Biogeophysical effects of land use on climate: Model simulations of radiative forcing and large-scale temperature change

Changes in land cover affect climate through the surface energy and moisture budgets. Here we assess the importance of these biogeophysical effects for present-day climate, and quantify the radiative forcing of historical climate change by land use change for comparison with radiative forcings due to anthropogenic changes in greenhouse gases and aerosols. We also discuss the implications of biogeophysical effects for the use of forestry as a tool for mitigating climate change through carbon sequestration. Our model results suggest that since most historical deforestation has taken place in temperate regions where the main climatic effect is an increase in surface albedo, the dominant biogeophysical effect of past land cover change has been a cooling. The northern mid-latitude agricultural regions are simulated to be approximately 1–2 K cooler in winter and spring in comparison with their previously forested state. This conflicts with the suggestion that land use change is responsible for the warming observed over the 20th century. The increase in albedo by 1750 is simulated to exert a negative radiative forcing of approximately −2 W m−2 locally over Europe, China and India, suggesting a potential anthropogenic influence on climate before fossil fuel burning began. The present-day global mean radiative forcing by anthropogenic surface albedo change relative to the natural state is simulated to be −0.2 W m−2, which is comparable with the estimated forcings relative to pre-industrial times by stratospheric and tropospheric ozone, N2O, the halocarbons, and the direct effect of anthropogenic aerosols. In cold regions, afforestation or reforestation would decrease the surface albedo and induce a positive radiative forcing (warming) which could partly or completely offset the negative forcing (cooling) due to carbon sequestration. This suggests that carbon sink plantations could be less effective than expected at reducing warming, and could even cause further warming. However, we note that reforestation (or avoided deforestation) in tropical regions could exert a double cooling effect through carbon sequestration and increased evaporation and cloud cover.