3.1.1.3, 3.1.1.5 - Changing carbon stores and budgets

Wildfire and volcanic activity

Natural disturbances redistribute carbon, but their immediate release and subsequent recovery have different timescales.

Wildfire combusts biomass and litter, transferring carbon rapidly to the atmosphere. Some remains as char; dead material can continue decomposing after the flames end. Regrowth may take up carbon over years or decades, but recovery depends on climate, soil damage and repeated fire. A forest repeatedly burned or converted to another land use may not regain its earlier stock. Some wildfires are naturally ignited and others are human ignited; the combustion mechanism does not identify their cause.

Volcanic activity releases geological carbon dioxide during degassing and eruptions. This connects the slow geological cycle to the atmosphere. Eruption aerosols can temporarily reduce incoming solar energy, but that cooling mechanism is different from carbon-dioxide release and its greenhouse effect. A dramatic local eruption is not evidence that volcanic carbon exceeds sustained global human emissions: compare matching global totals and time periods before drawing that conclusion.

The scale of the disturbance matters. A single burning stand may become a source during one season while surrounding regrowth remains a sink. Regional drought can increase fire, reduce photosynthesis and change decomposition together. An annual net flux is the combined result of these processes, rather than a count of fires alone.

Human transfers from land and fuels

Human activity can release existing carbon and reduce the future uptake that would otherwise replace it.

Extracting and burning hydrocarbons transfers fossil carbon from geological stores to the active atmosphere–ocean–land system. Fuel combustion releases carbon dioxide; production and transport can also leak methane. These flows add carbon that was isolated over geological timescales, whereas tree regrowth cannot immediately reverse fossil extraction at its original rate.

Deforestation removes biomass. Burning releases carbon rapidly; harvested products and unburned residues release it over different lifetimes. Losing trees also reduces later photosynthetic uptake. Soil disturbance can expose organic matter to decomposition and erosion. Conversion to pasture or crops generally replaces the original vegetation with a different carbon stock, but its direction and size depend on the previous ecosystem and management.

Farming can increase losses through drainage of peat, soil disturbance and removal of organic inputs. Cover crops, organic additions and reduced disturbance can increase soil inputs or reduce some losses. A soil practice's benefit must be measured against its baseline and cannot be assumed to continue at the same rate indefinitely as soil storage approaches a new balance. Land-use change can also include afforestation and restoration, which may increase uptake. Food production, tenure and displacement of activity to other land affect whether a local gain becomes a global gain.

Accounting for a global carbon budget

A global carbon budget separates emissions to the atmosphere from the carbon accumulating there and the carbon transferred into sinks.

The Global Carbon Budget 2025, published in its final journal form in 2026, estimates these 2024 flows:

Total anthropogenic emissions are 11.6$\text{GtC yr}^{-1}$. Substitution gives $11.6-7.9-3.4-1.9=-1.6$ using these rounded displayed values. The publication reports an imbalance of −1.7 from its underlying estimates. Do not silently force rounded values to equal that reported figure. The residual reflects independently estimated and uncertain terms; carbon has not vanished or been created.

Carbon mass differs from carbon-dioxide mass. Multiplying by $44/12$ converts a mass of carbon into the corresponding carbon-dioxide mass: 11.6$\text{GtC}$ is about 42.5$\text{GtCO}_2$ from rounded inputs; the publication reports 42.4 from underlying values. Atmospheric concentration in parts per million is another quantity, not a mass flux. For 2024 the reported global annual mean concentration is 422.8 ppm.

The Global Carbon Budget article, abstract and budget tables supplies the dated estimates and uncertainties. Its decade-average budget is closer to balance than this single-year result; one unusual year should not be treated as a fixed future partition.

Consequences on land, in oceans and in air

The destination of emitted carbon determines different environmental consequences, even when all transfers belong to one global budget.

Land uptake adds carbon to vegetation and soils, but depends on water, nutrients, temperature and disturbance. Higher carbon dioxide can support photosynthesis under suitable conditions; it does not remove drought or nutrient limits. Heat, wildfire and permafrost thaw can weaken storage or increase releases. A global land sink can coexist with regional losses and does not mean every ecosystem is accumulating carbon.

Ocean uptake reduces the atmospheric increase that would otherwise occur. Dissolved carbon dioxide also changes carbonate chemistry, increasing hydrogen-ion concentration and lowering pH. This ocean acidification can make calcification harder for some organisms. The ocean remains alkaline overall; 'acidification' describes the direction of pH change, not a claim that seawater has become an acid. Uptake is therefore not a consequence-free disposal route. Warming and circulation changes can alter future ocean exchange.

Carbon accumulating in the atmosphere strengthens the greenhouse effect and changes the climate conditions affecting both land and ocean sinks. The budget is an accounting framework, not a prediction that sinks will always absorb the same fraction. Sustained emissions can increase atmospheric storage despite sizeable sinks. Evaluate a claimed solution by whether it reduces the input, increases durable removal or merely redistributes carbon between short-lived stores.