15 November 2012

Little Change in Drought Over 60 Years

UPDATE: Courtesy @jfleck a pointer to another new paper from Hoerling et al. in JOC: "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."

A new paper out in the current issue of Nature finds little evidence to support claims that drought has increased globally over the past 60 years. The authors write:
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 warming1–3. 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 warming4,5. 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 change6. 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.
What does this mean?

For one, it means that a widely accepted and oft-repeated consensus position expressed in the IPCC 2007 now appears to have been incorrect. This should not be unexpected as a consensus position is a snapshot of perspectives, and in science, perspectives can change based on new evidence and study. The IPCC SREX, published earlier this year had already stepped back from the conclusions of the IPCC AR4.

A second important conclusion from this paper is that we simply don't know is drought has become worse over the past 60 years. This places drought into a category with tropical cyclones, floods, tornadoes and other phenomena where the evidence does not support claims that things are progressively getting worse -- with more frequent and intense extreme events on climate time scales. Once again the lesson is that if you are looking for a signal of human-caused climate change, it is best not to look at such extremes.

Finally, the paper leads to questions about predictions of future changes in drought. A companion essay in Nature explained:
[The] findings imply that there is no necessary correlation between temperature changes and long-term drought variations, which should warn us against using any simplifications regarding their relationship.
All this said, human-caused climate change remains a reality. However, what is also a reality is that there is very little evidence to support claims that the influence of such changes can be observed in the observational record of extreme events. Advocates who justify action on climate change by appeals to the latest extreme event go well beyond what science can support, and in the process undercut the very cause that they are advocating for.