“Pollution ‘chokes’ monsoon” by G.S. Mudur
Mudur, G.S. “Pollution ‘chokes’ monsoon.” The Telegraph. 30 September 2011.
Pollution ‘chokes’ monsoon
By G.S. Mudur
Friday, September 30, 2011
New Delhi, Sept. 29: Air pollution over India is choking the Indian monsoon and may explain an observed drop in monsoon rainfall over the past 50 years, US scientists said today, rekindling a debate on the impact of pollution on rains.
The scientists said their climate simulation experiments suggest that the observed 5 per cent decrease in monsoon rainfall over the past five decades may be attributed to human-induced aerosols, or tiny particles, emitted by industrial and vehicular emissions and burning wood.
Aerosols tend to reduce the levels of warmth provided by the Sun by reflecting sunlight back into outer space. The reduced warmth, many scientists have proposed, is likely to weaken the atmospheric processes that generate the annual monsoon rainfall.
“The aerosols reduce the power of the engine that drives the summer monsoon,” Yi Ming, a climate scientist at Princeton University involved in the simulation experiments, told The Telegraph in a telephone interview. A paper describing the new results by Ming and his collaborators will be published on Friday in the US journal Science.
Climate simulations over the past decade have thrown up inconsistent results — some have suggested that pollution will lead to more rainfall, others have predicted reduced rainfall, and one study predicted a wetter monsoon in early summer followed by a dry period.
“We’ve used a state-of-the-art sophisticated climate model,” Ming said.
Some climate change models have in the past predicted that the global warming caused by greenhouse gases is likely to warm the sub-continental region and lead to an increase in the rainfall. But Ming and his colleagues say aerosols may have “substantially masked” the rise in rainfall that would have occurred as a result of rising levels of greenhouse gases.
But some scientists caution that it is still premature to predict the impact of aerosols on the monsoon through such simulations. “Aerosols are complex, their properties depend critically on what they are made up of — dust or organic compounds,” said Jayaraman Srinivasan, chairperson of the Divecha Centre for Climate Change at the Indian Institute of Science (IISc), Bangalore. “We’ll need more research to determine whether these results will hold,” he said.
One scientist said the results of any computer simulation depend on the assumptions fed into the model. “The properties they have given to the aerosols in this study assume a warming that is less than the actual warming likely in the atmosphere even with the aerosols,” said Sridharan Krishna Satheesh, a professor of atmospheric sciences at the IISc.
“There are clearly some very good reasons of human health to reduce aersols which can cause respiratory problems,” Satheesh said. “But the effect of aersols on the monsoon is still unclear.”