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May 2007

Study predicts permanent drought in Southwest

drought

The swath of North America between Kansas, California, and northern Mexico can expect long-term drought conditions in the future due to warming global temperatures. The Southwest is already stressed from a drought that has affected the region since 1999. Here, an arroyo in northern Mexico sits dry. (Photo by Dave Gochis.)

ASP postdoc Jian Lu contributes to research

Aridity has always been the defining feature of the American Southwest, even as large-scale hydraulic engineering has allowed cities such as Phoenix and Las Vegas to burst from the desert floor.

But according to a sobering new study, the Southwest’s aridity is about to get worse. Published in the April 9 issue of Science, “Model Projections of an Imminent Transition to a More Arid Climate in Southwestern North America” predicts that climate change will permanently alter the landscape of the Southwest so severely that conditions reminiscent of the Dust Bowl days of the 1930s could become the norm within a few decades.

“Our study suggests a perpetual arid condition over the American Southwest,” says Jian Lu, a postdoctoral researcher in ASP/CGD who is an author of the study.

Of the 19 different computer models that the research team used for the study, all but one showed a drying trend in the swath of North America between Kansas, California, and northern Mexico. The models predicted an average 15% decline in runoff for the Southwest between 2021 and 2040, compared to the average surface moisture between 1950 and 2000.

The Southwest’s future droughts are expected to be of a different nature than those that have afflicted the region in the past. Scientists attribute past droughts to variations in sea surface temperatures caused by El Niño and La Niña events in the Pacific Ocean. La Niña is especially influential as it tends to shift precipitation belts north, leaving the Southwest thirsty.

Jian

Jian Lu.

As the climate warms, however, the basic dynamics of the atmosphere change, particularly in regard to the Hadley cell, a powerful circulation pattern that drives weather in the tropics and subtropics. “Our confidence in our projection is built upon our understanding of the fundamental dynamics of the Hadley cell,” Jian says.

Warm, moist air from near the equator normally rises into the atmosphere until it reaches the stratosphere, the second layer of Earth’s atmosphere. The air then spreads north and south toward the poles, descends over the subtropics, and flows back toward the Equator in the form of trade winds, completing the cell. Because the descending air over the subtropics suppresses rain by drying the lower atmosphere, many of Earth’s great deserts are located in these regions.

As the atmosphere warms from climate change, scientists expect the Hadley cell to expand its reach, bringing hot, dry air to a larger swath of the Middle East, Mediterranean, and North America, including the Southwest. “In the future warmed climate,” Jian explains, “the Hadley cell and the subtropical high should expand poleward, which tends to block rain coming through from the Pacific.”

For the study, the research team assumed that greenhouse gases would continue to rise from today’s level of 380 parts per million until beginning a decline around 2050, measuring 720 parts per million in 2100.

Jian stresses that it’s difficult to predict the onset or magnitude of the drought. “The timing of the drought is very uncertain, and given the uncertainties in the model physics and sizable spread across different models, we are not very sure about the magnitude of drought in the future,” he says.

The Southwest is one of the fastest-growing regions in the country. Increased aridity would put enormous strain on the Colorado River, a lifeline for the seven states in its basin (Colorado, Utah, Wyoming, Nevada, New Mexico, Arizona, and California) and northern Mexico.

Already, the region is experiencing changes that scientists link to climate change, including more severe wildfires, earlier winter snowmelt, the destruction of heat-weakened trees by beetles, and a loss of biodiversity in southern Arizona’s high-elevation “sky islands.”


In this issue...

Study predicts permanent drought in Southwest

NCAR scientists contribute to climate change assessments

Random profile: Karla Edwards

What is the color of space?

Center Green Idol

A Wirth-while talk

Remembering Jeanne Adams

PACDEX Update

Just One Look


 

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