December 14, 2010

US Southwest Could See Decades-long Drought: Study

An unprecedented, decades-long combination of heat and drought could be headed to the Southwest United States sometime this century, according to a new study by researchers at the University of Arizona.

The scientists reviewed previous studies of temperature changes and droughts in the region over the past 1,200 years, and concluded that a 60-year drought similar to the one that occurred during the 12th Century could be in our future.

"Major 20th century droughts pale in comparison to droughts documented in paleoclimatic records over the past two millennia," the researchers wrote, referring to the elevated temperatures combined with lengthy and widespread droughts that occurred during the Medieval period.

By determining the dates and duration of coinciding drought and warm temperatures in the past, the researchers identified plausible worst-case scenarios for the future. 

Such scenarios can often help water and other resource managers plan for the future, the scientists said.

"We're not saying future droughts will be worse than what we see in the paleo record, but we are saying they could be as bad," said Connie Woodhouse, a UA associate professor of geography and regional development and the study's lead author.

"However, the effects of such a worst-case drought, were it to recur in the future, would be greatly intensified by even warmer temperatures."

There have been several periods of intense, sustained drought that affected much of western North America over the past 2,000 years.

David Meko, the study's co-author and an associate research professor in the UA's Laboratory of Tree-Ring Research, noted that droughts that are accompanied by warm temperatures have more severe impacts on ecosystems.

During the Medieval period, temperatures were about 1.8 degrees Fahrenheit (1 C) above the long-term average.

However, average temperatures in the Southwest U.S. have been warmer than that since 1990, and are projected to rise at least another 3.6 F (2 C) by 2100, Woodhouse added.

The most severe warm-climate drought in the Southwest during the past 1,200 years occurred during the mid-12th century and lasted 60 years, covering most of the modern day western U.S. and northern Mexico.

For 25 years during that drought, the Colorado River flow averaged 15 percent below normal, the researchers said. 

The Colorado River supplies water for agriculture and cities, including Los Angeles, Las Vegas, Denver, Phoenix, Tucson and Albuquerque, in seven western U.S. states and two states in northwestern Mexico.

Over the past decade, sampling shows that the river is at its lowest point since 1906, when records first began being kept.

"As this drought unfolds you can't really evaluate it until you're looking back in time," said Woodhouse.

In the future, the Colorado River flow is projected to decrease between two and eight percent for every 1.8 degree Fahrenheit (1 C) of warming, wrote Woodhouse and her colleagues.

"Even without warming, if you had one of those medieval droughts now, the impact would be devastating," she said.

"Our water systems are not built to sustain us through that length of drought."

"The bottom line is, we could have a Medieval-style drought with even warmer temperatures."

The study is part of a special feature entitled "Climate Change and Water in Southwestern North America", which appears in the December 13 online edition of the Proceedings of the National Academy of Sciences.

The paper by Woodhouse and her colleagues is entitled "A 1,200-year perspective of 21st century drought in the southwestern North America."  Co-authors include Glen MacDonald of the University of California, Los Angeles, Dave Stahle of the University of Arkansas in Fayetteville and Edward Cook of Lamont-Doherty Earth Observatory of Columbia University.


Image 1: Annual tree rings record a detailed history of drought (narrow rings) and wetness (wide rings). This sample from a dead Douglas-fir tree in the Santa Catalina Mountains near Tucson, Ariz., has nearly 400 rings and dates back to the year 1600. Stress cracks, visible in the foreground of the image, occur as the dead wood dries and contracts. Credit: Copyright Daniel Griffin

Image 2: A core extracted from a living Douglas-fir tree in the Santa Rita Mountains south of Tucson, Ariz. Scientists use such cores to study the annual rings of trees, visible on the core as banding. Collecting such cores causes only temporary injury to the tree. Credit: Copyright 2009 Daniel Griffin


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