The carbon cycle is the interconnected set of physical, chemical, geological, and biological processes through which carbon moves among the atmosphere, oceans, rocks, soils, sediments, fossil deposits, and living organisms. Carbon is fundamental to life because it forms the structural basis of carbohydrates, lipids, proteins, nucleic acids, and many other organic molecules. In the atmosphere, carbon occurs mainly as carbon dioxide, with smaller quantities of methane and other carbon containing gases.
Plants, algae, and some microorganisms remove atmospheric carbon dioxide through photosynthesis and incorporate carbon into organic compounds. Respiration by plants, animals, fungi, and microorganisms returns carbon dioxide to the atmosphere or water. Decomposition transfers carbon from dead organic matter into soils, sediments, the atmosphere, and other reservoirs. Oceans exchange carbon dioxide with the atmosphere and contain enormous quantities of dissolved inorganic carbon.
Geological processes operate over much longer timescales, storing carbon in carbonate rocks and sediments and returning some carbon through weathering, volcanic activity, and tectonic processes. Human extraction and combustion of fossil fuels and changes in land use have rapidly transferred geological and biological carbon into the atmosphere, substantially altering the modern carbon balance.
Photosynthesis is one of the central biological processes in the carbon cycle. During photosynthesis, plants, algae, and cyanobacteria use light energy to convert carbon dioxide and water into energy rich organic compounds, with oxygen released as a byproduct in oxygenic photosynthesis. The carbon incorporated into sugars can then be transformed into starch, cellulose, lipids, amino acids, nucleotides, and many other molecules. Carbon moves through food webs when herbivores consume plants and predators consume other organisms.
Respiration reverses part of this transfer by oxidising organic molecules and releasing carbon dioxide while producing usable cellular energy. Decomposition performs a similar function for dead organisms and organic waste. Bacteria, fungi, archaea, and other decomposers break down complex organic matter, releasing carbon dioxide or, under oxygen limited conditions, sometimes methane. Some carbon remains in soils as relatively persistent organic matter. The balance between photosynthetic uptake, respiration, decomposition, burial, and transport determines whether particular ecosystems act as net carbon sinks or sources over specified periods.
The ocean is one of the largest active carbon reservoirs and participates in the carbon cycle through physical and chemical processes as well as biological activity. Atmospheric carbon dioxide dissolves in seawater, where it participates in a series of chemical equilibria involving dissolved carbon dioxide, carbonic acid, bicarbonate, and carbonate ions. Marine organisms use inorganic carbon to construct organic matter and, in some cases, calcium carbonate shells or skeletons.
When organisms die, some carbon is decomposed near the surface while some sinks into deeper waters and sediments. Ocean circulation transports dissolved carbon between surface and deep waters, creating important differences in carbon storage and exchange rates. Increased atmospheric carbon dioxide generally causes increased oceanic uptake, but this also changes seawater chemistry and contributes to ocean acidification. Ocean acidification refers to the decline in average ocean pH associated primarily with increased absorption of carbon dioxide and the resulting chemical reactions. The carbon cycle therefore links atmospheric composition with marine chemistry, marine biology, ocean circulation, and long term carbon storage.
The geological carbon cycle operates over timescales ranging from thousands to millions of years and provides a major long term regulator of atmospheric carbon dioxide. Carbon dioxide dissolved in rainwater contributes to chemical weathering of rocks, particularly silicate minerals. Weathering products can be transported by rivers to the ocean, where carbon can eventually become incorporated into carbonate sediments. Burial and lithification can store carbon in sedimentary rocks for geological periods.
Tectonic processes can return some carbon to the surface, and volcanic activity can release carbon dioxide. Organic carbon can also be buried under conditions that prevent complete decomposition and, over geological time, contribute to fossil fuel deposits. The geological cycle operates far more slowly than modern fossil fuel combustion.
This difference in rates is crucial because human activities can transfer carbon from geological reservoirs to the atmosphere in decades or centuries, whereas natural geological removal processes generally operate much more slowly. Carbon cycle analysis therefore requires attention to timescale, because a process that is rapid in an ecosystem may be extremely rapid compared with geological carbon storage and removal.
Human activities have substantially changed the modern carbon cycle. Burning coal, oil, and natural gas transfers carbon that was stored underground for geological periods into the atmosphere as carbon dioxide. Cement production also releases carbon dioxide through chemical transformation of carbonate minerals. Deforestation, land conversion, soil disturbance, and other land use changes can release carbon from vegetation and soils while reducing future biological carbon uptake.
Some ecosystems nevertheless continue to absorb significant amounts of anthropogenic carbon dioxide, although the persistence and capacity of such sinks can vary. Reforestation, improved soil management, wetland conservation, and ecosystem restoration can influence carbon storage, while carbon capture and storage technologies seek to prevent some industrial carbon dioxide emissions from entering the atmosphere or remove carbon dioxide from air. Carbon accounting must distinguish gross flows from net changes. A forest can absorb large amounts of carbon through photosynthesis while simultaneously releasing large amounts through respiration and decomposition. What matters for net storage is the balance among these processes over the relevant period.
Carbon is not merely an atmospheric greenhouse gas but a fundamental component of ecosystem structure and metabolism. Carbon enters food webs primarily through autotrophic carbon fixation and then moves through organisms as biomass and organic compounds. Herbivores, predators, decomposers, and detritivores redistribute carbon through feeding and waste production. Soil carbon influences fertility, aggregation, water retention, and microbial activity, although carbon storage does not automatically indicate high soil quality in every context.
Forests store carbon in trunks, branches, leaves, roots, litter, and soils. Grasslands often store substantial carbon below ground in roots and soils. Peatlands can accumulate exceptionally carbon rich organic matter because waterlogged conditions slow decomposition. Permafrost regions can also store large amounts of organic carbon that may become more vulnerable to decomposition as soils warm and thaw. These differences demonstrate why carbon cycle assessments must consider ecosystem type, climate, disturbance, soil conditions, and residence times rather than treating all vegetation as equivalent carbon storage.
Carbon cycle science is central to understanding climate change because carbon dioxide and methane influence Earth's radiative balance. Carbon dioxide absorbs infrared radiation and is a long lived greenhouse gas, while methane is more powerful per unit mass over shorter comparison periods but has a shorter atmospheric lifetime. The climate system responds not only to the concentration of greenhouse gases but also to feedbacks involving vegetation, soils, oceans, ice, temperature, and moisture.
Warming can alter respiration, decomposition, wildfire frequency, permafrost stability, ocean chemistry, and plant productivity, potentially changing carbon fluxes. Some carbon cycle feedbacks can amplify warming, while others can partially offset it. Scientific measurement uses atmospheric observations, ocean measurements, ecological experiments, remote sensing, isotope analysis, inventories, and Earth system models to estimate carbon stocks and fluxes. Because carbon moves among reservoirs on very different timescales, the distinction between temporary uptake and durable storage is essential in evaluating carbon management strategies.
A common misconception is that the carbon cycle is simply the exchange of carbon dioxide between plants and animals. That biological exchange is important but represents only part of a much larger system involving soils, oceans, sediments, rocks, fossil fuels, microbes, and geological processes. Another misconception is that carbon storage in vegetation is automatically permanent. Fire, drought, disease, harvesting, decomposition, and land use change can return stored carbon to the atmosphere.
Conversely, some carbon can remain in soils, sediments, or geological formations for very long periods. The carbon cycle also differs from the greenhouse effect. The carbon cycle describes carbon movement among reservoirs, whereas the greenhouse effect describes the influence of atmospheric gases on the Earth's energy balance. These processes interact but are not synonymous. Modern carbon cycle research therefore considers mass balance, chemical transformations, biological metabolism, geological storage, ocean chemistry, atmospheric composition, human emissions, and feedbacks across multiple timescales.
