Climate-driven megadrought is emerging in western US, says study

https://www.sciencedaily.com/releases/2020/04/200416151750.htm?fbclid=IwAR3eNo30k2isoHSKsCrWzWLAOl58ioeUGQb2okJUEHUWKg-fPOXs-BDnQ0c

Date:
April 16, 2020
Source:
Earth Institute at Columbia University
Summary:
A new study says a megadrought worse than anything known from recorded history is very likely in progress in the western United States and northern Mexico, and warming climate is playing a key role.

With the western United States and northern Mexico suffering an ever-lengthening string of dry years starting in 2000, scientists have been warning for some time that climate change may be pushing the region toward an extreme long-term drought worse than any in recorded history. A new study says the time has arrived: a megadrought as bad or worse than anything even from known prehistory is very likely in progress, and warming climate is playing a key role. The study, based on modern weather observations, 1,200 years of tree-ring data and dozens of climate models, appears this week in the leading journal Science.

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Reliable modern observations date only to about 1900, but tree rings have allowed scientists to infer yearly soil moisture for centuries before humans began influencing climate. Among other things, previous research has tied catastrophic naturally driven droughts recorded in tree rings to upheavals among indigenous Medieval-era civilizations in the Southwest. The new study is the most up-to-date and comprehensive long-term analysis. It covers an area stretching across nine U.S. states from Oregon and Montana down through California and New Mexico, and part of northern Mexico.

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with global warming proceeding, the authors say that average temperatures since 2000 have been pushed 1.2 degrees C (2.2 F) above what they would have been otherwise. Because hotter air tends to hold more moisture, that moisture is being pulled from the ground. This has intensified drying of soils already starved of precipitation.

All told, the researchers say that rising temperatures are responsible for about half the pace and severity of the current drought. If this overall warming were subtracted from the equation, the current drought would rank as the 11th worst detected -- bad, but nowhere near what it has developed into.

"It doesn't matter if this is exactly the worst drought ever," said coauthor Benjamin Cook, who is affiliated with Lamont and the Goddard Institute for Space Studies. "What matters is that it has been made much worse than it would have been because of climate change." Since temperatures are projected to keep rising, it is likely the drought will continue for the foreseeable future; or fade briefly only to return, say the researchers.

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The effects are palpable. The mighty reservoirs of Lake Mead and Lake Powell along the Colorado River, which supply agriculture around the region, have shrunk dramatically. Insect outbreaks are ravaging dried-out forests. Wildfires in California and across wider areas of the U.S. West are growing in area. While 2019 was a relatively wet year, leading to hope that things might be easing up, early indications show that 2020 is already on a track for resumed aridity.

"There is no reason to believe that the sort of natural variability documented in the paleoclimatic record will not continue into the future, but the difference is that droughts will occur under warmer temperatures," said Connie Woodhouse, a climate scientist at the University of Arizona who was not involved in the study. "These warmer conditions will exacerbate droughts, making them more severe, longer, and more widespread than they would have been otherwise."

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