Drought Under Global Warming

The largest impact of global warming will likely come from changes in precipitation and associated extreme events such as drought and floods. Here we examine historical changes since around 1950 in streamflow, soil moisture, and the (improved) Palmer Drought Severity Index (PDSI), a widely-used drought index that considers the cumulative effect of precipitation, temperature, radiation, humidity and wind speed on surface moisture conditions. We also analyze and compare with IPCC AR4 multi-model simulated changes in precipitation, soil moisture, runoff, and PDSI (computed using model-simulated climate changes). It is found that decreases in precipitation during the last several decades over most Africa, East Asia and eastern Australia have resulted in wide-spread drought conditions over these regions, as reflected by decreases in streamflow, soil moisture and PDSI. Large warming over northern middle and high-latitude land areas since the 1980s appears to have enhanced aridity over these regions. A large part of the recent precipitation decreases over Africa, East Asia and eastern Australia was caused by natural variations in tropical SST that is not related to increased greenhouse gases. However, for model-predicted 21st century climate, both PDSI and soil moisture changes suggest severe drying over most of the middle and low-latitude land areas, including the United State. Furthermore, drying could occur over regions with increased precipitation such as Southeast Asia due to increased evaporation and runoff.