This dissertation has two main components. The first is a longterm regional climate modeling study of the effects of different types of land use changes on Southeast Asian climate under present-day climate conditions and under future projected climate conditions at the end of the 21st Century. The focus of the second component is to estimate daily heat index for projected extreme temperatures at the end of the 21st Century and projecting the number of people affected by those heat conditions.

The first component of this study uses a high-resolution regional climate model centered on the Southeast Asian region to compare two land use change scenarios under modern climate and future projected climate conditions. Results from experiments under modern climate conditions indicate that changes in regional climate including widespread surface cooling, increased precipitation, and increased latent heat flux are primarily due to deforestation. As expected from other studies, future climate projections indicate increasing surface temperature and total precipitation. However, the combination of increasing global temperatures and irrigation appears to increase latent heat flux and evapotranspiration, leading to decrease in the surface temperature nearly the same magnitude, increasing both specific humidity and relative humidity. The increasing relative humidity causes low clouds to form, and the net surface solar absorbed flux decreases in response, which further cools the surface. These results imply that deforestation and irrigation have differing complex regional climate responses and the presence of irrigation could mask future surface temperature increases, at least in the short term and reinforce the importance of incorporating land use changes, particularly irrigation, into any studies of future regional climate.

The second component of this study uses global daily maximum heat indices derived from future climate future climate simulations for 2098 and projected population density to estimate how many people will be affected by rising temperatures. Our results show that over 4 billion people annually will experience prolonged periods of Danger heat index conditions, under which heat exhaustion and heat stroke are likely. In addition, a majority of people subjected to prolonged high heat stress conditions are located in tropical developing nations, such as those in south and Southeast Asia, where population density is high and large numbers of people work outdoors. Many countries in these regions lack the resources to mitigate the impact of heat stress on the large numbers of people likely to experience heat-related illness and death.