Tuesday, January 22, 2013

Revised Estimates of Radiative Forcing Due To Black Carbon

A 232 page paper with 29 authors is in press at Journal  of  Geophysical  Research-­Atmospheres titled: "Bounding the  role of black  carbon in the climate system: A Scientific Assessment." The bottom line is that previous studies have underestimated the global warming effect (radiative forcing) of black carbon by about three times. With the authors' new much higher estimate it is second only to carbon dioxide in its positive contribution to radiative forcing. If this is true, I think it is good news as it means that the climate sensitivity due to greenhouse gases based on recent data is then probably less than previously thought. Projections from the RCP exercise, show black carbon emissions declining in the future.

The problem is that this effect is very uncertain both in terms of emissions and the radiative forcing effect. From p2 of the paper:

"Total global emissions of black carbon using bottom-­up inventory methods are 7500Gg yr-­1 in the year 2000 with an uncertainty range of 2000 to 29000. However, global atmospheric absorption attributable to black carbon is too low in many models, and should be increased by a factor of almost three. After this scaling, the best estimate for the industrial-­era (1750 to 2005) direct radiative forcing of atmospheric black carbon is +0.71Wm-­2 with 90% uncertainty bounds of (+0.08, +1.27) Wm-­2."

A 95% confidence interval would stretch from roughly -0.04 to +1.38. In other words we can't reject the hypothesis that the effect is zero at the 5% significance level.

The lead author Tami Bond has a two page summary available.

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