A 60-year drought that scorched the Southwest during the 12th century may be a harbinger of things to come as greenhouse gases warm the Earth, according to research published yesterday in the Proceedings of the National Academy of Sciences.

The study's authors used tree rings to reconstruct a portrait of droughts that struck the Southwest over a 1,200-year period stretching back to 900 A.D. They believe that understanding the droughts of the past could help water managers plan for future dry periods that are expected to become more intense as climate change worsens.

Portions of the Southwest have suffered prolonged drought since 2001. But the medieval drought, which peaked along the Colorado River between 1146 and 1155, stands as the worst drought in the region for at least 1,200 years, according to the tree ring records.

Still, there are similarities to present-day conditions. The medieval drought occurred during a period from 900 to 1300 A.D. when the Southwest was about 1 degree Celsius warmer than average. Temperatures in the Southwest have been more than 1 degree Celsius warmer than average since 1990, and climate models suggest greater warming by the end of the century.

It's not a perfect comparison, said the study's lead author, University of Arizona paleoclimatologist Connie Woodhouse. Temperatures are warmer now than they were in the 12th century, when it was drier than it is today. But Woodhouse says the medieval scorcher represents a "conservative" worst-case scenario for future Southwest droughts, and the region's water managers should take heed.

"There's no reason to believe that we won't have a drought like that in the future," she said, "but we're going to have warming on top of that."

A region where the cost of inaction will be 'particularly high'

Woodhouse's work is one of several papers on climate change and drought in the Southwest published yesterday by PNAS. Together they sketch a portrait of a region where a combination of climate change and explosive population growth could bring about a water crisis in coming decades.

"Because climate warming will exacerbate water sustainability problems, the Southwest is likely to experience some of the highest economic expenses and environmental losses," said Glen MacDonald, director of the University of California, Los Angeles' Institute of the Environment, and an author of one of the new papers. "The ultimate costs of inaction in curbing greenhouse gas emissions will be particularly high for the Southwest."

Explosive population growth over the past century has pushed the Southwest's relatively meager water supply to unsustainable levels of use, the PNAS studies conclude.

The region's population grew from 2.1 million to more than 50 million during the 20th century. The U.S. Census Bureau estimates that, by 2030, the Southwest will be home to more than 67 million people. Within 50 to 100 years, the current population could double.

Even without the added pressure of climate change, that would stress the region's water system, said John Sabo, a senior fellow at Arizona State University's Global Institute of Sustainability, who served as lead author for one of the new studies.

His work shows the Southwest currently uses 76 percent of its surface water, a number that could rise to 86 percent when the region's population doubles. That's not enough to support the Southwest's growing communities and agriculture sector while also leaving enough water in its rivers to support healthy ecosystems.

"Part of the challenge we face in the Southwest is old-style thinking," said Peter Gleick, president of the Pacific Institute and an author of another analysis. "We brought to the Southwest very European ideas about water, developed in water-rich areas. ... That worked OK for a while, although not really. But now it's clear that green lawns and unlimited swimming pools and inefficient irrigated agriculture can't be sustained."

Gleick says the solution lies in a combination of encouraging the development of untraditional water sources, such as reclaimed wastewater, policies to encourage more efficient water use, efforts to coordinate water policy at local, state and federal levels, and planning to help water utilities adapt to climate change.

Best-case scenario 'is not too good'

"I don't consider the Southwest unique," he said. "I consider them the first dying canary in the coal mine. ... There is more and more evidence that climate changes are going to be felt in the Southwest early and deeply."

Still, it's not yet clear whether climate change is a factor in the current drought, scientists said.

"A lot of what we're seeing is caused by natural variability," said Richard Seager, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory. "There is this background steady drying of the region that is occurring due to rising greenhouse gases, and variability is moving around that."

Over the past three decades, a trend toward more frequent La Niña weather patterns has helped drive Southwestern drought, Seager said, by influencing sea surface temperatures in the tropical Pacific Ocean.

Whether that trend will continue in coming decades isn't clear. If the pattern switches to favor El Niño systems, which tend to send more storms through the Southwest, droughts could be gentler -- at first.

"In the worst-case scenario, it looks to me like models are going over more towards permanent drought-like conditions by the early part of the middle of this century," Seager said. If an El Niño trend emerges, "it just takes longer to get there," the scientist said of the drought. "They all get drier, so even the best-case scenario is not too good."

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