Is a Transition to Semipermanent Drought Conditions Imminent in the US Great Plains?

How Great Plains climate will respond under global warming continues to be a key unresolved question. There has been, for instance, considerable speculation that the Great Plains is embarking upon a period of increasing drought frequency and intensity that will lead to a semipermanent Dust Bowl in the coming decades. This view draws on a single line of inference of how climate change may affect surface water balance based on sensitivity of the Palmer drought severity index (PDSI). A different view foresees a more modest climate change impact on Great Plains surface moisture balances. This draws on direct lines of analysis using land surface models to predict runoff and soil moisture, the results of which do not reveal an ominous fate for the Great Plains. The authors' study presents a parallel diagnosis of projected changes in drought as inferred from PDSI and soil moisture indicators in order to understand causes for such a disparity and to shed light on the uncertainties. PDSI is shown to be an excellent proxy indicator for Great Plains soil moisture in the twentieth century; however, its suitability breaks down in the twenty-first century, with the PDSI severely overstating surface water imbalances and implied agricultural stresses. Several lines of evidence and physical considerations indicate that simplifying assumptions regarding temperature effects on water balances, especially concerning evapotranspiration in Palmer's formulation, compromise its suitability as drought indicator in a warming climate. The authors conclude that projections of acute and chronic PDSI decline in the twenty-first century are likely an exaggerated indicator for future Great Plains drought severity.