New Delhi, Aug. 17: Increasing air pollution and loss of forest cover in South Asia may trigger a sudden breakdown of the Indian monsoon and sharply reduce rainfall in the subcontinent within decades, climate scientists said.
Researchers at Germany’s Potsdam Institute for Climate Impact Research who set out to explore the stability of the summer monsoon over India have discovered that it’s particularly vulnerable to human influences.
“The stage seems to be set for a man-made reduction of the Indian summer monsoon,” climate physicist Kirsten Zickfeld and her colleagues said in a paper published in the latest issue of the journal, Geophysical Research Letters.
The scientists have shown that tiny particles called aerosols, spewed into the atmosphere during the burning of coal and firewood, reduce monsoon rainfall, while rising greenhouse gases such as carbon dioxide tend to increase rainfall.
A steady rise in aerosols will reduce the amount of sunlight striking land and lower land temperatures. This will decrease the supply of moisture-laden air from the sea and lower rainfall. Shrinking forest cover will also have the same effect.
“When these effects exceed a critical threshold, the monsoon could experience a sudden breakdown,” Zeckfeld told The Telegraph in a telephone interview. “We’d then see a very weak monsoon consistently over many years,” she said.
“This is something we should be concerned about,” said Sourendra Bhattacharya, a climate scientist at the Physical Research Laboratory in Ahmedabad.
Climate records have revealed that the Indian monsoon has undergone abrupt changes over the past 8,000 years. “There have been some dramatic ups and downs in the monsoon,” said Bhattacharya who specialises in ancient climate studies.
“While those ancient changes in rainfall were driven by natural phenomena, we’re now anticipating changes influenced by human activities,” he said.
Zeckfeld said her study could not quantify the reduction in the rainfall, nor predict when exactly the breakdown of the monsoon might occur.
But with rising aerosol levels, it could happen over the next few decades, she said.
If India adopts new technologies to reduce its aerosol emissions, Zeckfeld said, the monsoon may display a “roller-coaster” behaviour. After a suppression over the next decades, as aerosol-reduction policies begin to take effect and economic growth drives up carbon dioxide emissions, the monsoon may abruptly intensify and re-establish the “wet-monsoon” regime within a few years.
“It might be hard for India to have the monsoon doing really weird things ' reducing and then abruptly increasing,” she said. The study has also cautioned that India might find it a challenge to adapt to such a roller-coaster scenario.
This is not the first study to signal the danger that aerosols pose to the monsoon. A study by climate scientist Murari Lal, formerly at the Indian Institute of Technology, New Delhi, in 1995 was the first to show that aerosols can weaken rainfall.
Bhattacharya said the impact of aerosols would depend on what type they were. Some aerosols such as soot absorb sunlight and will tend to increase rainfall, while sulphur-containing particles will reflect sunlight and tend to reduce rainfall.
“We might also have a scenario where the effects of aerosols and greenhouse gases cancel each other out,” Bhattacharya said.