A simulation of radiative forcing and zonal mean temperature effects is presented for the Laacher See eruption (10,900 B.C.). A climate model which also covers the middle atmosphere is forced with background conditions representative of the Younger Dryas period. The Laacher See eruption is simulated by the injection of 15 Mt of SO2 to the lower stratosphere. The resulting sulfate loading in the stratosphere is comparable with post-Pinatubo (1991) observations, but clearly restricted to the Northern Hemisphere. The sulfate aerosol decays with an e-folding time of approximately 11 months, that is, also similar to recent observations. The high concentration of aerosol in the high latitudes leads to strongly intensified cooling in the polar night and to an intensified polar vortex. This produces midlatitude continental winter warming in the troposphere as observed after tropical eruptions. After tropical eruptions, strong absorption of terrestrial and near-infrared solar radiation in the lower latitudes is responsible for the strengthening of the polar vortex. However, for midlatitude and high-latitude eruptions it is the intense irradiance of longwave radiation from the aerosol-containing layer at polar latitudes that produces the meridional thermal gradient enhancing the polar vortex. ¿ 2001 American Geophysical Union