The concentration of carbon dioxide (CO2) in Earth's atmosphere is approximately 392 ppm (parts per million) by volume as of 2011[update][1] and rose by 2.0 ppm/yr during 2000–2009. 40 years earlier, the rise was only 0.9 ppm/yr, showing not only increasing concentrations, but also a rapid acceleration of concentrations.[1][2] The increase of concentration from pre-industrial concentrations of 280 ppm has again doubled in just the last 33 years.[2] Carbon dioxide is essential to photosynthesis in plants and other photoautotrophs, and is also a prominent greenhouse gas. Despite its relatively small overall concentration in the atmosphere, CO2 is an important component of Earth's atmosphere because it absorbs and emits infrared radiation at wavelengths of 4.26 µm (asymmetric stretching vibrational mode) and 14.99 µm (bending vibrational mode), thereby playing a role in the greenhouse effect in addition to other factors such as water vapour.[3] The present level is higher than at any time during the last 800 thousand years,[4] and likely higher than in the past 20 million years.[5]
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In 2009, the CO2 global average concentration in Earth's atmosphere was about 0.0387% by volume, or 387 parts per million by volume (ppmv).[1][6] 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 CO2 concentration because it has much greater land area and plant biomass than the Southern Hemisphere. Concentrations peak in May as the Northern Hemisphere spring greenup begins and reach a minimum in October when the quantity of biomass undergoing photosynthesis is greatest.[7]
Natural sources of atmospheric carbon dioxide include volcanic outgassing, the combustion of organic matter, 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 convert carbon dioxide to carbohydrates during a process called photosynthesis. They gain the energy needed for this reaction through the absorption of sunlight by pigments such as Chlorophyll. The resulting gas, oxygen, is released into the atmosphere by plants, which is subsequently used for respiration by heterotrophic organisms and other plants, forming a cycle.
Most sources of CO2 emissions are natural. For example, the natural decay of organic material in forests and grasslands, such as dead trees, results in the release of about 220 gigatonnes of carbon dioxide every year. 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.[8][9][10] 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,[11] which is less than 1% of the amount released by human activities.[12]
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 CO2 into and out of the biosphere and oceans.[13] In the pre-industrial era these fluxes were largely in balance. Currently about 57% of human-emitted CO2 is removed by the biosphere and oceans.[14] The ratio of the increase in atmospheric CO2 to emitted CO2 is known as the airborne fraction (Keeling et al., 1995); this varies for short-term averages but 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.[15]
Burning fossil fuels such as coal and petroleum is the leading cause of increased anthropogenic CO2; deforestation is the second major cause. In 2008, 8.67 gigatonnes of carbon (31.8 gigatonnes of CO2) were released from fossil fuels worldwide, compared to 6.14 gigatonnes in 1990.[16] In addition, land use change contributed 1.20 gigatonnes in 2008, compared to 1.64 gigatonnes in 1990.[16] 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.[17]
This addition, about 3% of annual natural emissions as of 1997[update], is sufficient to exceed the balancing effect of sinks.[18] As a result, carbon dioxide has gradually accumulated in the atmosphere, and as of 2009[update], its concentration is 39% above pre-industrial levels.[2]
Various techniques have been proposed for removing excess carbon dioxide from the atmosphere in carbon dioxide sinks.
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 caps. The most widely accepted of such studies come from a variety of Antarctic cores and indicate that atmospheric CO2 levels were about 260–280 ppmv immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 years (10 ka). In 1832 Antarctic ice core levels were 284 ppmv.[19]
One study disputed the claim of stable CO2 levels during the present interglacial of the last 10 ka. Based on an analysis of fossil leaves, Wagner et al.[20] argued that CO2 levels during the period 7–10 ka 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 CO2.[21] 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.[22]) believe the variations in Greenland cores result from in situ decomposition of calcium carbonate dust found in the ice. When dust levels in Greenland cores are low, as they nearly always are in Antarctic cores, the researchers report good agreement between Antarctic and Greenland CO2 measurements.
The longest ice core record comes from East Antarctica, where ice has been sampled to an age of 800 ka.[4] During this time, the atmospheric carbon dioxide concentration has varied by volume between 180–210 ppm during ice ages, increasing to 280–300 ppm during warmer interglacials.[23][24] 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.[25]
On long timescales, atmospheric CO2 content 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 CO2. The rates of these processes are extremely slow; hence they are of limited relevance to the atmospheric CO2 response to emissions over the next hundred years.
Various proxy measurements have been used to attempt to determine atmospheric carbon dioxide levels 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 CO2 volume concentrations between 200 and 150 Ma of over 3,000 ppm and between 600 and 400 Ma of over 6,000 ppm.[5] In more recent times, atmospheric CO2 concentration continued to fall after about 60 Ma. About 34 Ma, the time of the Eocene-Oligocene extinction event and when the Antarctic ice sheet started to take its current form, CO2 is found to have been about 760 ppm,[26] and there is geochemical evidence that volume concentrations were less than 300 ppm by about 20 Ma. Carbon dioxide decrease, with a tipping point of 600 ppm, was the primary agent forcing Antarctic glaciation.[27] Low CO2 concentrations may have been the stimulus that favored the evolution of C4 plants, which increased greatly in abundance between 7 and 5 Ma.
The Earth's oceans contain a huge amount of carbon dioxide in the form of bicarbonate and carbonate ions — much more than the amount in the atmosphere. The bicarbonate is produced in reactions between rock, water, and carbon dioxide. One example is the dissolution of calcium carbonate:
Reactions like this tend to buffer changes in atmospheric CO2. Since the right-hand side of the reaction produces an acidic compound, adding CO2 on the left-hand side decreases the pH of sea water, a process which has been termed ocean acidification (even though pH remains alkaline). Reactions between carbon dioxide 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;[28] 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 carbon dioxide gas. This, along with higher temperatures, would mean a higher equilibrium concentration of carbon dioxide in the air.
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 carbon emissions were to completely cease, atmospheric temperatures are not expected to decrease significantly.[29][30][31][32]
Carbon dioxide concentrations are growing rapidly and accelerating. The observed concentration rise is through multiple lines of evidence directly attributable to the use of gas, oil and coal. Of any emitted carbon dioxide, about 40% remains semipermanent in the atmosphere. According to a 2007 report by the Intergovernmental Panel on Climate Change, "About 50% of a CO2 increase will be removed from the atmosphere within 30 years, and a further 30% will be removed within a few centuries. The remaining 20% may stay in the atmosphere for many thousands of years."[33]
Three longer term processes are recognized to redistribute and eventually dissipate currently emitted carbon dioxide.[33] The first will be ocean invasion (300 years), which can only reduce concentration by a factor of ~4, because of the establishment of a new equilibrium. The second will be a new equilibrium with calcium carbonate, which can reduce the concentration by a factor of ~3 over a 5,000-year timescale. The third stage is eventual reaction with igneous rock with a time-constant of 400,000 years. These processes are so slow, that practically zero-emissions are at some point unavoidable in order not to exceed any practical carbon dioxide concentration limit.
To avoid a global warming of 2.1°C, it is estimated that a concentration of less than 450 ppm needs to be maintained if other gasses were to return to pre-industrial levels. Currently, a global warming of 0.7°C is measured, with another 0.6°C increase expected even without any further increased concentrations because the oceans are still being warmed along with the atmosphere. At the current accelerated growth rate, exponentially extrapolating the Keeling curve, this concentration will be reached in 22 years. Even with constant concentration growth, with the current 2.2 ppm/yr, this concentration will be reached in (450-390 ppm)/(2.2 ppm/yr)=27 years. These timescales are so short with respect to the timescale of the evolution that there is little doubt these concentrations will be reached soon barring any drastic behavior changes. The lifetime of power plants for instance can be 40 to 60 years.[34] To avoid dangerous climate change, a reduction of the concentration increase of 3.5% per year needs to be achieved for the foreseeable future. Reducing the concentration increase can be done by restricting emissions or with carbon sequestration. The concentration increase is dominated by human emissions.
The current increase to 386 ppm from 280 ppm causes a radiative forcing of 1.66 W/m2, and 1.34 W/m2 from increases in other gases, totaling 3.00 W/m2.[35] The current concentration of greenhouse gases already has a heating power equaling that of a concentration of (386−280)×3.00/1.66 + 280 = 472 ppm C02-eq (carbon dioxide equivalent). Therefore, the current concentrations are high enough for a temperature rise of more than 2° C.
To be able to reduce carbon dioxide concentration by Carbon sequestration back to pre-industrial levels, (390−280 ppm)/390ppm/(50%/100) = 70% of all the CO2 in the air needs to be removed, where 50% is the percentage of carbon dioxide residing in the atmosphere (and not in the oceans), removing about (390−280 ppm)/(50%/100) = 0.03% of the air — an immense task.