The effect on regional and global climate of expansion of the world's deserts

An atmospheric general-circulation model with realistic land-surface properties is used to investigate the climatic effect of doubling the extent of the earth's deserts. Control and anomaly integrations are performed for 10 years. In the anomaly case, deserts are expanded over northern and southern Africa, Australia, south-central Asia, south-western North America, and parts of South America. In the anomaly case, the troposphere is cooler across most of the tropics and subtropics, including all areas where desertification occurs. Remote effects in the winter circulation include a pronounced trough over northern Europe, and increased geopotential heights over the southern oceans. The response of climate in deforested areas is not uniform between the regions. The magnitude and seasonality of the changes, particularly in precipitation, seem to be functions of the monsoon regime. Northern Africa suffers a strong year-round drought, suggesting this area is most sensitive to desertification, and southern Africa has a somewhat weaker year-round drought. Rainfall over desertified parts of Asia, Australia and western South America decreases only during the summer, and not during the winter. North America and the Nordeste region of Brazil show the least sensitivity to desertification. Most regions show a notable correlation between decreases in evapotranspiration and resulting precipitation. However, regional land-sea distributions in the vicinity of each desertified area appear to determine the sensitivity and response of the local climate via induced changes in moisture flux convergence. Thus, different regions exhibit different responses of climate to desertification. Surface temperature decreases over most desertified areas, owing to reduced absorption of short-wave radiation by the brighter surface. Some regions, particularly the Sahel, show an increase in surface temperature caused by decreased soil moisture and latent-heat flux.