How the Carbon Cycle Connects Air, Land and Ocean
Carbon is part of living cells, ocean water, soil, fuels and the air. It does not stay in one place. The carbon cycle follows how carbon moves between major stores through biological, chemical and geological processes.
Photosynthesis moves carbon into living things
Carbon dioxide in the atmosphere can enter plants, algae and some bacteria through photosynthesis. These organisms use light energy to build carbon-containing sugars. The carbon becomes part of leaves, roots, wood and other biological molecules.
Animals obtain carbon by eating plants or other animals. Inside cells, respiration breaks down fuel molecules and releases energy. Carbon dioxide is produced and returns to the air or water. Plants respire too, so they both absorb and release carbon.
Dead organisms and waste contain carbon that decomposers can use. Bacteria and fungi break the material down, releasing carbon dioxide through respiration. In waterlogged soils or oxygen-poor environments, some decomposition can also produce methane.
Oceans and rocks store large amounts
Carbon dioxide moves between the atmosphere and the ocean surface. Some dissolves directly in seawater and takes part in chemical reactions that form bicarbonate and carbonate ions. Marine organisms use dissolved carbon to build tissues and, in some cases, shells.
When organisms die, part of their carbon sinks. Much is recycled near the surface, but some becomes buried in sediment. Over very long periods, carbon can become part of sedimentary rock or fossil fuels. Weathering, erosion and the movement of Earth's crust continue shifting it.
Volcanoes return some geological carbon to the atmosphere. Rock formation and tectonic recycling occur far more slowly than photosynthesis and respiration. The carbon cycle therefore contains fast loops measured in days or years and slow loops measured in thousands or millions of years. A carbon atom can therefore remain in one store for a moment or for longer than recorded human history.
Human activity changes the balance
Burning coal, oil and natural gas moves carbon from long-term geological stores into the atmosphere quickly. Cutting forests can release stored carbon and reduce the amount removed by future photosynthesis. Cement production also releases carbon dioxide.
Oceans and land ecosystems absorb part of these emissions, but not all of them. The remaining carbon dioxide increases its concentration in the atmosphere and strengthens the greenhouse effect. Ocean absorption also changes seawater chemistry.
Follow the major transfers:
- •Photosynthesis moves carbon from air into organisms.
- •Respiration and decomposition return carbon.
- •Oceans exchange and store dissolved carbon.
- •Burial stores carbon in sediments and fuels.
- •Combustion rapidly returns geological carbon to the air.
The takeaway
The carbon cycle links atmosphere, life, ocean and rock through transfers that operate at very different speeds. Natural processes continually exchange carbon, while human activity is moving long-stored carbon into the air unusually quickly. Keep the stores separate from the transfers, and the cycle becomes a readable map rather than an intimidating knot of arrows.