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Droughts in India to worsen with climate change -- study
mongabay.com
May 12, 2006


Climate change may be weakening India's monsoon, producing droughts and potentially causing significant consequences in the world's most populous region, according to a new study by scientists at Scripps Institution of Oceanography at the University of California, San Diego.

In a study published in the May 15 issue of the American Meteorological Society's Journal of Climate, Chul Eddy Chung and V. Ramanathan of Scripps Oceanography say that cooler-than-normal temperatures in the northern part of the Indian ocean have weakened the region's natural climate circulation and monsoon conditions, resulting in reduced rainfall over India and increased rainfall over the Sahel area south of the Sahara in Africa.

According to the authors, drier climate conditions result from varying rate at which parts of the Indian Ocean are warming -- tempartures in the tropical Indian Ocean are climbing faster than the northern Indian Ocean.

"It appears that the whole tropical region in this area is being pulled in different directions," said Ramanathan, director of the Center for Clouds, Chemistry and Climate at Scripps. "The observed trend of reduced sunlight reaching the Earth's surface, with compensating solar heating aloft from the pollution, also called the ‘brown haze,' appears to be masking the greenhouse warming in the northern Indian Ocean, while the greenhouse warming continues unabated in the southern Indian Ocean. We are starting to see that the air pollution affects sunlight and is potentially having a major disruption of the rain patterns, with some regions getting more and some less."

The amount of sunlight decreased by pollution, or "brown clouds," is highlighted by watts per square meter of Earth's surface. The gray and green areas have less of the dimming effect than the blue and dark blue regions. For example, the brown clouds over India have decreased sunlight by -12 to -20 watts per square meter. The climatological value of sunlight at the surface is about 200 watts per square meter, which means the sunlight has decreased by about 6 to 10 percent. (Graphic from Chung, Ramanathan, Kim and Podgorny, 2005 Journal of Geophysical Research)

"The greenhouse gases are pushing in one direction, warming the ocean and trying to make more rain, and the aerosols are pushing in another direction for cooler oceans and less rain. The net effect is to drive the monsoon rain system away from South Asia into the equatorial and southern oceans," said Ramanathan. "Some years the aerosols might win and in some years the greenhouse effect may win. So we are concerned that in coming decades the variability between the two will become large and it will be difficult to cope with rapid changes from year to year."

The authors say that similar pollution clouds can be seen in other parts of the world, including the United States.

"About five to 10 years ago we used to think about pollution as an urban problem," said Ramanathan. "Now we have discovered, with new observations including satellite data, that these pollution clouds travel quickly and can cover an entire ocean. Scientists have shown that in a matter of five days pollution traveled from China to the United States, and in a matter of three to four days it can travel from the U.S. to Europe."

Chung and Ramanathan say that monsoonal rainfall over India has decreased by approximately five to eight percent since the 1950s.



This article is based on a news release from Scripps Institution of Oceanography at the University of California, San Diego.