The winds in the polar stratosphere affect the weather in the mid-latitudes, making it important to understand potential changes in response to global warming. However, climate model projections disagree on how this so-called polar vortex will change in the future. Here we show that sea ice loss in the Barents and Kara (BK) seas plays a central role in this. The time when the BK seas become ice-free differs between models, which explains some of the disagreement regarding vortex projections.