Air pollution is stunting India’s monsoon. Marshall, Michael, 30 Sep 2011. NewScientist.
1.5 billion people, 20% of the world’s population, depends on the summer monsoon that brings 80% of India’s rain.
In the future, monsoons may see-saw between too weak leading to drought and too strong, flooding crops.
Air pollution has been drying out India for over 50 years. Droughts reduce food production.
Half of the air pollution causing this is local, but the other half is far away, hovering over Europe. This cools the continent below because air pollution reflects sunlight back to space. This in turn causes global wind currents to slow down because there’s less hot – cold differences between the northern and southern hemispheres. The end result is far less wind, which picks up less moisture to drop as rainfall in south Asia.
A countervailing trend — global warming from greenhouse gases — is tilting monsoons towards too much rainfall and consequent flooding of crops.
This erratic trend is even “more worrisome” than less rainfall
Another form of pollution – greenhouse gas emissions – is pushing the monsoon in the other direction, towards greater rainfall, says Ramanathan. The competing forces of the greenhouse effect and air pollution may lead to a much more variable monsoon, with drought one year followed by floods the next. He says this erratic behaviour is “more worrisome” than the overall decrease in rainfall according to Veerabhadran Ramanathan (University of California, San Diego).
Journal reference: Science, DOI: 10.1126/science.1204994