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How reliable are drought predictions? Study finds flaw in popular tool.

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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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