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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.

By Staff writer / November 15, 2012

A central Illinois farmer cultivates his cornfield last month, in Savoy Ill. A popular drought prediction tool has led some studies to overstate a trend over the past several decades toward more frequent, intense droughts, according to a new analysis.

Seth Perlman/AP/File

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The inappropriate use of a metric for measuring drought has led some studies to overstate a trend over the past several decades toward more frequent, intense droughts, according to a new analysis. The trend has been attributed in no small part to global warming.

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The authors of the paper caution that the metric's continued use is likely to overstate future effects as well, making it more difficult for water resource managers and others to get reasonable projections of future drought hazards as the climate warms.

The study does not make those projections. Nor does it focus on assessing recent or current droughts, such as this year's drought in the US – the most extensive and severe in at least 25 years, according to the US Department of Agriculture. Instead, the study is a cautionary tale about using the right tool for the right job.

IN PICTURES: Drought in the USA

More broadly, and consistent with a wide range of other research, "our work on 21st-century drought [shows] that there are regions of the world that are going to experience more drought" as global average temperatures continue to rise during this century, says Eric Wood, a professor of civil and environmental engineering at Princeton University in Princeton, N.J., and a co-author of the study, which appears in Thursday's issue of the journal Nature.

But the key questions for resource managers and their political overseers in assessing future drought risk, he continues, center on the extent, intensity, and frequency of drought. Some drought projections, which use the approach the new study challenges, yield results where "you'd think it will never rain again," Dr. Wood says.

At issue is a measure for gauging drought, known as the Palmer Drought Severity Index (PDSI). Its most publicly recognizable form is found in drought maps based on the index – with their yellow, tan, red, and brown splotches signifying the severity of the drought in different parts of the country. These maps have appeared on countless newspaper and web pages over the past two years of widespread drought in the United States.

The problem, according to the research team, led by Princeton's Justin Sheffield, lies in the index's original design. It was developed to track drought conditions from one week to the next, or one season to the next, with a particular focus on croplands in the center of the United States. It was not designed to track trends globally over decades or centuries. Yet some researchers continue to used it for that purpose, Dr. Wood says. The reasons range from the index's relative simplicity to a lack of confidence in the quality of more-recently available data that allow for more-sophisticated calculations.

It's an issue not lost on many in the wider community of researchers focusing on drought and global warming.

"We've known for quite a long time that the PDSI calculation is prone to problems dealing with climate change," says Richard Seager, a researcher who focuses on drought and climate change at Columbia University's Lamont-Doherty Earth Observatory, located in Palisades, N.Y. "Rising temperatures drive it haywire."

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