Water vapor plays a significant role in the chemistry and radiation budget of the stratosphere. One of the strongest signals in global change is the observed long term increase of water vapor in the stratosphere by ~ 2 ppmv, i.e. nearly a doubling since the mid-1950s1,2. This increase is only partly understood and several processes have been suggested to contribute to the stratospheric water vapor change. Here we suggest a mechanism that links increasing anthropogenic SO2 emissions in southern and eastern Asia with an increase in stratospheric water. Trajectory studies and model simulations suggest that the SO2 increase results in the formation of more sulfuric acid aerosol particles in the upper tropical troposphere and, as a consequence, to more ice crystals of smaller size in the tropical tropopause layer, which are lifted into the stratosphere more readily. Our studies suggest that the effect of increasing SO2 emissions since the mid-1950s may have caused up to 50% of the currentlyunexplained stratospheric water vapor increase.