Carbon constitutes 49 percent of the dry weight of organisms and is next only to water. 71 percent of carbon is found dissolved in oceans. This ocean reservoir regulates the amount of carbon dioxide in the atmosphere. Fossil fuels also represent a reservoir of carbon. Carbon cycling occurs through the atmosphere, ocean, and living and dead organisms. 4 x 10¹³ kg of carbon is fixed in the biosphere through photosynthesis annually.

A considerable amount of carbon returns to the atmosphere as CO₂ through respiratory activities of the producers and consumers. Decomposers also contribute substantially to the CO₂ pool by their processing of waste materials and dead organic matter of land or oceans. Some amount of fixed carbon is lost to sediments and removed from circulation. Burning of wood, forest fire and combustion of organic matter, fossil fuels, volcanic activity are additional sources for releasing CO₂ into the atmosphere.

Human activities have significantly influenced the carbon cycle. Rapid deforestation and the massive burning of fossil fuels for energy and transport have significantly increased the rate of release of carbon dioxide into the atmosphere.