Climate forcing by anthropogenic aerosols has recently excited growing interest [Charlson et al. 1987; Charlson et al., 1990; Wigley, 1991; Charlson et al., 1992>. It has earlier been shown that increased cloud droplet concentrations can increase cloud albedos [e.g. Twomey et al., 1984>. Wigley [1991> and Charlson et al. [1990> have pointed out that the Northern Hemisphere may be warming more slowly than the Southern Hemisphere. According to them this is mainly doe the direct and indirect effects of sulphate aerosols. In this report we discuss an alternative mechanism which could enhance indirect climate forcing by aerosols. Increased concentrations of condensable vapours (such as HNO3 and HCl) in the atmosphere could activate an increased number of pre-existing aerosol particles to act as cloud condensation nuclei [Kulmala et al., 1993>. This would cause similar effects on optical properties of clouds as an increase of aerosol particles in the atmosphere. ¿ American Geophysical Union 1995