The carbon cycle is the network of processes that transfers carbon among the atmosphere, biosphere, oceans, soils, sediments, and rocks of Earth. It is a biogeochemical cycle, involving biological activity, chemical transformations, and geological processes. Carbon is continually reused rather than consumed: its atoms move between organic compounds, dissolved substances, gases, and minerals. These exchanges support life and influence climate, while operating on timescales ranging from minutes to millions of years. (oceanservice.noaa.gov)
Reservoirs, fluxes, and timescales
A carbon reservoir, or pool, is a component of the Earth system that stores carbon. Most of Earth’s carbon is held in rocks and sediments. Other important reservoirs include ocean water, vegetation, soil organic matter, and atmospheric carbon dioxide (CO₂). Carbon also occurs in methane and numerous organic compounds. Burial and geological alteration of organic remains have produced deposits of fossil fuels, including coal, oil, and natural gas. (oceanservice.noaa.gov)
A flux is a transfer of carbon between reservoirs. A reservoir acts as a net sink when it receives more carbon than it releases over a specified interval, and as a net source when the reverse occurs. A large carbon stock does not necessarily imply a large annual uptake. Scientists distinguish relatively fast exchanges involving organisms and surface waters from slower transfers involving deep oceans, sediments, and geological reservoirs; these are interconnected processes rather than separate, closed loops. (gml.noaa.gov)
Biological cycling on land
Photosynthesis brings carbon into living systems. Plants and other photosynthetic organisms use sunlight to convert inorganic carbon into organic compounds. This carbon fixation supplies material for growth and underlies much biological production. Carbon subsequently moves among organisms through feeding relationships within a food web. (science.nasa.gov)
Cellular respiration returns part of this carbon to the environment as CO₂. When organisms die, decomposition transfers carbon into soils, microbial biomass, and gases. Some soil carbon persists much longer than leaves or other rapidly cycling plant material. Fires also release stored carbon. Whether an ecosystem gains or loses carbon therefore depends on the balance between photosynthetic uptake and losses through respiration, decomposition, disturbance, and material transport. (science.nasa.gov)
Seasonal changes in vegetation produce a recognizable atmospheric signal. Northern Hemisphere spring and summer growth generally draws down CO₂, while reduced uptake and continuing respiration contribute to its subsequent increase. This seasonal oscillation occurs alongside the longer-term rise caused by human emissions. (gml.noaa.gov)
Ocean exchange and storage
CO₂ continuously crosses the air–sea boundary in both directions. The net exchange depends on differences between atmospheric and surface-water CO₂, as well as temperature, winds, and ocean circulation. Cold water generally dissolves more CO₂ than warm water. Mixing and the sinking of water masses transfer dissolved carbon below the surface, where it can remain isolated from the atmosphere for much longer periods. (science.nasa.gov)
Marine organisms provide another pathway, known as the biological carbon pump. Photosynthetic plankton incorporate carbon into organic matter; sinking particles then carry some of it into deeper waters. Much is decomposed before reaching the seabed, returning carbon to dissolved forms, while a smaller portion becomes buried in sediments. Physical circulation and biological transport jointly shape ocean carbon storage. (science.nasa.gov)
Dissolved CO₂ participates in reactions that produce bicarbonate, carbonate, and hydrogen ions. Additional CO₂ uptake increases hydrogen-ion concentrations and lowers pH, causing ocean acidification. It also reduces carbonate-ion availability, affecting the chemical conditions under which organisms form calcium-carbonate shells and skeletons. (oceanservice.noaa.gov)
Geological cycling
The slow carbon cycle connects surface environments with rocks and Earth’s interior. During chemical weathering, CO₂ dissolved in water helps react with minerals. Rivers transport dissolved products to the ocean, where carbon can ultimately enter carbonate sediments. Burial of these sediments and organic matter creates long-lived geological stores. (science.nasa.gov)
Plate tectonics transports carbon-bearing material into Earth’s interior. Heating and geological reactions can release CO₂, which returns to surface environments through volcanic and other geological emissions. Silicate weathering and carbonate burial help regulate atmospheric CO₂ over geological timescales, but respond too slowly to balance rapid human emissions on annual or decadal timescales. (science.nasa.gov)
Human influence and climate feedbacks
Since the Industrial Revolution, fossil-fuel combustion and land-use changes, including deforestation, have redistributed carbon into the atmosphere. Land and ocean sinks absorb a substantial portion of these emissions, but the remainder accumulates. CO₂ is a greenhouse gas, so this accumulation contributes to climate change. (gml.noaa.gov)
Carbon–climate feedbacks alter these exchanges. Higher CO₂ can increase photosynthetic uptake, although nutrients and water constrain the response. Warming can increase decomposition, reduce ocean CO₂ solubility, and promote carbon release from thawing permafrost. Their combined effects influence future sink strength and atmospheric CO₂ trajectories. (ipcc.ch)
Observation and attribution
Researchers study the cycle using atmospheric measurements, ocean observations, and models. Carbon isotopes help distinguish sources: fossil fuels contain effectively no carbon-14 and generally have less carbon-13 relative to carbon-12 than atmospheric CO₂. Combining isotopic evidence with concentration measurements and emissions inventories allows researchers to identify fossil-fuel contributions and constrain exchanges between major reservoirs. (science.nasa.gov)