South Asia has a cloud over its head  an unpleasant, unhealthy and climate-affecting soup of sooty haze that envelops the region, particularly in winter.

Scientists have studied what’s called the “brown cloud” for years, yet there has always been uncertainty about it. How much of the soot and other carbon-containing aerosols that make up the haze comes from the burning of fossil fuels in cars, power plants and the like, and how much comes from burning wood and other biomass for cooking and agriculture?

Image Researchers found that the burning of biomass was the top culprit behind a brown haze in South Asia. Credit... AAAS/Science

Orjan Gustafsson of Stockholm University and colleagues have now removed the cloud of uncertainty hanging over the brown cloud. Burning of biomass, they report in Science, is the greater culprit.