The strength of the Atlantic Meridional Overturning Circulation, a key indicator of the climate state, is maintained by the subduction of dense water that feeds the deep southwards branch. At present, this subduction occurs almost entirely in the subpolar region, in the Labrador, Irminger and Nordic seas; however, whether this will continue under climate change is unknown. Here we use a quantitative Lagrangian diagnostic applied to climate model output to show that, in response to warming, the main source regions of this mixed-layer subduction shift northwards to the Arctic Basin and southwards to the subtropical gyre. These shifts are explained by changes in background stratification, mixed-layer depth and ocean circulation, highlighting the need to consider the full three-dimensionality of the circulation and its changes to accurately predict the future climate state.