Radiative Forcing by Anthropogenic Aerosol - Best Estimates and Uncertainties

The impact of a modified element in the Earth's Atmosphere is commonly quantified by changes it imposes on the Earth's radiative energy balance, also termed its 'radiative forcing'. Of particular interest, in understanding current and future climates is the radiative forcing that can be attributed to man-made atmospheric modifications. However, its quantification requires proper atmospheric representations not only for current conditions but also for a (human-) undisturbed reference state. Since the reference usually cannot be measured our understanding is largely derived from model simulations: Enhancements to greenhouse gas concentrations have retained (to the Earth-Atmosphere-System) radiative energy, to 'warm' our planet. The impact of enhanced anthropogenic aerosol, in contrast, is less clear and spatially much more diverse. On a global annual basis (anthropogenic) aerosol is expected to slow greenhouse gas warming. Even when excluding indirect (feedback) effects on clouds or the hydrological cycle, the impact uncertainty solely due to the presence of anthropogenic aerosol has remained large. This 'direct forcing' uncertainty, however, results not only from difficulties to quantify anthropogenic aerosol but also from the difficulties to quantify environmental properties at sufficient accuracy (e.g. clouds, surface below). Based on a hybrid approach, which incorporates quality data-sets on aerosol and environmental properties into typical properties offered by global modeling, new estimates for the aerosol direct radiative forcing estimates are provided (e.g. -0.4W/m2 at the top of the atmosphere, -2.4W/m2 at the surface). This approach then serves as the basis for sensitivity studies to study the impact of uncertainty to individual input properties, which are eventually summarized to provide estimates for the overall uncertainty of the aerosol 'direct forcing'. These sensitivity studies are further applied to provide insights on biases introduced by simplifying assumptions, in particular frequent assumptions associated with the terms 'anthropogenic' and 'all-sky'.