1 Introduction
The West Antarctic Ice Sheet (WAIS) is known to be a geometrically unstable system – thin near its modern margins but thick in the interior, where the ice sheet sits in deep marine basins (Morlighem et al., 2020). Because the flux of ice to the ocean by deformation and sliding scales non-linearly with ice thickness, forced retreat into the thick ice sheet interior triggers a positive feedback, increasing ice discharge and driving a self-sustaining mass imbalance that will proceed without further forcing (the Marine Ice Sheet Instability, or MISI; Weertman, 1974).
At present, WAIS is bounded by floating ice shelves, which play a critical role in buttressing ice stored in the continental interior (Dupont & Alley, 2005; Weertman, 1974). While ice shelf change over the early 21st century has been dominated by ocean driven melt from below, the disintegration of the Larsen B Ice Shelf (LBIS) by surface melt and ice-shelf hydrofracture has demonstrated that other modes of failure are possible (Glasser & Scambos, 2008), leading to a range of studies about calving dynamics and the future of Antarctic ice shelves in a warmer atmosphere (Bell et al., 2018; Kingslake et al., 2017; Lai et al., 2020).
Calving occurs most frequently by the concentration of longitudinal stresses in ice shelves at crevasse tips (Benn et al., 2007). Should WAIS experience widespread ice shelf break-up, thick glaciers could experience stresses at their termini that exceed the fracture toughness of ice. Such configurations lead to cliff failure, a specialized calving process controlled by differential stresses at the free surface of marine terminating ice fronts (Parizek et al., 2019). Cliff failure is thought to proceed faster than traditional calving, but the rate-limiting process is uncertain (DeConto & Pollard, 2016).
The “marine ice cliff instability (MICI)” is the idea that cliff failure, once initiated, accelerates with increasing cliff height. Like with MISI, forced retreat into the WAIS interior by fracture processes would then be self-sustaining. The inclusion of MICI into ice sheet models results in a sea level rise by 2100 equal to eight times the values predicted by models that do not include it (Bulthuis et al., 2019; DeConto & Pollard, 2016; Golledge et al., 2019). While it is not the only source of spread in ice sheet projections, MICI does represent the largest source of uncertainty (Fox-Kemper et al., 2021). As a result, there is a community desire to evaluate the likelihood of MICI and constrain its initiation thresholds, a focus of this work.
Recent models of cliff failure built from physics first principles (Bassis et al., 2021; Bassis & Walker, 2012; Clerc et al., 2019; Crawford et al., 2021; Parizek et al., 2019; Schlemm & Levermann, 2021; Ultee & Bassis, 2016) support MICI’s theoretical validity, but the modes of failure and controlling parameters differ across them. Depending on the prescribed (but uncertain) material strength of ice, cliff-failure thresholds in the literature span from 60 to 540 m high cliffs (Clerc et al., 2019). Ice-sheet wide models rely heavily on process models (and their underlying assumptions about ice strength) to justify parameterizations of ice-terminus dynamics.
There are few geological or geophysical observations capable of improving our understanding of ice cliff dynamics. The paleo-sea-level record may provide indirect evidence of MICI; during several periods in Earth’s history, ice sheet models have difficulty reproducing the observed rapid sea level rise without it (Wise et al., 2016), but there is disagreement in the literature about whether MICI is required to explain historical Antarctic ice loss (Edwards et al., 2019). The Intergovernmental Panel on Climate Change (IPCC) names Crane Glacier’s response to the LBIS collapse as the only potential evidence for MICI behavior in the satellite record (Oppenheimer et al., 2019). Crane Glacier has significantly influenced community thinking about MICI, with its post-LBIS-collapse ice calving rate used to constrain some parameterizations of cliff failure (DeConto & Pollard, 2016). But our knowledge of Crane Glacier’s geometry and behavior following the LBIS collapse is limited by sparse geophysical data, challenging evaluation of whether MICI-style cliff failure occured.
By both identifying what we do know and what we cannot know based on available geophysical data, we can more precisely use Crane Glacier’s behavior following the LBIS collapse event to refine our understanding of ice cliff failure. In this work, we evaluate Crane’s retreat behavior and terminal characteristics against process models, to both identify whether or not unstable retreat by cliff failure was theoretically possible at Crane, and if so, what unknown model parameters must be for failure to occur there.