Contribution of aerosols to climate change results from two effects: clear-sky and cloudy-sky forcing. The clear-sky climate forcing by carbonaceous aerosols from biomass burning and fossil fuel burning depends on the relative contribution of scattering and absorption by the aerosols which in turn depends on the fraction of aerosol mass associated with black carbon and its size distribution. This paper reviews estimates for the emission of carbonaceous aerosols, placing these estimates in the context of estimates for the emissions of anthropogenic and natural sulfate aerosols and natural sources of organic particulate matter. The cloudy-sky forcing from carbonaceous aerosols is difficult to estimate because, among other factors, it depends on the amount of absorption by the aerosols in the cloud. It is also highly sensitive to the assumed pre-existing, natural aerosol abundance. An upper limit for this cloudy-sky forcing is -4.4 W/m{sup 2}, but may range as low as -2.4 W/m{sup 2}, depending on background aerosol concentrations. These estimates do not yet account for absorption of radiation by black carbon associated with cloud or the presence of pre-existing dust particles
