A permanent drought seen for Southwest

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.