US megadrought worst in past 1,200 years, researchers say

The megadrought that has gripped the southwestern United States for the past 22 years is the worst over the past 12 centuries, according to a new study.

The research, which suggests that the past two decades in the American Southwest have been the driest period in 1,200 years, pointed to human-caused climate change as a major reason for the current drought's severity.

The findings were published Monday in the journal Nature Climate Change.

Jason Smerdon, one of the study's authors and a climate scientist at Columbia University's Lamont-Doherty Earth Observatory, said global warming has made the megadrought more extreme because it creates a "thirstier" atmosphere that is better able to pull moisture out of forests, vegetation and soil.

"It's a slow-motion train wreck," he said. "What we showed in the paper is that increasing temperatures in the Southwest contributed about 42 percent to the severity of this drought."

Over the past two decades, temperatures in the Southwest were around 1.64 degrees Fahrenheit higher than the average from 1950 to 1999, according to the researchers. Globally, the world has warmed by about 2 degrees Fahrenheit since the late 1800s.

In the study, Smerdon and his colleagues analyzed tree rings to piece together water availability throughout history, using data that stretches back to the year 800 A.D. The scientists focused on a swath of North America from southern Montana to northern Mexico, and from the Pacific Ocean east to the Rocky Mountains.

Tree rings are one way that scientists can estimate soil moisture conditions in past climates. Researchers can build detailed chronologies by looking at the thickness of tree rings, with thinner rings indicating drier years with less moisture, and wider rings signifying wetter years.

The results can then be compared to other paleoclimate signatures, including from sediment samples and archaeological records, as well as observational data throughout history, to forensically fill out a timeline of Earth's climate.

The researchers found that several significant megadroughts have occurred in the region over the past 12 centuries, some even lasting up to 30 years. Before the current megadrought, the region had not experienced such dry conditions since medieval times, in the late 1500s.

But while droughts occur naturally throughout history, climate change is making them both more frequent and more intense, the scientists said. And compared to other megadroughts in the historical record, what's surprising is that this current drought shows no signs of letting up, said A. Park Williams, a climate scientist at the University of California, Los Angeles, and the study's lead author.

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