carbon cycle

The carbon cycle is necessary for the continuation of life. Since there is only a limited amount of carbon in the Earth and its atmosphere, carbon must be continuously recycled if life is to continue. Other chemicals necessary for life – nitrogen, sulphur, and phosphorus, for example – also circulate in natural cycles.

Sequence by which carbon circulates and is recycled through the natural world. Carbon is usually found in a carbon compound of one sort or another and so the carbon cycle is really about the cycling of carbon compounds. Carbon dioxide is released into the atmosphere by most living organisms as a result of respiration. The CO₂ is taken up and converted into high-energy chemicals – glucose and other carbohydrates – during photosynthesis by plants; the oxygen component is released back into the atmosphere. Some glucose is used by the plant and some is converted into other carbon compounds, making new tissues. However, some of these compounds can be transferred to other organisms. An animal may eat the plant and that animal may be eaten and so on down the food chain. Carbon is also released through the decomposition of dead plant and animal matter, and the burning of fossil fuels such as coal and oil, which produce carbon dioxide that is released into the atmosphere. The oceans absorb 25–40% of all carbon dioxide released into the atmosphere.

The carbon cycle is in danger of being disrupted by the increased burning of fossil fuels, and the destruction of large areas of tropical forests. The rising levels of carbon dioxide in the atmosphere are probably increasing the temperature on Earth (enhanced greenhouse effect). It is thought that by limiting the production of carbon dioxide through human activities we can slow the rate at which temperatures on Earth will rise.

carbon cycle

In astrophysics, sequence of nuclear fusion reactions in which carbon atoms act as a catalyst to convert four hydrogen atoms into one helium atom with the release of energy. The carbon cycle is the dominant energy source for ordinary stars of mass greater than about 1.5 times the mass of the Sun.

Nitrogen and oxygen are also involved in the sequence so it is sometimes known as the carbon-nitrogen-oxygen cycle or CNO cycle.