Fifty-Year Drop in Asian Monsoons Linked to Fossil-Fuel Use

Sept. 29 – The reduction in seasonal rainfall in South Asia over the past 50 years may be a result of tiny chemicals emitted into the atmosphere by burning fossil fuels, U.S. scientists said.

Computer models incorporating the effect of aerosols emitted from car exhaust pipes, power plants and cooking stoves may explain the 10 percent drop in monsoon rainfall in parts of India over the last five decades, according to a study yesterday in Sciencexpress, the online version of the journal Science.

As much as 80 percent of South Asia’s rain falls during the June-to-September monsoon. Seasonal winds fluctuate widely and scientists have been developing new models that may help farmers prepare for water-supply disruptions and mitigate loss of life and property. Record monsoons last year caused floods in Pakistan that displaced almost 20 million people and caused more than $9 billion in damage.

“Our data shows that the 50-year trend of decreasing monsoon rains is not a natural variation and is likely because of the particles from burning fossil fuels,” study co-author Yi Ming, a scientist at the Washington-based National Oceanic and Atmospheric Administration, said in an interview. “By better understanding the past, we can be more confident about predicting future climate change.”

The researchers ran data on pollutants compiled by the United Nations’ International Panel on Climate Change in computer models to determine the effect on monsoon precipitation. They then compared the results with 50 years of rain data from India and Pakistan.

Microscopic particles like sulfates and carbon released from coal-burning power plants and automobiles can reflect sunlight back into space, producing a cooling effect. This can in turn influence global wind circulation patterns that underlie the monsoon system, Ming said.

Monsoons are large-scale winds in the Indian Ocean and southern Asia that blow from the southwest in summer and from the northeast in winter. During the four-month wet season, they bring heavy rains accompanied by lightning and thunderstorms.

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