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What is the Carbon Cycle?
Publication Date: Summer 2001
Carbon in its many forms flows between the biosphere, the atmosphere, and the oceans. This global flow is known as the carbon cycle.
Scientists study many processes through which carbon is exchanged among these elements, and many places where carbon is stored for long periods of time. Because the amounts of carbon involved are so huge, scientists use the term gigaton as a unit of measure. A single gigaton is equal to 1 billion tons of carbon.
The largest stores of carbon exist in fossil and sedimentary deposits underground and at the bottom of the oceans. About 44,000 gigatons of carbon is trapped in these stores. Fossil fuel emissions, land clearing, and other human activity releases about 6 gigatons of carbon from these stores into the atmosphere each year.
The atmosphere holds about 750 gigatons of carbon in the form of carbon dioxide. Scientists say that atmospheric CO2 is on the rise. Current levels are 25 percent higher than levels that existed before the industrial revolution.
Plants absorb carbon dioxide from the atmosphere during photosynthesis. They also release CO2 back into the air through respiration. About 800 gigatons of carbon is dissolved in the surface layers of the world’s oceans. Marine plants use that dissolved atmospheric CO2.
Plants and animals also store carbon in their bodies. About half the weight of a mature tree is elemental carbon. Scientists estimate that 550 gigatons of carbon exist in living plant and animal matter. Another 1,300 gigatons of carbon is trapped in dead leaves, twigs, branches, other ground litter, and soils.
These measurements are estimates, of course. While scientists have a general understanding of the carbon cycle’s pathways, they have not totally accounted for the rates of change between the atmosphere, land, and ocean. About 20 percent of the CO2 released each year—between 1 and 2 gigatons—cannot be accounted for. Scientists are still working to discover where this disappearing carbon goes.—Diane Boudreau