CARBON CYCLE FEEDBACK EFFECTS ON GLOBAL WARMING

A simple Earth system model, which couples a box model of the global carbon cycle to an energy balance approximation of global temperature, has been used to assess feedback mechanisms between atmospheric CO₂, surface temperature and land and ocean carbon cycling when forced by anthropogenic CO₂ emissions from fossil fuel burning and land-use change. Land and ocean carbon sinks are predicted to be growing at present and hence buffering the rate of rise of atmospheric CO₂. However, future global warming is predicted to suppress the ocean carbon sink and cause soil respiration to overtake the CO₂ fertilisation of net primary productivity, generating a transient land carbon source. This amplifies peak levels of atmospheric CO₂ and global warming by ~10% under 'business as usual' emissions. Land carbon source size depends on the rate and magnitude of forcing, and is sensitive to uncertainties in the functional response of terrestrial ecosystem processes. If forest re-growth or nitrogen deposition are significant contributors to the current land carbon sink, this implies a weaker CO₂ fertilisation effect on photosynthesis and a larger future carbon source. Peak CO₂ levels are sensitive by ±8% to upper and lower limits on the temperature responses of photosynthesis, plant respiration and soil respiration. The land surface should warm more rapidly than the ocean surface and this will increase peak CO₂ levels by up to 14% relative to uniform warming, because of increased carbon loss from the land. High latitudes should continue to warm more rapidly than low latitudes, with compensatory effects on land carbon storage. On a millennial timescale, the apportioning of carbon between the atmosphere, ocean and land will return close to steady state. If the entire know fossil fuel resource has been emitted, the remaining airborne fraction of CO₂ (~30%) will make the world at least 5°C warmer.