Carbon cycle

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Carbon cycle

The carbon cycle describes the movement of carbon in the atmosphere, where it exists as carbon dioxide gas, through organisms, and then back into the atmosphere and the oceans following the deomposition of the dead organisms.

Carbon is a central element of the huge diversity of organic chemicals found in living things, such as the many kinds of carbohydrates, proteins, and fats. Energy is contained in the chemical bonds that hold the atoms of carbon and other elements together in these organic compounds. Organisms use chemical energy from organic compounds to carry out all the processes necessary to life.

Carbon is released into the atmosphere through three major processes: cellular respiration, the burning of fossil fuels, and volcanic eruptions. In each of these processes, carbon is returned to the atmosphere or to the ocean.

Plants convert the carbon in atmospheric carbon dioxide into carbon-containing organic compounds such as sugars, fats, and proteins. Plants take in carbon dioxide through stomata—microscopic openings in their leaves. They combine atmospheric carbon with water and manufacture organic compounds, using energy trapped from sunlight in a process called photosynthesis. The byproduct of photosynthesis is oxygen, which plants release into the atmosphere through the stomata.

Animals that eat plants, or that eat other animals, incorporate the carbon in the sugars, fats, and proteins derived from the ingested biomass into their bodies. Inside their cells, energy is extracted from the food in a process called cellular respiration. Cellular respiration requires oxygen (which is the byproduct of photosynthesis) and it produces carbon dioxide, which is used in photosynthesis. In this way, photosynthesis and cellular respiration are linked in the carbon cycle.

Photosynthesis requires atmospheric carbon, while cellular respiration returns carbon to the atmosphere, and vice versa for oxygen. The global rates of photosynthesis and cellular respiration influence the amount of carbon dioxide in the atmosphere. In the summer, the high rate of photosynthesis uses up much of the carbon dioxide in the atmosphere, and the amount of atmospheric carbon dioxide decreases. In the winter, when the rate of photosynthesis is low, the amount of atmospheric carbon dioxide increases.

Another way that cellular respiration releases carbon into the atmosphere is through the actions of decomposers. Decomposers, such as bacteria and fungi, derive their nutrients by feeding on the remains of plants and animals. The bacteria and fungi use cellular respiration to extract the energy contained in the chemical bonds of the decomposing organic matter, and so release carbon dioxide into the atmosphere.

In ecosystems such as tropical rainforests, decomposition is accomplished quickly, and carbon dioxide is returned to the atmosphere at a relatively fast rate. In other ecosystems such as northern forests and tundra, decomposition proceeds more slowly. In some places, such as bogs and the deep ocean, the organic matter of plants and animals may accumulate in deep sediments, where decomposers cannot function well because of the lack of oxygen. Over millions of years, the carbon-rich materials are converted into carbon-rich fossil fuels, such as petroleum, natural gas, and coal. Also in marine environments, carbon-containing matter such as calcium carbonate is incorporated into the shells and other hard parts of aquatic organisms. When these organisms die, the carbon-rich hard parts

sink to the ocean bed. There they become buried in sediment, and eventually are transformed into rocks such as limestone and dolomite.

Release of carbon into the atmosphere

When fossil fuels are burned, their organic carbon is released into the atmosphere. During the past 100 years, fossil fuel consumption has increased dramatically, and this has led to a huge amount of carbon dioxide being released into the atmosphere. In addition, the widespread clearing of forests is resulting in the emission of huge quantities of carbon dioxide into the atmosphere. (Forests store large amounts of organic carbon in their biomass, most of which is emitted through decomposition and fire when deforestation occurs.) The increasing concentrations of atmospheric carbon dioxide are a cause for concern, since they may be responsible for global warming and associated climatic and ecological disruptions. During the middle of the nineteenth century, the atmospheric concentration of carbon dioxide was about 270 parts per million (ppm; equivalent to one microliter per liter), but in 2000 it had increased to 365 ppm. Just four years later, in 2006, the recorded carbon dioxide level had reached 381 ppm. The latter level is 100 ppm above the atmospheric level at the beginning of the industrial revolution.

Another way that carbon is released into the atmosphere is through volcanic eruptions. When a volcano erupts, it sends huge amounts of ash and soot high into the atmosphere. Some of this ash and soot is derived from ancient carbon-rich sediments, and the cloud of debris that results from a volcanic eruption returns large amounts of carbon to the atmosphere.

The carbon cycle in land and sea

The cycling of carbon takes place in oceans and other aquatic ecosystems as well as in terrestrial environments. The world’s oceans contain about 50% more carbon than does the atmosphere. The oceans are able to absorb some of the carbon dioxide currently being released by the burning of fossil fuels, thus offsetting global warming. However, the oceans cannot do this as quickly as the carbon dioxide is being released to the atmosphere, and this time lag is resulting in increasing concentrations in the atmosphere.

In aquatic environments, carbon cycling is more complex because carbon interacts with water. When carbon dioxide is released by cellular respiration, it combines with water to form carbonic acid and bicarbonate. Therefore, in aquatic environments most inorganic carbon is in the form of bicarbonate rather than carbon dioxide. Carbon dioxide from the atmosphere readily diffuses into water, and it is quickly converted to bicarbonate.

Importance of the carbon cycle

The carbon cycle is important in ecosystems because it moves carbon, a life-sustaining element, from the atmosphere and oceans into organisms and back again to the atmosphere and oceans. If the balance between these latter two reservoirs is upset, serious consequences, such as global warming and climate disruption, may result. Scientists are currently looking into ways in which humans can use other, non-carbon containing fuels for energy. Nuclear power, solar power, wind power, and water power are a few alternative energy sources that are being investigated.

Resources

BOOKS

Kondratyev, Kirill Y., Vladamir F. Krapivin, and Costas A. Varotsos. Global Carbon Cycle and Climate Change. New York: Springer, 2003.

MacKenzie, Susan H., Chris Field, and Michael Raupach. The Global Carbon Cycle: Integrating Humans, Climate, and the Natural World. Washington: Island Press, 2004.

Wigley, T.M.L., and D.S. Schimel. The Carbon Cycle. Cambridge: Cambridge University Press, 2005.

Kathleen Scogna

The Gale Encyclopedia of Science