In order to understand the response of the Earth-atmosphere system to anthropogenic aerosol forcing, it is essential to know the relative impact of various aerosol species on the radiative budget. Most of the models used for estimating the direct radiative forcing have assumed that the various aerosol species are mixed externally or internally. However, it is possible that one aerosol species may be coated over another species to form core-shell structure and resulting radiative impact can be significantly different than those of the externally-mixed or internally-mixed aerosols. Our study suggests that changes in the state of mixing of black carbon aerosols may be one of the possible causes for 'excess' atmospheric absorption reported by many investigators. We show that past estimates of climate forcing due to anthropogenic black carbon aerosols represent the lower bound and that the actual values may be larger than the current estimates.