BioEd Online

The drought that spawned the great American Dust Bowl of the 1930s may become the new climatic norm for much of the southwestern United States and other subtropical regions of the world. In a report published today, researchers in the United States and Israel project an imminent increase in aridity in subtropical regions over the next century, which will affect several important agricultural regions.

The results indicate that growing drought in the southwest is a problem that is likely to affect agriculture. "This is something that is already under way. It's not an end of the twenty-first century thing where we have the luxury to sit around and wait," says Richard Seager, a climatologist with the Lamont Doherty Earth Observatory in New York, who led the 13-member research team.

Seager and his colleagues looked at data from a number of computer climate models used in the International Panel on Climate Change's (IPCC) most recent report. Under the IPCC's 'business as usual' emission scenario, in which carbon dioxide emissions increase until mid-century and level off at around 720 parts per million by 2100, these models had already predicted a general increase in global mean temperature and in the likelihood of droughts over the twenty-first century. But Seager's team delved deeper, to see how arid the subtropical regions are likely to become.

They focused specifically on the southwestern United States and northern Mexico, because those areas are already showing signs of drying. But their findings are also applicable to the arid and semi-arid regions of southern Europe, Mediterranean northern Africa and the Middle East.

The result, published in Science¹, shows a reduction in moisture in these areas — on the order of 15% over the next two or three decades, says Seager. Moisture is defined as the mean annual rainfall minus the amount of water expected to be lost to evaporation, a measure that reflects the amount of water useful for agriculture.

"It's important to remember that for the Dust Bowl, the precipitation over the western United States was only about 15% less than normal — so you don't need a lot," says Seager.

Seager points out that this does not mean that the southwest will necessarily relive the disastrous effects of the 1930s drought. The catastrophic dust storms of the Dirty Thirties were more the result of poor farming practice and land management, he says, than the drought alone.

Winds of change

The study attributes the increased aridity to the poleward expansion of the Hadley cell, an atmospheric circulation system that today transports moist, warm air from the equator to the northern and southern mid-latitudes. The result is an expansion of subtropical arid regions, and a poleward push of the rain bands that provide precipitation in the higher latitudes. "This is not part of a regional peculiarity," says Seager, "but part of a hemispheric pattern."

Lending credence to Seager's study is evidence that the Hadley cell is already expanding².

"This is a robust prediction that's been backed by observation," says Dennis Hartmann, a climatologist in the Department of Atmospheric Sciences at the University of Washington, Seattle. "It seems like a reasonable scenario for the future."

The implications for such a drying are far-reaching. California, for instance, accounts for approximately 16% of all US agricultural exports. Seager suggests that North American farmers need to rethink certain agricultural practices, including adopting more water-efficient irrigation systems such as those being used in Israel.

And, where there are water shortages, there is also the potential for political conflict. "As Mark Twain wrote, 'Whiskey is for drinking. Water is for fighting over'," says Seager. Mexico depends on water originating from the Colorado River, he notes, whereas Iraq and Syria depend on the waters of the Euphrates, which originate in Turkey. Cross-border conflicts are likely to arise as these streams of water dry up.