BLACK CARBON – IMPACT ON CLIMATE AND TARGETS FOR MITIGATION

Assessing the climatic benefits of black carbon mitigation
Research team: Denise Mauzerall, Robert Kopp (post-doc)

To slow the rate of climate warming and avoid “dangerous anthropogenic interference with the climate system”, it will likely be necessary to reduce emissions not only of greenhouse gases but also of air pollutants with high radiative forcing (RF), particularly black carbon (BC) which is commonly known as “soot”. There are large uncertainties, however, in the magnitude of black carbon’s RF. In our recent PNAS paper (Kopp & Mauzerall, 2010) we performed a meta analysis of key recent studies to obtain a best estimate of effective RF which we use to analyze the benefits of mitigating BC emissions.  We found that failing to reduce carbonaceous aerosol emissions from contained combustion would require CO2 emission cuts about 8 years (range of 1 to 15 years) earlier than would be necessary with full mitigation of these emissions in order to achieve the 500ppmv CO2 equivalent. Organic carbon (OC) aerosols are often co-emitted with BC. OC aerosols are reflective and hence have negative radiative forcing while BC aerosols absorb radiation and hence have positive radiative forcing.  We used the OC/BC emission ratio from a variety of sources along with the RF estimates in our meta-analysis to characterize the likelihood that emissions from specific source types contribute to warming. Based on our analysis we conclude that policies that reduce BC emissions from contained combustion, particularly sources like diesel engines, will reduce climate warming and provide needed additional time to reduce the carbon intensity of our energy economy (Kopp & Mauzerall, 2010). Although just published, this paper has already received wide circulation in both the scientific and policy communities domestically and internationally.

Publication
Kopp, RE and DL Mauzerall. Assessing the climatic benefits of black carbon mitigation. Proceedings of the National Academy of Sciences, 2010.

Origin and climate impact of Black Carbon reaching the Himalayas
Research team: Denise Mauzerall, Monika Kopacz (post-doc) and collaborators

BC deposited on glaciers in the remote and high elevation regions of central Asia likely increase their melting rates. In order to target black carbon emission reduction efforts to locations where they would have the largest climate benefit it is helpful to know from where the BC reaching the glaciers originates. We have used the global chemical tracer model GEOS-Chem with its adjoint to identify the location from which BC arriving at a variety of glacial locations in the Himalayas and Tibetan Plateau originates (Kopacz et al., 2010). We find that emissions from northern India and central China contribute the majority of BC to the region, although the precise location varies with season and receptor location. Contributions from areas as varied as Nepal, African biomass burning and Middle Eastern fossil fuel combustion are also significant. We compute radiative forcing in the snow-covered regions and estimate that the forcing due to the BC induced snow-albedo effect is an order of magnitude larger than the RF from the direct effect, and with significant seasonal variation in the northern Tibetan Plateau. This positive radiative forcing accelerates glacier melting which has implications for water supplies in countries of the region including India, Pakistan, Nepal and China. Our analysis can help inform mitigation efforts to slow the rate of glacial melt by identifying regions that make the largest contributions to BC deposition in the region.

Publication
Kopacz, M, DL Mauzerall, J Wang, DK Henze, E Leibensperger. Origin and radiative forcing of black carbon transported to the Himalayas and Tibetan Plateau, Atmospheric Chemistry Physics Discussions, 2010.