We use our detailed radiative transfer model and the latitude-altitude distributions of zonal mean optical properties for the Pinatubo aerosol to calculate the time evolution of its instantaneous radiative forcing. We represent the zonal mean of this radiative forcing in terms of the zonal mean optical depth of the Pinatubo aerosol, together with the solar insolation, effective emitting temperature, daylight fraction of a day, and the planetary albedo in the absence of the aerosol. We use this representation, together with the volcano optical depths compiled by Sato et al. <1993>, to calculate the instantaneous and adjusted radiative forcing by volcanic aerosols from 1850 to 1994. ¿ 1999 American Geophysical Union