THE world has been suffering more droughts in recent decades, and climate change will bring many more, according to received wisdom. Now it is being challenged by an analysis that questions a key index on which it is based.
Predictions of megadroughts affecting Africa and the western side of North America may be wrong. We could even be headed for wetter times, says Justin Sheffield of Princeton University.
This potential handbrake turn for climate forecasts hangs on the accuracy of our main measure of drought, the Palmer Drought Severity Index. The Intergovernmental Panel on Climate Change’s 2007 science assessment cited studies using the PDSI to conclude that “droughts have become more common since the 1970s” as the world has warmed – a position we take to be true in this week’s cover story (see “Climate downgrade: Arctic warming“). The report predicted droughts will increase with global warming.
The problem with the PDSI, says Sheffield, is that it does not directly measure drought. Instead, it looks at the difference between precipitation and evaporation. But since evaporation rates are hard to determine, it uses temperature as a proxy, on the assumption that evaporation rises as it gets hotter.
Sheffield points out that temperature is only one factor influencing evaporation. He inferred evaporation rates using the Penman-Monteith equation, which includes factors such as wind speed and humidity, and found “little change in global drought over the past 60 years” (Nature, DOI: 10.1038/nature11575). His new calculations back up his own previous analysis that the most significant of recent droughts mostly occurred in the 1950s and 60s, before global warming got going.
The PDSI was created in the 1960s by US meteorologist Wayne Palmer to help allocate aid to drought-hit farmers, and was then widely adopted by climate scientists for its simplicity. Sheffield says he finds its continued use “a little strange”.
Roger Pielke Jr of the University of Colorado in Boulder says that since the PDSI uses a formula that assumes higher temperatures cause more droughts, it was hardly surprising that it finds a link.
Simon Brown of the UK Met Office in Exeter says Sheffield’s analysis is probably right. “There has been a growing acknowledgement that the PDSI should not be trusted when doing climate change studies,” he says. But one of the lead authors of parts of the 2007 IPCC report, Kevin Trenberth of the US National Center for Atmospheric Research in Boulder, is sceptical. He backs work by Aiguo Dai of the State University of New York, Albany, who reported last year that using the Penman-Monteith equation “only slightly reduces the drying trend”.
Sheffield’s findings raise important questions, says Steve Running at the University of Montana in Missoula. “If global drought is not increasing, if warmer temperatures are accompanied by more rainfall and lower evaporation rates, then a warmer wetter world would [mean] a more benign climate.”