In a paper published online today by the Proceedings of the National Academy of Sciences, James E. Hansen of the NASA Goddard Institute for Space Studies and Larissa Nazarenko of Columbia University report that soot may have caused fully 25 percent of the warming observed since 1880. The researchers focused on the effects of black carbon on snow albedo---that is, its ability to reflect sunlight back into space. As it turns out, dark, sooty snow absorbs significantly more solar energy than clean, white snow does. This creates positive feedback, with the black carbon melting the snow, thus concentrating the soot density, which in turn leads to the absorption of even more heat. Indeed, Hansen and Nazarenko determined that the contaminating particles raise temperatures twice as effectively as carbon dioxide does. This activity, they observe, may be partly responsible for the trend toward earlier springs in the Northern Hemisphere, thinning Arctic sea ice and shrinking glaciers.
Hansen notes that greenhouse gases are still the primary cause of climate warming during the past century and will continue to be the predominant agent of warming in the future. The good news is that soot emissions may be more easily lowered than the greenhouse gas variety. "Technology is within reach that could greatly reduce soot," the researchers write, "restoring snow albedo to near-pristine values, while having multiple other benefits for climate, human health, agricultural productivity, and environmental esthetics."