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May 2007
Study predicts permanent
drought in Southwest
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 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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