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The Carbon Cycle. The concentration of carbon in living matter (18%) is almost 100 times greater than its concentration in the earth (0.19%). So living things extract carbon from their nonliving environment. For life to continue, this carbon must be recycled. . What is the Carbon Cycle?

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The concentration of carbon in living matter (18%) is almost 100 times greater than its concentration in the earth (0.19%). So living things extract carbon from their nonliving environment. For life to continue, this carbon must be recycled.


What is the Carbon Cycle?

The Carbon Cycle is a complex series of processes through which all of the carbon atoms in existence rotate. The same carbon atoms in your body today have been used in countless other molecules since time began. The wood burned just a few decades ago could have produced carbon dioxide which through photosynthesis became part of a plant. When you eat that plant, the same carbon from the wood which was burnt can become part of you. The carbon cycle is the great natural recycler of carbon atoms. Unfortunately, the extent of its importance is rarely stressed enough. Without the proper functioning of the carbon cycle, every aspect of life could be changed dramatically.


Sample carbon cycle and how carbon atoms move through our natural world

Plants, animals, and soil interact to make up the basic cycles of nature. In the carbon cycle, plants absorb carbon dioxide from the atmosphere and use it, combined with water they get from the soil, to make the substances they need for growth. The process of photosynthesis incorporates the carbon atoms from carbon dioxide into sugars. Animals, such as the rabbit pictured here, eat the plants and use the carbon to build their own tissues. Other animals, such as the fox, eat the rabbit and then use the carbon for their own needs. These animals return carbon dioxide into the air when they breathe, and when they die, since the carbon is returned to the soil during decomposition. The carbon atoms in soil may then be used in a new plant or small microorganisms. Ultimately, the same carbon atom can move through many organisms and even end in the same place where it began.


The carbon cycle is the biogeochemical cycle by which carbon is exchanged between the biosphere, geosphere, hydrosphere, and atmosphere of the earth. (Other planetary bodies may have carbon cycles, but little is known about them.)

The cycle is usually thought of as four main reservoirs of carbon, interconnected by pathways of exchange. These reservoirs are the atmosphere, terrestrial biosphere, oceans, carbonate rocks, and sediments (as organic matter, including fossil fuels). The movement of carbon—the carbon exchanges between reservoirs—occurs because of various chemical, physical, geological, and biological processes. Overall, the carbon cycle reveals the harmonious coordination between different biotic and abiotic elements on Earth.


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 that pool or reservoir is functioning as a source or sink for carbon over different time scales.

The carbon cycle is central to understanding issues related to climate change. In resolving the divergent positions with respect to increases of carbon dioxide in the atmosphere and global warming, it is important that scientists maintain integrity in collecting, analyzing, and presenting data in the face of often strong political, commercial, and environmental agendas.


Carbon in the atmosphere

Carbon exists in the Earth's atmosphere primarily as the gas carbon dioxide (CO2). Although it comprises a very small part of the atmosphere overall (approximately 0.04 percent), it plays an important role in supporting life. Other gases containing carbon in the atmosphere are methane and chlorofluorocarbons (the latter are entirely artificial and are now strictly prohibited under the Montreal Protocol).


Carbon exchange with the atmosphere, biosphere, and oceans


Utilizing light from the sun, plants and algae perform photosynthesis to convert carbon dioxide, water, and sunlight into carbohydrates (C6H12O6, releasing oxygen in the process. This process removes carbon dioxide from the atmosphere and stores it in plant biomass, which may eventually get buried in sediments after the plant dies.



Respiration occurs when the biomass from photosynthetic plants and algae is consumed by animals, fungi, or bacteria, either while the plant is alive, or after it has died.

This is essentially the reverse process of photosynthesis, releasing CO2 back into the atmosphere. However, more material is photosynthesized than is respired (since a portion of the organic matter is buried in the sediments), thus more oxygen enters the atmosphere than does carbon dioxide as a result of these two processes.



Outgassing of volcanoes and mid-ocean ridges is the largest source of carbon dioxide in the atmosphere, releasing carbon dioxide from deep within the Earth that had been trapped there since the planet's creation. CO2 is released from subduction zones through metamorphism of carbonate rocks subducting with the ocean crust. Not all of this CO2 enters the atmosphere. Some of it dissolves in the oceans and some remains in biomass of organisms.



Weathering is a mechanism that removes carbon from the atmosphere. When carbon dioxide dissolves in water, it forms carbonic acid. This acid is used to weather rocks, yielding bicarbonate ions in addition to other ions (depending on the mineral content of the rock). The bicarbonate ion enters oceans through fresh water systems, and in the ocean, the bicarbonate ion combines with a calcium ion to form calcium carbonate and a by product of carbon dioxide and water. The calcium carbonate is used by marine organisms to form calcareous shells, and corals use it in their exoskeletons.


Solubility pump

  • The solubility pump is a physico-chemical process that transports carbon (as dissolved inorganic carbon) from the ocean's surface to its interior.
  • The solubility pump is driven by the coincidence of two processes in the ocean:
  • The solubility of carbon dioxide is a strong inverse function of seawater temperature (i.e. solubility is greater in cooler water)
  • The thermohaline circulation, ocean circulation driven by density differences in salinity and temperature, is driven by the formation of deep water at high latitudes where seawater is usually cooler and more dense

Since deep water (that is, seawater in the ocean's interior) is formed under the same surface conditions that promote carbon dioxide solubility, it contains a higher concentration of dissolved inorganic carbon than one might otherwise expect. Consequently, these two processes act together to pump carbon from the atmosphere into the ocean's interior.

One consequence of this is that when deep water upwells in warmer, equatorial latitudes, it strongly outgasses carbon dioxide to the atmosphere because of the reduced solubility of the gas.


Carbon in the biosphere

Carbon is an essential part of life on Earth. It plays an important role in the structure, biochemistry, and nutrition of all living cells. 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. 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, chemosynthesis. The most important autotrophs for the carbon cycle are trees in forests on land and phytoplankton in the Earth's oceans.


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. 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).


Carbon may also leave 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.

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 (Bennett 2005). Because of their size and composition, these houses are rarely collected in such traps, so most biogeochemical analyses have erroneously ignored them.


Carbon in the oceans

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 down welling transfer carbon (CO2) from the atmosphere to the ocean. When CO2 enters the ocean, carbonic acid is formed: CO2 + H2O <—> H2CO3

This reaction has a forward and reverse rate; that is it achieves a chemical equilibrium.

Another reaction important in controlling oceanic pH levels is the release of hydrogen ions and bicarbonate.


The Carbon Cycle and the Climate

Carbon dioxide and methane are two carbon compounds that act as greenhouse gases in Earth's atmosphere, insulating the planet and making it a comfortable place for organisms to survive.

The carbon cycle responds to perturbations through a series of feedbacks so that temperatures never get too hot or too cold, within certain bounds. For example, if CO2 outgassing from volcanoes and mid-ocean ridges increases as a result of increased tectonic activity, atmospheric temperatures will rise. Rising temperatures and increased amounts of dissolved CO2 will result in increased rates of weathering of crustal rocks, which will use up the surplus CO2, decrease atmospheric CO2 levels, and bring temperatures back down. On the other hand, if global cooling occurred, weathering would slow down and CO2 would build up in the atmosphere and temperatures would rise again.


The recent debate about anthropogenic (human-induced) climate change has been centered around the release of thousands of tons of carbon dioxide from the burning of fossil fuels and its effect on global climate. Some scientists, using carbon cycle climate models, argue that with the "business as usual" scenario, atmospheric temperatures will rise over the next century (Cox et al. 2000). Other studies suggest that ocean uptake of CO2 will slow because of increased stratification of the ocean (less deep mixing) (Sarmiento et al. 1998). In addition, increased global temperatures would warm the oceans, decreasing the solubility of CO2 in ocean water. All of these factors are considered to cause a build up of CO2 in the atmosphere.