Earth Carbon Cycle and Human Impacts

Carbon cycle as a biogeochemical exchange system

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 and exchange pathways

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