"Carbon is the key element for living things" (Ritter, 2001, pg.62). The carbon cycle is "the matter cycle in which, through the processes of photosynthesis, digestion, cellular respiration, decomposition, and combustion, carbon atoms move from an inorganic form in the air, water, or soil, to an organic form in living things, and then back to an inorganic form; all organic compounds contain carbon" (Ritter, 2001, pg.709). Carbon usually moves from rocks to air to living organisms to water and back to rocks (The Carbon Cycle, 2003, The Carbon Cycle). "Most of the carbon that forms living organisms is returned to the atmosphere or water as carbon dioxide from body wastes and when the dead organisms decay," during this cycle (Ritter, 2001, pg.709). If the decay process is delayed, then the organic matter may be converted into rock or fossil fuels such as coal, petroleum, and natural gas (Ritter, 2001, pg.63). This carbon can only be released by uplifting and weathering, or by burning as fuels (combustion), which releases carbon dioxide into the atmosphere.
Organic carbon is stored in bodies of living things as fats, proteins, and sugars are all organic. But deaths of living things and decomposition "returns the carbon to the cycle in inorganic form" (Ritter, 2001, pg.63).
Inorganic carbon is found in three reservoirs on Earth: the atmosphere, the oceans, and Earth's crust. Carbon dioxide makes up 0.03% of the gases that are taken in by living things. It is easily accessible by land plants who need carbon dioxide for photosynthesis. Carbon dioxide is also dissolved in oceans, and it is also used for photosynthesis by aquatic plants. It may also react with sea water to produce compounds that make shells and sediments. Volcanic activity in the Earth's crust release carbon dioxide into the atmosphere (Ritter, 2001, pg.63).
Humans have impacted the carbon cycle by mining and burning fossil fuels trapped in Earth's crust, and by burning forests. People are also clearing away vegetation, which increases the amount of carbon dioxide in the inorganic reservoir of the atmosphere. These actions complicate photosynthesis, which result in the excess production of carbon dioxide (Ritter, 2001, pg.64).