Carbon Cycle

THE CARBON CYCLE describes the biogeochemical cycle, or routes by which carbon atoms are exchanged through nested networks of environmental systems from the atmosphere into the biosphere, through photosynthesis and back again with respiration, decomposition, and biomass burning. Elemental carbon is a traditional component of the hydrosphere, the atmosphere, the geosphere (rocks, such as limestone; coal; and soils), as well as the biosphere (all living things).

The carbon cycle also involves the process of removal and uptake of carbon on a global scale. This process involves components in food chains, in the atmosphere, as carbon dioxide; in the hydrosphere; and in the geosphere. The major movement of carbon results from photosynthesis and from respiration. Carbon is present in the planet in the following major reservoirs: as the gas, carbon dioxide (CO2) in the atmosphere; as organic matter in soils; as organic molecules in living and dead organisms found in the biosphere; in the lithosphère as fossil fuels and sedimentary rock deposits such as limestone, dolomite and chalk; and in the oceans as dissolved atmospheric CO2 and as calcium carbonate shells in marine organisms.

The carbon cycle has a large impact on Earth, both globally and locally. At the global scale, the carbon cycle influences Earths climate by regulating the amount of CO2, a principal greenhouse gas, in the atmosphere. Terrestrial ecosystems store as much carbon as the atmosphere, so plants and soils play an important role in regulating climate. The carbon cycle also plays a primary role in keeping ecological systems in balance, since it is involved in basic ecological processes such as plant growth and accumulation, and the death and decay of plant material.

As a principal building block of organic matter, carbon is utilized by biotic components of an ecosystem, especially by organisms for structural growth: a portion of elemental carbon that a living thing takes in is usually incorporated into its tissues. Thus, the carbon cycle is one of the most important biogeochemical cycles to humans, because it is a vehicle through which one of the primary elements required for the formation of human tissues is cycled, and also because it is a means through which elemental carbon is introduced to plants, the basis of human food.

The carbon cycle is also important to the human climate system because it sets the background for the environment, through CO2 and methane (CH4), which are major drivers of global climate temperatures. The carbon atoms present in the atmosphere, hydrosphere, biosphere, and the geosphere are able to move from one of these environmental systems to another as part of the carbon cycle.

The carbon cycle is sometimes conceptualized as four major carbon sinks, interrelated by nested systems of pathways for transport of carbon atoms. The carbon reservoirs are the air (atmosphere), considered as the starting point of the cycle; the sea water and oceans (hydrosphere), which include biotic and abiotic marine biota; the sediments (fossil fuels); and the terrestrial biosphere, which includes fresh water systems (lenthic or lotie), as well as nonliving organic substances, such as soil carbon. The carbon cycle is primarily controlled by a series of biological, physical, chemical, and geological processes within the environment.

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