conventional scientific wisdom has it that irrigation works against global warming. But there is evidence now to shake us out of complacency. Scientists from the University of California, Merced, have shown that irrigated fields have their limits in masking effects of greenhouse gas emissions. Specially, when land under agriculture isn't going up and global warming is getting abetted by a host of factors. The scientists also suggest that studies linking irrigation to climate change be accompanied with those dealing with effects of land use change.
Celine Bonfils and David Lobell of the University of California's School of Natural Sciences observed temperature and irrigation trends in California between 1915 and 2000. Irrigation-induced cooling in the top irrigated state in the us with 3.3 million hectares was palpable between 1915 and 1979, they observed.This was the period when land under irrigation more than doubled in California. "But in comparison, there was no clear effect of irrigation on temperatures over 1980-2000 when there was no net growth of irrigation," Bonfil and Lobell said. Their paper was published in the Proceedings of the National Academy of Sciences (Vol 104, No 34).
Studies did try to find the contrast between pre- and post-irrigation temperature trends but they did not document the rate of irrigation growth. This lack fostered a certain amount of smugness about irrigation's cooling effects. "Our results in California suggest that the rate at which irrigation evolved, and not just the presence of irrigation, explains the cooling effect and its attendant effect on global warming better," the scientists say.
A K Gosain, who works on hydrological cycles at the Indian Institute of Technology, Delhi, however agrees with the University of California scientists. "Irrigation has an impact on the micro-climate. If the water status of the local area is high, there will be higher evapo-transpiration and the relative humidity of the place will be higher. The temperature of that place will dip to some extent, but it has a very regional effect," he explains.
The Bonfils and Lobell study has shown that irrigation is an important variable but in a constellation of factors. So, it might not be able to bail us out of global warming. In California, for example, irrigation expansion is on the wane with an increase in urbanisation and increase in water demand from other sources. In the us, irrigation has decreased by 2 per cent between 1998 and 2003.
Bonfils summed the issue aptly when she told the media "Throughout the major irrigated regions of the world, the cooling influence of irrigation on daytime maximum temperatures will be much smaller in the next 50 years than in the past century, and irrigation will not continue to curb the effects of greenhouse warming anymore."
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