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The carbon cycle is a biogeochemical exchange system that moves carbon among Earth’s biosphere, pedosphere (soils), geosphere (rocks), hydrosphere (waters), and atmosphere. It includes both rapid recycling of carbon through living organisms and ecosystems and long-term sequestration and release from carbon sinks. Carbon is essential for biological compounds and is also stored in major rock types such as limestone, making the cycle fundamental to sustaining life and regulating Earth’s climate. To describe its dynamics, the cycle is often separated into a fast (biological) carbon cycle and a slow (deep/geological) carbon cycle. The fast cycle can operate on timescales of years, transferring carbon between the atmosphere and biosphere via processes like photosynthesis, respiration, and decomposition, and it also involves exchanges among terrestrial ecosystems, oceans, and sediments. The slow cycle can take millions of years, moving carbon through Earth’s crust and mantle via rock formation, weathering, sedimentation, subduction, and volcanic degassing. Natural exchanges among major carbon reservoirs (atmosphere, terrestrial biosphere, oceans, sediments, and Earth’s interior) are often roughly balanced, but human activities—especially land-use change and burning fossil fuels—have increased atmospheric CO2 substantially, driving global warming and ocean acidification. Major reservoirs and pathways include the atmosphere (mainly CO2 and methane), the terrestrial biosphere (living biomass and soils), the ocean (surface and deep layers with dissolved inorganic carbon and biological matter), and the geosphere (sedimentary carbonates, kerogens, and other long-lived stores). The ocean’s largest active near-surface pool exchanges carbon with the atmosphere relatively quickly, while deep-ocean circulation exchanges more slowly over centuries. Carbon can enter the ocean through dissolution of atmospheric CO2 and river transport, then be transformed by marine organisms and transported to deeper waters and sediments through processes such as the biological pump. In the geosphere, most carbon is stored inertly in lithospheric reservoirs, and it returns to the atmosphere and ocean through geological processes like metamorphism and volcanism; humans can also extract and burn fossil carbon, directly adding CO2 to the atmosphere.
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Carbon reservoirs are major stores of carbon interconnected by exchange pathways. The principal reservoirs are the atmosphere, terrestrial biosphere, oceans, sediments and fossil fuels, and the Earth’s crust and mantle. The ocean is the largest actively cycling near-surface reservoir, while most of Earth’s total carbon is stored over long periods in carbonate rocks, kerogens and other geological materials. Terrestrial carbon is held in living and dead biomass and soils, with soils storing carbon for years to thousands of years. Carbon moves between reservoirs through biological, physical, chemical and geological processes. In the fast carbon cycle, photosynthesis removes carbon dioxide from the atmosphere, while respiration, decomposition, combustion and water transport return carbon to the atmosphere or carry it into aquatic systems. The ocean absorbs carbon dioxide directly from the atmosphere and through rivers; marine organisms convert it into organic matter and carbonate shells, some of which sink through the biological pump and become stored in deep waters or sediments. The slow carbon cycle operates over thousands to millions of years through rock weathering, sedimentation, subduction, metamorphism, volcanic activity and tectonic uplift. Human activities have disrupted the balance between reservoirs by burning fossil fuels, producing cement and changing land use. These actions transfer geological and terrestrial carbon to the atmosphere, increasing carbon dioxide concentrations and contributing to global warming. Greater oceanic carbon uptake also forms carbonic acid, lowering ocean pH and altering marine chemistry, while deforestation and ecosystem degradation reduce the land biosphere’s capacity to store carbon.
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