| Summary: | Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 46, (2019): 12108-12116, doi:10.1029/2019GL084183. 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. We thank Greg Hirth, Brad Hager, and Bill Durham for their useful comments. The manuscript benefited from constructive reviews by Dan Martin and an anonymous reviewer and editorial handling by Mathieu Morlighem. This work was supported by an NSF‐GRFP (Fiona Clerc), and NSF Awards OPP‐1739031 (Brent Minchew) and EAR‐19‐03897 (Mark Behn). Code reproducing our results is found at this address (https://doi.org/10.5281/zenodo.3379074). 2020-04-21
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