Tipping point in ice-sheet grounding-zone melting due to ocean water intrusion

https://www.nature.com/articles/s41561-024-01465-7

Abstract
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. 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. Additionally, this tipping point may not be easily detected with early warning indicators. Although completely unbounded intrusions are not expected in practice, this suggests a mechanism for dramatic changes in grounding-zone behaviour, which are not currently included in ice-sheet models. 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.

Main
There is growing evidence suggesting that ice-sheet models lack representation of important physical processes driving ice sheet retreat (for example, refs. 1,2,3,4,5,6,7), rendering their projections of sea-level rise less sensitive to climatic changes than they should be. Point-wise observations1,2 suggest that ice-shelf basal melt rates are considerably smaller than those typically required by models to reproduce observed retreat rates. Moreover, ice-sheet models systematically underestimate recent ice loss7 and struggle to reproduce observationally constrained sea-level highstands from previous interglacials3,4. Palaeoclimate ice-sheet reconstructions have largely been able to reproduce low-end estimates only when mechanisms to boost sensitivity to climatic forcing are invoked8,9,10. Recently, evidence from diverse sources has emerged suggesting that relatively warm ocean water can intrude long distances upstream of ice-shelf grounding lines5,6,11,12,13,14; such long intrusions have dramatic consequences for sea-level-rise contributions from ice sheets5,15, making them a candidate mechanism to reconcile modelled and observed sea-level rise. Here we investigate how previously ignored feedbacks between melting and the confining ice geometry make this intrusion mechanism even more powerful.
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