The Carbon Cycle describes the movement and transformation of carbon among the atmosphere, oceans, terrestrial ecosystems, soils, rocks, sediments and living organisms. Carbon is one of the fundamental elements of life because it forms stable covalent bonds capable of producing an enormous diversity of molecules, including carbohydrates, lipids, proteins, nucleic acids and many other organic compounds. In the atmosphere, carbon occurs primarily as carbon dioxide and smaller quantities of methane and other compounds.
Plants, algae and certain microorganisms remove atmospheric carbon dioxide through photosynthesis, converting inorganic carbon into organic matter. Carbon subsequently moves through food webs when organisms consume one another, returns to the atmosphere through respiration and decomposition, and can become stored in soils, sediments, biomass or geological formations. The cycle operates across timescales ranging from seconds in cellular metabolism to millions of years in rock formation, burial and geological uplift.
The short term biological carbon cycle involves rapid exchange among atmospheric carbon dioxide, vegetation, animals, microorganisms and soils. Photosynthesis converts carbon dioxide and water into organic compounds using energy derived from sunlight, while cellular respiration reverses much of this transformation by oxidising organic matter and releasing carbon dioxide. Decomposition transfers carbon from dead organisms and waste products into microbial biomass, soil organic matter, dissolved carbon and atmospheric gases.
Some organic carbon becomes stabilised within soils or sediments and may persist for centuries or longer, depending on chemical composition and environmental conditions. Fire can rapidly transfer vegetation carbon to atmospheric carbon dioxide and particulate material, although subsequent ecosystem recovery may restore some carbon stocks. The balance between production, respiration, decomposition, disturbance and storage determines whether an ecosystem functions as a net carbon source or net carbon sink over a particular period.
The ocean is a major component of the Carbon Cycle and contains carbon in several inorganic and organic forms. Atmospheric carbon dioxide dissolves into seawater and participates in chemical equilibria involving dissolved carbon dioxide, carbonic acid, bicarbonate and carbonate ions. Marine organisms use inorganic carbon for photosynthesis, shell formation and other biological processes.
Phytoplankton convert dissolved inorganic carbon into organic matter, some of which is consumed near the surface while another fraction is transported into deeper waters through sinking particles, dissolved organic matter and physical circulation. Ocean circulation moves carbon between surface and deep reservoirs, while upwelling can return carbon rich deep water to the surface. The ocean therefore acts simultaneously as a carbon reservoir, chemical buffer, biological system and transport mechanism, and changes in temperature, circulation, biological productivity and carbonate chemistry can alter the strength and character of its carbon exchanges.
The geological Carbon Cycle operates much more slowly than biological and atmospheric exchanges. Carbon dioxide is removed from the atmosphere through chemical weathering of rocks, particularly silicate minerals, with reactions ultimately transporting dissolved carbon through rivers to the ocean. Marine organisms can incorporate carbonate into shells and skeletons, and carbon bearing sediments may become buried and lithified into sedimentary rocks.
Over geological time, tectonic processes can subduct carbon containing material into Earth's interior, while volcanic activity and metamorphic reactions can return carbon dioxide to the atmosphere. Organic carbon can also be buried in sediments and transformed into fossil carbon under appropriate pressure, temperature and geological conditions. This slow cycle contributes to long term regulation of atmospheric carbon dioxide and climate, although its natural adjustment timescale is vastly longer than the timescale over which modern industrial carbon emissions have altered atmospheric composition.
Human activities have significantly changed the Carbon Cycle by transferring geological carbon into the atmosphere and modifying terrestrial and marine carbon reservoirs. Coal, oil and natural gas contain carbon that was stored through geological processes over extremely long periods, while combustion converts much of that carbon rapidly into atmospheric carbon dioxide. Cement production releases carbon dioxide through the chemical transformation of carbonate minerals as well as through associated energy use.
Deforestation and land degradation can reduce vegetation and soil carbon stocks, while reforestation, improved soil management and ecosystem restoration can increase biological carbon storage under suitable conditions. Human activities therefore affect both carbon sources and sinks. The resulting imbalance is particularly important because atmospheric carbon dioxide interacts with radiation, oceans and ecosystems, creating climatic and biogeochemical consequences that extend beyond the original emission source.
Carbon reservoirs and carbon fluxes are central concepts for analysing the cycle. A reservoir is a stock of carbon held in a particular component of the Earth system, whereas a flux describes the rate at which carbon moves between reservoirs. Reservoir size and flux magnitude are not equivalent measures of importance because a large reservoir can exchange carbon relatively slowly while a smaller reservoir can experience rapid turnover.
Carbon budgets quantify sources, sinks and changes in atmospheric, oceanic and terrestrial stocks, but estimates contain uncertainties arising from incomplete measurements and natural variability. Isotopes of carbon, particularly carbon 12 and carbon 13 and the radioactive isotope carbon 14, provide valuable information about carbon sources, age and cycling processes. Carbon 14 is particularly useful for investigating relatively recent carbon reservoirs and processes because it is produced naturally in the atmosphere and decays at a known rate.
Understanding the Carbon Cycle is important in ecology, climate science, geology, agriculture, oceanography and environmental management. Carbon availability influences plant productivity, soil fertility and microbial metabolism, while carbon storage affects ecosystem resilience and greenhouse gas concentrations. Changes in ocean carbon chemistry can influence marine organisms that construct carbonate structures, while alterations in terrestrial carbon storage can affect biodiversity and nutrient cycling.
Climate change can in turn modify the Carbon Cycle through temperature effects on respiration, changes in precipitation, vegetation shifts, permafrost thaw and disturbances such as wildfire. These feedbacks mean that the Carbon Cycle is not a passive response to climate but an active component of the coupled Earth system. Scientific understanding therefore requires simultaneous consideration of physical transport, chemical reactions, biological processes and geological transformations.
Carbon Cycle research uses atmospheric measurements, ocean observations, ecosystem experiments, soil sampling, remote sensing, isotopic analysis, geological records and numerical models. Carbon fluxes can be estimated from direct measurements or inferred using mass balance and modelling approaches. Important applications include greenhouse gas accounting, ecosystem management, agricultural planning, climate modelling, carbon sequestration assessment and reconstruction of past environmental conditions.
Carbon sequestration refers broadly to processes that increase the storage of carbon in biomass, soils, oceans or geological formations, but different storage mechanisms have different capacities, permanence and environmental consequences. A forest, for example, can accumulate substantial carbon while growing but can release some or much of that carbon through fire, harvesting or decomposition. Geological storage can offer greater permanence but requires suitable formations, monitoring and assessment of leakage risks. The Carbon Cycle therefore provides the conceptual framework for understanding where carbon resides, how it moves, how rapidly it moves and how human activity changes these relationships.
