If black carbon is responsible for trapping so much heat, then reducing soot may be an effective way to slow down the planet’s warming. It’s even more attractive because black carbon washes quickly out of the atmosphere, and so reducing soot emissions would lead to a fast fall in the concentration of black carbon in the atmosphere. Carbon dioxide, by contrast, lingers for centuries in the atmosphere.
James Hansen of the Goddard Institute for Space Studies has been arguing for such a strategy for over a decade. But the new study reveals a paradox in reducing soot to fight global warming. If tomorrow we could shut down every brick kiln, every burning farm field, and every other source of soot, we would, on balance, have no effect on global warming whatsoever.
How can this be? Because when things burn, black carbon is not the only thing they produce. A forest fire produces black carbon as well as organic carbon molecules. The forest fire black carbon helps to warm the planet, but the organic carbon creates a haze that blocks sunlight, cooling the atmosphere. The two emissions cancel each other out. “In the real world you can’t just get rid of black carbon emissions,” says Doherty. “You get rid of other things as well.”
But Doherty and her colleagues found that some sources of soot — including coal and diesel fuel — produce a lot of warming with very little compensating cooling. They suggest that these sources should be the top priority for efforts to fight global warming.