Carbon—the element that defines life—enters the biota through photosynthesis while the oxygen released in the process makes possible aerobic respiration of all living things. Molecules that contain carbon are the major constituents of living tissues, but the amount of carbon in active biosphere cycling is minor compared to the amount held in abiotic reservoirs such as sedimentary rocks, fossil fuel deposits, and deep sea sediments.
Respiration and photosynthesis are the driving forces of the carbon cycle. Carbon enters the biosphere as atmospheric carbon dioxide (CO 2), which is incorporated by photosynthetic organisms into carbohydrates. It leaves, also as CO 2, through the respiration of organisms. The carbon cycle is thus bound with that of oxygen and, through oxidation‐reduction reactions, to other elements of importance to organisms. Removal of CO 2 from the atmosphere by terrestrial vegetation in photosynthesis is balanced by the return due to respiration of plants and decomposer organisms in the soil. Plankton organisms in the surface waters of the oceans also remove and return atmospheric carbon in about the same proportions, although a minor portion of dissolved CO 2 is lost in the deep ocean and buried in the sediments. The atmosphere links all compartments in the cycle, and the changes in atmospheric CO 2 are a measure of the health of the ecosystem.
A disturbing feature of the current carbon cycle is the net atmospheric increase of CO 2. In the last 50 years of accurate measurements, annual increases of about 0.4 percent (1.5 ppm) have occurred consistently—indicating that a cycle in place for the last 100,000 years no longer is in balance. An increase of as much as 30 percent in the last 200 years (since the beginning of the Industrial Revolution) is circumstantial evidence that the burning of fossil fuels together with human alterations to the natural vegetation cover of large areas of the globe may be the cause. It is suspected that the increased levels of CO 2 will trap more reflected radiation in the atmosphere and increase the temperature on the surface of the Earth causing a greenhouse effect.