Air pollution is reducing rainfall in Mawsynram, Meghalaya’s rain-soaked jewel that has for decades held the title of the world’s wettest place.
Scientists have found that Mawsynram’s annual rainfall has declined from 10,200mm to 8,800mm over the past decade, coinciding with a near-15 per cent rise in aerosols — airborne particles originating from natural dust, vehicle exhausts, biomass burning and industrial activities.
Their study is the first to examine how aerosols might be influencing rainfall in Mawsynram, a town in the East Khasi Hills where moisture-laden winds from the Bay of Bengal forced upward by the terrain cool rapidly and typically release extraordinary amounts of rainfall. Earlier studies have pointed to mildly weakening rainfall trends across parts of northeast India.
“Our findings show that pollution from faraway places can affect local rainfall patterns — even in a pristine place like Mawsynram,” Sumit Mishra, senior principal scientist and aerosol specialist at the National Physical Laboratory in New Delhi, who led the study, told The Telegraph.
The researchers analysed rainfall data alongside aerosol measurements gathered between 2014 and 2023 using an instrument aboard a Nasa Earth-observation satellite. The satellite data allowed them to track year-to-year changes in aerosol concentrations over the Khasi Hills, including seasonal variations.
They found that aerosol levels over Mawsynram increased by nearly 15 per cent during the study period, with the sharpest rises occurring in the winter and pre-monsoon months.
These particles trap heat in the lower atmosphere, warming the air above the surface and disrupting the delicate processes that allow clouds to grow and produce rain.
Years with higher aerosol concentrations consistently recorded lower rainfall totals, the study found.
The study, published in the Journal of the Indian Society of Remote Sensing, also shows that aerosols increased the rate at which the air warms by as much as 0.5 to 0.7 degrees Celsius per day in the months leading up to the monsoon. This added warming weakens the upward movement of air needed for cloud formation, suppressing rainfall before the monsoon even begins.
While clouds require tiny airborne particles to act as seeds for moisture, Mishra said, aerosols produced by human activities often work against rainfall. High concentrations of such particles lead to the formation of many small cloud droplets rather than fewer large ones, making it harder for droplets to merge, grow heavy and fall as rain.
Much of the public attention on air pollution in India has focused on winter smog across the Indo-Gangetic plains and its health impacts. But when winds carry those pollutants away from their source regions, they can have unexpected effects elsewhere, Mishra said. Mawsynram appears to be one such location.
But not all of the pollution affecting the region, however, comes from distant sources. Local biomass burning associated with shifting cultivation also contributes to aerosol levels in parts of Meghalaya. An independent study by environmental scientist Pratibha Deka of Tezpur University in Assam has found that biomass burning in the
West Garo Hills can raise fine particulate pollution to as much as 16 times pre-burning levels, adding another layer of stress
to an already changing
atmosphere.
The NPL study’s other coauthors are Mamta Devi and Kartika Pandey and collaborators at the Indian Institute of Tropical Meteorology, Pune, and the Indian Institute of Technology, New Delhi.