Aerosol particles scatter and absorb radiation and interact with cloud particles. The net aerosol forcing since preindustrial times is negative and has offset part of the greenhouse gas warming. It dominates the uncertainty of the overall anthropogenic forcing. This large uncertainty results from gaps in our knowledge on the underlying aerosol and cloud microphysical processes and aerosol-cloud interactions as well as their representation in global coupled aerosol-climate models. A recent paper by Nazarenko et al. (2017) illustrates how the anthropogenic aerosol forcing, especially the effective radiative forcing that includes aerosol-cloud and aerosol-radiation interactions, depends on climate feedbacks and on the employed aerosol model.