The carbon cycle, alongside the cycles of nitrogen, oxygen and water, is one of the four major global cycles on which life on earth depends. It describes the movements of carbon between the atmosphere, the oceans, the biosphere, including soils, plants, animals and humans, and the lithosphere which includes rocks, sediments and fossil fuels. The carbon cycle is the one that is creating great interest currently because of its influence on global warming and climate change.
There are two main sub-cycles that make up the total carbon cycle, the geological cycle and the biological cycle. These differ from each other greatly in the time scale over which they operate and the rate of movement of the carbon. The geological carbon cycle operates on a time scale of millions of years. Since the earth began to form millions of years ago carbon dioxide from the atmosphere, and water, have combined to produce carbonic acid which reacts with existing rocks and minerals to form carbonates. During geological history many of the sediments containing carbonates have been converted into rocks such as limestone, dolomite and chalk which 'lock up' carbon for millions of years. Many such deposits make up the core rocks beneath the land as well as beneath the oceans. Very little of this carbon gets back into the atmosphere.
The biological carbon cycle concerns the use of carbon to support life. It operates on a timescale of days to several decades. In one part of cycle, by the process of photosynthesis, green plants remove carbon dioxide (CO2) from the atmosphere. The carbon is used to make carbohydrates such as sugar or starch, and these are converted into vital parts of plants such as cell walls, pigments and proteins. Virtually all vegetation and food crops depend on this process. The other part of the biological cycle concerns the processes that help carbon return back to the atmosphere. The process of respiration, somewhat the reverse of photosynthesis, involves the intake of oxygen and the release of carbon dioxide to the atmosphere. The organic substances that are a product of the photosynthesis process are broken down in the respiration and metabolism of plants, releasing carbon dioxide back to the atmosphere. It is this biological cycle that we can influence most to modify levels of CO2 in the atmosphere.