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The concentration of carbon dioxide (CO2) in Earth's atmosphere determines its contribution to the greenhouse effect and the rates of plant and algal photosynthesis. The concentration has increased markedly in the 21st century, at a rate of 2.0 ppm/yr during 2000–2009 and faster since then. It was 280 ppm in pre-industrial times, and has risen to 400 ppm (parts per million) as of May 2013[update], with the increase largely attributed to anthropogenic sources. About 57% of the CO2 emissions go to increase the atmospheric level, with much of the remainder contributing to ocean acidification. Carbon dioxide is used in 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. The present level appears to be the highest in the past 800,000 years and likely the highest in the past 20 million years, but well below 10% of its 500-million-year peak.
In 2009, the CO2 global average concentration in Earth's atmosphere was about 0.0387%, or 387 parts per million (ppm). 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. 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.
Sir Brian Hoskins of the Royal Society said that the 400ppm milestone should "jolt governments into action". The National Geographic noted that the level 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."
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 CO2 emissions are natural, and are balanced to various degrees by natural CO2 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 CO2 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 CO2 is removed by the biosphere and oceans. 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 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.
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 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 CO2 containing carbon of different isotopic ratios to those of living plants, enabling them to distinguish between natural and human-caused contributions to CO2 concentration.
Burning fossil fuels such as coal and petroleum is the leading cause of increased anthropogenic CO2; deforestation is the second major cause. In 2010, 9.14 gigatonnes of carbon (33.5 gigatonnes of CO2) 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.
False-color image of smoke and ozone pollution from Indonesian fires, 1997.
|Nuclear||Various generation II reactor types||16|
|Solar thermal||Parabolic trough||22|
|Geothermal||Hot dry rock||45|
|Solar PV||Polycrystalline silicon||46|
|Natural gas||Various combined cycle turbines without scrubbing||469|
|Coal||Various generator types without scrubbing||1001|
|Country||Carbon dioxide emissions per|
year (106 Tons) (2006)
|Percentage of global total||Avg. emission|
per km2 of its land (tons)
|Carbon dioxide emissions per year|
(Tons per person) (2007)
|Country||Carbon dioxide emissions per year|
(Tons per person) (2006)
|United Arab Emirates||32.8|
|Trinidad and Tobago||25.3|
According to Washington DC, Carbon Monitoring for Action (CARMA).[notes 1] In the United States, power generators produce 40% of all CO2 emissions. Globally power generators contribute 25 per cent of emissions. Carbon dioxide accounted for the 95 per cent of direct greenhouse gas emissions (GGE).  In November 2007 the Scherer Plant was the only American power plant on the list of the world’s top 25 carbon dioxide emitters ranking 20th out of 25. Scherer Plant in Juliette, Georgia is the largest CO2 emitter in the U.S.
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 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.
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. 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. 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.) 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. 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 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.
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. 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 CO2 concentration over the next hundreds, thousands, or millions of 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. 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, 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. 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.
In billion-year timescales, it is predicted that plant, and therefore animal, life on land will die off altogether as 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. 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.
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 (pH of the ocean becomes more acidic although the pH value remains in the alkaline range). 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; 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 human carbon dioxide emissions were to completely cease, atmospheric temperatures are not expected to decrease significantly in the short term.