Mitigation needed to avoid unprecedented multi-decadal North Atlantic Oscillation magnitude

The North Atlantic Oscillation (NAO) dominates winters in Western Europe and eastern North America. Future climate model projections of the NAO are highly uncertain due to both modelled irreducible internal variability and different model responses. Here we show that some of the model spread in multi-decadal NAO simulations is caused by climatological water vapour errors, and develop an emergent constraint that reveals a substantial response of the NAO to volcanic eruptions and greenhouse gases (GHGs). Taking account of the signal-to-noise paradox apparent in these simulations suggests that under the high-emissions scenario the multi-decadal NAO will increase to unprecedented levels that will likely cause severe impacts, including increased flooding and storm damage. This can be avoided through mitigation to reduce GHG emissions. Our results suggest that taking model projections at face value and seeking consensus could leave society unprepared for impending extremes.