Researchers examining ancient tree ring records have linked prolonged periods of epic drought in the West with warmer temperatures, suggesting that global warming could promote long-term drought in the interior West.
Analyzing North American tree ring data from the last 1,200 years, the research team found that severe, decades-long droughts settled over the West during the "Medieval Warm Period," a time of unusual warmth in parts of the world.
"Whether increased warmth in the future is due to natural variables or greenhouse [gases], it doesn't matter," said Edward R. Cook of Columbia University's Lamont-Doherty Earth Observatory, the lead author of a study published Thursday in the online edition of the journal Science. "If the world continues to warm, one has to worry we could be going into a period of increased drought in the western U.S. I'm not predicting that. [But] the data suggests that we need to be concerned about this."
The study, in which Cook was joined by scientists from the National Oceanic and Atmospheric Administration as well as the universities of Arizona and Arkansas, maps a 400-year period of recurring mega-droughts that make the West's current five-year dry spell look puny.
"Compared to the earlier 'mega-droughts' that are reconstructed to have occurred around AD 936, 1034, 1150 and 1253, the current drought does not stand out as an extreme event because it has not yet lasted nearly as long," the authors wrote. "This is a disquieting result because future droughts in the West of similar duration to those seen prior to AD 1300 would be disastrous."
Cook called the centuries between 900 and 1300 "the most persistently dry period on record in the last 1,200 years." Large portions of the West were gripped by droughts that lasted two or three decades at a time, dwarfing the current drought that, despite its comparative brevity, has dramatically shrunk reservoirs and raised the possibility of water shortages in the Colorado River Basin.
"I think the impact of the current drought indicates how vulnerable a good part of the West can be," Cook said. "Tack on another five years and I think the scenario is grim."
The research team plotted tree ring data across North America from the last 1,200 years, painting the broadest picture yet of past drought conditions on the continent. Prior reconstructions, the authors said, dealt with smaller areas and shorter time frames.
To back up the tree ring record, the team looked at other data from the same period. Fire scars on sequoias, wildfire-deposited charcoal in ancient lake beds and elevated lake salinity levels all reflected arid conditions in the West during the late Middle Ages.
The dry conditions roughly coincided with a period believed to have been warmer in North America. That ancient coincidence, said co-author and NOAA paleoclimatologist C. Mark Eakin, is in accord with climate modeling that indicates warmer temperatures in the tropical Pacific Ocean have led to the upwelling of cooler waters in the eastern Pacific, causing drier, La Niña conditions.
"So if we see warming in the future, that could lead to the same sort of cooler, eastern Pacific, drier West as we've seen in the past," Eakin said.
But Alan Hamlet, a research scientist with the Climate Impacts Group at the University of Washington, said global warming won't necessarily lead to drier times. "We have high confidence things are getting warmer and will probably continue to get warmer. What is still uncertain is what will happen with precipitation," he said. "I have not seen compelling evidence that just because it gets warmer, it gets drier.
"I don't think the paleo record sheds a lot of light on what's going to happen" under global warming, he added.
The study published in Science is the second released in recent months to suggest that the West had experienced far longer droughts than the current one, which is the most severe in the Colorado River basin since record-keeping began in 1906.
In August, researchers from the University of Nevada and Scripps Institution of Oceanography published a paper that concluded this drought was the seventh worst to hit the Upper Colorado River Basin in the past 500 years.
"The current drought is bad, but it could be worse," they concluded.
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