Carbon cycle: reservoirs, fluxes, and impacts

Carbon cycle as a biogeochemical exchange system

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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Carbon reservoirs across Earth’s spheres

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