Carbon Cycle

The carbon cycle involves the circulation of carbon dioxide (CO₂) from the atmosphere into plants and other living organisms; the transfer of car­bon from these organisms into other temporary storage pools, living or nonliving, containing organic and inorganic carbon compounds; and the return of CO₂ to the atmosphere through respiration or combustion processes. The carbon cycle provides a unifying framework for examin­ing exchanges or storage of carbon associated with photosynthesis and energy assimilation by organisms, respiration and metabolism, produc­tivity and biomass accumulation, and the decay and recycling of organic matter at the level of a single organism, an ecosystem, or the global bios­phere.

Analysis of the carbon cycle in a forest ecosystem, for example, requires the estimation of pools of carbon in live biomass, dead wood, decaying lit­ter (branches and leaves), and soil organic matter. This information is com­bined with estimates of major transfers within the cycle such as carbon fixation via photosynthesis, CO2 release by respiration, carbon flow to the soil as litterfall and root turnover, and carbon flow through grazing and de­composer food chains.

On a global scale, the primary carbon storage pools are the oceans and marine sediments, fossil fuels and shale deposits, terrestrial plants and soils, and the atmosphere. The global carbon cycle is characterized by large ex­changes of carbon between Earth and its atmosphere. Photosynthesis and ocean uptake processes remove CO2 from the atmospheric carbon pool, whereas CO₂ is returned to the atmosphere by biological respiration, de­forestation and land clearing, forest fires, and fossil fuel combustion asso­ciated with human activities. As of 2001, the atmosphere is experiencing a net gain of 3 billion tons of carbon per year from CO₂ emissions derived from human combustion of coal, oil, and gas, as well as from deforestation and land clearing activities. This imbalance in the global carbon cycle is reflected in the rising concentration of atmospheric CO₂, which has in­creased 15 percent from 320 ppm (parts per million) to 368 ppm since the mid-1960s.

References

Botkin, Daniel, and Edward Keller. Environmental Science. New York: John Wiley & Sons, 1995.

Schlesinger, William H. Biogeochemistry: An Analysis of Global Change. New York: Aca­demic Press, 1991.