What Is the Carbon Cycle?

The carbon cycle is the set of biogeochemical processes that move carbon among the atmosphere, ocean, land biosphere, soils, freshwater systems, and geological reservoirs. Carbon circulates in several chemical forms, principally carbon dioxide and methane in the atmosphere, dissolved inorganic carbon and carbonate minerals in the ocean, and organic compounds in living tissue, soil, and sedimentary rock. The cycle is studied as a coupled system of stocks, measured in gigatonnes of carbon, and fluxes, measured in gigatonnes per year, because the atmospheric concentration at any moment reflects the imbalance between inputs and removals rather than either quantity alone.

Carbon cycle science draws on atmospheric chemistry, oceanography, ecology, soil science, and isotope geochemistry. Its practical importance is that carbon dioxide is the dominant human influence on the planetary energy balance, so quantifying where emitted carbon goes determines how much of it accumulates in the atmosphere and for how long.

Reservoirs and Fluxes

The cycle is conventionally split into a fast component and a slow component. The fast cycle exchanges carbon between the atmosphere, surface ocean, vegetation, and soils on timescales of days to centuries, driven by photosynthesis, respiration, decomposition, air-sea gas exchange, and fire. Gross fluxes in this component are large, on the order of 100 gigatonnes of carbon per year in each direction between atmosphere and land, so the net change is a small residual between two big numbers. The slow cycle operates over hundreds of thousands to millions of years through silicate weathering, carbonate deposition, burial of organic matter, and volcanic outgassing, and it sets the long-term baseline. Fossil fuel combustion effectively transfers carbon from the slow reservoir into the fast one at a rate the slow processes cannot match.

Ocean and Land Sinks

Roughly half of anthropogenic CO2 emissions are absorbed each year by the ocean and the terrestrial biosphere, and the remainder accumulates in the atmosphere. Ocean uptake proceeds through dissolution and the carbonate buffer system, with the biological pump exporting organic carbon to depth, and it carries the side effect of falling seawater pH. Land uptake reflects CO2 fertilization of photosynthesis, nitrogen deposition, forest regrowth, and climate effects on growing seasons, offset by deforestation, drought, and fire. The annual accounting of these terms is published in the Global Carbon Budget assessment, which reconciles emissions estimates against observed atmospheric growth and closes the residual as a budget imbalance. Sink strength is not fixed. Both land and ocean uptake efficiency are expected to decline as warming proceeds, a feedback that shapes projected atmospheric trajectories.

Observation and Modeling

Quantifying fluxes requires sustained measurement. The NOAA Global Monitoring Laboratory carbon cycle greenhouse gas program operates a global reference network of flask and in situ sampling sites tracking carbon dioxide, methane, and nitrous oxide, providing the calibrated record against which regional estimates are tested. Eddy covariance flux towers measure ecosystem exchange directly, ocean cruises and autonomous floats sample dissolved inorganic carbon, and satellite instruments retrieve column-averaged CO2. Isotopic ratios of carbon-13 and carbon-14 distinguish fossil carbon from biospheric carbon, a technique described in NASA Earth Observatory's account of the global carbon cycle. Atmospheric inverse models combine these observations with transport simulations to infer surface fluxes that measurements alone cannot resolve.

Applications

Carbon cycle science has applications in a range of fields, including:

• Earth system and climate model development, including carbon-climate feedback representation
• National greenhouse gas inventory verification using atmospheric measurements
• Forest and soil carbon management, including afforestation and agricultural practice change
• Carbon dioxide removal assessment, covering ocean alkalinity enhancement and mineralization
• Ocean acidification monitoring and marine ecosystem impact studies
• Remote sensing instrument design for atmospheric composition missions