Boulder Scientist Has Grim Drought Forecast For West

Boulder scientist has grim drought forecast for West
Expert says some could be worse than Dust Bowl

http://www.rockymountainnews.com/drm...006901,00.html

By Jim Erickson, Rocky Mountain News
September 20, 2006

LONGMONT - Future Western droughts could last an average of 12 years, spanning half of the region and severely reducing Colorado River flows that supply millions of people, according to climate projections from a Boulder scientist.

Eighteen of the world's most powerful computer climate models were used in the study, presented Tuesday by Martin Hoerling, of the National Oceanic and Atmospheric Administration.

Hoerling said he was startled by his own findings. But he cautioned that his results are preliminary and have not been published in a peer-reviewed scientific journal.

"The numbers are pretty striking," he said Tuesday at a drought conference sponsored by the Geological Society of America. "Climate change is moving us in the direction of a perpetual state that is of the Dust Bowl type."

The models forecast a temperature increase of 5.4 degrees Fahrenheit by 2060 in the interior West, largely because of the buildup of heat-trapping gases emitted by fossil-fuel combustion.

But little or no precipitation increase is foreseen in the West. The result: Increased evaporation and drier soils, leading to more severe and frequent droughts, Hoerling said.

How severe? Some droughts could be 25 percent worse than the 1930s Dust Bowl, Hoerling said.

How frequent? On average, half of the interior West will suffer from severe drought each year, he said.

But several other scientists said that Hoerling's projections should be taken with several grains of salt, even though previous studies have concluded that more frequent and severe droughts are likely in a warming West.

"Past studies have shown smaller changes, not on the order of what Marty is showing," said Brad Udall, director of the Western Water Assessment at the University of Colorado.

"The (climate) models have a lot of problems," Udall said. "They get a lot of things right, and they're really good at the global level. But they're not so good when you try to step them down into the regional level."

Hoerling found that the Upper Colorado River Basin is likely to take the hardest hit from an increase in drought severity and frequency.

Currently, 13.5 million acre feet of Colorado River water are consumed each year. About 9 million acre feet are used by the Lower Colorado River Basin states: Arizona, California and Nevada. About 4.5 million acre feet go to the upper basin states: Colorado, Utah, Wyoming and New Mexico.

The dividing line between the two basins is at Lees Ferry, near the Utah-Arizona border, just below Lake Powell.

Hoerling's computer simulations show that the flow at Lees Ferry could drop to 6 million acre feet by 2050, creating huge problems for water managers trying to meet obligations to the Lower Colorado River Basin.

"The decrease in runoff at Lees Ferry is pretty dramatic," said Connie Woodhouse, NOAA paleoclimatologist. "If he's right, we're going to have to rethink how we use water in this area."

But Woodhouse cautioned that other studies of future Colorado River runoff are in the works, and that preliminary results from one of them show "not as sharp a decrease" in runoff as Hoerling found.

Denver Museum of Nature & Science geologist Bob Raynolds echoed Udall's concerns about the uncertainties of climate models. But he said he was alarmed by Hoerling's findings.

"It's something to be concerned about, but it's not something to be scared of," he said. "We're going to have to adapt our survival strategies to coping with less water."

Last year in the journal Nature, a team led by U.S. Geological Survey hydrologist Christoper Milly used 12 state-of-the-art climate models to study global-warming associated reductions in river runoff worldwide.

Milly and his colleagues found that by 2050, drier conditions could lead to a 10 to 20 percent drop in runoff from rivers in the U.S. West.

Milly said Tuesday that his results appear to jibe with Hoerling's findings.

"I think those drought numbers that he's giving you are probably realistic numbers," Milly said in a telephone interview. "What he's saying does not seem far-fetched."

NOAA researcher Jon Eischeid collaborated with Hoerling on the study.