Fewer April Showers for U.S. Southwest as Climate Changes

Things could get uglier for desert flowers looking to bloom in May--and for the region's water supply, year-round

by David Biell, Scientific American, August 20, 2008

STORM TRACKER: The jet stream that carries moisture from Pacific storms over the U.S. has shifted north in recent decades, making the arid Southwest even drier. COURTESY OF STEPHANIE MCAFEE/UNIVERSITY OF ARIZONA/2008
The already parched U.S. Southwest is drying up even more, at least in early spring, because of climate change. A new study in Geophysical Research Letters shows that since 1978, the jet stream that brings rainstorms from the Pacific over the western U.S. has been shifting northward—and so has the rain and snow.

"That northward shift in the storm track is tied to reduced early spring precipitation, especially over the southwest U.S.," says atmospheric scientist Stephanie McAfee of the University of Arizona (U.A.) in Tucson, who led the research identifying the loss of a few storms per season. "It looks like the northern Great Plains seem to get a little bit more rain."

The total amount is only a fraction of an inch, McAfee notes, drawing on precipitation and storm data from the U.S. National Oceanic and Atmospheric Adminstration. But this is lengthening "the dry season in parts of the country that are already quite arid," she says.

Greenhouse gases emitted by burning fossil fuels are piling up in the atmosphere, warming its lowest layer. At the same time, human-produced chemicals have eaten holes in the ozone layer over the poles, ultimately cooling the uppermost part of the atmosphere. This temperature differential creates pressure differences that have been shifting the powerful high-altitude winds known as jet streams in both the Northern and Southern hemispheres closer to the poles since the 1980s. "It's man-made either way you look at it," says U.A. biogeochemist Joellen Russell, senior author of the study.

Climate change has arrived in the Southwest in the form of an earlier seasonal shift to summerlike weather, bringing an end to April showers in the lowlands and spring snowfalls in the mountains. "The change so far in total precipitation is pretty small," Russell notes. But "we expect that this may continue or even get worse."

The Southwest is already suffering through an extended drought that has lowered water levels in Lake Mead and threatens agriculture. McAfee says the climatic shift may also decrease the mountain snowpack that provides water for cities as well as dry out soils, thereby starting fire season earlier in the year, although she has no data on those phenomena yet. "People are going to need to change a little bit in their expectations of what a season is going to be like," adds McAfee, who is now looking at how this is impacting area vegetation. "If we keep doing this, the climate response becomes more extreme."

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