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The carbon cycle is a biogeochemical exchange system in which carbon is transferred among Earth’s biosphere, pedosphere (soils), geosphere (rocks), hydrosphere (waters), and atmosphere. It includes both rapid recycling processes that move carbon between the atmosphere and living ecosystems (and back) and long-term geological sequestration and release that store carbon in sinks such as sediments, fossil fuels, and Earth’s interior. Natural fluxes between major carbon reservoirs are often roughly balanced, but human activities—especially land-use change and the mining and burning of ancient carbon (coal, petroleum, and gas)—have increased atmospheric CO2 substantially, driving climate change and altering ocean chemistry (including ocean acidification). The cycle is commonly described using major carbon pools: the atmosphere (mainly CO2 and methane), the terrestrial biosphere (living biomass and soils), the ocean (dissolved inorganic carbon and marine biota, with surface and deep layers), sediments (including fossil fuels and non-living organic material), and Earth’s interior (mantle and crust). Fast cycling occurs through biological processes such as photosynthesis and respiration, while slow cycling occurs through rock weathering, sediment formation, subduction, and volcanic degassing. The ocean is a key active reservoir near the surface and a much larger deep pool that exchanges with the atmosphere over centuries; CO2 dissolves into water, forms carbonic acid, and can be stored as carbonates or transported through the food web and sinking organic matter. Overall, the carbon cycle links biological activity, chemical reactions, and geological processes into a system that regulates Earth’s long-term climate and supports life.
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Earth’s carbon is stored and exchanged among several major reservoirs across the atmosphere, biosphere, hydrosphere, pedosphere, and geosphere. The atmosphere contains carbon mainly as carbon dioxide and methane. The terrestrial biosphere stores carbon in living and dead organisms and soils, while the oceans contain the largest active near-surface carbon pool, including dissolved inorganic carbon, organic carbon, and marine organisms. Sediments and fossil fuels store carbon over longer periods, and the lithosphere—including Earth’s crust and mantle—contains enormous quantities in limestone, other carbonate rocks, and kerogens. These reservoirs are connected by biological, physical, chemical, and geological processes. Photosynthesis transfers atmospheric carbon dioxide into ecosystems, while respiration, decomposition, combustion, rivers, and ocean-atmosphere exchange return carbon to the atmosphere or transport it to aquatic systems. The fast carbon cycle operates over years to decades through organisms, soils, and surface waters, whereas the slow carbon cycle moves carbon through rocks, sediments, tectonic activity, weathering, and volcanism over thousands to millions of years. Human activities, especially fossil-fuel combustion, cement production, and land-use change, have increased atmospheric carbon dioxide and disrupted the natural balance among reservoirs.
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