A simplified representation of the contemporary global carbon cycle is shown in the center of this figure. Values in parentheses are estimates of the main carbon reservoirs in gigatons (GT) as reported in Houghton (2007). The natural flux between the terrestrial biosphere and the atmosphere is about 120 GT of carbon per year, and that between the oceans and atmosphere is about 90 GT per year (IPCC 2007). In the terrestrial biosphere, photosynthesis removes about 120 GT of carbon from the atmosphere; decomposition of biological material and respiration from plants and soil microbes return 120 GT of carbon. In the oceans, the marine biosphere does not take up CO2 directly from the atmosphere. Each year the oceans absorb and release about 90 GT of carbon largely via diffusion across the air-ocean interface. The physical processes controlling the sinking of CO2 into colder, deeper waters (where CO2 is more soluble) and the mixing of ocean water at intermediate depths are known collectively as the solubility pump. Phytoplankton photosynthesis converts CO2 into organic carbon that is largely returned to ocean water as CO2 via microbial respiration and decomposition. The biological pump refers to the small fraction of organic carbon that forms into degradation-resistant clumps and sinks to the ocean floor. Together the solubility and biological pumps control the amount of carbon transported to ocean depths and the exchange of CO2 between ocean and atmosphere. Human activities (primarily fossil fuel use) emit about 9 GT of carbon each year. About 4 GT of this human-contributed carbon remain in the atmosphere; 3 GT are taken up by natural terrestrial processes, and another 2 GT are removed by the ocean (Canadell et al. 2007).
Credit or Source: Office of Biological and Environmental Research of the U.S. Department of Energy Office of Science. science.energy.gov/ber/
U.S. DOE. 2008. Carbon Cycling and Biosequestration: Report from the March 2008 Workshop, DOE/SC-108, U.S. Department of Energy Office of Science. (p. 2-3) (website)