Large-scale changes in land cover affect near-surface energy, moisture and momentum fluxes owing to changes in surface structure (referred to as biogeophysical effects) and the atmospheric CO2 concentration owing to changes in biomass (biogeochemical effects). Here we quantify the relative magnitude of these processes as well as their synergisms by using a coupled atmosphere-biosphere-ocean model of intermediate complexity. Our sensitivity studies show that tropical deforestation tends to warm the planet because the increase in atmospheric CO2 and hence, atmospheric radiation, outweighs the biogeophysical effects. In mid and high northern latitudes, however, biogeophysical processes, mainly the snow-vegetation-albedo feedback through its synergism with the sea-ice-albedo feedback, win over biogeochemical processes, thereby eventually leading to a global cooling in the case of deforestation and to a global warming, in the case of afforestation. ¿ 2001 American Geophysical Union