Precipitation has dropped owing to human-influenced aerosols: study

Human activity that spews out fine particles into the atmosphere could be taking a toll on the Indian monsoon, leading to a decline in recent decades, say researchers in a paper being published online by the journal Science.

With the rising levels of these fine particles, known as aerosols, over the Indian subcontinent and neighbouring China, there has been considerable scientific debate over the impact they might be having on the climate. Soot, produced by burning biomass and fossil fuels, has aroused particular concern. Some industries and vehicles can emit sulphate particles. In addition, there are natural aerosols such as dust and sea salt.

Global model

In their study, Massimo Bollasina of Princeton University in the U.S., along with Yi Ming and V. Ramaswamy of the Geophysical Fluid Dynamics Laboratory at Princeton, used a state-of-the-art global climate model to simulate what was happening in the ocean and the atmosphere with and without human-generated aerosols.

“We find that the observed precipitation decrease can be attributed mainly to human-influenced aerosols emissions,” they state in the paper.

This drying was most pronounced over central and northern India, said Dr. Ming in an email. It had also led to a reduction in the monsoon rainfall over India in the past 50 years and above.

But some Indian scientists have expressed reservations over the paper and its findings.

“I am not convinced of their argument that the Indian monsoon rainfall is decreasing,” remarked M. Rajeevan, who was director of the India Meteorological Department's National Climate Centre at Pune and is currently with the National Atmospheric Research Laboratory at Gadanki in Andhra Pradesh.

Data did not show any significant trend in nationwide monsoon rainfall over the past 100 years. But it has long been known that such rainfall pattern had 30-year cycles. The decline in nationwide rainfall observed in recent decades could be the result of such natural variability.

Besides, while rainfall in parts of central and northern India had shown a significant declining trend in the last 25 to 30 years, it was increasing in Maharashtra, northern Karnataka and along the Western Ghats, he told The Hindu. So if human-produced aerosols were held to be the cause of the former, how was the latter explained?

The levels of soot used as input for the climate model was far less than the values observed across India, said S.K. Satheesh of the Indian Institute of Science in Bangalore who studies aerosols. As a result, the atmospheric warming produced by soot was under-estimated in the model. If appropriate values for soot were incorporated, “they will get normal rain or excess rain.”

Many models, including the one used in this study, assumed that soot and other aerosols remained separate particles. But soot could coat dust particles and sulphate could envelope particles of soot.

Such mixed particles had vastly different characteristics than when they are separate. Understanding and incorporating such mixing into the models was essential for predicting the impact that aerosols would have on the monsoon, he said.

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