The global carbon cycle as a system; its major stores and fluxes; the fast and slow carbon cycles; and the human modification of the cycle through combustion and land-use change.

What this dot point is asking

Eduqas wants you to explain the global carbon cycle as a system, identify its major stores and fluxes, distinguish the fast and slow carbon cycles, and explain how human activities (combustion and land-use change) modify the cycle.

The answer

Stores and fluxes

Carbon is held in four main stores. The lithosphere holds the most by far, as carbonate rocks, sediments and fossil fuels, but releases it only very slowly. The oceans are the largest active store, holding dissolved carbon dioxide, carbonate and marine life. The biosphere holds carbon in vegetation and especially in soils. The atmosphere holds the smallest amount, as carbon dioxide and methane, yet it is the store whose change drives climate. Carbon moves between them by photosynthesis (atmosphere to biosphere), respiration and decomposition (biosphere to atmosphere), air-sea exchange (atmosphere and ocean), and over geological time by weathering, sedimentation, burial and volcanism.

The fast and slow carbon cycles

This distinction is central to understanding human impact. Carbon locked in fossil fuels is part of the slow cycle, removed from the atmosphere over hundreds of millions of years. Burning it transfers that carbon into the fast cycle and the atmosphere in a matter of decades, far faster than the slow processes of weathering and sedimentation can return it to the lithosphere. That imbalance is why atmospheric carbon dioxide is rising and the cycle is no longer in equilibrium.

Human modification of the cycle

Human activities shift carbon between stores and unbalance the cycle. Fossil-fuel combustion for energy, transport and industry transfers lithosphere carbon to the atmosphere, the single largest human flux, and links the carbon cycle directly to energy security and the global energy mix. Deforestation and land-use change reduce the biosphere store and its capacity to sequester carbon, and burning or decaying cleared vegetation releases stored carbon. Agriculture, cement production and the draining of peatlands and wetlands add further releases. The net effect is a rising atmospheric carbon dioxide concentration and enhanced greenhouse warming, with mitigation requiring a shift away from fossil fuels and the protection and restoration of natural carbon sinks.

Examples in context

Example 1. Tropical rainforest as a carbon store (the Amazon). The Amazon rainforest is a vast biosphere carbon store, holding carbon in its dense biomass and soils and sequestering more through rapid photosynthesis. Deforestation for cattle, soy and logging both releases stored carbon (through burning and decay) and removes the sink, so the forest's net capacity to absorb carbon dioxide falls, and parts of the southern Amazon have shifted towards being a net source. The Amazon is the standard Eduqas case linking land-use change to the carbon cycle and to climate, and it ties the carbon cycle to the water cycle through the moisture the forest recycles.

Example 2. Peatlands and the soil carbon store. Peatlands store enormous amounts of carbon in waterlogged, partly decayed organic soil, far more per hectare than forest, because waterlogging slows decomposition. Draining peatlands for agriculture or forestry exposes the peat to oxygen, so decomposition resumes and carbon is released to the atmosphere, turning a long-term sink into a source. Restoring (rewetting) peatlands halts the release and resumes sequestration. Peatlands are a powerful example of how the biosphere/soil store can be either protected or released depending on management, central to carbon-cycle mitigation answers.

Try this

Q1. Name the four major stores of the global carbon cycle. [2 marks]

• Cue. The lithosphere (rocks and fossil fuels), the oceans, the atmosphere, and the biosphere (vegetation and soils).

Q2. Explain why burning fossil fuels unbalances the carbon cycle. [3 marks]

• Cue. Fossil fuels are part of the slow carbon cycle, locked in the lithosphere over millions of years; burning them transfers that carbon to the atmosphere in decades, far faster than slow processes can return it, so atmospheric carbon dioxide rises.