Black carbon is 'black'!

Black carbon is the second largest man-made contributor to global warming and its influence on climate has been greatly underestimated, according to the first quantitative and comprehensive analysis of this issue. Black carbon has a much greater (twice the direct) climate impact than reported in previous assessments. Black carbon ranks “as the second most important individual climate-warming agent after carbon dioxide”. Cleaning up diesel engines and some wood and coal combustion could slow the warming immediately.

Published in the Journal of Geophysical Research-Atmospheres the study says the direct influence of black carbon, or soot, on warming the climate could be about twice previous estimates.  Accounting for all of the ways it can affect climate, black carbon is believed to have a warming effect of about 1.1 Watts per square meter (W/m²), approximately two thirds of the effect of the largest man made contributor to global warming, carbon dioxide. Co-lead author David Fahey from the U.S. National Oceanic and Atmospheric Administration (NOAA) said, “This study confirms and goes beyond other research that suggested black carbon has a strong warming effect on climate, just ahead of methane.”  The study, a four-year, 232-page effort, led by the International Global Atmospheric Chemistry  (IGAC) Project, is likely to guide research efforts, climate modeling, and policy for years to come.

The international team urges caution because the role of black carbon in climate change is complex.  “Black carbon influences climate in many ways, both directly and indirectly, and all of these effects must be considered jointly”, says co-lead author Sarah Doherty of the University of Washington, an expert in snow measurements. The dark particles absorb incoming and scattered heat from the sun (solar radiation); they can promote the formation of clouds that can have either cooling or warming impact; and black carbon can fall on the surface of snow and ice, promoting warming and increasing melting.  In addition, many sources of black carbon also emit other particles whose effects counteract black carbon, providing a cooling effect.

The study suggests mitigation of black carbon emissions for climate benefits must consider all emissions from each source and their complex influences on climate. Black carbon and its contribution to global warming has been a bone of contention between some academic groups with some believing that it is merely a hype to focus the spotlight on the poor nations where burning wood and charcoal could be seen as a major contributor!