The change in radiative fluxes introduced by sudden increases of stratospheric aerosol associated with volcanic eruptions has been a topic of renewed interest since the eruption of Pinatubo. Directly observed impacts have been documented in a special issue of Geophysical Res. Letters, 19, [1992]. Surface and satellite observations [ Valero and Pilewskie, 1992; Dutton and Christy, 1992; Dutton et al., 1994; Minnis et al., 1993] now provide us with some idea of the magnitude of the radiative forcing associated with the Pinatubo event.
An interesting hypothesis developed from the analyses of Minnis et al. [1993] which follows from the lidar analyses of Sassen [1992] and the model study of Jensen and Toon [1992] is that stratospheric aerosol may directly impact on the formation of cirrus clouds and thus introduce an indirect climate forcing much in the way of the Twomey effect.