We illustrate how climate forcing by stratospheric aerosols depends on aerosol properties. The climate forcing is a function of aerosol size distribution, but the size dependence can be described well by a single parameter: the area-weighted mean radius, reff. If reff is greater than about 2&mgr;m, the global average greenhouse effect of the aerosols exceeds the albedo effect, causing a surface heating. The aerosol climate forcing is less sensitive to other characteristics of the size distribution, the aerosol composition, and the altitude of the aerosols. Thus, stratospheric aerosol forcing can be defined accurately from measurements of aerosol extinction over a broad wavelength range. ¿American Geophysical Union 1992