13.1.1  Global carbon cycle

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A mere 0.1% of all the known carbon reserves on earth is cycling in our atmosphere, oceans and terrestrial biosphere combined. On a geological time scale (106–109 years) these three pools can be regarded as well mixed and in equilibrium with each other, cycling with the 99.9% of the earth’s carbon incorporated in rocks. Such geologic cycling occurs via calcium carbonate and formation of reduced organic carbon in global oceans (including carbon derived from fossil fuels) and by subduction of rocks into the earth’s magma via plate tectonics. Carbon (C) re-emerges as gaseous CO2 from volcanoes, fumaroles and other leakage points, at about 0.1 Gt C year–1 globally, and for hundreds of millions of years following subduction. Prior to industrialisation, the distribution of carbon within this dynamic fraction (0.1% of the global total) was approximately oceans 95%, vegetation and soils 4% and atmosphere 1.5%. Over the past two centuries a tiny fraction of the huge pool of reduced carbon in rocks has been extracted and burnt as fossil fuels. This releases at about 6 Gt C year–1 as CO2 directly into our atmosphere and is so fast relative to the natural processes of cycling that atmospheric levels are building up at about 0.5% year–1. This perturbation will take many thousands of years to redistribute into the oceans and tens of millions of years to redistribute into rocks, even if fossil fuel burning should cease. Meanwhile we will have to cope with continuing repercussions from this human impact on global cycles.