Carbon cycle

1. Reservoirs, fluxes and the natural carbon budget

Carbon is a basic constituent of carbohydrates, proteins, fats and nucleic acids. It circulates as atmospheric carbon dioxide and methane, organic matter in organisms and soils, dissolved inorganic carbon in water, and carbonates or organic carbon in rocks. The carbon cycle is therefore both biogeochemical and climatic: biological, chemical and geological processes redistribute an element whose gaseous forms influence Earth's radiation balance.

Major reservoirs differ greatly in size and turnover time. Rocks and sediments hold the greatest quantities over geological periods. The ocean contains about 38,000 GtC, mainly as dissolved inorganic carbon, whereas the atmosphere contains roughly 900 GtC, with the amount continuing to rise. Soils store more organic carbon than living terrestrial vegetation, although estimates depend on soil depth and whether peatlands and permafrost are included.

A reservoir is not automatically a sink. A mature forest may hold a large stock yet have only a modest net annual uptake; a disturbed forest can become a source. Fluxes are expressed as carbon transferred per unit time, commonly GtC per year. Natural exchanges are large in both directions, so even a comparatively smaller persistent anthropogenic addition can substantially alter atmospheric concentrations.

• Stock: the quantity of carbon stored at a particular time.
• Flux: the rate of carbon movement between reservoirs.
• Net sink or source: the balance between carbon entering and leaving a reservoir over a defined period.

2. The fast biological cycle on land

The fast carbon cycle operates through organisms, soils and exchanges with the atmosphere, from daily to centennial timescales. Green plants, algae and cyanobacteria use photosynthesis to convert CO2 into organic compounds using sunlight. Carbon then passes through food webs as herbivores consume plants and predators consume other organisms. Respiration by plants, animals and microorganisms releases CO2 while making energy available for cellular processes.

Gross primary productivity, or GPP, is the total carbon fixed through photosynthesis. Net primary productivity, or NPP, equals GPP minus plant respiration. NPP supports plant growth and supplies organic matter to consumers and decomposers. However, NPP is not equivalent to net ecosystem carbon sequestration: microbial respiration, fire, harvesting and other losses must also be considered when estimating the ecosystem's carbon balance.

Dead biomass, roots and animal wastes enter litter and soil organic matter pools. Decomposers return much of this carbon to the atmosphere, while some persists through association with minerals or protection within soil aggregates. Waterlogging can slow decomposition and support peat formation. Under anaerobic conditions, microbial processes may generate methane; oxygenated soils and waters can support methane oxidation.

Land-use change alters both stocks and fluxes. Deforestation removes biomass, releases carbon through burning or decomposition, and can reduce future uptake. Tillage, erosion and drainage can deplete soil carbon. Conversely, protecting natural ecosystems, restoring degraded land and improving soil management can conserve or increase carbon stocks. Such gains remain vulnerable to drought, fire, warming and subsequent land conversion.

3. Ocean carbon uptake and the slow geological cycle

CO2 moves between the atmosphere and the surface ocean according to the difference in its partial pressure across the air–sea boundary. Cold water generally dissolves more CO2 than warm water. Ocean circulation transfers some absorbed carbon to deeper layers through the solubility pump, where it may remain isolated from the atmosphere for centuries or longer.

The biological pump begins when phytoplankton fix dissolved CO2 through photosynthesis. Some organic material sinks as dead organisms, faecal pellets and aggregates. Most is consumed or decomposed before reaching the seabed, but a smaller fraction reaches deep waters and sediments. Thus, high marine productivity does not automatically imply equally high permanent carbon burial.

In seawater, dissolved CO2 participates in reactions involving carbonic acid, bicarbonate and carbonate ions. Bicarbonate is the dominant dissolved inorganic carbon form at normal seawater pH. Additional CO2 increases hydrogen ion concentration and reduces carbonate availability, making calcification more difficult for many corals and shell-forming organisms. Ocean acidification means declining pH; the open ocean remains alkaline overall.

The slow cycle links chemical weathering, rivers, sedimentation, burial, tectonics and volcanism. Weathering of silicate rocks consumes CO2, and rivers transport dissolved products to oceans. Over long timescales, carbonate formation and burial can lock carbon into sedimentary rocks. Subduction, metamorphism and volcanic degassing return carbon to the surface system. Carbonate-rock weathering alone should not be treated as an equivalent permanent sink, because subsequent carbonate precipitation can offset its CO2 uptake.

4. Human disturbance and climate–carbon feedbacks

Burning coal, petroleum and natural gas rapidly transfers geological carbon into the active atmosphere–ocean–biosphere system. Cement manufacture also releases CO2 when limestone is calcined, in addition to emissions from energy use. Deforestation and other land-use changes add emissions and modify uptake. Atmospheric CO2 has consequently increased from about 280 parts per million before industrialisation to more than 420 parts per million.

Land and ocean sinks slow atmospheric accumulation but do not neutralise fossil-fuel emissions. Their uptake varies with climate, ecological conditions and ocean circulation. Moreover, absorbed carbon can produce other impacts: ocean uptake moderates atmospheric warming while causing acidification. The atmospheric increase is the residual of human emissions after net land and ocean uptake.

A positive feedback amplifies an initial change. Warming can accelerate soil decomposition, thaw permafrost and increase wildfire risks, releasing additional CO2 or methane. Warmer surface waters also tend to hold less dissolved CO2. CO2 fertilisation can increase plant growth under suitable conditions, but nutrient limitations, water stress and heat constrain this response. These competing processes make the future strength of natural sinks uncertain.

• Methane belongs to the carbon cycle, but its lifetime and radiative effects differ from those of CO2.
• CO2 has no single atmospheric lifetime because different removal processes operate over very different timescales.

5. Carbon-cycle knowledge in climate governance

The UNFCCC, adopted in 1992, recognises the importance of conserving and enhancing greenhouse-gas sinks and reservoirs. The 2015 Paris Agreement seeks a balance between anthropogenic greenhouse-gas emissions by sources and removals by sinks in the second half of this century. This broader greenhouse-gas balance must be distinguished from net-zero CO2, which balances anthropogenic CO2 emissions and removals specifically.

National inventories account for emissions and removals from land use, land-use change and forestry, often abbreviated LULUCF. India's updated nationally determined contribution of 2022 retained the goal of creating an additional carbon sink of 2.5–3 billion tonnes of CO2 equivalent through additional forest and tree cover by 2030. This is a sink target, not simply a target for plantation area.

Credible carbon projects require additionality, reliable baselines, monitoring, avoidance of double counting and attention to leakage and permanence. A forest offset can be reversed by fire or clearing, whereas geological storage may provide much longer retention if appropriately selected and managed. Carbon capture from a fossil-fuel facility avoids some emissions but is not automatically atmospheric carbon removal. Bioenergy is likewise not automatically carbon-neutral: outcomes depend on land-use effects, supply-chain emissions and the time required for regrowth.

Carbon reservoirs, fluxes and climate governance

The carbon cycle connects reservoirs through transfers called fluxes. Photosynthesis moves atmospheric carbon into biomass, while respiration, decomposition and combustion return carbon to the atmosphere. Oceans exchange carbon dioxide with the air and store carbon mainly in dissolved forms; marine organisms also contribute to biological transfers and carbonate formation. Weathering, sediment burial and volcanic activity connect these processes to a slower geological cycle. A reservoir’s size is not the same as its annual uptake: a large carbon store need not be a strong net sink.

Climate governance seeks to limit the human-driven transfer of carbon into the atmosphere and protect or enhance sinks. Fossil-fuel combustion introduces carbon from geological stores into the active cycle, while land-use change can release vegetation and soil carbon. In India, forest conservation, ecosystem restoration and soil management can support mitigation, but outcomes depend on ecological conditions, land rights and management. Carbon-cycle illustrations explain mechanisms, not verified quantities: they cannot establish a project’s additional carbon removal, storage permanence or eligibility for credits.

• Distinguish stocks from flows: carbon storage is an accumulated quantity; sequestration is uptake over time. A forest can hold substantial carbon while losing it overall.
• Ocean uptake moderates atmospheric carbon dioxide but also drives acidification. Absorption does not make fossil-fuel emissions environmentally harmless.
• For carbon markets, examine additionality, leakage, permanence and double counting. Biological storage can be reversed by fire or clearing and should not be equated automatically with avoided fossil emissions.

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