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The carbon cycle is the biogeochemical system that exchanges carbon among Earth’s biosphere, pedosphere (soils), geosphere (rocks and interior), hydrosphere (waters), and atmosphere. It is essential for sustaining life because it recycles carbon through living organisms and also includes long-term processes that store carbon in sinks and later release it. Carbon exists in many forms and reservoirs (carbon pools), such as the atmosphere (mainly CO2 and methane), the ocean (dissolved inorganic carbon and marine biota), terrestrial ecosystems and soils, sediments and fossil fuels, and Earth’s interior. Within the Earth system, the carbon cycle is often described as two interacting timescales: the fast (biological) carbon cycle and the slow (deep/geological) carbon cycle. The fast cycle operates through photosynthesis, respiration, decomposition, and exchange between atmosphere, land ecosystems, and the ocean, typically completing within years to decades. The slow cycle moves carbon through rocks and Earth’s crust and mantle, involving weathering, sedimentation, subduction, and volcanic degassing, and can take millions of years. Natural carbon flows are often roughly balanced, but human activities—especially fossil fuel extraction and burning and land-use change—redistribute large amounts of geologic carbon into the atmosphere, increasing atmospheric CO2 and altering ocean chemistry (including ocean acidification), thereby affecting climate and ecosystem functioning.
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The global carbon cycle consists of interconnected carbon reservoirs, or pools, that exchange carbon through biological, physical, chemical, and geological processes. The major reservoirs are the atmosphere; the terrestrial biosphere, including vegetation, organisms, and soils; the ocean, including dissolved inorganic carbon and marine organisms; sediments and fossil fuels; and the Earth's crust and mantle. The ocean is the largest actively cycling near-surface reservoir, while sedimentary rocks, carbonates, kerogens, and the deep Earth contain the greatest long-term stores. Carbon moves rapidly among the atmosphere, land ecosystems, surface ocean, soils, and marine life through photosynthesis, respiration, decomposition, dissolution, ocean circulation, runoff, and sedimentation. These fast exchanges can occur over days to decades, whereas the slow geological cycle transfers carbon through rock weathering, burial, subduction, volcanic activity, and tectonic uplift over thousands to millions of years. Under natural conditions, exchanges are broadly balanced, but fossil-fuel combustion, cement production, and land-use change have transferred additional carbon from geological reservoirs to the atmosphere, increasing atmospheric carbon dioxide and contributing to ocean acidification and climate change.
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