The Downside to Cleaning Up Air Pollution: Devastating Monsoons

Polluted airborne particles have been messing with India and Southeast Asia’s summer monsoon seasons, suppressing rains over the last half century.

The culprit? Scientists have singled out aerosols released from the burning of fossil fuels and auto emissions as responsible for hindering the region’s rainfall. But as countries clean up their air, the region could get an increase in rain not seen in decades—and that could lead to flooded agricultural fields, washed-out roads, disease, displacement, and death.

“It’s very hard to overstate the importance of the monsoon, which is the main source of water for several billion people,” said Yi Ming, a climate scientist at Princeton University.

Monsoon rainfall in India, Southeast Asia, and parts of Africa declined from the 1950s to the 1980s, affecting crops and worrying farmers. In recent decades, rainfall in these regions has increased but hasn’t returned to precipitation levels seen before the middle of the 20th century.

Scientists wondered whether greenhouse gases, volcanoes, or natural climate variability might account for the drier monsoon period, but a recent study published in the journal Geophysical Research Letters singled out aerosols.

“This research explicitly ruled out other explanations and showed that only anthropogenic aerosols could explain the observed changes,” said Dr. Debbie Polson, a climate scientist at the University of Edinburgh and coauthor of the study.

Aerosols hold back rain by blocking sunlight, lowering air temperatures, and reducing evaporation, meaning the air contains less water. Typically, the effects of rising concentrations of greenhouse gases lead to increased precipitation—which it has in most of the world’s tropics and subtropics—but the aerosols could be responsible for masking the effects of global warming in areas that rely on monsoons for water.

Researchers expect aerosol levels to further decrease this century, and that could mean quickly increasing water levels. Although the study did not look at how monsoon rains would be affected by falling aerosols and rising greenhouse gases, “it is possible the monsoon rains may eventually exceed levels seen at any point during the previous century,” said Polson.

From the 1950s to the 1980s, the monsoon rains decreased by 7 percent to 15 percent, depending on the regions, according to Polson’s research. By the late 1980s, the United States and Europe had started cleaning up their aerosol emissions, and Polson said the recovery in precipitation seen in the 1990s was at least partly attributable to this reduction in aerosols.

While the global decline in aerosols appears to have an effect on monsoons, other studies have found that the South Asian monsoon is weakened mainly by local aerosols. “Still, one cannot rule out the possible influence of remote aerosols,” said Ming. “Man-made aerosols could be key to understanding regional precipitation change, yet future aerosol emissions are notoriously uncertain, as are their climate impacts.”