Land and ocean carbon cycle feedback effects on global warming in a simple Earth system model

Abstract

A simple Earth system model is developed by coupling a box model of the global carbon cycle to an energy-balance approximation of global temperature. The model includes a range of feedback mechanisms between atmospheric CO₂, surface temperature and land and ocean carbon cycling. It is used to assess their effect on the global change being driven by anthropogenic CO₂ emissions from fossil fuel burning and land-use change. When tuned to reach the 1990 level of atmospheric CO₂, the model CO₂ predictions for 1832–1990 are reasonably close to ice-core and instrumental records, observed global warming of ~0.6 K from 1860–1990 is accurately predicted and the land and ocean carbon sinks for the 1980s are close to IPCC central estimates. The ocean sink is reduced by ~0.3 GtC yr^-1 when the ocean surface is assumed to warm at the same rate as global surface temperature. Land and oceanic carbon sinks are predicted to be growing at present and hence buffering the rate of rise of atmospheric CO₂. In the basic model, the current land carbon sink is assumed to be due to CO₂ fertilisation of photosynthesis. The slight warming that has occurred enhances soil respiration (carbon loss) and net primary productivity (carbon uptake) by similar amounts. When the model is forced with a “business as usual”(IS92a) emissions scenario for 1990–2100 followed by a linear decline in emissions to zero at 2200, CO₂ reaches a peak of 985 ppmv in 2170 and temperature peaks at +5.5 K in 2180. Peak CO₂ is ~135 ppmv higher than suggested by IPCC for the same forcing, principally because global warming first suppresses the land carbon sink then generates a land carbon source. When warming exceeds ~4.5 K, soil respiration “overtakes” the CO₂ fertilisation of NPP, triggering a release of ~70 GtC from terrestrial ecosystems over ~100 years. When the effects of temperature on photosynthesis, respiration and soil respiration are removed, peak levels of CO₂ are reduced by ~100 ppmv and peak temperature by ~0.5 K. Distinguishing separate soil carbon pools with different residence times does not significantly alter the timing of the switch to a land carbon source or its effect on peak CO₂, but it causes the source to persist for longer. If forest re-growth or nitrogen deposition are assumed to contribute to the current land carbon sink, this implies a weaker CO₂ fertilisation effect on photosynthesis and generates a larger future carbon source. Peak CO₂ levels are also sensitive by about ±80 ppmv to upper and lower limits on the temperature responses of photosynthesis, plant respiration and soil respiration. By forcing the model with a range of future emission scenarios it is found that the creation of a significant land carbon source requires rapid warming, exceeding ~4.5 K, and its magnitude increases with the rate of forcing. The carbon source is greatest for the most rapid burning of the largest reserve of fossil fuel. It is concluded that carbon loss from terrestrial ecosystems may significantly (~10%) amplify global warming under “business as usual” or more extreme scenarios.