How well can we quantify global black carbon radiative effects?

Atmospheric aerosols play an important role in the global climate system. Carbonaceous aerosols stand out through their potential to warm (through absorption and semi-direct effects) and cool (through scattering and indirect effects) climate, depending on their microphysical properties, regional distribution and their vertical profile. Current global aerosol models vary drastically in simulated abundance, transport and radiative properties of black carbon and show significant biases when compared to observations. At the same time, "host" models used for the calculation of black carbon radiative forcing show significant differences in components relevant for the assessment of forcing, such as clouds, surface albedos and radiative transfer schemes. This presentation will review the current state of the art in the global assessment of black carbon radiative effects from aerosol models and observationally based forcing calculations, with focus on uncertainties. Particular attention will be given to novel observational constraints arising from advances in measurement technologies and observational strategies as well as to uncertainties in the radiative forcing calculations, as highlighted in the direct forcing experiments of the recent Phase II of the AeroCom aerosol intercomparison project. The identified uncertainties in the process chain, from point of emission through microphysical transformation and transport to the actual radiative transfer, could serve as guidance for future measurement strategies as well as for model improvements aiming to reduce the remaining significant uncertainties in the black carbon radiative effects.