Importance of background climate in determining impact of land-cover change on regional climate

Humans have modified the Earth's climate through emissions of greenhouse gases and through land-use and land-cover change (LULCC). Increasing concentrations of greenhouse gases in the atmosphere warm the mid-latitudes more than the tropics, in part owing to a reduced snow-albedo feedback as snow cover decreases. Higher concentration of carbon dioxide also increases precipitation in many regions, as a result of an intensification of the hydrological cycle. The biophysical effects of LULCC since pre-industrial times have probably cooled temperate and boreal regions and warmed some tropical regions. Here we use a climate model to show that how snow and rainfall change under increased greenhouse gases dominates how LULCC affects regional temperature. Increased greenhouse-gas-driven changes in snow and rainfall affect the snow-albedo feedback and the supply of water, which in turn limits evaporation. These changes largely control the net impact of LULCC on regional climate. Our results show that capturing whether future biophysical changes due to LULCC warm or cool a specific region therefore requires an accurate simulation of changes in snow cover and rainfall geographically coincident with regions of LULCC. This is a challenge to current climate models, but also provides potential for further improving detection and attribution methods. © 2011 Macmillan Publishers Limited. All rights reserved.