Background

CO2 is the most important of the anthropogenic greenhouse gases. Its concentration in the atmosphere has increased by about 30% during the industrial period and is almost certain to reach double pre-industrial concentrations in the second half of the twenty-first century.

On time-scales of millions of years, geological processes dominate the carbon cycle. Atmospheric concentrations of CO2 are determined by the balance among erosion, sedimentation and volcanic emissions. Sundquist has described a hierarchical classification of geological exchanges of carbon on the basis of time-scale. On any particular time-scale, any faster processes make the reservoirs involved appear to be in a well-mixed equilibrium state whereas any slower processes appear as almost-fixed boundary conditions. Geological processes act too slowly to cause significant changes to atmospheric concentrations of CO2 on time-scales less than millennia. On timescales of centuries or less, the dominant fluxes of CO2 to and from the atmosphere come from the oceans and the terrestrial biosphere, i.e. living biota, dead and decaying biota and active carbon reservoirs in soil. It is these ‘active’ reservoirs that we need to consider when we are analysing anthropogenic changes in the carbon cycle. Figure 14.1 gives a schematic representation of the exchanges between these reservoirs, showing the large natural cycle in approximate balance and a comparatively small anthropogenic perturbation.