The driest periods of the last century could become the norm in the Southwest United States within decades due to global warming, according to a new study.

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The driest periods of the last 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 new study.

The research suggests the transformation could be happening already. 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 Thursday 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.

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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?”

Assessing global warming

The future impact of global warming is the subject of a United Nations report being released today in Brussels, Belgium. It is the second installment of four 2007 reports on warming’s region-by-region impact.

The document is the product of the Intergovernmental Panel on Climate Change (IPCC), a U.N. network of 2,000 scientists as authors and reviewers, along with representatives of more than 120 governments.

The first 2007 report from the IPCC was released in February. It declared that global warming had become a “runaway train” and that human activities were “very likely” to blame.

That landmark report helped shift the long and rancorous political debate over climate change, from whether human-caused warming is real to what can be done about it.

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Seager said the projections, based on 19 separate computer models, show a surprising level of agreement regarding the reduction in rainfall. “There is only one model that does not have a drying trend,” he said. The models used data dating back to 1860 and projected into the future.

Philip Mote, an atmospheric scientist at the University of Washington, who was not involved in the study, said: “There is a convergence of the models that is very strong and very worrisome.”

The finding aligns with past studies that suggest “the Southwest is Ground Zero for a drying effect, and that is a critical issue for the Colorado River,” said University of Washington hydrologist Dennis Lettenmaier, who also was not a member of the study group.

While much of the nation west of the Mississippi River is likely to get drier because of the buildup of greenhouse gases in the atmosphere, the greatest effect will be experienced in the arid areas on both sides of the U.S.-Mexico border. By the end of the century, the climate researchers predicted, rainfall in that region will have declined by 10 to 20 percent annually.

Water fight

For the United States, 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 and 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.

Inevitably, water be would reallocated from agriculture, which uses most of the West’s supplies, 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 about how to handle the current drought, which has left the Colorado River’s big reservoirs — Lake Powell and Lake Mead — roughly half-empty. Without some very wet years, federal water managers say Lake Mead may never completely refill.

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A 15 percent drop led to the conditions that caused the Dust Bowl in the Great Plains and the northern Rockies during the 1930s, resulting in one of the nation’s largest mass migrations as farmers abandoned their parched land and moved away in search of jobs.

“Dramatic” changes

Previously, much of the concern in the Southwest has involved decreasing snowpack, said atmospheric scientist Steven Ghan, of the Pacific Northwest National Laboratory in Richland, Wash. “But this is news because it suggests a shift in the precipitation itself.”

A similar drying out of the “subtropical” belt above and below the equator will hit the Mediterranean region and parts of Africa, South America and South Asia, the report says, as the overall warming of the oceans and surface air transforms basic wind and precipitation patterns around the Earth.

The prediction of a drier Southwest was made by climate computer models assembled by the Intergovernmental Panel on Climate Change (IPCC), the international scientific effort to assess the impact of global warming, which is releasing a new report today.

The drought results were analyzed separately in the Science report. That analysis also predicted regions outside the drying belt will get more rain.

“It’s a situation of the poor getting poorer and the rich getting richer when it comes to rainfall,” said Yochanan Kushnir, of the Lamont-Doherty Earth Observatory at Columbia University, and one of the paper’s authors. “From a climate perspective, these changes are quite dramatic.”

He said the authors of the new paper had a very high level of confidence that the droughts will develop and that they will be the result of increases in atmospheric greenhouse gases created through burning fossil fuels and other human activities.

The researchers said future droughts in the affected regions will be different from those in the past, which were caused by local weather conditions and the effects of El Niño and La Niña ocean-temperature variations.

As the planet warms, the researchers said, basic climate dynamics will change. Currently, hot air from the equatorial tropics rises eight to 12 miles until it hits the stratosphere and is blocked, and spreads to the north and south and remains aloft until it passes 10 to 30 degrees latitude before cooling and descending again.

The computer models show that with more carbon dioxide and other greenhouse gases making the planet hotter, the area where the hot air remains aloft — and suppresses rainfall — will widen. Dry areas will become drier, and the arid areas will expand.

Water woes

The prospect of a drier Southwest is particularly troublesome because the region has some of the nation’s fastest-growing cities, including Las Vegas and Phoenix.

Seager, also from Lamont-Doherty, said the region will have to rethink how it uses the available water. Governments “need to plan for this right now, coming up with new, well-informed and fair deals for allocation of declining water resources,” he said.

Most water in the Southwest is now used in agriculture, but the urban population of the region is growing and so the water needs of people are growing too, he said.

“So, in a case where there is a reduced water supply, there will have to be some reallocation between the users,” Seager said. “The water available is already fully allocated.”

He said adjustments can be made to deal with the change, perhaps by withdrawing some land from production and by conserving water in urban areas.

“But it’s something that needs to be planned for,” Seager said. “It’s time to start thinking how to deal with that.”

Uncertainty remains

The climate models generally assumed a gradually increasing amount of carbon dioxide in the atmosphere until 2050, at which point they assume the nations of the world will have found ways to replace fossil fuels as the main source of energy. Because climate responds steadily but slowly to the buildup, however, the full effect on precipitation changes would not be experienced until 2100.

The changes are happening and will not be stopped for decades even by dramatic reductions in greenhouse-gas emissions, the researchers said.

The drought that has affected much of the Southwest since 2000 may be the result of global warming as much as regional weather patterns, they said. For instance, Kushnir said, the drought continued last year even though there was a significant El Niño effect, which normally produces increased rainfall in the area.

Climate scientists have debated whether the increased dryness projected is a function of greater evaporation as a result of higher temperatures or of decreases in rainfall. The consensus from the 19 new climate models puts the blame on decreased rainfall, Kushnir said.

While the models show the drying has started, they are not accurate enough to know if 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 paleoclimatologist at the University of Arizona. “It may well be one of the first events we can attribute to global warming.”

Material from The Associated Press and Gannett News Services included in this report.

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