Definition · Plain-language
Carbon cycle
The carbon cycle is the continuous exchange of carbon between the air, living things, the oceans and the Earth’s rocks.
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How carbon circulates
Carbon is one of the building blocks of life, and it constantly moves between four great reservoirs: the atmosphere, living things (the biosphere), the oceans and the rocks and soils. In the carbon cycle, the same carbon atoms are exchanged among these stores rather than being used up. Some exchanges are fast — a leaf taking up carbon dioxide today may release it within days — while others are extremely slow, locking carbon into rock for millions of years. This balance of fast and slow pathways keeps the amount of carbon in the air relatively stable over the short term.
Photosynthesis, respiration and decay
The biological heart of the cycle is the exchange between photosynthesis and respiration. Plants, algae and some microbes use photosynthesis to take carbon dioxide from the air and, using sunlight, build it into sugars, locking carbon into living tissue. Respiration does the reverse: plants, animals and microbes break down those sugars for energy and release carbon dioxide back to the air. When organisms die, decomposers break down their remains, returning yet more carbon. Combustion — the burning of wood or fuel — also releases stored carbon as carbon dioxide. These flows pass carbon constantly between life and the atmosphere.
Long-term stores and human impact
Beyond the fast biological loop, carbon is held in slow, long-term stores. The oceans dissolve and absorb vast quantities of carbon dioxide, and marine organisms build shells that settle and form carbon-rich rock such as limestone. Over millions of years, buried plant and plankton remains can become coal, oil and gas — fossil fuels. Burning those fossil fuels releases ancient carbon that had been locked away, adding carbon dioxide to the atmosphere far faster than natural processes can reabsorb it. This extra carbon dioxide strengthens the greenhouse effect, which is the main driver of present-day climate change.
Key facts
At a glance
- Definition: the continuous movement of carbon between air, life, oceans and rock
- Main stores: the atmosphere, living things, the oceans and rocks
- Takes carbon out of air: photosynthesis by plants and algae
- Returns carbon to air: respiration, decay and combustion
- Long-term store: oceans, limestone and fossil fuels
- Human impact: burning fossil fuels adds carbon dioxide faster than it is removed
Common misconceptions
What people often get wrong
Often heard: Carbon dioxide is a pollutant that has no natural place in the environment.
Actually: Carbon dioxide is a natural and essential part of the carbon cycle — plants depend on it. The concern is the extra carbon dioxide humans add by burning fossil fuels, which unbalances the cycle.
Often heard: Only living things take part in the carbon cycle.
Actually: Carbon also moves through the oceans, the atmosphere and rocks. Limestone, fossil fuels and dissolved ocean carbon are huge non-living stores that exchange carbon over long periods.
Often heard: Plants and trees can quickly absorb all the carbon dioxide humans release.
Actually: Natural carbon uptake is real but limited and slow. Burning fossil fuels releases carbon that was locked away for millions of years, faster than plants and oceans can reabsorb it.
