Anthropogenic aerosols are composed of a variety of aerosol types and components including water-soluble inorganic species (e.g., sulfate, nitrate, ammonium), condensed organic species, elemental or black carbon, and mineral dust. Previous estimates of the clear sky forcing by anthropogenic sulfate aerosols and by organic biomass-burning aerosols indicate that this forcing is of sufficient magnitude to mask the effects of anthropogenic greenhouse gases over large regions. Here, the uncertainty in the forcing by these aerosol types is estimated. The clear sky forcing by other anthropogenic aerosol components cannot be estimated with confidence, although the forcing by these aerosol types appears to be smaller than that by sulfate and biomass-burning aerosols. The cloudy sky forcing by anthropogenic aerosols, wherein aerosol cloud condensation nuclei concentrations are increased, thereby increasing cloud droplet concentrations and cloud albedo and possibly influencing cloud persistence, may also be signific... Anthropogenic aerosols are composed of a variety of aerosol types and components including water-soluble inorganic species (e.g., sulfate, nitrate, ammonium), condensed organic species, elemental or black carbon, and mineral dust. Previous estimates of the clear sky forcing by anthropogenic sulfate aerosols and by organic biomass-burning aerosols indicate that this forcing is of sufficient magnitude to mask the effects of anthropogenic greenhouse gases over large regions. Here, the uncertainty in the forcing by these aerosol types is estimated. The clear sky forcing by other anthropogenic aerosol components cannot be estimated with confidence, although the forcing by these aerosol types appears to be smaller than that by sulfate and biomass-burning aerosols. The cloudy sky forcing by anthropogenic aerosols, wherein aerosol cloud condensation nuclei concentrations are increased, thereby increasing cloud droplet concentrations and cloud albedo and possibly influencing cloud persistence, may also be signific...