Black carbon – common soot -- not only causes smog but is the number two contributor to global warming after carbon dioxide, a U.S. researcher says. A four-year assessment by an international panel that included University of Washington atmospheric scientist Sarah Doherty found black carbon, the soot particles in smoke and smog, contributes about twice as much to global warming as previously estimated. The estimate is far greater than that made in 2007 by the Intergovernmental Panel on Climate Change. \"We were surprised at its potential contribution to climate,\" said Doherty, one of four coordinating lead authors of the study. Some previous research had suggested black-carbon emissions from sources like open burning of forests, crops and grasslands, and from energy-related emissions in Southeast Asia and East Asia, were being underestimated, she said. A bright spot in the study is the finding that since soot only remains in the atmosphere a short time, controlling soot emissions can result in more immediate climate benefits than trying to control carbon dioxide, which can linger in the atmosphere for years, Doherty said. \"We hope reducing black-carbon emissions buys us some time,\" she said. \"But it doesn\'t replace cutting back on CO2 emissions.\" The study has been published in the Journal of Geophysical Research-Atmospheres.