DIRECT IMPACTS OF ATMOSPHERIC CO2 CONCENTRATION ON GLOBAL VEGETATION DISTRIBUTION

The equilibrium vegetation model BIOME4 is used to explore the relative importance of the direct physiological impact of changes in atmospheric CO2 concentration [CO2] and in climate as experienced in the past and predicted for the future. Simulations driven by modern climatology but with [CO2] varying from minimum glacial values (180-200 ppmv) to values four times higher than our control run (1300 ppmv) suggest the response to [CO2] is non-linear. In tropical regions, changes in the balance of forest and non-forest vegetation resulting from changes in [CO2] over the range 280-1000 ppmv are muted, but there are large increases in grassland/shrubland when [CO2] reach glacial levels. The reduction of temperate/boreal forest when [CO2] is lower than today is less pronounced than the reduction in tropical forest, but the increase in forest in response to increased [CO2] is dramatic.

BIOME4 simulations of vegetation at the last glacial maximum with and without the direct physiological effects of [CO2], driven by output from 17 climate simulations from the Palaeoclimate Modelling Intercomparison Project, suggest the direct impact of low [CO2] was as important as the change towards colder/drier climates in restricting the extent of tropical forest. Colder and drier climate was the major cause of forest reduction in the extratropics, although reduced [CO2] also contributed. Comparison of the simulated vegetation distribution with reconstructed palaeovegetation patterns show that forest extent is overestimated when climate changes alone are used to drive BIOME4, and estimated more accurately when the physiological effects of [CO2] are included.

We compared the direct effect of increased [CO2] and the impact of potential future climate changes caused by increased [CO2] using climate simulations made with the ECHAM4-OPYC and HADCM3 models, assuming the IPCC IS92a greenhouse gas scenario. These simulations show the direct impact of increased [CO2] is significant, particularly in subtropical/temperate regions, and counteracts climate-induced changes in vegetation distribution in some regions.