In any given year, tens of billions of tons of carbon move between the atmosphere, hydrosphere, and geosphere. Human activities add about 5.5 billion tons per year of carbon dioxide to the atmosphere. The illustration above shows total amounts of stored carbon in black, and annual carbon fluxes in purple. (Illustration courtesy NASA Earth Science Enterprise).
The carbon cycle is the global process by which the element carbon is stored and exchanged between the air, oceans, earth, and living things. The cycle is usually thought of as four main reservoirs of carbon interconnected by pathways of exchange. The reservoirs are the atmosphere, terrestrial biosphere (usually includes freshwater systems), oceans, and sediments (includes fossil fuels). The annual movements of carbon, the carbon exchanges between reservoirs, occur because of various chemical, physical, geological, and biological processes. The ocean contains the largest pool of carbon near the surface of the Earth, but most of that pool is not involved with rapid exchange with the atmosphere.
In the carbon cycle, carbon, obtained from the atmosphere as carbon dioxide, is absorbed by green plants, synthesized into organic compounds, and then returned to the atmosphere as carbon dioxide. The organic compounds, particularly carbohydrates, are synthesized in plants from carbon dioxide and water in the presence of chlorophyll and light by a process known as photosynthesis. The carbohydrates are then broken down to carbon dioxide and water either by the plant during respiration or after death by putrefying bacteria and fungi.