Putting Lipstick on a PIG: Modeling Pine Island Glacier (PIG) Shear Margin Collapse with Compressive Arch Failure and Observations

Abstract

Pine Island Glacier (PIG) drains 10\% of the West Antarctic Ice Sheet and has undergone rapid change in the observational record, contributing to uncertainty in sea level rise projections. The Pine Island Ice Shelf (PIIS), which provides a key buttressing force that slows the flux of ice across the grounding line, has accelerated 800 m/yr (an approximate 20\% increase in speed) between 2015 and 2024, accompanied by a visible increase in damage in the Southern shear margin, indicating a partial loss of buttressing. We examine this loss of buttressing to determine the mechanisms through which ice shelves collapse. Buttressing allows an ice shelf to increase in thickness to a point at which the stresses within the ice would exceed the tensile yield strength without the compression provided by buttressing. Following the Compressive Arch Theory proposed by \textcite{doake_breakup_1998}, we hypothesize that when a calving event decouples the ice shelf from a buttressing region, the thicker ice shelf is thrown into tension and rapidly collapses, as happened with the Larsen B Ice Shelf in 2002. We use the Ice-sheet and Sea-level System Model to investigate the instantaneous stress response to loss in buttressing on an idealized glacier, with the goal of finding the changes in shear margin buttressing that most accurately recreate observed changes. In our model, we are only able to replicate observed changes in stress regime by decoupling both shear margins, suggesting the PIIS is currently providing negligible buttressing, allowing PIG to accelerate, thin, and retreat. We construct a timeline of shear margin evolution and collapse over the PIIS from 2015 to 2024 using model outputs of stress field response to changes in buttressing, coupled with observed changes in velocity, effective and principal strain rates, and calving events. Despite losing buttressing from both shear margins, the PIIS is still intact, contrary to our initial hypothesis on compressive arch failure. We re-frame Compressive Arch Theory to better capture the timescales involved in loss of buttressing. We posit that compressive arch failure from loss of buttressing on short time scales leads to rapid ice shelf disintegration, whereas compressive arch failure occurring on longer time scales allows the ice to viscously relax, leading to ice shelf thinning instead of collapse. This new framework for investigating loss of buttressing allows us to better assess the stability of ice shelves and more accurately model future Antarctic contributions to sea level rise.