Carbon cycle

Carbon cycle is the circulation of the element carbon among Earth’s atmosphere, land, oceans, interior, and organisms (living things). Carbon is an essential element of living creatures. It is also a major ingredient in the natural energy resources: coal, natural gas, and petroleum. The carbon cycle helps to determine levels of heat-trapping carbon dioxide (CO₂) gas in the atmosphere. The cycle thus has a major effect on Earth’s climate. The total amount of carbon on Earth is constant. The cycle does not create or destroy carbon. Instead, it simply exchanges carbon among different places and forms.

On land, certain organisms draw carbon, in the form of carbon dioxide, from the air. This carbon is then used in photosynthesis, a process in which plants, algae, and some other organisms use energy from sunlight to make food and the building blocks for growth. The carbon thus becomes part of the plants’ bodies.

When plants die or shed leaves, they deposit their trapped carbon into the soil. There, certain bacteria may break down some of the plant matter. These bacteria release carbon back into the atmosphere as CO₂. Animals also eat some plant matter for energy and growth. The animals exhale some of the carbon from the plant matter as CO₂. When animals die, as with plants, soil bacteria may consume them, releasing CO₂ back into the atmosphere. Animals’ remaining carbon can become buried.

In the oceans, algae, plants, and other organisms use CO₂ from the atmosphere for photosynthesis. These organisms may be broken down by bacteria or consumed by other organisms, or they may sink to the bottom.

CO₂ can also dissolve directly into the oceans from the atmosphere. Some organisms, such as coral and oysters, can use this carbon to create shells. When these organisms die, the shells may settle to the ocean floor and form limestone. Limestone sediments represent Earth’s largest store of carbon. Ocean currents can bring dissolved carbon from the deep ocean back to the surface. There, it may return to the atmosphere.

Over millions of years, once-living matter that remains in the ground can transform into the fossil fuels: coal, natural gas, and petroleum. When people burn fossil fuels for energy, the carbon that was captured by plants or eaten by animals millions of years ago is returned to the atmosphere. Since the mid-1800’s, human activity has significantly altered the carbon cycle. Fossil fuels that would have remained deep underground are instead burned for energy. Burning fossil fuels has raised the concentration of CO₂ in the atmosphere. It has also raised the acidity of the oceans.

See also Global warming (Causes of global warming); Global warming (Capturing emissions); Global warming (Sequestration in plants and soils); Greenhouse effect.