Now there’s one less ‘dirty weather’ claim for the bloviator in chief, Al Gore.
A new paper just published and available for preview for the upcoming issue of Nature demonstrates that hyped claims that drought has increased aren’t founded in science. Plotting the Palmer Drought Severity Index globally over the past 60 years they show little change in drought.
Here’s the abstract:
Drought is expected to increase in frequency and severity in the future as a result of climate change, mainly as a consequence of decreases in regional precipitation but also because of increasing evaporation driven by global warming. Previous assessments of historic changes in drought over the late twentieth and early twenty-first centuries indicate that this may already be happening globally. In particular, calculations of the Palmer Drought Severity Index (PDSI) show a decrease in moisture globally since the 1970s with a commensurate increase in the area in drought that is attributed, in part, to global warming. The simplicity of the PDSI, which is calculated from a simple water-balance model forced by monthly precipitation and temperature data, makes it an attractive tool in large-scale drought assessments, but may give biased results in the context of climate change. Here we show that the previously reported increase in global drought is overestimated because the PDSI uses a simplified model of potential evaporation7 that responds only to changes in temperature and thus responds incorrectly to global warming in recent decades. More realistic calculations, based on the underlying physical principles8 that take into account changes in available energy, humidity and wind speed, suggest that there has been little change in drought over the past 60 years.
Hmmm where have we seen this before?
And here is another:
Hoerling et al. in Journal of Climate: Is a Transition to Semi-Permanent Drought Conditions Imminent in the U.S. Great Plains?
“We conclude that projections of acute and chronic PDSI decline in the 21st Century are likely an exaggerated indicator for future Great Plains drought severity.”
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 semi-permanent Dust Bowl in 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. Our 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 20th Century; however, its suitability breaks down in the 21st 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. We conclude that projections of acute and chronic PDSI decline in the 21st Century are likely an exaggerated indicator for future Great Plains drought severity.
Corresponding author address: M. Hoerling, NOAA/Earth System Research Laboratory