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The Antarctic Ice Sheet (AIS) response to past warming consistent with the 1.5–2°C "safe limit" of the United Nations Paris Agreement is currently not well known. Empirical evidence from the most recent comparable period, the Last Interglaciation, is sparse, and transient ice‐sheet experiments are few and inconsistent. Here, we present new, transient, GCM‐forced ice‐sheet simulations validated against proxy reconstructions. This is the first time such an evaluation has been attempted. Our empirically constrained simulations indicate that the AIS contributed 4 m to global mean sea level by 126 ka BP, with ice lost primarily from the Amundsen, but not Ross or Weddell Sea, sectors. We resolve the conflict between previous work and show that the AIS thinned in the Wilkes Subglacial Basin but did not retreat. We also find that the West AIS may be predisposed to future collapse even in the absence of further environmental change, consistent with previous studies. Plain Language Summary: Ice sheets can respond to climatic warming in complex ways, commonly only reaching a new state of balance many hundreds or even thousands of years after the initial change in climate has occurred. Here, we investigate how the Antarctic Ice Sheet (AIS) responded to a period of prolonged warmer‐than‐present climate that took place around 125,000 years ago. At this time the global climate was only around 1–2°C above present, but geological records show that the global sea level was at least 6 m, or maybe even as much as 9–11 m, higher than today. Our study shows that around 4 m of this could have come from Antarctica. Our model agrees well with geological evidence of enhanced ice discharge both close to the ice sheet and further afield. Applying this model to the future our experiments suggest that the West AIS may already have been sufficiently destabilized to trigger a long‐term sea‐level contribution of up to 4 m, even without further greenhouse gas emissions. Key Points: We present data‐constrained simulations of the Antarctic Ice Sheet through the Last InterglaciationOur model predicts a maximum contribution to global mean sea level of 4 m at 126 ka BPLoss of much of the present‐day West Antarctic Ice Sheet is already committed under current climate [ABSTRACT FROM AUTHOR]

Increased ocean heat supply to the Antarctic continental shelves is projected to cause accelerated ice sheet loss and contribute significantly to global sea-level rise over coming decades. Changes in temperature or salinity of dense shelf waters around Antarctica, resulting from increased glacial meltwater input, have the potential to significantly impact the location and structure of the global Meridional Overturning Circulation, with seabed irregularities such as submarine canyons, driving these flows toward the abyss. Submarine canyons also influence the location of intruding warm water currents by acting as preferential routes for rising Circumpolar Deep Water. These global changes have implications for large-scale effects to atmospheric and oceanic circulation. The ability for numerical modellers to predict these future behaviours is dependent upon our ability to understand both modern and past oceanic, sedimentological and glaciological processes. This knowledge allows ocean models to better predict the flux and pathways of Circumpolar Deep Water delivery to the shelf, and consequently to ice shelf cavities where melt is concentrated. Here we seek to understand how dense shelf water and other continental slope processes influence submarine canyon morphology by analysing newly collected geophysical and oceanographic data from a region of significant and prolonged dense shelf water export, the Hillary Canyon in the Ross Sea. We find that cascading flows of dense shelf water do not contribute to significant gully incision at the shelf edge during interglacial periods, however, are strong enough to prevent gully infilling and contribute to canyon-levee aggradation down-slope. We find buried paleo-gullies beneath gullies incising the modern seafloor. Paleo-gullies occur as single gullies and in complexes indicating that gully activity was continuous over multiple glacial cycles and formed an important role in the development of the shelf edge and upper slope. Glacial cycles likely drive large-scale shifts in canyon head processes with periods of intense seafloor erosion and significant gully incision likely occurring when ice grounded near to the shelf edge, during glacial and deglacial periods, when sediment-laden subglacial meltwater was released at the shelf edge. We put slope morphology observed at the Hillary Canyon head into global perspective to show that cascading flows of dense shelf water do not exert consistent patterns of erosion on high-latitude continental margins. Unlabelled Image • New geophysical, sediment and oceanographic data show Antarctic canyon dynamics. • Irregular gully distribution along shelf edge show varying ice extent. • Dense shelf water does not cause significant gully erosion. • Glacial cycles drive large-scale shifts in canyon head processes. [ABSTRACT FROM AUTHOR]