5 January 2004

A new paper on the role of black carbon, or soot, emissions on climate change—published by James Hansen and Larissa Nazarenko of the US NASA’s Goddard Institute—suggests that black carbon (BC) emissions directly contribute to worldwide melting of ice, a phenomenon that has been usually attributed solely to global warming. The ice melting effect is caused by reducing the snow albedo (i.e., its ability to reflect sunlight) by soot.

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This phenomenon, in turn, further magnifies the global warming effect, especially in areas at high latitude in the Northern Hemisphere, during winter months. Polar ice and snow reflect light from the sun. The amount of reflected sunlight is decreased as the ice and snow covered areas diminish through melting, as well as due to the increased energy absorption by the darker, soot-covered snow. The sunlight that is not reflected into space is absorbed into land and oceans, raising the overall temperature. The contribution of black carbon to global warming has been recognized for many years, but scientists disagree on its magnitude. According to Hansen, the role of black soot has been more prominent than previously thought. Black soot may be responsible for as much as 25% of observed global warming over the 1880-2000 period, concludes Hansen in his paper.

The study was widely publicized in the press and on the internet, with some reports misquoting the NASA findings and linking the effect in its entirety to diesel particulate emissions. The paper deals with total BC emissions, originating from various sources, which change geographically and also historically. However, the relative contribution of diesel engines to BC emissions, although not precisely known, is considered significant, especially in developed countries. Burning of biofuels is believed to be a significant BC contribution in Asia.

Hansen also notes that “soot contributions to climate change do not alter the conclusion that anthropogenic greenhouse gases have been the main cause of recent global warming and will be the predominant climate forcing in the future”. Restoring snow albedos to their pristine values would introduce a future cooling tendency, partially opposing the greenhouse gas warming effect. It should be noted that BC is believed to have a short lifetime in the atmosphere, on the order of weeks or months, compared to some 100 years for carbon dioxide. Thus, control of BC emissions could bring immediate environmental benefit, while greenhouse gas emission effects would linger for generations.

James Hansen is one of the pioneer scientists who started the current debate on global warming and recognized the role of man-made emissions in climate change. “With a high degree of confidence we could associate the warming with the greenhouse effect,” said Hansen back in 1998, when speaking before the US Senate Energy Committee.