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Carbon dioxide (CO
2) is an important long-lived trace gas in Earth's atmosphere currently constituting about 0.04% (400 parts per million) of the atmosphere. Despite its relatively small overall concentration, CO
2 is a potent greenhouse gas and plays a vital role in regulating Earth's surface temperature through radiative forcing and the greenhouse effect: CO
2 absorbs and emits infrared radiation at wavelengths of 4.26 µm (asymmetric stretching vibrational mode) and 14.99 µm (bending vibrational mode).
Carbon dioxide is essential to life on Earth and is an integral part of the carbon cycle, a biogeochemical cycle in which carbon is exchanged between the Earth's oceans, soil, rocks and biosphere. Biologically, plants and other photoautotrophs extract carbon from the atmosphere in the form of carbon dioxide by the process of photosynthesis and use it as an energy source and for the construction of their body parts. Therefore, Earth wouldn't have a present-day biosphere without atmospheric CO
2. Carbon dioxide is well mixed in the Earth's atmosphere and reconstructions show that concentrations of CO
2 in the atmosphere varied from as high as 7,000 parts per million during the Cambrian period about 500 million years ago in ancient-Earth biospheres to as low as 180 parts per million during the Quaternary glaciation of the last two million years.
The recent phenomenon of global warming has been attributed primarily to increasing atmospheric CO
2 concentrations in Earth's atmosphere. The global annual mean concentration of CO
2 in the atmosphere has increased markedly since the Industrial Revolution, from 280 ppm to 395 ppm as of 2013, with the increase largely attributed to anthropogenic sources, particularly the burning of fossil fuels. The daily average at Mauna Loa first exceeded 400 ppm on 10 May 2013. It is currently rising at a rate of approximately 2 ppm/year and accelerating. An estimated 30–40% of the CO
2 released by humans into the atmosphere dissolves into oceans, rivers and lakes. which contributes to ocean acidification. The present concentration of CO
2 in Earth's atmosphere is the highest in the past 800,000 years and likely the highest in the past 20 million years.
As recently as 2007 scientists reasoned that CO
2 concentrations could be safely allowed to reach 550 parts per million, but more recent research produces a scientific consensus that "urges the world to reduce atmospheric carbon dioxide concentration CO
2 to about 300 parts per million by volume"  NASA climatologist Gavin Schmidt, however, once told statistician and writer Nate Silver that neither of them would ever see a year in which carbon dioxide concentrations have gone down, not ever, nor would their children.
The global average concentration of CO
2 in Earth's atmosphere is about 0.0397%, or 397 parts per million (ppm). There is an annual fluctuation of about 3–9 ppmv which roughly follows the Northern Hemisphere's growing season. The Northern Hemisphere dominates the annual cycle of CO
2 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 when the quantity of biomass undergoing photosynthesis is greatest.
During the recent geologic history of the planet, CO
2 concentrations have been very stable. Over the past 400,000 years, CO
2 concentrations have varied regularly from about 180 parts per million during the deep glaciations of the Holocene to 280 parts per million during the interglacial periods. In the very recent geologic history, the atmospheric CO
2 concentration has increased to over 390 parts per million and continues to increase, causing the phenomenon of global warming which is mostly attributed to human CO
Because global warming is attributed primarily to increasing atmospheric CO
2 concentrations, scientists closely monitor atmospheric CO
2 concentrations and their impact on the present-day biosphere. At the scientific recording station in Mauna Loa, the concentration reached 0.04% or 400 ppm for the first time in May 2013, although this level had already been reached in the Arctic in June 2012. Sir Brian Hoskins of the Royal Society said that the 400 ppm milestone should "jolt governments into action". The National Geographic noted 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", and according to the global monitoring director at the National Oceanic and Atmospheric Administration's Earth System Research Lab, "it's just a reminder to everybody that we haven't fixed this, and we're still in trouble." As of April 2014 carbon dioxide concentration in the atmosphere was 401.3 ppm. The current concentration may be the highest in 20 million years.
Carbon dioxide concentrations have varied widely over the Earth's 4.7 billion year history. Carbon dioxide is believed to have existed during Earth's first atmosphere which dates back to shortly after Earth's formation. Earth's second atmosphere emerged after many of the lighter gasses like hydrogen escaped to space or were bound up in molecules and is thought to have consisted largely of nitrogen, carbon dioxide and inert gases produced by outgassing from volcanism, supplemented by gases produced during the late heavy bombardment of Earth by huge asteroids. Cyanobacteria converted some of the carbon dioxide in the atmosphere to oxygen which 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 had dropped to 7,000 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.
On long timescales, atmospheric CO
2 concentration is determined by the balance among geochemical processes including organic carbon burial in sediments, silicate rock weathering, and volcanism. The net effect of slight imbalances in the carbon cycle over tens to hundreds of millions of years has been to reduce atmospheric CO
2. 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 proceeds further). The rates of these processes are extremely slow; hence they are of no relevance to the atmospheric CO
2 concentration over the next hundreds, thousands, or millions 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 CO
2 by radioactivity-driven tectonic activity will have continued to slow down. The loss of plant life would also result in the eventual loss of oxygen. Some microbes are capable of photosynthesis at concentrations of CO
2 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.
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. While these measurements give much less precise estimates of carbon dioxide concentration than ice cores, there is evidence for very high CO
2 volume 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. In more recent times, atmospheric CO
2 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, CO
2 is found to have been about 760 ppm, and there is geochemical evidence that concentrations were less than 300 ppm by about 20 million years ago. Carbon dioxide decrease, with a tipping point of 600 ppm, was the primary agent forcing Antarctic glaciation. Low CO
2 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.
The most direct method for measuring atmospheric carbon dioxide concentrations for periods before direct 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 CO
2 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. In 1832, Antarctic ice core levels were 284 ppmv.
The longest ice core record comes from East Antarctica, where ice has been sampled to an age of 800,000 years. During this time, the atmospheric carbon dioxide concentration has varied between 180–210 ppm during ice ages, increasing to 280–300 ppm during warmer interglacials. The beginning of human agriculture during the current Holocene epoch may have been strongly connected to the atmospheric CO
2 increase after the last ice age ended, a fertilization effect raising plant biomass growth and reducing stomatal conductance requirements for CO
2 intake, consequently reducing transpiration water losses and increasing water usage efficiency.
Ancient-Earth climate reconstruction is a vibrant field with numerous studies and reconstructions that sometimes reinforce one another and sometimes disagree with each other. Academically, one study disputed the claim of stable CO
2 concentrations during the present interglacial of the last 10,000 years. Based on an analysis of fossil leaves, Wagner et al. argued that CO
2 levels during the last 7,000–10,000 year period were significantly higher (~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 CO
2. Relevant to this dispute is the observation that Greenland ice cores often report higher and more variable CO
2 values than similar measurements in Antarctica. However, the groups responsible for such measurements (e.g. H. J Smith et al.) 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 CO
Earth’s natural greenhouse effect makes life as we know it possible and carbon dioxide plays a significant role in providing for the relatively warm temperature that the planet enjoys. The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface and the lower atmosphere, it results in an elevation of the average surface temperature above what it would be in the absence of the gases.
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 atmosphere might help explain this faint young sun paradox. When Earth first formed, Earth's atmosphere may have contained more greenhouse gases and CO
2 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 CO
2 and water vapor, may have been more prevalent as well, with a mixing ratio of 10−4 (100 parts per million by volume).
Without the greenhouse effect, the Earth's temperature would be about −18 °C (-0.4 °F) . The surface temperature would be 33 °C (57.6 °F) below Earth's actual surface temperature of approximately 14 °C (57.2 °F). The mechanism that produces this difference between the actual surface temperature and the effective temperature is due to the atmosphere and is known as the greenhouse effect.
Atmospheric carbon dioxide plays an integral role in the Earth's carbon cycle whereby carbon dioxide is removed from the atmosphere by some natural processes and added back to the atmosphere by other natural processes. 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 carbon cycles are intrinsically interconnected and atmospheric gaseous carbon dioxide facilitates the carbon cycle.
Natural sources of atmospheric carbon dioxide include volcanic outgassing, the combustion of organic matter, wildfires and the respiration processes of living aerobic organisms. Man-made sources of carbon dioxide 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.
Most sources of CO
2 emissions are natural, and are balanced to various degrees by natural CO
2 sinks. For example, the natural decay of organic material in forests and grasslands and the action of forest fires results in the release of about 439 gigatonnes of carbon dioxide every year, while new growth entirely counteracts this effect, absorbing 450 gigatonnes per year. Although the initial carbon dioxide in the atmosphere of the young Earth was produced by volcanic activity, modern volcanic activity releases only 130 to 230 megatonnes of carbon dioxide each year, which is less than 1% of the amount released by human activities (at approximately 29 gigatonnes). These natural sources are nearly balanced by natural sinks, physical and biological processes which remove carbon dioxide from the atmosphere. For example, some is directly removed from the atmosphere by land plants for photosynthesis and it is soluble in water forming carbonic acid. There is a large natural flux of CO
2 into and out of the biosphere and oceans. In the pre-industrial era these fluxes were largely in balance. Currently about 57% of human-emitted CO
2 is removed by the biosphere and oceans. From pre-industrial era to 2010, the terrestrial biosphere represented a net source of atmospheric CO
2 prior to 1940, switching subsequently to a net sink.The ratio of the increase in atmospheric CO
2 to emitted CO
2 is known as the airborne fraction (Keeling et al., 1995); this varies for short-term averages and is typically about 45% over longer (5 year) periods. Estimated carbon in global terrestrial vegetation increased from approximately 740 billion tons in 1910 to 780 billion tons in 1990.
Carbon dioxide in the Earth's atmosphere is essential to life and to the present planetary biosphere. Over the course of Earth's geologic history CO
2 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. Cyanobacteria appeared later, and the excess oxygen they produced contributed to the oxygen catastrophe, which rendered the evolution of complex life possible. In recent geologic times, low CO
2 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. At current atmospheric pressures photosynthesis shuts down when atmospheric CO
2 concentrations fall below 150 ppm and 200 ppm although some microbes can extract carbon from the air at much lower concentrations. Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts, which is about six times larger than the current power consumption of human civilization. Photosynthetic organisms also convert around 100–115 thousand million metric tonnes of carbon into biomass per year.
Photosynthetic organisms are photoautotrophs, which means that they are able to synthesize food directly from CO
2 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 CO
2, as a source of carbon. 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 CO
2 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 a source of energy to drive this process, and the electrons needed to convert CO
2 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 CO
2 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.
The Earth's oceans contain a large amount of CO
2 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 CO
2. Since the right-hand side of the reaction produces an acidic compound, adding CO
2 on the left-hand side decreases the pH of sea water, 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 CO
2 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 CO
2. Over hundreds of millions of years, this has produced huge quantities of carbonate rocks.
Ultimately, most of the CO
2 emitted by human activities will dissolve in the ocean; 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 CO
2. This, along with higher temperatures, would mean a higher equilibrium concentration of CO
2 in the air.
The recent phenomenon of global warming has been attributed primarily to increasing atmospheric carbon dioxide concentrations in Earth's atmosphere. While CO
2 absorption and release is always happening as a result of natural processes, the recent rise in CO
2 levels in the atmosphere is known to be mainly due to human activity. Researchers know this both by calculating the amount released based on various national statistics, and by examining the ratio of various carbon isotopes in the atmosphere, as the burning of long-buried fossil fuels releases CO
2 containing carbon of different isotopic ratios to those of living plants, enabling them to distinguish between natural and human-caused contributions to CO
Burning fossil fuels such as coal and petroleum is the leading cause of increased anthropogenic CO
2; deforestation is the second major cause. In 2010, 9.14 gigatonnes of carbon (33.5 gigatonnes of CO
2) were released from fossil fuels and cement production worldwide, compared to 6.15 gigatonnes in 1990. In addition, land use change contributed 0.87 gigatonnes in 2010, compared to 1.45 gigatonnes in 1990. In 1997, human-caused Indonesian peat fires were estimated to have released between 13% and 40% of the average carbon emissions caused by the burning of fossil fuels around the world in a single year. In the period 1751 to 1900, about 12 gigatonnes of carbon were released as carbon dioxide to the atmosphere from burning of fossil fuels, whereas from 1901 to 2008 the figure was about 334 gigatonnes.
This addition, about 3% of annual natural emissions, as of 1997[update], is sufficient to exceed the balancing effect of sinks. As a result, carbon dioxide has gradually accumulated in the atmosphere, and as of 2013[update], its concentration is almost 43% above pre-industrial levels. Various techniques have been proposed for removing excess carbon dioxide from the atmosphere in carbon dioxide sinks.
Carbon dioxide has unique long-term effects on climate change that are largely "irreversible" for one thousand years after emissions stop (zero further emissions) even though carbon dioxide tends toward equilibrium with the ocean on a scale of 100 years. 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.
False-color image of smoke and ozone pollution from Indonesian fires, 1997.