Antarctic Ice Sheet projections show the highest sensitivity to increased basal melting in the Amundsen Sea sector. However, little is known about the processes controlling future increase in melt rates. We build an ensemble of three ocean–sea-ice–ice-shelf simulations for both the recent decades and the late 21st century, constrained by regional atmosphere simulations and the multi-model mean climate change of the 5th Climate Model Intercomparison Project under the RCP8.5 scenario. The ice shelf melt rates are typically multiplied by 1.4 to 2.2 from present day to future, for a total basal mass loss increased by 347Gt/yr. This is approximately equally explained by advection of warmer water from remote locations and by regional changes in Ekman downwelling and in the ice-shelf melt-induced circulation, while increased iceberg melt plays no significant role. Our simulations suggest that high-end melt projections previously used to constrain recent sea level projections may have been significantly overestimated.