INTRODUCTION Carbon Cycle (ecology)
Carbon Cycle (ecology), in ecology, the cycle of carbon usage by which energy flows through the earth's ecosystem. The basic cycle begins when photosynthesizing plants (see Photosynthesis) use carbon dioxide (CO2) found in the atmosphere or dissolved in water. Some of this carbon is incorporated in plant tissue as carbohydrates, fats, and protein; the rest is returned to the atmosphere or water primarily by aerobic respiration.
Carbon is thus passed on to herbivores that eat the plants and thereby use, rearrange, and degrade the carbon compounds. Much of it is given off as CO2, primarily as a by-product of aerobic respiration, but some is stored in animal tissue and is passed on to carnivores feeding on the herbivores. Ultimately, all the carbon compounds are broken down by decomposition, and the carbon is released as CO2 to be used again by plants.
AIR-WATER EXCHANGES
On a global scale the carbon cycle involves an exchange of CO2 between two great reservoirs: the atmosphere and the earth's waters. Atmospheric CO2 enters water by diffusion across the air-water surface. If the CO2 concentration in the water is less than that in the atmosphere, it diffuses into water, but if the CO2 concentration is greater in the water than in the atmosphere, CO2 enters the atmosphere. Additional exchanges take place within aquatic ecosystems.
Excess carbon may combine with water to form carbonates and bicarbonates. Carbonates may precipitate out and become deposited in bottom sediments. Some carbon is incorporated in the forest-vegetation biomass (living matter) and may remain out of circulation for hundreds of years. Incomplete decomposition of organic matter in wet areas results in the accumulation of peat. Such accumulation during the Carboniferous period created great stores of fossil fuels: coal, oil, and gas.
TOTAL CARBON POOL
The total carbon pool, estimated at about 49,000 metric gigatons (1 metric gigaton equals 109 metric tons), is distributed among organic and inorganic forms. Fossil carbon accounts for 22 percent of the total pool. The oceans contain 71 percent of the world's carbon, mostly in the form of bicarbonate and carbonate ions. An additional 3 percent is in dead organic matter and phytoplankton. Terrestrial ecosystems, in which forests are the main reservoir, hold about 3 percent of the total carbon. The remaining 1 percent is held in the atmosphere, circulated, and used in photosynthesis.
ADDITIONS TO ATMOSPHERE
Because of the burning of fossil fuels, the clearing of forests, and other such practices, the amount of CO2 in the atmosphere has been increasing since the Industrial Revolution. Atmospheric concentrations have risen from an estimated 260 to 300 parts per million (ppm) in preindustrial times to more than 350 ppm today. This increase accounts for only half of the estimated amount of carbon dioxide poured into the atmosphere. The other 50 percent has probably been taken up by and stored in the oceans. Although terrestrial vegetation may take up considerable quantities of carbon, it is also an additional source of CO2.
Atmospheric CO2 acts as a shield over the earth. It is penetrated by short-wave radiation from outer space but blocks the escape of long-wave radiation. As increased quantities of CO2 are added to the atmosphere, the shield thickens and more heat is retained, increasing global temperatures. Although such increases have not yet been great enough to cancel out natural climatic variability, projected increases in CO2 from the burning of fossil fuels suggest that global temperatures could rise some 2° to 6° C (about 4° to 11° F) by early in the 21st century. This increase would be significant enough to alter global climates and thereby affect human welfare. See also Air Pollution; Greenhouse Effect.
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