The global carbon cycle as a system of stores and fluxes, the fast biological and slow geological cycles, the biological and physical ocean pumps, terrestrial stores and the greenhouse effect, ocean acidification and planetary health.

What this dot point is asking

Edexcel wants you to explain the global carbon cycle as a system of stores and fluxes, distinguish the fast biological and slow geological cycles, explain the biological and physical ocean pumps and terrestrial stores, and explain the greenhouse effect, ocean acidification and why the carbon cycle matters for planetary health.

Carbon stores and fluxes

Carbon moves by fluxes: photosynthesis removes atmospheric CO2 into plants; respiration, decomposition and combustion return it; ocean-atmosphere exchange transfers CO2 across the sea surface; and over long timescales volcanic outgassing, sedimentation and burial move carbon through rocks. Residence time (store size divided by flux) ranges from years in the atmosphere to millions of years in rocks.

The fast and slow carbon cycles

The two cycles are linked: weathering of rock slowly draws CO2 down, while volcanism and the burning of fossil fuels release it. The danger is one of rate: combustion returns buried carbon to the atmosphere in centuries, far faster than the slow cycle can re-bury it.

The biological and physical ocean pumps

The oceans are central to regulation through two pumps. The biological pump: phytoplankton at the surface fix CO2 by photosynthesis; when they die or are eaten, organic matter and shells sink to the deep ocean, locking carbon away from the atmosphere for centuries. The physical or solubility pump: CO2 dissolves more readily in cold, dense water, which sinks at high latitudes and carries dissolved carbon into the deep ocean through thermohaline circulation, returning it where upwelling brings deep water to the surface.

Terrestrial stores, the greenhouse effect and planetary health

Major terrestrial stores include tropical rainforest, boreal (taiga) forest, peatlands and soils, which together hold more carbon than the atmosphere; deforestation and peat drainage release it. The natural greenhouse effect is essential: CO2, CH4, N2O and water vapour trap outgoing longwave radiation and keep Earth about 33 degrees Celsius warmer than it would otherwise be. The enhanced greenhouse effect is the human-amplified version that drives warming.

Ocean acidification is the chemical consequence of rising CO2: as oceans absorb more, surface pH falls, reducing the carbonate ions that corals and shell-forming organisms need. This threatens reefs, shellfish and the base of marine food webs. Because carbon regulates climate, ocean chemistry and ecosystem services, the carbon cycle is fundamental to planetary health.

Examples in context

Example 1: the Amazon rainforest. The Amazon is one of the largest terrestrial carbon stores, holding tens of billions of tonnes of carbon in its biomass and soils and removing CO2 through photosynthesis. Deforestation and fire turn parts of it from a sink into a source, and prolonged drought reduces its uptake. It shows the scale and fragility of terrestrial storage and its sensitivity to human and climatic disturbance.

Example 2: the Great Barrier Reef, Australia. Rising CO2 has lowered surface ocean pH and, with marine heatwaves, driven repeated mass coral bleaching across the reef since 2016. Acidification reduces the carbonate available for coral skeletons, slowing reef growth. It demonstrates ocean acidification and warming acting together to damage a major marine ecosystem and the services it provides.

Try this

Q1. State the formula for the residence time of carbon in a store. [2 marks]

• Cue. Residence time = store size / flux (rate of input or output).

Q2. Explain how the biological pump transfers carbon to the deep ocean. [4 marks]

• Cue. Phytoplankton fix CO2 by photosynthesis; dead organisms and shells sink, carrying carbon to the deep ocean for centuries.