The carbon cycle as a closed global system of stores and fluxes; the biological, geological and oceanic sub-cycles; carbon sequestration over short and long timescales; and the natural and human factors that change carbon stores and fluxes.

What this dot point is asking

OCR wants you to describe the carbon cycle as a closed global system of stores and fluxes, explain the biological, geological and oceanic sub-cycles, explain carbon sequestration over short and long timescales, and explain the natural and human factors that change carbon stores and fluxes.

The answer

The carbon cycle as a closed system of stores and fluxes

Carbon moves between these stores through processes that operate at very different rates. Because the system is closed, a change is always a redistribution: adding carbon to the atmosphere means removing it from another store, classically the geological store via fossil-fuel combustion. The key fluxes are photosynthesis (carbon dioxide to plant tissue), respiration and decomposition (organic carbon back to carbon dioxide), combustion (biomass or fossil fuel to carbon dioxide), ocean-atmosphere exchange (dissolution and outgassing), weathering (carbon dioxide drawn into solution and into carbonate rock) and volcanic outgassing (returning rock carbon to the atmosphere).

The biological, geological and oceanic sub-cycles

The biological (fast) sub-cycle links the atmosphere, biosphere and soils: plants fix carbon by photosynthesis, animals and microbes release it by respiration and decomposition, and the store turns over in days to centuries. The oceanic sub-cycle exchanges carbon dioxide across the sea surface (the solubility pump, with cold water absorbing more) and via the biological pump, where plankton fix carbon that sinks to the deep ocean. The geological (slow) sub-cycle operates over millions of years: weathering of silicate and carbonate rock consumes atmospheric carbon dioxide, carbon is deposited and buried as marine sediment and lithified into limestone, and volcanism eventually returns it to the atmosphere.

Carbon sequestration over short and long timescales

Sequestration is the capture and storage of carbon out of the atmosphere. Short-term sequestration holds carbon in biomass (forests, vegetation) and soils for years to centuries; it is reversible, released by fire, decomposition or land-use change. Long-term sequestration locks carbon into sedimentary rock and the deep ocean for geological timescales through the biological pump and burial. The balance between sequestration and release sets whether a store is a sink (net uptake) or a source (net release). Forests, soils and oceans are major natural sinks; this matters directly to climate policy, since protecting and enhancing these sinks is a key mitigation route.

Natural and human factors changing carbon stores and fluxes

Natural factors drive variation over many timescales: wildfire and volcanism release carbon; vegetation growth and ocean uptake absorb it; and over long periods tectonics and climate shift the balance through weathering and outgassing rates. Milankovitch cycles modulate carbon storage over glacial cycles. Human factors now dominate the rapid change: fossil-fuel combustion transfers geological carbon to the atmosphere; deforestation and land-use change release biosphere and soil carbon and reduce future uptake; agriculture and cement production add further fluxes. The result is a measurable rise in atmospheric carbon dioxide and a disruption of the natural balance that the climate-management strand then addresses.

Examples in context

Example 1. The ocean as a carbon sink and the acidification cost. The oceans absorb roughly a quarter of human carbon dioxide emissions through the solubility and biological pumps, buffering atmospheric warming. But this sequestration has a cost: dissolved carbon dioxide forms carbonic acid, lowering surface-ocean pH (ocean acidification), which stresses calcifying organisms such as corals and shellfish. This shows a natural sink limiting atmospheric change while transferring the disruption into the marine carbon and ecosystem system, a direct synoptic link to the Exploring Oceans and Climate Change debates.

Example 2. Deforestation switching a sink to a source. Tropical forests such as the Amazon are large short-term carbon stores that normally act as net sinks. Where clearance and fire are extensive, parts of the basin have been measured switching to a net carbon source, releasing stored biomass and soil carbon and cutting future photosynthetic uptake. This illustrates how a human land-use change can reverse the direction of a major flux, releasing carbon faster than the slow cycle can re-bury it, and underpins the case for forest protection in climate management.

Try this

Q1. Name the two pumps by which the ocean takes up carbon. [2 marks]

• Cue. The solubility pump (carbon dioxide dissolving in cold surface water) and the biological pump (plankton fixing carbon that sinks to the deep ocean).

Q2. Explain why burning fossil fuels disrupts the carbon cycle. [4 marks]

• Cue. It transfers carbon locked in the slow geological cycle into the fast cycle and atmosphere over decades, far faster than weathering and sedimentation return carbon to rock, so carbon dioxide accumulates in the atmospheric store.