- The five-year drought parching much of the western United
States could continue for years and may be just the beginning of a long
era of drier times due to global warming, a new study suggests.
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- "Global warming has already warmed the tropical
oceans and could be contributing to the current drought in the West,"
said Edward Cook of the Lamont-Doherty Earth Observatory in Palisades,
New York, who led the research.
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- While the current drought is unprecedented in the last
century, much longer and more severe droughts occurred hundreds of years
ago, according to the study, published Thursday on Science Express.
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- Using tree-ring data -- the width of some trees' growth
rings varies with moisture availability -- from trees that lived hundreds
of years ago, scientists discovered an approximately 400-year-long period
of dry conditions in the West, spanning the years 900 to 1300. This time
span roughly corresponds to the Medieval Warm Period, a well-documented
era when the global climate was somewhat warmer due to increased solar
activity, Cook said.
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- Cook, who avoids making any strong statements regarding
his findings, uses the term global warming very carefully because there
is "lots of distortion of information about global warming,"
he said.
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- This is the first detailed reconstruction of the climate
of the western United States during the Medieval Warm Period. While it's
important to know that long-lasting droughts have happened in the past,
the big questions are how the climate got locked into a 400-year-long drought
and whether something similar could happen again, he said.
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- If droughts are a natural response to a warmer climate,
the study said, "then any trend toward warmer temperatures in the
future could lead to a serious long-term increase in aridity over western
North America."
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- Evidence exists that droughts between 1998 and 2002 were
linked to warmer tropical oceans. When data from those years and the new
tree-ring data are plugged into computerized climate models, the resulting
simulations show that warmer oceans and other climatic factors 1,100 year
ago "are surprisingly similar to what's happening currently,"
Cook said.
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- "This isn't predictive," he added. "The
drought could go for another five years or it could end tomorrow."
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- However, many computer climate models used to predict
the impacts of global warming suggest that as greenhouse-gas emissions
increase, the interior of North America will get progressively drier during
the summer due to warmer temperatures in the region.
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- If these and Cook's results hold up, then the 400-year
drought of the past "might truly be a harbinger of things to come
in the West," the study concludes.
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