Carbon dioxide in the atmosphere is a 'greenhouse gas'
- it traps some of the radiation that would otherwise be lost
to space, and causes the Earth's atmosphere to be warmer than
it would otherwise be. Man-made emissions of carbon dioxide have
caused the amount in the atmosphere to increase by about 30% since
pre-industrial times, and this is a major cause of global warming.
Therefore it is important for us to understand how the carbon
cycle works in order for us to be able to predict how it may behave
in the future.
Carbon is continuously cycled between reservoirs in the ocean, on the land, and in the atmosphere, where it occurs primarily as carbon dioxide. On land, carbon occurs primarily in living biota and decaying organic matter. In the ocean, the main form of carbon is dissolved carbon dioxide and small creatures, such as plankton. The largest reservoir is the deep ocean, which contains close to 40,000 Gt C, compared to around 2,000 Gt C on land, 750 Gt C in the atmosphere and 1,000 Gt C in the upper ocean. The atmosphere, biota, soils, and the upper ocean are strongly linked. The exchange of carbon between this fast-responding system and the deep ocean takes much longer (several hundred years). The ocean takes up carbon dioxide when it is cold, at higher latitudes, and releases it near the tropics. Photosynthesis takes carbon dioxide from the atmosphere and transfers it to vegetation, while respiration releases carbon dioxide back into the atmosphere. These processes are shown schematically in the figure below. Although natural transfers of carbon dioxide are approximately 20 times greater than those due to human activity, they are in near balance, with the magnitude of carbon sources closely matching those of the sinks. The additional carbon resulting from human activity is the cause of atmospheric carbon dioxide rises over the last 150 years.
Schematic representation of the global carbon cycle, showing the stores of carbon and the fluxes (exchanges) of carbon between them. Changes in climate have a significant effect on the carbon cycle. Increases in atmospheric carbon dioxide concentration increase plant photosynthesis and the amount of carbon stored in vegetation. However, increases in temperature also lead to increases in plant and soil respiration rates, which tend to reduce the size of the terrestrial carbon store. In some regions, the changes in climate (such as decreased rainfall) can also reduce plant photosynthesis and reduce the ability of vegetation to sequester carbon. |
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