The driest periods of the past century - the Dust Bowl of the 1930s and the droughts of the 1950s - could become the norm in the Southwest United States within decades due to global warming, according to a study released yesterday.
The research suggests the transformation could already be happening. Much of the region has been in a severe drought since 2000, which the study's analysis of computer climate models shows as the beginning of a long drying period.
The study, published online in the journal Science, predicted a permanent drought by 2050 throughout the Southwest - one of the fastest-growing regions in the nation.
The data tell "a story which is pretty darn scary and very strong," said Jonathan Overpeck, a climate researcher at the University of Arizona, who was not involved in the study.
Richard Seager, a climate modeler at Columbia University and the lead author of the study, said the changes will force an adjustment of the social and economic order from Colorado to California.
"There are going to be some tough decisions on how to allocate water," he said. "Is it going to be the cities or is it going to be agriculture?"
Seager said that the projections, based on 19 computer models, show a surprising level of agreement. "There is only one model that does not have a drying trend," he said.
Philip Mote, an atmospheric scientist at the University of Washington, who was not involved in the study, added: "There is a convergence of the models that is very strong and very worrisome."
The future impact of global warming is the subject of a United Nations' report being released today in Brussels, the second of four installments being unveiled this year.
The first installment from the Intergovernmental Panel on Climate Change was released in February. It declared that global warming had become a "runaway train" and that human activities were "very likely" to blame.
The landmark report helped shift the long and rancorous political debate over climate change from whether man-made warming is real to what can be done about it.
The mechanics of drought in the Southwest have been well studied through the years.
During the last period of significant prolonged droughts - the Medieval Climate Optimum from about 900 to 1300 - the region experienced dry periods lasting as long as 20 years, scientists have determined.
Drought research has largely focused on the workings of air currents that arise from variations in sea surface temperature in the Pacific Ocean known as El Niqo and La Niqa.
The most significant in terms of drought is La Niqa. During La Niqa years, precipitation belts shift northward, parching the Southwest.
The latest study investigated the possibility of a broader, global climatic mechanism that could cause drought. Specifically, they looked at the Hadley Cell, one of the planet's most powerful atmospheric circulation patterns, driving weather in the tropics and subtropics.
Within the cell, air rises at the equator, moves toward the poles and descends over the subtropics.
Increasing levels of greenhouse gases, the researchers said, warms the atmosphere, which expands the poleward reach of the Hadley Cell. Bone-dry air, which suppresses precipitation, then descends over a wider expanse of the Mediterranean region, the Middle East and North America.
All of those areas would be similarly affected, though the study examined only the effect on North America, in a swath reaching from Kansas to California and south into Mexico.
The researchers tested a "middle of the road" scenario of future carbon dioxide emissions to predict rainfall and evaporation. They assumed that emissions would rise until 2050 and then decline. The carbon dioxide concentration in the atmosphere would be 720 parts per million in 2100, compared with about 380 parts per million today.
They models, on average, found about a 15 percent decline in surface moisture, which is calculated by subtracting evaporation from precipitation, from 2021 to 2040 as compared to the average from 1950 to 2000.
A 15 percent drop led to the conditions that caused the Dust Bowl in the Great Plains and the northern Rockies during the 1930s.
Even without the circulation changes, global warming intensifies existing patterns of vapor transport, causing dry areas to get drier and wet areas to get wetter. When it rains, it is likely to rain harder, but scientists said that is unlikely to make up for losses from a shifting climate.
Kelly Redmond, deputy director of the Western Regional Climate Center in Reno, Nev., who was not involved in the study, said he believed the region would still experience periodic wet years that are part of the natural climate variation.
But he added: "In the future we may see fewer such very wet years."
While the models show the drying has started, they are not accurate enough to know whether the current drought is the result of global warming or simply a result of natural variation.
"It's really hard to tell," said Connie Woodhouse, a paleo-climatologist at the University of Arizona. "It may well be one of the first events we can attribute to global warming."
The biggest problem in a future of chronic drought would be water shortages. The seven Colorado basin states - Colorado, Wyoming, Utah, Nevada, New Mexico, Arizona, California - would battle each other for diminished Colorado River flows. Mexico, which has a share of the river under a 1944 treaty and has complained of U.S. diversions in the past, would join the conflict.
"This is a situation that is going to cause water wars," said Kevin Trenberth, a scientist at the National Center for Atmospheric Research in Boulder, Colo.
Alan Zarembo and Bettina Boxall write for the Los Angeles Times.