Carbon cycling is a general term that covers the flux of carbon from inorganic forms to organic compounds and back to inorganic molecular states. The term may cover very different processes dependent on the time scale. Carbon cycling in the form of gas exchange, photosynthesis, and biochemical transformations takes place in seconds while it takes millions of years from formation to weathering of carbonate rocks representing the longest time scale of carbon cycling.
In recent decades, the term carbon cycling has been most widely used in the context of understanding the sources and sinks of atmospheric carbon dioxide (CO2). CO2 is a greenhouse gas and the global climate is linked to its atmospheric concentration, which is why it is of great importance that we understand the global carbon cycle in order to understand global climate. It is well known that atmospheric CO2 concentration is increasing rapidly as a consequence of anthropogenic emissions and that it is now at a concentration (360 ppm) that is more than 25% higher than in preindustrial times (about AD 1700). The atmospheric concentration of another carbon-carrying and very strong greenhouse gas, methane (CH4), has been increasing by more than 100% since the early part of the 18th century. The increasing CO2 concentration is expected to have important consequences for ecosystem productivity (as CO2 is the substrate for plant growth), and the general increase in the concentrations of greenhouse gases in the atmosphere is expected to cause climate warming (see Climate Change; Global Change Effects).
Figure 1 shows a schematic illustration of the global modern carbon cycle. Carbon is being fixed as CO2 through photosynthesis (gross primary production, GPP), and organic matter and oxygen are produced following the simplified chemical reaction
At a global scale, this process is responsible for the fixation of 120x1015g C/yr of atmospheric CO2
(Figure 1). The CO2 is returned to the atmosphere through the following generalized process of respiration:
Roughly half the carbon fixed in the process of GPP is returned to the atmosphere from plant respiration and the other half from decomposers. Where there is a slight imbalance between the net primary productivity (NPP=GPP-plant respiration) and the decomposition, carbon accumulates in the soils. Peat bogs are a good example of this, where NPP in general exceeds respiration and organic material (peat) accumulates in the ground (see Peatlands and Bogs) This is a phenomenon of special relevance for high northern latitudes as a large fraction (about 30%) of the global soil organic material 1500 x 1015 g C (Figure 1) is found in northern boreal and tundra soils.
The oceans are the largest single reservoir of carbon amounting to 38,000 x 1015 g C, although only a tiny fraction of this amount, around 90 x 1015 g C/yr, is in active exchange with the atmosphere. This compared to the stock-size, small exchange of carbon between global oceans and the atmosphere is, however, extremely important for understanding the global carbon cycle. Using the oceanographers' best estimates, the global oceans are namely a net sink for atmospheric CO2 taking up around 2 x 1015 g C/yr or one-third of the global anthropogenic emissions. In this respect, the oceans are "helping" us in taking up some of the extra CO2 that we add to the atmosphere and that otherwise could accelerate climate warming further. The Arctic has a special role here in that the sink functioning of the oceans is largely taking place under cold oceanic conditions where the solubility of CO2 in sea water is the highest and the northern oceans are also where the mixing of surface and deep waters (which constitutes the real sink for atmospheric CO2) takes place.
Man-made fossil fuel emissions amount to a total of about 6 x 1015 g C/yr and deforestation (mainly in the tropics, for example, Amazonia) amounts to an estimated further 1 x 1015 g C emitted as CO2 annually to the atmosphere (Figure 1). If we try and trace what happens to these emitted amounts, we discover an important discrepancy between known sources and sinks. The sinks constitute the oceanic 2 x 1015 g C mentioned above and, with the traditional assumption that the terrestrial biosphere is in equilibrium, the only other sink is the actual increasing concentration in the atmosphere. Due to detailed measurements of the rate of increase in atmospheric CO2 at several monitoring stations around the globe and the fact that the size of the atmosphere is well known, it is possible to estimate the amount of C that goes into the atmospheric increase with great accuracy. Such a calculation leads to a sink of about 3 x 1015 g C/yr. The best-known sinks (oceans + atmospheric increase) therefore only amount to a total of approximately 5 x 1015 g C/yr while the total sources (fossil + deforestation) are estimated at around 7 x 1015 g C/yr. The difference of 2 x 1015 g C/yr has been termed the "missing sink" in the global carbon budget and it has been subject to intensive global biogeochemical research over recent years to locate where the numbers in the calculation are wrong or where possibly there is a significant global sink for carbon not accounted for in the budget. The most recent consensus among carbon cycling researchers is that the residual "missing" sink is to be found in the terrestrial biosphere, and it is a significant extra uptake by the terrestrial biota in particular at high latitudes that is needed for the closure of the global carbon budget. The increased uptake by the terrestrial vegetation may be induced by a so-called fertilization effect caused by the rising CO2 concentration in itself. CO2 is, as shown above, the substrate for photosynthesis, and increasing its atmospheric concentration may induce higher rates of NPP in some ecosystems without affecting the respiration to the same extent. This would lead to an enhanced sink activity of the terrestrial biosphere. Respiration, on the other hand, may be more sensitive to warming, and in climate warming scenarios the balanced effect of rising CO2 concentrations and temperature on global photosynthesis versus respiration will be of crucial importance for the fate of the stored soil carbon that potentially could be released as extra CO2 in the atmosphere. This issue has particular implications in the circumpolar Arctic, as this is where the predicted warming is expected to be the greatest and also where significant amounts of carbon are stored as soil organic matter.
Torben R. Christensen
See also Global Warming; Peatlands and Bogs; Soil Respiration
Chapin, F.S. et al. (editors), Arctic Ecosystems in a Changing Climate: An Ecophysiological Perspective, San Diego: Academic Press, 1992 Christensen, T.R., S. Jonasson,A. Michelsen, T.V. Callaghan & M. Havström, "Environmental controls on soil respiration in the Eurasian and Greenlandic Arctic". Journal of Geophysical Research, 103(D22) (1998): 29015-29021 Oechel, W.C. et al. (editors), Global Change and Arctic Terrestrial Ecosystems, Berlin and New York: Springer, 1997 Prentice, I.A. et al., "The Carbon Cycle and Atmospheric Carbon Dioxide". In Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, edited by J.T. Houghton et al., Cambridge and New York: Cambridge University Press, 2001 Schlesinger, W.H., Biogeochemistry: An Analysis of Global Change, San Diego: Academic Press, 1997
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