JOHN STEINBECK鈥橲 novel The Grapes of Wrath immortalised the plight of migrants who left behind the parched American prairies to travel west into a verdant promised land. Now these greener pastures may themselves be heading for a prolonged drought more severe than the one that created the dust bowl from which the migrants fled 70 years ago.
For the past seven years, states in the south-western US have been drying up. Rising temperatures, a decline in precipitation and an increasing population have combined to leave major water sources running perilously low. Now an analysis by Richard Seager of Columbia University in New York and his team suggests that the region is in the early stages of a profound climate shift.
From 19 major climate models used by the Intergovernmental Panel on Climate Change they predict that sea surface temperatures within 40 degrees of the equator will continue to rise. Storms will follow the warming waters, dragging behind them the dry air that now settles over the world鈥檚 deserts.
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The models predict prolonged drought conditions in the south-western US, with annual rainfall diminishing by about 3.6 centimetres by 2050. At present, average annual rainfall in most places in this region ranges from 12.7 to 38.1 centimetres, so the projected reduction would have a serious impact (Science, DOI: 10.1126/science.1139601). 鈥淲e think of drought as being an occasional thing, but it鈥檚 not going to be like that in the future,鈥� says team member Ming Fang Ting. 鈥淚t鈥檚 going to be dry all the time.鈥� Semi-arid areas in Mexico, eastern Australia, southern Africa and the Mediterranean are also at risk.
The response of plants to these drier conditions will only make matters worse. Vegetation is crucial to sustaining atmospheric moisture in dry climates, says Gemma Narisma of the University of Wisconsin, Madison. Plants react to a reduction in available water by feeding less moisture into the atmosphere, which further reduces rainfall.
To better understand this feedback loop, Narisma has studied some 30 regions around the world that endured droughts of more than five years during the past century (Geophysical Research Letters, vol 34, p L06710). She reckons that the plant effect could amplify the drought and trigger the onset of the dust-bowl-like conditions also predicted by climate models. The resulting changes to the atmospheric circulation would then sustain drought for decades.
鈥淧lants could amplify the drought and trigger the dust-bowl-like conditions also predicted by climate models鈥�
Whether this effect has been at work in causing the drought of the past seven years is still uncertain, as it goes against recent longer-term trends. 鈥淥ver the last 50 years, much of the US, including the west has [seen] increased precipitation,鈥� says Aiguo Dai of the National Center for Atmospheric Research in Colorado.
Isaac Held of the National Oceanic and Atmospheric Administration is also cautious about the models鈥� predictions. 鈥淭here is a hint that the current drying may be caused by what the models suggest, but it鈥檚 only a hint right now.鈥�
