The driest periods of the last century -- the Dust Bowl of the 1930s and the droughts of the 1950s -- may become the norm in the Southwest United States within decades because of global warming, according to a study released Thursday.
The research suggests that the transformation may already be underway. 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 dry 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 research scientist at Lamont-Doherty Earth Observatory at Columbia University and the lead author of the study, said the changes would force an adjustment to 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 the projections, based on 19 computer models, showed 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 effect of global warming is the subject of a United Nations report to be released today in Brussels, the second of four installments being unveiled this year.
The first report 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 was real to what could be done about it.
The mechanics and patterns of drought in the Southwest have been the focus of increased scrutiny in recent years.
During the last period of significant, prolonged drought -- the Medieval Climate Optimum from about the years 900 to 1300 -- the region experienced dry periods that lasted as long as 20 years, scientists say.
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 Nino and La Nina.
The most significant in terms of drought is La Nina. During La Nina years, precipitation belts shift north, 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. 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.
The computer models, on average, found about a 15% decline in surface moisture -- which is calculated by subtracting evaporation from precipitation -- from 2021 to 2040, as compared with the average from 1950 to 2000.
A 15% 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 was unlikely to make up for losses from a shifting climate.
Kelly Redmond, deputy director of the Western Regional Climate Center in Reno, who was not involved in the study, said he thought the region would still have periodic wet years that were part of the natural climate variation.
But, he added, "In the future we may see fewer such very wet years."
Although the computer models show the drying has already started, they are not accurate enough to know whether the drought is the result of global warming or a natural variation.
"It's really hard to tell," said Connie Woodhouse, a paleoclimatologist at the University of Arizona. "It may well be one of the first events we can attribute to global warming."
The U.S. and southern Europe will be better prepared to deal with frequent drought than most African nations.
For the U.S., the biggest problem would be water shortages. The seven Colorado River Basin states -- Colorado, Wyoming, Utah, Nevada, New Mexico, Arizona and California -- would battle each other for diminished river flows.
Mexico, which has a share of the Colorado River under a 1944 treaty and has complained of U.S. diversions in the past, would join the struggle.
Inevitably, water would be reallocated from agriculture, which uses most of the West's supply, to urban users, drying up farms. California would come under pressure to build desalination plants on the coast, despite environmental concerns.
"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.
"If there's not enough water to meet everybody's allocation, how do you divide it up?"
Officials from seven states recently forged an agreement on the current drought, which has left the Colorado River's big reservoirs -- Lake Powell and Lake Mead -- about half-empty. Without some very wet years, federal water managers say, Lake Mead may never refill.
In the next couple of years, water deliveries may have to be reduced to Arizona and Nevada, whose water rights are second to California.