Carbon Cycle: Fast and Slow Processes

Carbon cycle definition and Earth system context

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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Major carbon reservoirs and interconnected carbon pools

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