CONTRASTING PHYSIOLOGICAL AND STRUCTURAL VEGETATION FEEDBACKS IN CLIMATE-CHANGE SIMULATIONS

Anthropogenic increases in the atmospheric concentration of carbon dio xide and other greenhouse gases are predicted to cause a warming of th e global climate by modifying radiative forcing(1). Carbon dioxide con centration increases may make a further contribution to warming by ind ucing a physiological response of the global vegetation-a reduced stom atal conductance, which suppresses transpiration(2). Moreover, a CO2-e nriched atmosphere and the corresponding change in climate may also al ter the density of vegetation cover, thus modifying the physical chara cteristics of the land surface to provide yet another climate feedback (3-6). But such feedbacks from changes in vegetation structure have no t yet been incorporated into general circulation model predictions of future climate change. Here we use a general circulation model iterati vely coupled to an equilibrium vegetation model to quantify the effect s of both physiological and structural vegetation feedbacks on a doubl ed-CO2 climate. On a global scale, changes in vegetation structure are found to partially offset physiological vegetation-climate feedbacks in the long term, but overall vegetation feedbacks provide significant regional-scale effects.