Grounding-line retreat of Milne Glacier, Ellesmere Island, Canada over 1966–2023 from satellite, airborne, and ground radar data

Abstract

Milne Glacier is a marine-terminating glacier located on the northern coast of Ellesmere Island in the Canadian High Arctic, a region that has experienced extensive ice-mass loss over the last two decades. Milne Glacier flows into Milne Fiord where it transitions from grounded to floating at its grounding line. The glacier rests on a retrograde slope and is therefore potentially vulnerable to marine ice-sheet instability where enhanced basal melt and grounding-line retreat can trigger further deterioration of the glacier. Glacier changes that occur in the ice flexure zone, which spans from the hinge line, the inland limit of tidal flexure, past the grounding line to the landward limit of hydrostatic equilibrium, are critical for glacier dynamics. In this study, we quantify changes in the Milne Glacier grounding-line position from 1966 to 2023 using satellite, airborne, and ground radar observations. Double difference interferometric analysis of Synthetic Aperture Radar (DDInSAR) images acquired between 1992 and 2023 from European Remote Sensing (ERS-1/2) satellites, Sentinel-1 A/B, and RADARSAT Constellation Mission (RCM) was performed to delineate a timeseries of the hinge line. We used these hinge lines to quantify changes in the grounding-line position as their migration rates are directly correlated. RCM-derived results had the highest spatial resolution (10 m) and the best coherence between 4-day repeat acquisitions, which provided the most continuous and detailed hinge-line delineation across the glacier. We also used airborne and ground-based ice penetrating radar (IPR) data collected between 2014 and 2023 to calculate the normalized bed reflection (NBRP) and internal reflection power (NIRP) coefficients to distinguish between basal returns associated with water versus bed and assess signal attenuation within the ice column. Spatial patterns in NBRP and NIRP coefficients allowed us to reliably separate the floating and grounded parts of Milne Glacier. This alternate way of delineating the grounding line was in good agreement with our satellite-based DDInSAR results. Analysis of historical airborne radar surveys in 1966 and 1981 in conjunction with our more recent NBRP/NIRP analysis revealed a ∼3.1 km retreat (or ∼55 m yr−1) of the grounding line along the glacier centerline over the past 57 years. DDInSAR analysis provided additional details on this shift across the Milne Glacier. ERS and RCM images acquired in 3 and 4-day repeat cycles, respectively, revealed variability in hinge-line positions. This information allowed us to estimate the landward and seaward bounds, associated with the short-term hinge-line migration, that were used to quantify the grounding-line retreat over years. The grounding line retreat was highly asymmetric with the grounding-line retreating at over twice the average rate near the western margin (124 m yr−1) than at the center (53 m yr−1) of the glacier between 2011 and 2023. The calculated average retreat rates of grounding line showed a close association with changes in the ocean temperatures and subglacial discharge. Our study demonstrated that satellite-based monitoring of the hinge line at high spatiotemporal resolution is crucial to better assess the grounding line short-term positional variability and reliably quantify its long-term retreat. Airborne and ground-based radar observations can provide additional in-situ information to explain changes in the grounding line that affect glacier dynamics and viability.