The 1934 drought was 30 percent more severe than the runner-up drought -- in 1580 -- and extended across 71.6 percent of western North America, according to scientists from NASA and Lamont-Doherty Earth Observatory. Consider that the average extent of the 2012 drought was 59.7 percent.
     The scientists used a tree-ring-based drought record from the years 1000 to 2005 and modern records in their study, which will be published in the Oct. 17 edition of Geophysical Research Letters.
     Two sets of conditions led to the severity and extent of the drought. The first was a winter high-pressure system that hovered over the West Coast, turning away wet weather. Also, in the spring of 1934 rainfall was suppressed by dust storms, the result of poor land management practices.
     “In combination then, these two different phenomena managed to bring almost the entire nation into a drought at that time," said the study’s co-author, Richard Seager, professor at the Lamont-Doherty Earth Observatory of Columbia University in New York. "The fact that it was the worst of the millennium was probably in part because of the human role."
     Climate change is likely to make droughts in North America worse, according to the recent Fifth Assessment Report of the Intergovernmental Panel on Climate Change. The southwest is expected to become drier, as are summers in the central plains.
     The abnormal high-pressure system is one lesson from the past that informs scientists' understanding of the current severe drought in California and the western United States.

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