carbon cycle - Carbon in the atmosphere, Carbon in the biosphere, Carbon in the Oceans, Carbon cycle modelling

The cycle through which carbon is transferred between the biological (biotic) and nonbiological (abiotic) parts of the global ecosystem. It involves the fixation of gaseous carbon dioxide during photosynthesis to form complex organic molecules, as well as the subsequent processes through which it ultimately returns to the atmosphere by respiration and decomposition.

The carbon cycle is the biogeochemical cycle by which carbon is exchanged between the biosphere, geosphere, hydrosphere and atmosphere of the Earth (other astronomical objects may have similar carbon cycles, but nothing is yet known about them). The reservoirs are the atmosphere, the terrestrial biosphere (which usually includes freshwater systems and non-living organic material, such as soil carbon), the oceans (which includes dissolved inorganic carbon and living and non-living marine biota), and the sediments (which 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 active pool of carbon near the surface of the Earth, but the deep ocean part of this pool does not rapidly exchange with the atmosphere.

The global carbon budget is the balance of the exchanges (incomes and losses) of carbon between the carbon reservoirs or between one specific loop (e.g., atmosphere - biosphere) of the carbon cycle. An examination of the carbon budget of a pool or reservoir can provide information about whether the pool or reservoir is functioning as a source or sink for carbon dioxide.

Carbon in the atmosphere

Carbon exists in the Earth's atmosphere primarily as the gas carbon dioxide (CO2). Other gases containing carbon in the atmosphere are methane and chlorofluorocarbons (the latter is entirely artificial).

The quantity of carbon in the atmosphere, in the form of CO2, is around 810 gigatonnes.

Carbon is taken from the atmosphere in several ways:

When the sun is shining, plants perform photosynthesis to convert carbon dioxide into carbohydrates, releasing oxygen in the process. In upper ocean areas of high productivity, organisms form tissue containing reduced carbon, and some also form carbonate shells, tests, or other hard body parts. These are, respectively, oxidized (soft-tissue pump) and redissolved (carbonate pump) at lower average levels of the ocean than those at which they formed, resulting in a downward flow of carbon (see entry on biological pump). Unlike the previous two processes, this does not move the carbon into a reservoir from which it can readily return to the atmosphere.

Carbon can be released back into the atmosphere in many different ways,

Through the respiration performed by plants and animals. Fungi and bacteria break down the carbon compounds in dead animals and plants and convert the carbon to carbon dioxide if oxygen is present, or methane if not. Through combustion of organic material which oxidizes the carbon it contains, producing carbon dioxide (as well as other things, like smoke). At the surface of the oceans where the water becomes warmer, dissolved carbon dioxide is released back into the atmosphere Volcanic eruptions and metamorphism release gases into the atmosphere. These gases include water vapor, carbon dioxide and sulfur dioxide. so the two processes, which are the chemical reverse of each other, sum to roughly zero, and do not affect the level of atmospheric carbon dioxide on time scales of less than about 100,000 yr.

Carbon in the biosphere

Around 1900 gigatonnes of carbon are present in the biosphere. And life plays an important role in the carbon cycle:

Autotrophs are organisms that produce their own organic compounds using carbon dioxide from the air or water in which they live. Photosynthesis follows the reaction 6CO2 + 6H2O → C6H12O6 + 6O2 Carbon is transferred within the biosphere as heterotrophs feed on other organisms or their parts (e.g., fruits). When oxygen is present, aerobic respiration occurs, which releases carbon dioxide into the surrounding air or water, folowing the reaction C6H12O6 + 6O2 → 6CO2 + 6H2O. forest fires, wood used for heating) can also transfer substantial amounts of carbon to the atmosphere Carbon may also leave the biosphere when dead organic matter (such as peat) becomes incorporated in the geosphere.

Carbon storage in the biosphere is influenced by a number of processes on different time-scales: while Net primary productivity follows a diurnal and seasonal cycle, carbon can be stored up to several hundreds of years in trees and up to thousands of years in soils.

Carbon in the Oceans

The seas contain around 36000 gigatonnes of carbon, mostly in the form of bicarbonate ion. Inorganic carbon, that is carbon compounds with no carbon-carbon or carbon-hydrogen bonds, is important in its reactions within water. This carbon exchange becomes important in controlling pH in the ocean and can also vary as a source or sink for carbon. Carbon is readily exchanged between the atmosphere and ocean. In regions of oceanic upwelling, carbon is released to the atmosphere. Conversely, regions of downwelling transfer carbon (CO2) from the atmosphere to the ocean. This reaction controls large changes in pH:

H2CO3 ⇌ H

Carbon cycle modelling

Models of the carbon cycle can be incorporated into global climate models, so that the interactive response of the oceans and biosphere on future CO2 levels can be modelled. For example, Zeng et al. (GRL, 2004 ) find that in their model, including a coupled carbon cycle increases atmospheric CO2 by about 90 ppmv at 2100 (over that predicted in models with non-interactive carbon cycles), leading to an extra 0.6°C of warming (which, in turn, may lead to even greater atmospheric CO2).