Southwest May Get Even Hotter, Drier

By Marc Kaufman
Washington Post Staff Writer
Friday, April 6, 2007

Global warming will permanently change the climate of the American Southwest, making it so much hotter and drier that Dust Bowl-scale droughts will become common, a new climate report concludes.

Much of the nation west of the Mississippi River is likely to get drier because of the buildup of greenhouse gases in the atmosphere, but the greatest effect will be felt in already arid areas on both sides of the U.S.-Mexico border. By the end of the century, the climate researchers predict, annual rainfall in that region will have decline by a worrisome 10 to 20 percent.

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 said, 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 16 of 19 climate computer models assembled for the Intergovernmental Panel on Climate Change, 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 a paper published online yesterday by the journal Science, which also predicted that 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 of Columbia University, one of the paper's authors. "From a climate perspective, these changes are quite dramatic."

He said that the paper's authors have a high level of confidence that droughts will develop, and that they will result from increases in atmospheric greenhouse gases created through burning fossil fuels and other human activities.

The researchers said 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. The Southwest has had significantly below-average rainfall since 1999, and preliminary information suggests that global warming is already playing a role in the current drought.

As the planet warms, they said, basic climate dynamics will change. Currently, hot air from the equatorial tropics rises about eight to 12 miles until it hits the stratosphere and is blocked. It then 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.

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. Richard Seager, also from Lamont-Doherty and a lead author on the paper, said the region must 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.

Climate models generally assumed a gradually increasing amount of carbon dioxide in the atmosphere until 2050, at which point they assume that nations 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 felt until 2100.

The changes are already taking place 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 1999 may already be the result of global warming as much as regional weather patterns, the researchers 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 projected increased dryness is a function of greater evaporation as a result of hotter temperatures or of decreases in rainfall. The broad consensus from the 19 new climate models puts the blame on decreased rainfall, Kushnir said.

Independently of the new study, others note that the Southwest has been experiencing environmental changes besides drought that many believe are associated with global warming.

For instance, the 27 "sky islands" in southern Arizona -- small, green mountaintop climate zones that are generally 20 degrees cooler than the lower elevations around them-- have been gradually losing their rich diversity of life and have become far more vulnerable to major fires. More than two dozen species have been listed as endangered or threatened in the Coronado National Forest, which contains some of the most important "sky islands," and previously unseen lowland insects have begun to invade.

Those who think these changes are the result of long-term climate shifts note that during a decade-long drought in the 1950s, the sky islands continued to flourish.

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