconstructed temperature records for the last 420 million years indicate that atmospheric CO₂ concentrations peaked at ~2,000 ppm during the Devonian (~400 Ma) period, and again in the Triassic (220–200 Ma) period. 500 million years ago the CO₂ concentration was 20 times greater than today, and four times current levels during the Jurassic period.^[16]^[17]

Current concentration and future trends

Current situation

Since the start of the Industrial Revolution, atmospheric CO₂ concentration have been increasing, causing global warming and ocean acidification.^[8] As of May 2022, the average monthly level of CO₂ in Earth's atmosphere reached 421 parts per million.^[3]

Each part per million by volume of CO₂ in the atmosphere represents approximately 2.13 gigatonnes of carbon, or 7.82 gigatonnes of CO₂.^[18]

Annual fluctuation

There is an annual fluctuation – 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.^[19]^[20]

Concentrations also vary on a regional basis, most strongly near the ground with much smaller variations aloft. In urban areas concentrations are generally higher^[21] and indoors they can reach 10 times background levels.

Concentrations in the recent past

In 2001 it was stated that the current concentration may be the highest in the last 20 million years.^[22]
Data from 2009 found that the global mean CO₂ concentration is currently rising at a rate of approximately 2 ppm/year and accelerating.^[23]^[24]
The daily average concentration of atmospheric CO₂ at Mauna Loa Observatory first exceeded 400 ppm on 10 May 2013^[25]^[26] although this concentration had already been reached in the Arctic in June 2012.^[27] 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."^[28]
As of 2018, CO₂ constituted about 0.041% by volume of the atmosphere (equal to 410 ppm).^[23]^[29]

Measurement techniques

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.^[30] In 1958 the first ongoing measurements were started at Mauna Loa. These measurements have been ongoing at the same location ever since then. 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,^[31] WDCGG,^[32] and RAMCES.^[33] The NOAA/ESRL Baseline Observatory Network, and the Scripps Institution of Oceanography Network^[34] 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.^[35]

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.^[36]

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.^[37]
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.^[38]
Atmospheric O₂ levels are decreasing in Earth's atmosphere as it reacts with the carbon in fossil fuels to form CO₂.^[39]

Causes of the current increase

Anthropogenic CO₂ emissions

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.^[44] Anthropogenic carbon emissions exceed the amount that can be taken up or balanced out by natural sinks.^[45] As a result, carbon dioxide has gradually accumulated in the atmosphere, and as of May 2022, its concentration is 50% above pre-industrial levels.^[3]

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.^[46] This larger disruption to the natural balance is responsible for recent growth in the atmospheric CO₂ concentration.^[29]^[47] 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.^[48]

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.^[49] In addition, land use change contributed 0.87 GtC in 2010, compared to 1.45 GtC in 1990.^[49] 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.^[50]

Role in greenhouse effect

How Co2 causes the greenhouse effect.

Natural greenhouse effect

Earth'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 that the planet enjoys. 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.^[51]^[52]^[53] Without the greenhouse effect, the Earth's average surface temperature would be about −18 °C (−0.4 °F)^[54]^[55] compared to Earth's actual average surface temperature of approximately 14 °C (57.2 °F).^[56]

Carbon dioxide is believed to have played an important effect in regulating Earth's temperature throughout its 4.7 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. It has been suggested by scientists that 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 1,000 kPa (10 bar), because there was no bacterial photosynthesis to reduce the gas to carbon compounds and oxygen. Methane, a very active greenhouse gas which reacts with oxygen to produce CO₂ and water vapor, may have been more prevalent as well, with a mixing ratio of 10⁻⁴ (100 parts per million by volume).^[57]^[58]

Additional heat trapping due to human activities

Though water is responsible for most (about 36-70%) of the total greenhouse effect, 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%).^[59]

The concept of atmospheric CO₂ increasing ground temperature was first published by Svante Arrhenius in 1896.^[60] 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.^[59]

Role in carbon cycle

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.

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).^[62] 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.^[63] Natural sources are more or less balanced by natural sinks, in the form of chemical and biological processes which remove CO₂ from the atmosphere.

Changes due to human activities

Overall, there is a large natural flux of atmospheric CO₂ into and out of the biosphere, both on land and in the oceans.^[64] 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.^[65] In 2012, about 57% of human-emitted CO₂, mostly from the burning of fossil carbon, was taken up by land and ocean sinks.^[66]^[65]

The ratio of the increase in atmospheric CO₂ to emitted CO₂ is known as the airborne fraction (Keeling et al., 1995). This ratio varies in the short-term and is typically about 45% over longer (5-year) periods.^[65] Estimated carbon in global terrestrial vegetation increased from approximately 740 gigatonnes in 1910 to 780 gigatonnes in 1990.^[67]

Atmospheric CO₂ and photosynthesis

Carbon dioxide in the Earth's atmosphere is essential to life and to most of the planetary biosphere. 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.^[68] Cyanobacteria appeared later, and the excess oxygen they produced contributed to the oxygen catastrophe,^[69] 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.^[70] 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.^[71]^[72] Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts,^[73]^[74]^[75] which is about six times larger than the current power consumption of human civilization.^[76] Photosynthetic organisms also convert around 100–115 billion metric tonnes of carbon into biomass per year.^[77]^[78]

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.^[79] 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. However, there are some types of bacteria that carry out anoxygenic photosynthesis, which consumes CO₂ but does not release oxygen.

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. However, the two processes take place through a different sequence of chemical reactions and in different cellular compartments.

Most organisms that utilize photosynthesis to produce oxygen use visible light to do so, although at least three use shortwave infrared or, more specifically, far-red radiation.^[80]

Effects of current increase

CO₂ emissions have also led to the stratosphere contracting by 400 meters since 1980, which could affect satellite operations, GPS systems and radio communications.^[81]

Effects on plants and crops

Since global warming is attributed to increasing atmospheric concentrations of greenhouse gases such as CO₂ and methane, scientists closely monitor atmospheric CO₂ concentrations and their impact on the present-day biosphere.

A 1993 review of scientific greenhouse studies found that a doubling of CO₂ concentration would stimulate the growth of 156 different plant species by an average of 37%. Response varied significantly by species, with some showing much greater gains and a few showing a loss. For example, a 1979 greenhouse study found that with doubled CO₂ concentration the dry weight of 40-day-old cotton plants doubled, but the dry weight of 30-day-old maize plants increased by only 20%.^[82]^[83]

In addition to greenhouse studies, field and satellite measurements attempt to understand the effect of increased CO₂ in more natural environments. In free-air carbon dioxide enrichment (FACE) experiments plants are grown in field plots and the CO₂ concentration of the surrounding air is artificially elevated. These experiments generally use lower CO₂ levels than the greenhouse studies. They show lower gains in growth than greenhouse studies, with the gains depending heavily on the species under study. A 2005 review of 12 experiments at 475–600 ppm showed an average gain of 17% in crop yield, with legumes typically showing a greater response than other species and C4 plants generally showing less. The review also stated that the experiments have their own limitations. The studied CO₂ levels were lower, and most of the experiments were carried out in temperate regions.^[84] Satellite measurements found increasing leaf area index for 25% to 50% of Earth's vegetated area over the past 35 years (i.e., a greening of the planet), providing evidence for a positive CO₂ fertilization effect.^[85]^[86]

A 2017 Politico article states that increased CO₂ levels may have a negative impact on the nutritional quality of various human food crops, by increasing the levels of carbohydrates, such as glucose, while decreasing the levels of important nutrients such as protein, iron, and zinc. Crops experiencing a decrease in protein include rice, wheat, barley and potatoes.^[87]^{[scientific citation needed]}

Effects on oceanic carbon cycle

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:

CaCO
₃ + CO₂ + H
₂O ⇌ Ca²⁺
+ 2 HCO⁻
₃

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.

Ultimately, most of the CO₂ emitted by human activities will dissolve in the ocean;^[88] 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.^[89]^[90]

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.^[89] Lowering global temperatures more rapidly would require carbon sequestration or geoengineering.

Carbon moves between the atmosphere, vegetation (dead and alive), the soil, the surface layer of the ocean, and the deep ocean. A detailed model has been developed by Fortunat Joos in Bern and colleagues, called the Bern model.^[91] 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:^[92]

f(t)=0.217+0.338\exp(-t/18.51)+0.259\exp(-t/172.9)+0.186\exp(-t/1.186)

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).

Ocean acidification

Ocean acidification is the ongoing decrease in the pH of the Earth's ocean. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05.^[93] Carbon dioxide emissions from human activities are the primary cause of ocean acidification, with atmospheric carbon dioxide (CO₂) levels exceeding 422 ppm (as of 2024^[update]).^[94] CO₂ from the atmosphere is absorbed by the oceans. This chemical reaction produces carbonic acid (H₂CO₃) which dissociates into a bicarbonate ion (HCO−3) and a hydrogen ion (H⁺). The presence of free hydrogen ions (H⁺) lowers the pH of the ocean, increasing acidity (this does not mean that seawater is acidic yet; it is still alkaline, with a pH higher than 8). Marine calcifying organisms, such as mollusks and corals, are especially vulnerable because they rely on calcium carbonate to build shells and skeletons.^[95]

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 include ocean currents and upwelling zones, proximity to large continental rivers, sea ice coverage, and atmospheric exchange with nitrogen and sulfur from fossil fuel burning and agriculture.^[96]^[97]^[98]

Proposed solutions

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.

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.^[99]^: 2221 This process is also known as carbon removal, greenhouse gas removal or negative emissions. CDR is more and more often integrated into climate policy, as an element of climate change mitigation strategies.^[100]^[101] Achieving net 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 the net into net zero emissions"^[102]). In the future, CDR may be able to counterbalance emissions that are technically difficult to eliminate, such as some agricultural and industrial emissions.^[103]^: 114

Concentrations in the geologic past

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 4.54 billion year 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 CO
₂ was produced by outgassing from volcanism, supplemented by gases produced during the late heavy bombardment of Earth by huge asteroids.^[104] 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 before the present. 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 during the Quaternary glaciation of the last two million years.^[5]

Drivers of ancient-Earth CO₂ concentration

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.

In billion-year timescales, it is predicted that plant, and therefore animal, life on land will die off altogether, since by that time most of the remaining carbon in the atmosphere will be sequestered underground, and natural releases of CO₂ by radioactivity-driven tectonic activity will have continued to slow down.^[105]^{[better source needed]} The loss of plant life would also result in the eventual loss of oxygen. Some microbes are capable of photosynthesis at concentrations of CO₂ of a few parts per million and so the last life forms would probably disappear finally due to the rising temperatures and loss of the atmosphere when the sun becomes a red giant some four billion years from now.^[106]

Measuring ancient-Earth CO₂ concentration

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 ppmv immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 years.^[107] The longest ice core record comes from East Antarctica, where ice has been sampled to an age of 800,000 years.^[108] 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.^[109]^[110] The beginning of human agriculture during the current Holocene epoch may have been strongly connected to the atmospheric CO₂ increase after the last ice age ended, a fertilization effect raising plant biomass growth and reducing stomatal conductance requirements for CO₂ intake, consequently reducing transpiration water losses and increasing water usage efficiency.^[111]

Various proxy measurements have been used to attempt to determine atmospheric carbon dioxide concentrations millions of years in the past. These include boron and carbon isotope ratios in certain types of marine sediments, and the number of stomata observed on fossil plant leaves.^[70]

Phytane is a type of diterpenoid alkane. It is a breakdown product of chlorophyll and is now used to estimate ancient CO₂ levels.^[112] 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.^[112]

600 to 400 Ma

There is evidence for high CO₂ concentrations between 200 and 150 million years ago of over 3,000 ppm, and between 600 and 400 million years ago of over 6,000 ppm.^[22]

60 to 5 Ma

In more recent times, 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,^[113] 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.^[114] 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.^[70]

CO₂ possibly higher 10,000 - 7,000 years ago

Based on an analysis of fossil leaves, Wagner et al.^[115] argued that atmospheric CO₂ concentrations during the last 7,000–10,000 year period were significantly higher than 300 ppm and contained substantial variations that may be correlated to climate variations. Others have disputed such claims, suggesting they are more likely to reflect calibration problems than actual changes in CO₂.^[116] Relevant to this dispute is the observation that Greenland ice cores often report higher and more variable CO₂ values than similar measurements in Antarctica. However, the groups responsible for such measurements (e.g. H.J. Smith et al.^[117]) believe the variations in Greenland cores result from in situ decomposition of calcium carbonate dust found in the ice. When dust concentrations in Greenland cores are low, as they nearly always are in Antarctic cores, the researchers report good agreement between measurements of Antarctic and Greenland CO₂ concentrations.

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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)

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@@ Line 69: / Line 69: @@
 [[File:M15-162b-EarthAtmosphere-CarbonDioxide-FutureRoleInGlobalWarming-Simulation-20151109.jpg|thumb|upright=1.5|right|[[Carbon dioxide|CO<sub>2</sub>]] in [[Earth]]'s [[atmosphere]] if half of anthropogenic CO<sub>2</sub> emissions are ''not'' absorbed.<ref name="NASA-20151112-ab">{{cite web |last1=Buis |first1=Alan |last2=Ramsayer |first2=Kate |last3=Rasmussen |first3=Carol |date=12 November 2015 |title=A Breathing Planet, Off Balance |url=http://www.jpl.nasa.gov/news/news.php?feature=4769 |access-date=13 November 2015 |work=[[NASA]]}}</ref><ref name="NASA-20151112b">{{cite web |author=Staff |date=12 November 2015 |title=Audio (66:01) - NASA News Conference – Carbon & Climate Telecon |url=http://www.ustream.tv/recorded/77531778 |access-date=12 November 2015 |work=[[NASA]]}}</ref><ref name="NYT-20151110">{{cite news |last=St. Fleur |first=Nicholas |date=10 November 2015 |title=Atmospheric Greenhouse Gas Levels Hit Record, Report Says |work=[[The New York Times]] |url=https://www.nytimes.com/2015/11/11/science/atmospheric-greenhouse-gas-levels-hit-record-report-says.html |access-date=11 November 2015}}</ref> ([[NASA]] [[computer simulation]])]]
-While {{CO2}} absorption and release is always happening as a result of natural processes, the recent rise in {{CO2}} levels in the atmosphere is known to be mainly due to human (anthropogenic) activity.<ref name="AR5" /> Anthropogenic carbon emissions exceed the amount that can be taken up or balanced out by natural sinks.<ref name="BallantyneAlden2012">{{cite journal |last1=Ballantyne |first1=A.P. |last2=Alden |first2=C.B. |last3=Miller |first3=J.B. |last4=Tans |first4=P.P. |last5=White |first5=J.W.C. |year=2012 |title=Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years |journal=Nature |volume=488 |issue=7409 |pages=70–72 |bibcode=2012Natur.488...70B |doi=10.1038/nature11299 |issn=0028-0836 |pmid=22859203 |s2cid=4335259}}</ref> As a result, carbon dioxide has gradually accumulated in the atmosphere, and as of May 2022, its concentration is 50% above pre-industrial levels.<ref name=":0" />
+While {{CO2}} absorption and release is always happening as a result of natural processes, the recent rise in {{CO2}} levels in the atmosphere is known to be mainly due to human (anthropogenic) activity.<ref>Eyring, V., N.P. Gillett, K.M. Achuta Rao, R. Barimalala, M. Barreiro Parrillo, N. Bellouin, C. Cassou, P.J. Durack, Y. Kosaka, S.  McGregor, S. Min, O. Morgenstern, and Y. Sun, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter03.pdf Chapter 3: Human Influence on the Climate System]. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L.  Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 423–552, doi:10.1017/9781009157896.005.</ref> Anthropogenic carbon emissions exceed the amount that can be taken up or balanced out by natural sinks.<ref name="BallantyneAlden2012">{{cite journal |last1=Ballantyne |first1=A.P. |last2=Alden |first2=C.B. |last3=Miller |first3=J.B. |last4=Tans |first4=P.P. |last5=White |first5=J.W.C. |year=2012 |title=Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years |journal=Nature |volume=488 |issue=7409 |pages=70–72 |bibcode=2012Natur.488...70B |doi=10.1038/nature11299 |issn=0028-0836 |pmid=22859203 |s2cid=4335259}}</ref> As a result, carbon dioxide has gradually accumulated in the atmosphere, and as of May 2022, its concentration is 50% above pre-industrial levels.<ref name=":0" />
 As of year 2019 the extraction and burning of geologic fossil carbon by humans releases over 30&nbsp;gigatonnes of {{CO2}} (9&nbsp;billion tonnes carbon) each year.<ref name="gcb19">Friedlingstein, P., Jones, M., O'Sullivan, M., Andrew, R., Hauck, J., Peters, G., Peters, W., Pongratz, J., Sitch, S., Le Quéré, C. and 66 others (2019) "Global carbon budget 2019". ''Earth System Science Data'', '''11'''(4): 1783–1838. {{doi|10.5194/essd-11-1783-2019}}. [[File:CC-BY_icon.svg|50x50px]] Material was copied from this source, which is available under a [[creativecommons:by/4.0/|Creative Commons Attribution 4.0 International License]].</ref>  This larger disruption to the natural balance is responsible for recent growth in the atmospheric {{CO2}} concentration.<ref name="ESRLtrends" /><ref name="NOAA_CO2annual">{{cite web |last1=Dlugokencky |first1=E. |date=5 February 2016 |title=Annual Mean Carbon Dioxide Data |url=ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_annmean_gl.txt |access-date=12 February 2016 |website=Earth System Research Laboratory |publisher=[[NOAA]]}}</ref>  Currently about half of the carbon dioxide released from the [[Global warming|burning of fossil fuels]] is not absorbed by vegetation and the oceans and remains in the [[atmosphere]].<ref>{{Cite journal |author1=A.P. Ballantyne |author2=C.B. Alden |author3=J.B. Miller |author4=P.P. Tans |author5=J.W. C. White |year=2012 |title=Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years |journal=[[Nature (journal)|Nature]] |volume=488 |issue=7409 |pages=70–72 |bibcode=2012Natur.488...70B |doi=10.1038/nature11299 |pmid=22859203 |s2cid=4335259}}</ref>