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 analyze changes in 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 future 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. Although the predicted future changes in drought occurrence are essentially monotonic increasing globally and in many regions, they are generally not statistically different from natural variability for multiple decades, in contrast to primary climate variables, such as global mean surface air temperature and precipitation. On the other hand, changes in annual and seasonal means of terrestrial hydrologic variables, such as evaporation and soil moisture, are essentially undetectable within the 21st century. Changes in the extremes of climate and their hydrological impacts may therefore be more detectable than changes in their means.

Last Updated: 2007-07-26