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This diagram of the fast carbon cycle shows the movement of carbon between land, atmosphere, and oceans in billions of tons of carbon per year. Yellow numbers are natural fluxes, red are human contributions in gigatons of carbon per year. White numbers indicate stored carbon.

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. It is one of the most important cycles of the Earth and allows for carbon to be recycled and reused throughout the biosphere and all of its organisms.^{[citation needed]}

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. The carbon cycle was initially discovered by Joseph Priestley and Antoine Lavoisier, and popularized by Humphry Davy.^[1]

Relevance for the global environment[link]

Carbon-based molecules are crucial for life on earth, as all organisms use it extensively in various biological functions.^{[citation needed]} Carbon is also a major component of many minerals. Carbon also exists in various forms in the atmosphere. Carbon dioxide (CO₂) is partly responsible for the greenhouse effect and is the most important human-contributed greenhouse gas.^{[citation needed]}

In the past two centuries, human activities have seriously altered the global carbon cycle, most significantly in the atmosphere. Changes in the amount of atmospheric CO₂ are considerably altering weather patterns and indirectly influencing oceanic chemistry. ^[2]

Main Components[link]

The global carbon cycle is now usually divided into the following major reservoirs of carbon interconnected by pathways of exchange:

• The atmosphere
• The terrestrial biosphere
• The oceans, including dissolved inorganic carbon and living and non-living marine biota
• The sediments, including fossil fuels, fresh water systems and non-living organic material, such as soil carbon
• The Earth's interior, carbon from the Earth's mantle and crust. These carbon stores interact with the author components through geological processes

The carbon exchanges between reservoirs occur as the result of various chemical, physical, geological, and biological processes. The ocean contains the largest active pool of carbon near the surface of the Earth.^[2] The natural flows of carbon between the atmosphere, ocean, and sediments is fairly balanced, so that carbon levels would be roughly stable without human influence. ^[3]

Atmosphere[link]

2011 Carbon dioxide mole fraction in the troposphere.

Carbon in the Earth's atmosphere exists in two main forms: Carbon dioxide and methane. Both of these gases are partially responsible for the greenhouse effect, which makes life as we know it possible through trapping heat in the atmosphere. Methane produces a large greenhouse effect per volume as compared to carbon dioxide, but it exists in much lower concentrations and is more short-lived than carbon dioxide, making carbon dioxide the more important greenhouse gas of the two.^{[citation needed]}

Human activity over the past two centuries has significantly increased the amount of carbon in the atmosphere, mainly in the form of carbon dioxide, both by modifying ecosystems' ability to extract carbon dioxide from the atmosphere and by emitting it directly, e.g. by burning fossil fuels and manufacturing concrete.^[2]

Terrestrial biosphere[link]

Carbon is an essential part of life on Earth. About half the dry weight of most living organisms is carbon. It plays an important role in the structure, biochemistry, and nutrition of all living cells. Living biomass holds about 575 gigatons of carbon, most of which is wood.

Trees and other green plants such as grass convert carbon dioxide into carbohydratesduring photosynthesis, releasing oxygen in the process. This process is most prolific in relatively new forests where tree growth is still rapid. The effect is strongest in deciduous forests during spring leafing out. This is visible as an annual signal in the Keeling curve of measured CO₂ concentration. Northern hemisphere spring predominates, as there is far more land in temperate latitudes in that hemisphere than in the southern.

• Forests store 86% of the planet's terrestrial above-ground carbon and 73% of the planet's soil carbon.^[4]
• Autotrophs are organisms that produce their own organic compounds using carbon dioxide from the air or water in which they live. To do this they require an external source of energy. Almost all autotrophs use solar radiation to provide this, and their production process is called photosynthesis. A small number of autotrophs exploit chemical energy sources in a process called chemosynthesis. The most important autotrophs for the carbon cycle are trees in forests on land and phytoplankton in the Earth's oceans. Photosynthesis follows the reaction 6CO₂ + 6H₂O + Light Energy (Sun) → C₆H₁₂O₆ + 6O₂
• Carbon is transferred within the biosphere as heterotrophs feed on other organisms or their parts (e.g., fruits). This includes the uptake of dead organic material (detritus) by fungi and bacteria for fermentation or decay.
• Most carbon leaves the biosphere through respiration. When oxygen is present, aerobic respiration occurs, which releases carbon dioxide into the surrounding air or water, following the reaction C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Otherwise, anaerobic respiration occurs and releases methane into the surrounding environment, which eventually makes its way into the atmosphere or hydrosphere (e.g., as marsh gas or flatulence).
• Burning of biomass (e.g. forest fires, wood used for heating, anything else organic) can also transfer substantial amounts of carbon to the atmosphere
• Carbon may also be circulated within the biosphere when dead organic matter (such as peat) becomes incorporated in the geosphere. Animal shells of calcium carbonate, in particular, may eventually become limestone through the process of sedimentation.

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 in the terrestrial biosphere for up to several centuries, e.g. in wood.

Hydrosphere[link]

"Present day" (1990s) sea surface dissolved inorganic carbon concentration (from the GLODAP climatology)

The oceans contain around 36,000 gigatonnes of carbon, mostly in the form of bicarbonate ion (over 90%, with most of the remainder being carbonate). Extreme storms such as hurricanes and typhoons bury a lot of carbon, because they wash away so much sediment. For instance, a research team reported in the July 2008 issue of the journal Geology that a single typhoon in Taiwan buries as much carbon in the ocean—in the form of sediment—as all the other rains in that country all year long combined.^[5] 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. When CO₂ enters the ocean, it participates in a series of reactions which are locally in equilibrium:

Solution:

CO₂ (atmospheric) ⇌ CO₂(dissolved)

Conversion to carbonic acid:

CO₂ (dissolved) + H₂O ⇌ H₂CO₃

First ionization:

H₂CO₃ ⇌ H⁺ + HCO₃⁻ (bicarbonate ion)

Second ionization:

HCO₃⁻ ⇌ H⁺ + CO₃²⁻ (carbonate ion)

This set of reactions, which of each has its own equilibrium coefficient, determines the form that inorganic carbon takes in the oceans.^[6] The coefficients, which have been determined empirically for ocean water, are themselves functions of temperature, pressure, and the presence of other ions (especially borate). In the ocean the equilibria strongly favor bicarbonate. Since this ion is three steps removed from atmospheric CO₂, the level of inorganic carbon storage in the ocean does not have a proportion of unity to the atmospheric partial pressure of CO₂. The factor for the ocean is about ten: that is, for a 10% increase in atmospheric CO₂, oceanic storage (in equilibrium) increases by about 1%, with the exact factor dependent on local conditions. This buffer factor is often called the "Revelle Factor", after Roger Revelle.

In the oceans, dissolved carbonate can combine with dissolved calcium to precipitate solid calcium carbonate, CaCO₃, mostly as the shells of microscopic organisms. When these organisms die, their shells sink and accumulate on the ocean floor. Over time these carbonate sediments form limestone which is the largest reservoir of carbon in the carbon cycle. The dissolved calcium in the oceans comes from the chemical weathering of calcium-silicate rocks, during which carbonic and other acids in groundwater react with calcium-bearing minerals liberating calcium ions to solution and leaving behind a residue of newly formed aluminium-rich clay minerals and insoluble minerals such as quartz.

The flux or absorption of carbon dioxide into the world's oceans is influenced by the presence of widespread viruses within ocean water, that infect many species of bacteria. The resulting bacterial deaths spawn a sequence of events that lead to greatly enlarged respiration of carbon dioxide, enhancing the role of the oceans as a carbon sink.^[7]

• At the surface of the oceans towards the poles, seawater becomes cooler and morecarbonic acid is formed as CO₂ becomes more soluble. This is coupled to the ocean's thermohaline circulation which transports dense surface water into the ocean's interior (see the entry on the solubility pump).
• In upper ocean areas of high biological productivity, organisms convert reduced carbon to tissues, or carbonates to hard body parts such as shells and tests. 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 the biological pump).
• The weathering of silicate rock (see carbonate-silicate cycle). Carbonic acid reacts with weathered rock to produce bicarbonate ions. The bicarbonate ions produced are carried to the ocean, where they are used to make marine carbonates. Unlike dissolved CO₂ in equilibrium or tissues which decay weathering does not move the carbon into a reservoir from which it can readily return to the atmosphere.
• Much remains to be learned about the cycling of carbon in the deep ocean. For example, a recent discovery is that larvacean mucus houses (commonly known as "sinkers") are created in such large numbers that they can deliver as much carbon to the deep ocean as has been previously detected by sediment traps.^[8]Because of their size and composition, these houses are rarely collected in such traps, so most biogeochemical analyses have erroneously ignored them.

Pedosphere[link]

Soils hold approximately 1,500 gigatons,^[9] mostly in the form of organic carbon, with perhaps a third of that inorganic forms of carbon such as calcium carbonate.^[10]

Carbon can be stored for up to thousands of years in soils. Changes in this long term carbon pool (e.g. through de- or afforestation or through temperature-related changes in soil respiration) may thus affect global climate change.

See also[link]

References[link]

Further reading[link]

• The Carbon Cycle, updated primer by NASA Earth Observatory, 2011
• Appenzeller, Tim (2004). "The case of the missing carbon". National Geographic Magazine. http://environment.nationalgeographic.com/environment/global-warming/missing-carbon.html.  – article about the missing carbon sink
• Bolin, Bert; Degens, E. T.; Kempe, S.; Ketner, P. (1979). The global carbon cycle. Chichester ; New York: Published on behalf of the Scientific Committee on Problems of the Environment (SCOPE) of the International Council of Scientific Unions (ICSU) by Wiley. ISBN 0-471-99710-2. http://www.icsu-scope.org/downloadpubs/scope13/index.html. Retrieved 2008-07-08. 
• Houghton, R. A. (2005). "The contemporary carbon cycle". In William H Schlesinger (editor). Biogeochemistry. Amsterdam: Elsevier Science. pp. 473–513. ISBN 0-08-044642-6. 
• Janzen, H. H. (2004). "Carbon cycling in earth systems—a soil science perspective". Agriculture, ecosystems and environment 104 (3): 399–417. DOI:10.1016/j.agee.2004.01.040. 
• Millero, Frank J. (2005). Chemical Oceanography (3 ed.). CRC Press. ISBN 0-8493-2280-4. 
• Sundquist, Eric; Broecker, Wallace S.(editors) (1985). The Carbon Cycle and Atmospheric CO₂: Natural variations Archean to Present. Geophysical Monographs Series. American Geophysical Union. 

External links[link]

• Carbon Cycle Science Program – an interagency partnership.
• NOAA's Carbon Cycle Greenhouse Gases Group
• Global Carbon Project – initiative of the Earth System Science Partnership
• UNEP – The present carbon cycle – Climate Change carbon levels and flows
• NASA's Orbiting Carbon Observatory
• Carboscope, a website presenting maps of fluxes of greenhouse gases (CO2 and CH4)
• CarboSchools, a European website with many resources to study carbon cycle in secondary schools.
• Carbon and Climate, an educational website with a carbon cycle applet for modeling your own projection.