Growing drought

The Statesman , Monday, May 28, 2007
Correspondent : Staff Reporter
South-western America, northern Mexico, the arid and semi-arid regions of southern Europe, Mediterranean northern Africa and West Asia ~ they’re are all showing signs of becoming dusty landscapes

THE drought that spawned the great American Dust Bowl of the ’30s may become the new climatic norm for much of southwestern America and other sub-tropical regions of the world. In a report published in Science recently, US and Israeli researchers project an imminent increase in aridity in sub-tropical regions over the next century, which will affect several important agricultural regions.

The results indicate growing drought in the American 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 21st 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 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 21st century. But Seager's team dug deeper, to see how arid the sub-tropical regions were likely to become.

They focused specifically on south-western USA 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 West Asia.

The result shows a reduction in moisture in these areas — on the order of 15 per cent 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 USA was only about 15 per cent less than normal — so you don’t need a lot,” he says.

Seager points out that this doesn’t mean that the south-west 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.

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 sub-tropical 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, but part of a hemispheric pattern.”

Lending credence to Seager's study is evidence that the Hadley cell is already expanding. “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 in 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 per cent 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 originates in Turkey. Cross-border conflicts are likely to arise as these streams of water dry up.

 
SOURCE : The Statesman, Monday, May 28th 2007
 


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