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Carbon Cycle

The natural world works through cycles of matter, in which molecules are formed and reformed through chemical and biological reactions, which manifest themselves in physical changes in the materials - including carbon, nitrogen, phosphorus and sulphur. This article discusses some of the ways the carbon cycle works, referring to the impact of human activities. Carbon, in the form of carbon dioxide (CO2), is the major greenhouse gas, implicated in climate change. Atmospheric CO2 concentration has increased by about a third since the start of the industrial revolution. See Woods Hole Research centre site for discussion.

global carbon flowCarbon is crucial to all life on earth, indeed earth life is sometimes described as "carbon-based". It makes up about half of the dry weight of all plants and animals on earth. The oceans and rocks hold the major stores of carbon. The atmosphere and biosphere hold a much smaller proportionl. Fossil carbon is locked in rocks, particularly in carbonate sedimentary rocks such as limestone. Hydrocarbons - coal, oil and natural gas - are stored through processes of sedimentation and evaporation. They remain in rock for millions of years. Carbon is released naturally through the processes of weathering, vulcanism and sea floor spreading. However, human action has greatly increased the rate of flow from some of the lithospheric stores, for example, through burning fossil fuels - which liberates carbon through oxidation. This has led to a significant new flow of carbon, greatly increasing the natural atmospheric carbon store.

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 eat the plants and use the carbon to build their own tissues. (They may in turn be eaten by other creatures who 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 same atoms can be recycled for millennia.

The carbon cycle has four types of pools - the oceanic, the atmospheric, biological, and geological. The atmospheric pool links directly to the oceanic and biological pools, with rapid flux between these pools and the atmosphere. Linkage with the geological pool is less direct and the flux slower, although human actions over the past two centuries modified this rate markedly, with increasing implications for all biospheric processes.

Carbon content in organic matter includes both currently living plant and animal tissue. The vast majority of biological material is in, or derived from, autotrophs. One of the best indicators of the way in which organic debris will break down is the ratio of carbon to nitrogen (C/N.) Material in the range 100/1 is derived from cellulose rich product and is resistant to change. Material with a low C/N ratio - of about 10/1 - is much more susceptible to rapid and complete decomposition. The nature of the living vegetation that contributes to this debris is itself controlled by a whole complex of environmental factors, which are in turn inter-related. Climate and soil conditions shape a cyclical system of interactions between plants and soil which form the nutrient cycling system. The parent material affects the nutrient cycling. It influences the soil PH through the chemistry of the mineral fraction of the soil. The particle size of the mineral fraction - i.e. soil texture - also impacts significantly on the transmission and retention of water in the soil profile.

The carbon model helps explain the basic structures of natural communities. With each trophic level, less energy is available to larger individuals and thus the numbers of larger individuals decreases. Plants are very numerous because they receive their energy directly from the sun, but can only support successively fewer large animals. Hence, predators at the top of the food chain are always rare, where humans have not disturbed the "natural" cycles of carbon, etc.

On a global scale, climate change caused by human short-circuiting of the carbon cycle is having profound effects on the ecosystem. The climate changes associated with global warming have changed the geographical distribution and timing of patterns of rainfall. Through its effects on plant life, and because of the role of the water reservoir in biogeochemical cycles, disruption of the functioning of hydrological cycles will have a major impact on material ecosystems and on agriculture.


Jackson, A.R & J.M (1996) Environmental Science - the natural environment and human impact, Harlow, Longman.

Bunce, N.J. (1990) Environmental Chemistry, Winnipeg, Wurtz.

El-Hinnawi, E., Hashmi, M.H., (1982) Global Environmental Issues, United Nations Environment Programme, Dublin, Tycooly

Wheeling Jesuit University/NASA Classroom of the Future, 1997-2000:

Woods Hole Research Center:

The Concise Columbia Electronic Encyclopedia

Woodwell, G.M. (1978) The carbon dioxide question, Scientific American, 238, 1, 34-43

Wong, C.S. (1978) Atmospheric impact of carbon dioxide from burning wood, Science, 200, 197-9

Goudie, A. (1990) The Human Impact on the Natural Environment (3rd Ed) Oxford, Blackwell

Henderson-Sellers A. and Robinson P.J. (1986) Contemporary Climatology London, Longman.


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