Man-made pollution dries up monsoon: Study

Rainfall in India declined by about 4-5 per cent in last 50 years due to accumulation of aerosols

Over the last 50 years, monsoon rainfall in the country has declined by about 4-5 per cent and the trend is most visible in central and Gangetic plains.

The downfall could be attributed to accumulation of aerosols — fine dust particles suspended in air causing air pollutions — and its influence on the monsoon.

“Averaged over the whole country, Indian summer rainfall underwent a reduction of 4-5 per cent over the last 50 years. The drying trend in central-northern India can be attributed to aerosols of anthropogenic origin,” a team of researchers from US National Oceanic and Atmospheric Administration and Princeton University, USA, reported in the journal “Science”.

Aerosols are air pollutants originating from natural sources like volcanic ash, dust and sea salt as well as man-made sources like burning of woods and fossil fuels, emission of thermal power plants and vehicular pollution.

The impact of air pollution on weather is being researched. While the pollution haze hang over India and adjoining areas for months in the winter, they have a relative short-life of a couple of weeks in the summers.

Scientists use climate modeling to illustrate a complex interplay of natural and man-made factors adversely affecting the Indian monsoon in which aerosols accumulated in atmosphere in summer months stand out as a major culprit. The models indicate drying trend over central northern India due to aerosols. The region would have been wetter had other factors like greenhouse gas and ozone have an upper hand. There may be a slight increase in rainfall in southern and northwest India and Pakistan.

But other scientists are not in agreement with some of the other conclusions of the study.
“One part of it is bothersome. Their simulation of aerosol effects over China seems to be opposite to what has been observed. The model simulation suggests maximum drop in rainfall in China due to aerosols, whereas southern China has experiences a large increase,” pointed out V Ramanathan, a climate researcher at the University of California, San Diego.

“The simulated decrease over India is part of a large-scale decrease that is centered in China. If the simulation over the centre of action is incorrect, then we need to worry,” Ramanathan, who first waved the red flags on aerosol in the late 1990s, told Deccan Herald.

Ramanathan, who is now coordinating a pan-Indian initiative to assess the impact of air pollution, agrees with the study’s first conclusion of aerosols being responsible for rainfall drop. In a 2005 paper, he showed the similar trend using observed data.

The study suggests an energy imbalance caused by the distribution of aerosols between the northern (where concentrations are higher) and southern (where concentrations are lower) hemispheres produces a slowing of the vigorous conveyer belt-like atmospheric circulation that drives monsoon rainfall.