The carbon cycle is the continuous movement of carbon among the atmosphere, living things, soils, oceans, and rocks. Carbon moves through fast pathways, such as photosynthesis and respiration, and through slow pathways, such as sediment formation and geologic change over thousands to millions of years.
Carbon is a building block of life and an important part of Earth’s climate system. When carbon moves naturally, it helps support plants, animals, soils, and oceans. When humans rapidly add extra carbon dioxide to the atmosphere by burning fossil fuels and changing land use, the balance of the cycle is disrupted.
Carbon in the atmosphere is mostly found as carbon dioxide. It moves into plants during photosynthesis and returns to the air through respiration, decomposition, fires, and combustion.
Plants remove carbon dioxide from the air and use sunlight to convert it into sugars and plant tissue. This is one of the main ways carbon enters the living world.
Animals get carbon by eating plants or other animals. They release some of that carbon back to the atmosphere through respiration and return more to soils and waters through waste and decay.
Dead plants and animals break down in soils. Decomposers return some carbon to the atmosphere and store some in soil organic matter. A small fraction can be buried for very long periods.
The ocean absorbs carbon dioxide from the atmosphere. Some remains dissolved in seawater, some is used by marine life, and some eventually becomes part of shells, sediments, and rock.
Over millions of years, some buried organic carbon becomes coal, oil, and natural gas. When humans burn these fuels, carbon that was stored underground is quickly returned to the atmosphere.
Some carbon is locked away in limestone, marine sediments, and other rocks. These slow processes are part of the long-term carbon cycle and can store carbon far longer than the fast biological cycle.
Human activity changes the balance of the cycle by adding carbon dioxide faster than many natural systems can absorb it. This contributes to warming, ocean acidification, and ecosystem change.
Plants pull carbon dioxide from the air during photosynthesis.
Carbon enters food webs when animals eat plants or other animals.
Respiration returns carbon dioxide to the air.
Death and waste move carbon into soils, where decomposition begins.
Carbon dioxide constantly moves between the air and the sea surface.
Some carbon is buried and stored over long time scales.
Combustion rapidly releases long-stored carbon back into the air.
Shell-building organisms can help move carbon into long-term geologic storage.
These sources come from established scientific and educational organizations and are useful starting points for deeper study.