WASHINGTON - Summer monsoons that provide up to 80 percent of the water South Asia needs have gotten drier in the past half century, possibly due to aerosol particles spewed by burning fossil fuels, climate scientists said on Thursday.Monsoon rains are driven by looping air circulation patterns over India, and the aerosols appear to have interfered with these patterns, researchers reported in the journal Science.RELATED:Climate change to cost Canada many billions UN: Soot, smog curbs quick way to combat warming Between 1950 and 1999, the drying was most pronounced in central-northern India, with a 10 percent drop in average June-September rainfall, the researchers said. The rest of India experienced a decrease of about 5 percent over the same period, they added.This does not seem to be a direct consequence of greenhouse gas emissions, even though the burning of fossil fuels and biomass that produces the aerosol particles also emits climate-warming carbon dioxide, the researchers said.Particle pollution can increase the risk of heart disease, lung cancer and asthma attacks and interfere with the growth and function of the lungs, according to the American Lung Association, which has fought to curb these emissions in the United States.Over South Asia, aerosol particles have actually slowed down climate warming by reacting with sunlight and reflecting some of it back into space, said study co-author Yi Ming of the US National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey.That same cooling effect tends to slow down the north-south air circulation that delivers the monsoon rains, Ming said in a telephone interview. The Northern Hemisphere, including India, tends to emit more aerosol particles because it is more heavily developed than the Southern Hemisphere, Ming said. This could offer hope that the aerosol drying of the monsoon could be reversed.This is because developed countries such as the United States and much of Europe have taken steps to cut down on particle pollution, which means less gets into the air.Ming and his colleagues project that if India and other Asian countries continue to develop their economies, they too will cut back on particle emissions."The aerosol levels will be cut a lot out of concern for human health, like what happened in the US and Europe," Ming said. "Once countries are rich enough, they want to clean their air."This could begin to reverse the monsoon's drying trend in 20 to 30 years, he said. However, it would do nothing to reduce the emissions of carbon dioxide associated with more developed economies, and dealing with that, Ming said, "will be a very difficult policy challenge."