Man-made atmospheric brown clouds have dimmed the skies over hundreds of Asian cities 25% in 30 years. A particularly nasty, 2-mile thick layer of soot, sulfates, toxic aerosols and carcinogens blankets South Asia these days, especially in winter.
We know quite a lot about the South Asian brown cloud. For example, we know it costs China, Japan and Korea $5 billion per year in crop damage and we know it causes 340,000 excess deaths per year from cardiovascular and respiratory disease.
But one thing we didn’t know until now was what caused it.
And for what it’s worth, most people who had ventured an opinion on the matter guessed incorrectly.
If Stockholm University’s Orjan Gustafsson and colleagues are to be believed, that is.
Scientists had debated for decades whether the South Asian brown cloud arose primarily from burning fossil fuels in cars and power plants or from burning wood, dung and related biomass for agriculture, deforestation and cooking.
The smart money had been on the former.
Then, in 2006, Gustafsson’s team secured some high-grade atmospheric soot from the South Asian cloud and tested it for carbon-14, a clever thing to do since the radioactive substance has a half-life of 5,700 years and therefore would not be present in fossil fuels created several million years ago.
What they found was a ton of C-14 in the samples, indicating that a good 2/3 of the brown cloud came from biomass combustion.
The paper is in Science. The conclusion is that regulating agricultural burning and using better cooking technology might do more to brighten the day for 2 the billion people living under the South Asian brown cloud than restricting cars or building clean coal power plants.