Tipping point behaviour of submarine melting in ice-sheet grounding zones

Marine ice sheets are highly sensitive to submarine melting in their grounding zones, where they transition between grounded and floating ice. Recently published studies of the complex hydrography of grounding zones suggest that warm ocean water can intrude large distances beneath the ice sheet, with dramatic consequences for ice dynamics. Here, we develop a model to capture the feedback between intruded ocean water, the melting it induces, and the resulting changes in ice geometry. We reveal a sensitive dependence of the grounding zone dynamics on this feedback: as the grounding zone widens in response to melting, both temperature and flow velocity in the region increase, further enhancing melting. Intriguingly, we find that increases in ocean temperature can lead to a tipping point being passed, beyond which ocean water intrudes in an unbounded manner beneath the ice sheet, via a process of runaway melting. This suggests a mechanism for dramatic changes in grounding zone behavior, which are not currently included in ice sheet models, and may enable them to reproduce previous high warm-period sea levels. We consider the susceptibility of present-day Antarctic grounding zones to this process, finding that both warm and cold water cavity ice shelves may be vulnerable. Our results point towards a stronger sensitivity of ice sheet melting, and thus higher sea level rise contribution in a warming climate, than has been previously understood.