In his much-ballyhooed 1939 novel, The Grapes of Wrath, John Steinbeck describes the conditions of the southern American Great Plains, where a severe drought caused the devastating wind-swept storms of the 1930s dust bowl: "When June was half gone, the big clouds moved up out of Texas and the Gulf, high heavy clouds, rain-heads," Steinbeck wrote. "The rain-heads dropped a little spattering and hurried on to some other country. Behind them the sky was pale again and the sun flared. In the dust there were drop craters where the rain had fallen, and there were clean splashes on the corn, and that was all."

Unfortunately, severe droughts are not just the stuff of classic literature. A research team, led by a group at Columbia University's Lamont-Doherty Earth Observatory (LDEO) in Palisades, N.Y., reveal in this week's Science that southwestern North America will likely be saddled with increasingly arid conditions during the next century. This drying effect, the researchers say, is directly related to man-made climate change and will demand new methods for managing water resources in the region. They based their findings on 19 climate models, all of which contributed to the Intergovernmental Panel on Climate Change's (IPCC) Fourth Assessment Report released in Paris in early February

"The Southwest," in the current study, refers to the southwestern U.S. and northern Mexico. The researchers add that a similar pattern of drying is predicted for the region bounded by southern Europe, the Mediterranean Sea and the Middle East; wetter climes are expected for areas farther away from the tropics and closer to Earth's poles.

The output of the climate models used in the current analysis came in measures of future precipitation and evaporation. "[Those figures are] what matters in terms of the water that's available to the soil in the ground," says study co-author Mingfang Ting, a senior research scientist at LDEO. By subtracting the area's evaporation rate from its precipitation rate, the scientists were able to make projections on the net gain of water available in the ground. (Of the 19 models, 18 of them show this value to be negative by the latter half of the current century, indicating, according to Ting, that "precipitation is reduced or evaporation could be increased.")

On average, the models suggest that the cusp of the late 20th century–early 21st century ushered in a period of consistently arid climate. Soon, if not already, one quarter of the models predict that moisture will begin to disappear from the region at a rate of 0.1 millimeters per day. In fact, the researchers believe the current six-to-seven-year drought in the region is the beginning of this drying trend. "The current drought is related to the warming due to the greenhouse gases," says Ting. "In the past, El niño [would] disappate the drought, but now it's not able to stop the drought.''

Normally, the El Niño and La Niña weather systems are largely responsible for cyclical precipitation and drought in the Southwest. El Niño brings in moisture from the tropics (by the warming of the ocean, which condenses water into the lower atmosphere that is then shuttled into the subtropical regions), whereas La Niña essentially does the opposite, causing cold ocean temperatures in the equatorial eastern Pacific. The latter phenomenon is believed to be the culprit behind both the 1930s dust bowl and a widespread drought in the Southwest during the 1950s.

"The drought that we're taking about is not La Niña," Ting explains, referring to the current dry system. "That is associated with the greenhouse gas warming." While the consequences are similar, the actual effect on the oceans is very different, she says. Instead of a cooling in the tropics, there will be a uniform warming of the ocean, which will push the Pacific jet stream farther north. As a result, "Canada does get quite a lot more rain," Ting notes, whereas "the whole state of California, for example, will be much drier."

1 Comments

1. 1. DesertRat 09:59 AM 1/23/09

   Why are all SciAm.com articles so conclusionary? Of all sources, scientific magazines, even those catering to the general public, should exhibit a scientific approach that sources its findings and conclusions with referenced facts.

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