Carbon dioxide in the Earth's atmosphere
Carbon dioxideforms approximately 0.04% of the Earth's atmosphere. It is essential to photosynthesisin plants and other photoautotrophs, and is also a prominent greenhouse gasdue to its radiative forcingstrength.
As of November 2007, the CO2 concentration in
Earth's atmospherewas about 0.0384% by volume, or 384 ppmv. This is 100 ppm (35%) above the 1832 ice core levels of 284 ppm [ cite web | title=Historical CO2 record derived from a spline fit (20 year cutoff) of the Law Dome DE08 and DE08-2 ice cores | url=http://cdiac.ornl.gov/ftp/trends/co2/lawdome.smoothed.yr20 | accessdate=2007-06-12] [http://cdiac.ornl.gov/ftp/trends/co2/maunaloa.co2] . The National Oceanic and Atmospheric Administrationdefines the current levels in terms of "a dry mole fractiondefined as the number of molecules of carbon dioxide divided by the number of molecules of dry air multiplied by one million (ppm)." [cite web | title = Trends in Atmospheric Carbon Dioxide - Mauna Loa | url=http://www.esrl.noaa.gov/gmd/ccgg/trends/ | accessdate=2007-06-12 ]
There is an annual fluctuation of about 3-9 ppm in the measurements, which roughly follows the Northern Hemisphere's growing season. The
Northern Hemispheredominates 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 spring greenup begins and reach a minimum in October when the quantity of biomassundergoing photosynthesis is greatest. [ [http://cdiac.ornl.gov/pns/faq.html Carbon Dioxide Information Analysis Center (CDIAC) - Frequently Asked Questions ] ]
Despite its relatively small concentration overall in the atmosphere, CO2 is an important component of Earth's atmosphere because it absorbs
infraredradiation 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. [Petty, G.W.: "A First Course in Atmospheric Radiation", pages 229-251, Sundog Publishing, 2004] "See also " Carbon dioxide equivalent".
The initial carbon dioxide in the atmosphere of the young Earth was produced by volcanic activity. This was essential for a warm and stable climate conducive to life. Volcanic activity now releases about 130 to 230 teragrams (145 million to 255 million
short tons) of carbon dioxide each year, [Gerlach, T.M., 1992, Present-day CO2 emissions from volcanoes: Eos, Transactions, American Geophysical Union, Vol. 72, No. 23, June 4, 1991, pp. 249, and 254 – 255] which is less than 1% of the amount released by human activities. [U.S. Geological Survey, "Volcanic Gases and Their Effects" http://volcanoes.usgs.gov/Hazards/What/VolGas/volgas.html]
fossil fuels such as coaland petroleumis the leading cause of increased anthropogenicCO2; deforestationis the second major cause. As of 2004, around 27 gigatonnes of CO2 are released from fossil fuels per year worldwide, equivalent to about 7.4 gigatonnes of carbon (see List of countries by carbon dioxide emissions); in 2006 8.4 gigatonnes carbon were emitted [http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter2.pdf] . With some simple calculations based on the surface area of the Earth, normal atmospheric pressure, and an estimate of roughly 400ppmv atmospheric CO2 content one can calculate that the atmospheric CO2 content is currently approximately 3 teratonnes.
Carbon dioxide is released to the atmosphere by a variety of natural sources, and over 95% of total CO2 emissions would occur even if humans were not present on Earth. 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. This carbon dioxide alone is over 8 times the amount emitted by humans. These natural sources are balanced by natural sinks, which remove carbon dioxide from the atmosphere. [US Global Change Research Information Office, "Common Questions about Climate Change" http://www.gcrio.org/ipcc/qa/05.html] The increase in carbon dioxide concentration arises because the increase from human activity is not balanced by a corresponding sink.
In 1997, Indonesian
peatfires may have released 13% – 40% as much carbon as fossil fuel burning does in a single year. [ [http://www.ens-newswire.com/ens/nov2002/2002-11-08-06.asp Indonesian Wildfires Accelerated Global Warming ] ] [ [http://www.newscientist.com/article.ns?id=dn6613 Massive peat burn is speeding climate change - 06 November 2004 - New Scientist ] ] Various techniques have been proposed for removing excess carbon dioxide from the atmosphere in carbon dioxide sinks. Not all the emitted CO2 remains in the atmosphere; some is absorbed in the oceans or biosphere. 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 57% over longer (5 year) periods.
Increased amounts of CO2 in the atmosphere enhance the
greenhouse effect. It is currently the majority scientific opinion that carbon dioxide emissions are the main cause of global warmingobserved since the mid-20th century. The effect of combustion-produced carbon dioxide on climate is occasionally called the Callendar effect, after engineer and inventor Guy Stewart Callendarwho was one of the first to propose this association (in 1938).
Natural sources of atmospheric carbon dioxide include
volcanic outgassing, the combustionof organic matter, and the respiration processes of living aerobic organisms; man-made sources of carbon dioxide include the burning of fossil fuelsfor heating, power generation and transport. It is also produced by various microorganisms from fermentation and cellular respiration. Plants convert carbon dioxide to carbohydrates during a process called photosynthesis. They produce the energy needed for this reaction through the photolysisof water. The resulting gas, oxygen, is released into the atmosphere by plants, which is subsequently used for respiration by heterotrophicorganisms, forming a cycle.
During the 100,000 year ice age cycle, CO2 varies between a low of approximately 200 ppm during cold periods and a high of 280 ppm during interglacials. Recent human influences have increased this to above 380 ppm. There is a large natural flux of CO2 into and out of the biosphere and oceans. In the pre-industrial era these fluxes were largely in balance. Currently approximately 50% of human-emitted CO2 is removed; without this effect CO2 levels would be even higher.Fact|date=March 2008
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
Greenlandice 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 kyr).
ice corerecord comes from East Antarctica, where ice has been sampled to an age of 800 kyr BP (Before Present). [ [http://news.bbc.co.uk/2/hi/science/nature/5314592.stm BBC NEWS | Science/Nature | Deep ice tells long climate story ] ] 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. [ [http://pubs.acs.org/cen/news/83/i48/8348notw1.html Chemical & Engineering News: Latest News - Ice Core Record Extended ] ] The data can be accessed [http://www.ncdc.noaa.gov/paleo/icecore/antarctica/vostok/vostok_data.html here] .
Some studies have disputed the claim of stable CO2 levels during the present interglacial of the last 10 kyr. Based on an analysis of fossil leaves, Wagner et al. [cite journal | first = Friederike | last = Wagner | coauthors = Bent Aaby and Henk Visscher | title = Rapid atmospheric O2 changes associated with the 8,200-years-B.P. cooling event | journal = PNAS | volume = 99 | issue = 19 | year = 2002 | pages = 12011 – 12014 | doi = 10.1073/pnas.182420699 | pmid = 12202744 ] argued that CO2 levels during the period 7 – 10 kyr ago 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. [cite journal | first = Andreas | last = Indermühle | coauthors = Bernhard Stauffer, Thomas F. Stocker | title = Early Holocene Atmospheric CO2 Concentrations | journal = Science | volume = 286 | issue = 5446 | year = 1999 | pages = 1815 | doi = 10.1126/science.286.5446.1815a | url = http://www.sciencemag.org/cgi/content/full/286/5446/1815a | accessdate = May 26 | accessyear = 2005 ] 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., Smith et al. [cite journal | first = H.J. | last = Smith | coauthors = M Wahlen and D. Mastroianni | title = The CO2 concentration of air trapped in GISP2 ice from the Last Glacial Maximum-Holocene transition | journal = Geophysical Research Letters | volume = 24 | issue = 1 | year = 1997 | pages = 1 – 4 | doi = 10.1029/96GL03700 ] ) believe the variations in Greenland cores result from "in situ" decomposition of
calcium carbonatedust found in the ice. When dust levels in Greenland cores are low, as they nearly always are inAntarctic cores, the researchers report good agreement between Antarctic and Greenland CO2 measurements.
On longer timescales, various proxy measurements have been used to attempt to determine atmospheric carbon dioxide levels millions of years in the past. These include
boronand carbon isotoperatios in certain types of marine sediments, and the number of stomataobserved 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 myrBP of over 3,000 ppm and between 600 and 400 myr BP of over 6,000 ppm. [http://www.grida.no/climate/ipcc_tar/wg1/fig3-2.htm Climate Change 2001: The Scientific Basis ] ] On long timescales, atmospheric CO2 content is determined by the balance among geochemical processes including organic carbon burial in sediments, silicate rock weathering, and vulcanism. The net effect of slight imbalances in the carbon cycleover 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. In more recent times, atmospheric CO2 concentration continued to fall after about 60 myr BP, and there is geochemical evidence that volume concentrations were less than 300 ppm by about 20 myr BP. Low CO2 concentrations may have been the stimulus that favored the evolution of C4 plants, which increased greatly in abundance between 7 and 5 myr BP. Present carbon dioxide levels are likely higher now than at any time during the past 20 myr and certainly higher than in the last 800,000.
It must be noted that although CO2 concentrations have been significantly higher in the distant past than they are today, the energy output of the sun has also steadily increased over the same period. It is projected to continue to do so long into the future, according to theories concerning the
formation and evolution of the Solar System.
Relationship with oceanic concentration
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:
:CaCO3 + CO2 + H2O unicode|⇌ Ca2+ + 2 HCO3-
Reactions like this tend to buffer changes in atmospheric CO2. However, since it produces an acidic compound, the
pHof sea water is thought to go down with increasing carbon dioxide levels. Reactions between carbon dioxide and non-carbonate rocks also add bicarbonate to the seas, which 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.
The vast majority of CO2 added to the atmosphere will eventually be absorbed by the oceans and become bicarbonate ion, but the process takes on the order of a hundred years because most seawater rarely comes near the surface.
As the oceans warm, carbon dioxide solubility in the surface waters decreases markedly. However, the overall system is quite complex, as indicated above, and further details may be found in the article on the carbon solubility pump.
An unknown, though probably large, quantity of CO2 is in the ocean sediments as a methane-carbon dioxide-water clathrates, one of the family of gas hydrates.
List of countries by carbon dioxide emissions per capita
List of countries by carbon dioxide emissions
List of countries by ratio of GDP to carbon dioxide emissions
Avoiding Dangerous Climate Change- A Scientific Symposium on Stabilisation of Greenhouse Gases
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