Drivers of Pine Island Glacier speed-up between 1996 and 2016

Jan De Rydt,Fernando S Paolo,G Hilmar Gudmundsson,Ronja Reese

doi:10.5194/tc-15-113-2021

Abstract

Abstract. Pine Island Glacier in West Antarctica is among the fastest changing glaciers worldwide. Over the last 2 decades, the glacier has lost in excess of a trillion tons of ice, or the equivalent of 3 mm of sea level rise. The ongoing changes are thought to have been triggered by ocean-induced thinning of its floating ice shelf, grounding line retreat, and the associated reduction in buttressing forces. However, other drivers of change, such as large-scale calving and changes in ice rheology and basal slipperiness, could play a vital, yet unquantified, role in controlling the ongoing and future evolution of the glacier. In addition, recent studies have shown that mechanical properties of the bed are key to explaining the observed speed-up. Here we used a combination of the latest remote sensing datasets between 1996 and 2016, data assimilation tools, and numerical perturbation experiments to quantify the relative importance of all processes in driving the recent changes in Pine Island Glacier dynamics. We show that (1) calving and ice shelf thinning have caused a comparable reduction in ice shelf buttressing over the past 2 decades; that (2) simulated changes in ice flow over a viscously deforming bed are only compatible with observations if large and widespread changes in ice viscosity and/or basal slipperiness are taken into account; and that (3) a spatially varying, predominantly plastic bed rheology can closely reproduce observed changes in flow without marked variations in ice-internal and basal properties. Our results demonstrate that, in addition to its evolving ice thickness, calving processes and a heterogeneous bed rheology play a key role in the contemporary evolution of Pine Island Glacier.

Highlights

Introduction and motivationSince the 1990s, satellite measurements have comprehensively documented the sustained acceleration in ice discharge across the grounding line of Pine Island Glacier (PIG, Fig. 1) in West Antarctica (Rignot et al, 2002; Rignot, 2008; Rignot et al, 2011; Mouginot et al, 2014; Gardner et al, 2018; Mouginot et al, 2019b)
As detailed in Sect. 2.2.2, perturbations are split between four separate cases: (1) calving (EC3alv); (2) thinning of the ice shelf (EI3SThin); (3) thinning of the ice shelf and grounded ice (ET3hin), which includes associated movement of the grounding line and changes in basal traction; and (4) the combined impact of all the above (EC3alvThin)
Based on the flux changes through Gate 1 and 2, we find that (1) calving and ice thickness changes in combination with a spatially variable, predominantly plastic bed rheology account for 67 and 105 % of flux changes through Gate 1 and 2 respectively, compared to 28 and 64 % for a uniform non-linear viscous sliding law with exponent m = 3; that (2) calving and ice shelf thinning caused an almost identical response in ice dynamics upstream of the grounding line; and that (3) dynamic thinning and grounding line movement account for most of the flux changes between the years 1996 and 2016

Summary

Introduction

Introduction and motivationSince the 1990s, satellite measurements have comprehensively documented the sustained acceleration in ice discharge across the grounding line of Pine Island Glacier (PIG, Fig. 1) in West Antarctica (Rignot et al, 2002; Rignot, 2008; Rignot et al, 2011; Mouginot et al, 2014; Gardner et al, 2018; Mouginot et al, 2019b). The changes in flow speed are an observable manifestation of the glacier’s dynamic response to both measurable perturbations, such as calving and ice shelf thinning, and poorly constrained variations in physical ice properties and basal sliding. Evidence from indirect observations has indicated that changes in ice shelf thickness have occurred since at least some decades before the 1970s (Jenkins et al, 2010; Smith et al, 2017; Shepherd et al, 2004; Pritchard et al, 2012). Due to the dynamic connection between ocean-driven ice shelf melt rates and tropical climate variability (Steig et al, 2012; Dutrieux et al, 2014; Jenkins et al, 2016; Paolo et al, 2018), several model studies have focused on the important problem of simulating the response of PIG to a potential anthropogenic intensi-

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