IS CLIMATE DRYING OR IS DROUGHT INCREASING?

Less available water, perhaps due to both increased demand and reduced supply, will have severe societal consequences. What will be the character of future climate as it concerns such water availability? The threat to sustainability posed by a climate punctuated by increasingly severe chronic water shortages (droughts) is great. This would be true even if long-term average precipitation did not change. A climate system undergoing gradual, measured drying would be arguably less undesirable than one experiencing more drastic variations.

It is therefore with more than scientific interest that the climate community addresses the character of hydrologic responses to anthropogenic forcing. The capacity to do so owes to an increased realism of climate models used to predict climate change. An expectation for a more intense hydrologic cycle was already voiced in the 1995 Second Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Among the robust findings from experiments with the improved models of the 2001 Third Assessment Report was the likelihood for increased summer drying and associated drought risk over midlatitude continent interiors.

However, these early reports fail to quantify the changing character of droughts. Rather, droughts were seen as increasing in intensity and/or frequency as a simple consequence of regional drying and warming trends. Whether severe events such as the 1930s Dust Bowl would recur with greater frequency was not addressed. Using the new suite of 21st Century simulations now available as part of the Fourth Assessment Report, this talk diagnoses the statistics of 21st Century U.S. drought. We compute an objective drought measure, the so-called Palmer Drought Severity Index (PDSI) from the monthly statistics of model simulations. The changed frequency of various PDSI threshold exceedences (e.g., moderate, severe, extreme) are presented, as is a diagnosis of drought duration. Finally, the separate effects of anthropogenic changes in temperature and precipitation are discussed, and trends in mean climate are compared with changing statistics of drought.