Without them the planet would have an average temperature of about 5 degrees Fahrenheit, well below freezing. But future increases in greenhouse gases of the magnitude expected during the 21 st century must be considered seriously as these increases could affect the future economy and well-being of the nation. For this reason, enhanced study of the global carbon cycle deserves our highest attention and vigilance. The carbon cycle is the process by which the Earth exchanges huge amounts of carbon between three major reservoirs, the land, the oceans and the atmosphere. On land, carbon dioxide is taken up by trees and other plants during photosynthesis, and is sequestered in soils when plants die and undergo incomplete decay.
Carbon dioxide is given off during respiration of biological systems.
Oceans exchange carbon dioxide with air at the surface, although some of the dissolved carbon dioxide in the ocean water is not returned to the atmosphere. The amount of carbon dioxide that remains in the atmosphere is the difference between these huge carbon dioxide inputs and outputs of the land and oceans. We can now measure the atmospheric content of carbon dioxide on a global average quite accurately. We also know the magnitude of fossil fuel emissions. Deforestation in the tropics is less well known but can be estimated.