How reliable are drought predictions? Study finds flaw in popular tool.
Researchers say the Palmer Drought Severity Index, devised for monitoring short-term trends, has been misused for longer term analyses and is thrown off by higher temperatures from global warming.
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One example of the disconnect comes from the last major set of climate reports from the Intergovernmental Panel on Climate Change (IPCC), published in 2007. One study that factored heavily in the drought discussion in the volume that focused on the science of climate change showed widespread drying since 1970, with the extent of very dry areas around the world more than doubling – a conclusion that relied on the Palmer Drought Severity Index.Skip to next paragraph
In Pictures Dry Land: Drought in the US
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But in last year's volume on weather extremes, which builds on more-recent research, the IPCC authors noted that "there are still large uncertainties" in discerning global trends in observed droughts. They placed "medium confidence" that some of regions of the world were experiencing longer, stronger droughts. For other other regions however, including central North America, droughts were trending toward less frequent, less intense, or shorter.
In some ways the study represents a window on the sociology of science. Many researchers have concluded that the index has little or no value as a climate-change projection tool. Yet some still cling to it.
"There's an awful lot of inertia" in science, says Dr. Seager, who is not a member of the team that produced the PSDI analysis. "We often sit around and wonder: Why are people still doing this?"
The relatively recent availability of the right kind of data and doubts about its quality are factors. But so is science's general tendency to move slowly, he adds.
Dr. Sheffield and colleagues traced the problem with PDSI to the way it handles evaporation rates. Typically, the PDSI has used precipitation and temperature as the key drivers governing evaporation. But researchers have long known that other factors affect evaporation rates as well. These include relative humidity, the amount of the sun's energy reaching the surface, and wind speed.
When the team calculated the PDSI using the temperature-focused approach for evaporation and used the results to identify drought trends, the results indicated that between 1950 and 2008, dryness increased for 98 percent of the globe's land area. When the team used the more sophisticated approach to calculating evaporation, the picture was more mixed: 52 percent of the land area saw increased drying, while 42 percent saw a decrease.
Overall, both approaches show a drying trend, the team reports, but the increase in areas covered by drought using the more complete treatment of evaporation is seven times smaller than the more-simple approach. Focusing in the last decade, the more-complete calculation suggests that the global area covered by drought has increased slightly, although the increase isn't statistically significant, while soil moisture averaged over the globe essentially has remained unchanged.
While the study shows the difference the treatment of evaporation can make in the outcome, the more complete treatment doesn't necessarily redeem the use of the PDSI.
The Palmer Drought Severity Index "should not be used for climate-change projections," Seager says flatly.
Looking forward to the next set of major IPCC reports on climate, due out over 12 months starting next September, Wood says, "I think you will see work coming out of the community that will use appropriate methods" for projecting future drought patterns.
IN PICTURES: Drought in the USA