The glaciers of the Amundsen Sea Embayment (ASE) are some of the fastest-thinning in Antarctica. Future ice loss from this region depends on the trajectory of ocean warming. However, the response of glaciers to this warming depends to some extent on how they are initialised, or calibrated, to match observations. The relative importance of these dual factors of forcing and initialisation remains poorly understood. We carry out climate scenario-forced, synchronously coupled ice-ocean simulations of the ASE extending to the 23rd century. We conduct four experiments varying ice-sheet initialisation method (Snapshot versus Transient Calibration) and far-field ocean forcing (baseline climatology versus an RCP8.5 scenario). We find that the mode of ice-sheet initialisation dominates the trajectory of Thwaites Glacier volume loss throughout the 21st century, while climate forcing emerges as the primary control in subsequent centuries — a contrast explained by the low buttressing of Thwaites ice shelf prior to substantial grounding-line retreat. Under RCP8.5 forcing, Thwaites alone contributes up to 2.6 mm/a sea-level equivalent by 2200 following its retreat past Upper Thwaites Ridge, with the full ASE exceeding 3.2 mm/a. Under current ocean conditions, loss rates stabilise near 1.7 mm/a, partly due to growth of pinning points due to a thicker ice shelf compared to RCP8.5. Our results suggest that ASE ice loss remains sensitive to climate forcing even after retreat into the deep interior, implying that emissions mitigation could delay the trajectory of sea-level rise from this region – but that the effects of mitigation would not be felt for nearly a century.