Marine Ice Cliff Instability Mitigated by Slow Removal of Ice Shelves

Abstract

The accelerated calving of ice shelves buttressing the Antarctic Ice Sheet may form unstable ice cliffs. The marine ice cliff instability hypothesis posits that cliffs taller than a critical height (~90 m) will undergo structural collapse, initiating runaway retreat in ice‐sheet models. This critical height is based on inferences from preexisting, static ice cliffs. Here we show how the critical height increases with the timescale of ice‐shelf collapse. We model failure mechanisms within an ice cliff deforming after removal of ice‐shelf buttressing stresses. If removal occurs rapidly, the cliff deforms primarily elastically and fails through tensile‐brittle fracture, even at relatively small cliff heights. As the ice‐shelf removal timescale increases, viscous relaxation dominates, and the critical height increases to ~540 m for timescales greater than days. A 90‐m critical height implies ice‐shelf removal in under an hour. Incorporation of ice‐shelf collapse timescales in prognostic ice‐sheet models will mitigate the marine ice cliff instability, implying less ice mass loss.

Long‐term projections of sea‐level rise from ice sheets

“…Whether this constitutes an emerging consensus between ice sheet models remains to be seen, but it nonetheless illustrates that there is reasonable agreement between models that use traditional ice flow schemes. Since new studies investigating ice shelf hydrofracture (Robel & Banwell, ) and the structural stability of vertical ice cliffs (Clerc et al, ) now appear to show that both processes tend to proceed much less catastrophically, if at all, than initially proposed (Pollard et al, ), the role either of these mechanisms may play in AIS dynamics remains far from well understood.…”

“…Furthermore, while it is clear that both ice shelf hydrofracture and tidewater glacier calving do occur, they have never been observed at the scale and for the length of time proposed by DeConto and Pollard (). Since their simulations therefore represent a “no‐analogue” state that also requires unrealistically fast ice shelf collapse (Clerc, Minchew, & Behn, ; Robel & Banwell, ), it makes predictions based on these mechanisms very difficult to verify. Equilibrium ice sheet responses and the associated SLR during periods of past warmth (in particular, the mid‐Pliocene warm period, ca.…”

Early Last Interglacial ocean warming drove substantial ice mass loss from Antarctica

“…When used to drive ice sheet models, these climate anomalies are not sufficient to remove the floating ice shelves that buttress ice flow from central Antarctica (20). In an attempt to bypass these problems, ice sheet models have been driven by a wide range of prescribed climate scenarios; however, these suggest widely different sensitivities dependent on model physics and parameterization (21,22), with >2°C (and in some instances >4°C) ocean warming required for the loss of the WAIS, exceeding paleoclimate estimates (3,9,20,23) and different sensitivities of Antarctic ice sheet sectors (18,24,25).…”

A simple model of mélange buttressing for calving glaciers

“…The concept of cliff calving and a cliff calving instability is not without criticism. According to Clerc et al (2019), the lower part of the glacier terminus where shear failure is assumed to occur (Bassis and Walker, 2011;Schlemm and Levermann, 2019) is actually in a regime of thermal softening with a much higher critical stress and thus remains stable for large ice thicknesses.…”