The study supports existing evidence that air pollution

Lurching rains
Ming says his modelling suggests that the effect of European aerosol pollution accounts for about half the drop in the volume of monsoon rainfall – the other half is down to pollution over south Asia. In as-yet-unpublished experiments, he confirmed the important role that the European pollution plays in weakening the monsoon. He ran his models again, this time assuming no aerosol pollution over south Asia. Even so, India had a significantly weaker monsoon.

The study supports existing evidence that air pollution is weakening the monsoon, says Veerabhadran Ramanathan of the University of California, San Diego.

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.