Decomposition of Radiative Forcing by Black Carbon in CESM1

Black carbon particles can influence climate through a variety of mechanisms that alter the planetary energy balance, including direct effects, semi-direct effects, indirect effects, and effects on surface albedo. The short lifetime of black carbon in the atmosphere and its significant radiative forcing suggest that reductions in black carbon emissions might provide a rapid means for reducing global warming. However, uncertainty in estimates of radiative forcing by black carbon is great. Moreover, most processes that emit black carbon also emit scattering components such as organic carbon, which partially cancel the warming effect of black carbon. The treatment of aerosols in CESM1 is designed to represent all of these mechanisms in a physically-consistent manner. In this presentation we will first evaluate the present-day black carbon distribution and aerosol absorption optical depth simulated by the modal aerosol model in CESM1, and then use the model to provide estimates of each radiative forcing mechanism due to emissions from each of several important anthropogenic sources of black carbon and associated co-emitted species. Estimates with the three-mode and seven-mode representations of the aerosol size distribution will be compared for pre-industrial, present day, and several IPCC AR5 emissions scenarios for year 2100. These estimates will be normalized by emissions to provide estimates of the potential for mitigating global warming by reducing emissions from specific sectors.