In setting the stage for their intriguing paper on one of the most devastating extreme types of weather, Sheffield et al. (2012) note that drought is expected by many “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,” citing Sheffield and Wood (2008), Dai (2010) and Seneviratne et al. (2012), while noting that some people feel “this may already be happening globally.”
So is it happening? … and is it happening globally?
The three researchers state that previous assessments of historic changes in drought over the late twentieth and early twenty-first centuries indicate that what some have suggested in this regard may indeed “be happening globally,” based on calculations of the Palmer Drought Severity Index (PDSI) performed by Dai et al. (2004) and Briffa et al. (2009). However, they appear to have a problem with this suggestion.
Sheffield et al. explain themselves by first noting that “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 they say that it “may give biased results in the context of climate change,” citing the work of Roderick et al. (2009) and going on to show that “the previously reported increase in global drought is overestimated because the PDSI uses a simplified model of potential evaporation that responds only to changes in temperature and thus responds incorrectly to global warming in recent decades.”
Performing what they describe as “more realistic calculations, based on the underlying physical principles that take into account changes in available energy, humidity and wind speed,” they thus derive results that suggest “there has been little change in drought over the past 60 years,” which may help, in their words, “to explain why palaeoclimate drought reconstructions based on tree-ring data diverge from the PDSI-based drought record in recent years,” as revealed by the work of Fang et al. (2009) and de Grandpre et al. (2011).
Therefore, even in a situation where it had been thought to have been demonstrated that global warming promotes global drought, such is now known not to be correct.