The Climatic Effects of Deforestation in South and Southeast Asia

Deforestation can have strong effects on climate by altering the surface albedo, evapotranspiration and exchange of latent and sensible heat between the surface and the atmosphere, leading to modifications of surface energy and moisture budgets. These modifications can alter the existing spatial and temporal patterns of temperature and precipitation. These changes would then be further modulated or enhanced by the regional circulation. The Weather Research and Forecasting model (WRF) was employed, along with forcing from NCEP/NCAR Reanalysis data (NNRP), to study the effects of tropical deforestation in South and Southeast Asia. These are regions where the Asian monsoon plays an important role in determining the regional climate, and regions where deforestation is currently or potentially a major issue. The WRF simulations were done for three years: 1988, 1991 and 1993, representing a strong, weak and normal monsoon year respectively. Simulations were at 12 km resolution, with an additional 4 km domain over Sri Lanka. For each of the three years, a control run as well as two idealized runs for completely deforested and forested scenarios were carried out and analyzed. For the deforested and forested runs all land-use categories other than water were replaced with grassland for the former and evergreen broadleaf forest for the latter. These changes, while extreme, provided the maximum possible range of changes due to deforestation. The simulations show that in response to tropical deforestation, the land areas become warmer and drier, with precipitation, evapotranspiration and cloud cover all showing a decrease. Moisture convergence decreases over South Asia but shows an increase over Southeast Asia. Precipitation values far exceed the local evapotranspiration over all domains, demonstrating the importance of an external moisture source (e.g., the Indian Ocean). Further, the decrease in precipitation is greater than that of evapotranspiration. The reduction in the local moisture recycling capacity alone cannot explain the strong decrease in precipitation, but changes in the geopotential height at 500 mb shows an increase in pressure aloft which inhibits the formation of precipitation over the deforested regions. The wind direction does not change in response to deforestation but the wind speed (over land) increases over South Asia and weakens over Southeast Asia. The changes in wind speed during the monsoon season are not uniform. The warmer and drier conditions persist over both annual and seasonal (monsoon) scales but the circulation itself remains unmodified for the most part. Deforestation alters the climate by adding an extra forcing to the existing monsoon climate without modifying the regional circulation.