Monitoring changes in the position of the boundary where marine-terminating glaciers transition from grounded to floating, known as the grounding line, is critical for advancing our understanding of glacier stability. The grounding line is difficult to observe directly but can be inferred from the ice surface tidal-flexure zone. This zone spans from the inland limit of tidal flexure – the hinge line, across the grounding line to the hydrostatic line, where the glacier becomes freely floating. We explore flexure-zone changes on Milne Glacier, Nunavut, Canada from 2023 to 2024 at a high spatial resolution of 10 m using 4 d RADARSAT Constellation Mission (RCM) data acquisitions from four different orbits. The Double Difference Interferometric Synthetic Aperture Radar (DDInSAR) technique was employed to delineate 116 flexure zones through time, which were then examined with respect to SAR viewing geometry, tides and atmospheric pressure variability, glacier velocity changes, and ice bending, as well as ice surface and bed topography. The most reliable flexure-zone delineation (horizontal uncertainty of ±8 m) was achieved with ascending RCM tracks at low incidence angles during periods when the ice velocity was steady and bending was not substantial, resulting in a combined SAR line-of-sight error of 0.13 cm. A worst-case flexure-zone delineation uncertainty of ±222 m was noted when a descending track with a high incidence angle was used and glacier velocity became more variable, increasing the combined SAR line-of-sight error to 3.26 cm. Low-error DDInSAR images enabled the detection of a newly developed ∼2.4 km grounding-line retreat associated with the emergence of a new pinning point at the eastern margin of the glacier. Our results underscore that care must be taken to ensure that the low-error DDInSAR results are used when monitoring changes in grounding-line position and that pinning points alter the grounding-line short-term migration and its long-term retreat.
Thermokarst lakes are critical components of Arctic carbon cycling, yet their ice phenology remains poorly characterized at regional scales. We present the first comprehensive analysis of ice-on and ice-off timing across 30 774 lakes on the Alaska North Slope using Sentinel-1 synthetic aperture radar (SAR) classified by a Random Forest (RF) model trained using labels derived from Sentinel-2 optical imagery and ERA5 temperature data for the period 2019–2023. Our RF classifier achieved 94 % accuracy for ice state detection, enabling phenology retrieval for 97 % of lake-years. Results revealed a mean ice-free period of 114 d (standard deviation = 25 d), with ice-off occurring at day-of-year 159 (8 June) and ice-on at day-of-year 273 (30 September). Spatial analysis demonstrated strong latitudinal control on ice phenology, with ice-free duration decreasing by 23 d per degree northward. Lake morphology (area, circularity, convexity, and shoreline development index) showed modest but significant effects on ice timing after controlling for latitude effects, with shoreline development index and convexity each contributing ∼ 3 d variation across typical lake ranges. Comparison of the RF model and simplistic accumulated degree-day (ADD) model-detected ice phenology yielded a convincing match, where the offsets in ice phenology between the models fell within twice the per-detection observation gap for approximately 80 % of the lakes. Furthermore, these offsets exhibited the same strong latitudinal control and negligible effects of lake morphology. These lake-specific phenology dates provide timing and duration constraints for future methane studies using high-resolution sensors and provide baseline phenology data essential for understanding how continued Arctic warming will affect thermokarst lake dynamics and associated carbon cycle feedbacks.
Mountain glaciers provide an irreplaceable water resource in High Mountain Asia, with a significant proportion of water input to rivers coming from glacial meltwater. However, the volume of water held in these glaciers and their evolution over the coming decades is subject to great uncertainty. The reliability of existing glacier ice thickness estimates in High Mountain Asia is limited by the use of low-order models, known to be locally unreliable on mountain glaciers, to describe the relationship between ice velocity and thickness, and by the scarcity of measured thicknesses available for constraint and validation at the time those estimates were produced. We use the Instructed Glacier Model (v2.2.3), a deep-learning-based high-order ice flow model with the capability to invert observed glacier surface velocity for ice thickness, to construct an estimated thickness map of Bhote Kosi glacier. Our thickness inversion is constrained using data collected via a novel airborne radar method for measuring ice thickness. We perform an in-depth case study, carefully justifying inversion parameter choices and quantifying the accuracy of our results. We demonstrate that in the absence of thickness observations, results can be optimized via the use of an L-curve to select the regularization parameter, with significant bias in the unconstrained results, but comparable accuracy to leading thickness estimates. We find that while thickness-constrained inversions are able to correct the modelled thickness field where there is limited information from observed surface velocity, cross-validation experiments demonstrate that the “interpolative power” of thickness observations is weak.
Surge-type behaviour and low-angle glacier detachment are distinct but potentially linked forms of dynamic instability that remain insufficiently documented in small debris-covered mountain glaciers. This study investigates the 2025–2026 event at Dehdal Glacier in the northwestern Pamirs, which we interpret as a surge-detachment sequence, using daily PlanetScope imagery. Multi-epoch uncrewed aerial vehicle (UAV) orthomosaics and digital elevation model (DEM) differencing from 2019 to 2023 reveal pre-event mass redistribution, evidenced by surface thickening of the central tongue and surface lowering in both the upper glacier and the distal tongue, indicating pre-surge ice mass redistribution. Daily PlanetScope imagery documents visible surface deformation before the first detachment, renewed advance of the dynamically connected glacier tongue following the first detachment, and widespread acceleration coincident with the second detachment. Together, these observations support a surge–detachment sequence rather than an isolated glacier detachment. Two low-angle detachments displaced ice up to 4.7 km down-valley, depositing an estimated 4–5×106 m3 of ice on the valley floor. By 12 January 2026, the glacier had largely stabilized, with the primary glacier tongue covering approximately 1.83 km2 and a detached ice mass of about 0.26 km2 persisting farther downstream, resulting in a total ice area of roughly 2.09 km2. Notably, the recurrence interval of 9–10 years since the previous surge is substantially shorter than the earlier intervals of approximately 20–45 years. This shortened interval is unlikely to reflect complete reservoir regeneration, and no clear seismic trigger was identified. The event occurred during warmer, drier conditions than the 1981–2010 baseline, with mean monthly temperature anomalies reaching +1.11 °C. These findings provide new insights into glacier flow instabilities in High Mountain Asia and highlight the geomorphic impacts of surge-related acceleration followed by localized mechanical detachment in small debris-affected mountain glaciers.
Antarctic surface mass balance (SMB) projections are a major source of uncertainty in future sea-level rise estimates, and regional climate models (RCMs), thanks to their finer resolution and polar-tailored physics, are increasingly used to refine and complement Earth System Model (ESM) projections. Here, three recent downscalings of the ESM CESM2 with the RCMs MAR, RACMO, and HIRHAM under SSP5-8.5 produce consistent contemporary Antarctic SMB, but diverge strongly by 2100, especially over ice shelves. Projected SMB anomalies there range from a large decline in HIRHAM to a moderate decrease in MAR, while RACMO maintains near balance. These differences mainly reflect contrasting melt–albedo feedbacks and present-day melt and refreezing levels, particularly in HIRHAM, where melt and refreezing rates are about five and four times higher than both MAR and RACMO, reaching levels the other two models only attain by the end of the century, along with a persistent 1–2 °C temperature offset between MAR and RACMO. CESM2 shows a decline similar in magnitude to MAR, with high melt partly compensated by extensive refreezing. Over grounded ice, all models project increased SMB from higher snowfall, though runoff still drives their SMB spread. Despite identical boundary conditions and similar present-day SMB, the four models diverge substantially by 2100. Yet CESM2, despite its coarser resolution as an ESM, reproduces integrated behavior consistent with MAR. These results show that inter-model agreement on present-day SMB does not guarantee reliable future projections, underscoring the need for continued RCM development to better constrain the Antarctic contribution to sea-level rise.
