SOOT—also known as black carbon—heats up the atmosphere because it absorbs sunlight. Black things do. That is basic physics. But for years the institutions that focus on climate policy have played down the role of pollutants such as black carbon that stay in the atmosphere for a short time, and concentrated on carbon dioxide, which, once generated, tends to remain there. That may soon change.

On January 15th, the fifth day that smog-darkened Beijing’s air-quality index was registering “hazardous” (see article), the most comprehensive study of black carbon yet conducted was published in the Journal of Geophysical Research: Atmospheres. It concluded that the stuff was the second-most-damaging greenhouse agent after CO 2 and about twice as bad for the climate as had been thought until now. The implications are profound.

This study, a four-year affair conducted under the auspices of the International Global Atmospheric Chemistry Project, an umbrella group for research into such matters, is based on a lot more information about soot than was previously available, and a better understanding of how it affects the climate. It found that the black carbon around at the moment has a warming effect of about 1.1 watts per square metre of the Earth’s surface (W/m2). This is greater than that of methane and second only to the 1.7W/m2 of carbon dioxide. An earlier estimate by the United Nations Environment Programme (UNEP) put the black-carbon effect at only 0.3-0.6W/m2. The higher the figure, the worse the warming.

Black carbon is especially damaging to frozen regions, because when soot falls on snow and ice it increases the amount of light and heat they absorb. The new assessment may therefore help explain why the Arctic has been melting faster than anyone had expected. The study argues that warming is likely to be especially marked in the high latitudes of the northern hemisphere—northern Canada, Alaska, northern Europe and Siberia. It also gives a warning that black carbon, by changing regional precipitation patterns, may affect Asian monsoons.

Sweep it away

The biggest impact of soot, though, is not on the climate but on health—through lung and other diseases. The UNEP study reckoned that controlling emissions of black carbon could save 2.4m lives a year, regardless of any effects on the climate.

It might seem that the new study is one more item of bad environmental news. Not so. It should be easier to deal with black carbon than with carbon dioxide. Whereas CO 2 is long-lasting and an inevitable by-product of burning fossil fuels, soot drops out of the atmosphere within weeks. Stop putting it there and it will rapidly go away—a potentially easy win.

That win is made easier still by the fact that about 70% of emissions in Europe and the Americas come from diesel engines. Better exhausts, to trap carbon particles before they are emitted, and the scrapping of old, highly polluting vehicles could make an immediate impact. In other countries the problem is more often inefficient stoves and dirty fuel—again, things that are easy to deal with, at least in principle.