Effects of land cover conversion on surface climate

This study investigates the effects of large-scale human modification of land cover on regional and global climate. A general circulation model (Colorado State University GCM) coupled to a biophysically-based land surface model (SiB2) was used to run two 15-yr climate simulations. The control run used current vegetation distribution as observed by satellite for the year 1987 to derive the vegetation's physiological and morphological properties. The twin simulation used a realistic approximation of vegetation type distribution that would exist in the absence of human disturbance. In temperate latitudes, where anthropogenic modification of the landscape has converted large areas of forest and grassland to cropland, conversion cools canopy temperatures up to 0.7 degrees C in summer and 1.1 degrees C in winter. This cooling results from both (1) morphological changes in vegetation which increase albedo and (2) physiological changes in vegetation which increase latent heat flux of crops compared with undisturbed vegetation during the growing season. In the tropics and subtropics, conversion warms canopy temperature by about 0.8 degrees C year round. The warming results from a combination of morphological changes in vegetation offset by physiological changes that reduce latent heat flux of existing compared with undisturbed vegetation. If water efficient, tropical C4 grasses replace C3 vegetation, latent heat flux is further reduced. The overall effect of land cover conversion is cooling in temperate latitudes and warming in the tropics. Because the effects are opposite in sign in tropics and middle latitudes, they cancel each other when averaged globally. Over land, the surface temperature increased by 0.2 C in winter and remained essentially unchanged in summer. The effects on land surface hydrology were also small when averaged globally. The results suggest that the effects of land use change of the observed magnitude do not have a strong impact on the globally averaged climate but their signature at regional scales is significant and vary according to the type of land cover conversion.