Effects of land cover conversion on surface climate

This study investigates the effects of large-scale human modification of la nd cover on regional and global climate. A general circulation model (Color ado State University GCM) coupled to a biophysically-based land surface mod el (SiB2) was used to run two 15-yr climate simulations. The control run us ed current vegetation distribution as observed by satellite for the year 19 87 to derive the vegetation's physiological and morphological properties. T he twin simulation used a realistic approximation of vegetation type distri bution that would exist in the absence of human disturbance. In temperate l atitudes, where anthropogenic modification of the landscape has converted l arge areas of forest and grassland to cropland, conversion cools canopy tem peratures up to 0.7 degrees C in summer and 1.1 degrees C in winter. This c ooling results from both (1) morphological changes in vegetation which incr ease albedo and (2) physiological changes in vegetation which increase late nt heat flux of crops compared with undisturbed vegetation during the growi ng season. In the tropics and subtropics, conversion warms canopy temperatu re by about 0.8 degrees C year round. The warming results from a combinatio n of morphological changes in vegetation offset by physiological changes th at reduce latent heat flux of existing compared with undisturbed vegetation . If water efficient, tropical C4 grasses replace C3 vegetation, latent hea t flux is further reduced. The overall effect of land cover conversion is c ooling in temperate latitudes and warming in the tropics. Because the effec ts are opposite in sign in tropics and middle latitudes, they cancel each o ther when averaged globally. Over land, the surface temperature increased b y 0.2 C in winter and remained essentially unchanged in summer. The effects on land surface hydrology were also small when averaged globally. The resu lts suggest that the effects of land use change of the observed magnitude d o not have a strong impact on the globally averaged climate but their signa ture at regional scales is significant and vary according to the type of la nd cover conversion.