Changes in Global and Regional Drought: Retrospective Analysis and Future Projections

Recent and potential future increases in global temperatures are likely to be associated with impacts on the hydrologic cycle, including changes to precipitation and increases in extreme events such as droughts. We summarize recent analyses of historic reconstructions of drought, including trends over the second half of the 20th century, and projections of the future drought occurrence from IPCC AR4 multi-model, multi-scenario experiments. Global and regional trends in drought for 1950-2000 are analyzed using a soil moisture based drought index derived from a simulation of the terrestrial hydrologic cycle driven by a hybrid reanalysis-observation forcing dataset. There is an overall small wetting trend in global soil moisture, forced by increasing precipitation, which is weighted by positive trends over the western hemisphere. Regional variation is nevertheless apparent and significant drying over West Africa stands out. Trends in drought duration, intensity and severity are predominantly decreasing but statistically significant changes are limited in areal extent, depending on the variable and drought threshold. Concurrent changes in drought spatial extent are evident. Despite the overall wetting trend there is a switch since the 1970s to a drying trend, globally and in many regions, especially in high northern latitudes. This is shown to be caused, in part, by concurrent increasing temperatures. Although drought is driven primarily by variability in precipitation, projected continuation of temperature increases during the 21st century indicate the potential for enhanced drought occurrence. We analyze changes in future drought occurrence using soil moisture data for the SRES B1, A1B and A2 future climate scenarios from eight AOGCMs that participated in the IPCC AR4. The models show decreases in soil moisture globally for all scenarios with a corresponding doubling of the spatial extent of severe soil moisture deficits and frequency of short-term (4-6 month duration) droughts from the mid 20th century to the end of the 21st. Long-term (more than 12 month duration) droughts become three times more common. Regionally, the Mediterranean, west African, central Asian and central American regions show large increases most notably for long term frequencies as do mid latitude North American regions but with larger variation between scenarios. In general, changes under the higher emission scenarios, A1B and A2 are the greatest, and despite following a reduced emissions pathway relative to the present day, the B1 scenario shows smaller but still substantial increases in drought, globally and for most regions. Increases in drought are driven primarily by reductions in precipitation with increased evaporation from higher temperatures modulating the changes. In some regions, increases in precipitation are offset by increased evaporation.