Benjamin J. Wallis

Synthetic Aperture Radar (SAR) offset tracking is widely used to measure ice velocity on floating ice shelves, however, vertical displacement caused by ocean tides projects into the radar-range direction, introducing apparent horizontal motion unrelated to true ice flow. Here, we develop an observation-driven tide-correction for SAR-derived ice speed measurements using differential range offset tracking (DROT) applied to Sentinel-1 SAR data. The method estimates spatially and temporally varying tidal response coefficients directly from observed radar-range displacement variability and an ocean tide model. Application of the method to SAR-derived ice velocity measurements on Antarctic Ice Shelves shows spatial variability in the magnitude of the tide-induced ice-speed correction, with an Antarctic-wide mean tide-correction range of 106 m/yr, a standard deviation of 71 m/yr and maximum values exceeding 300 m/yr. The largest tide correction range predominantly occurs on freely floating ice shelves in regions of strong tidal forcing, particularly in the Weddell Sea and Antarctic Peninsula. Smaller tide corrections are observed near grounding zones and pinned regions where tidal motion is mechanically constrained. Time-series analyses show that the tide correction systematically reduces non-physical temporal variability in SAR-derived ice velocity products while preserving the large-scale spatial structure of ice flow. Independent validation using GNSS observations on Brunt Ice Shelf shows improved agreement between tide-corrected satellite-based ice speed and in situ measurements. Our results demonstrate that tide-induced apparent motion is a source of variability in SAR-derived ice velocity products on floating ice shelves, highlighting the importance of applying a correction when tide-induced signals are not required.

Marine-terminating glaciers in central and southeast Greenland are major contributors to Greenland’s mass loss making it essential to monitor their changing behaviour. Here, we use over a decade of Sentinel-1 satellite observations to measure the change in ice speed on 66 glaciers between 2014 and 2025. We observe widespread, synchronous acceleration between 2016 and 2020, during which two thirds of glaciers accelerate and 6 more than double their velocity, with many glaciers reaching their fastest speeds since at least 2000. This period is followed by widespread deceleration after 2020, although the slowdown is smaller in magnitude and more gradual, with most glaciers remaining faster than their pre-2016 values. The acceleration coincides with exceptionally warm surface ocean waters in 2016 (+3 °C anomaly), warming at depth in 2018 (+1 °C anomaly) and a prolonged reduction in regional sea-ice concentration, suggesting ocean-driven processes played a role in driving this change. Our results show that ice dynamic change in southeast Greenland has been more spatially extensive and larger magnitude than previously recognised, highlighting the sensitivity of these glaciers to environmental forcing and their susceptibility to rapid, synchronous change.

Synthetic Aperture Radar (SAR) offset tracking is widely used to measure ice velocity on floating ice shelves, however, vertical displacement caused by ocean tides projects into the radar-range direction, introducing apparent horizontal motion unrelated to true ice flow. Here, we develop an observation-driven tide-correction for SAR-derived ice speed measurements using differential range offset tracking (DROT) applied to Sentinel-1 SAR data. The method estimates spatially and temporally varying tidal response coefficients directly from observed radar-range displacement variability and an ocean tide model. Application of the method to SAR-derived ice velocity measurements on Antarctic Ice Shelves shows spatial variability in the magnitude of the tide-induced ice-speed correction, with an Antarctic-wide mean tide-correction range of 106 m/yr, a standard deviation of 71 m/yr and maximum values exceeding 300 m/yr. The largest tide correction range predominantly occurs on freely floating ice shelves in regions of strong tidal forcing, particularly in the Weddell Sea and Antarctic Peninsula. Smaller tide corrections are observed near grounding zones and pinned regions where tidal motion is mechanically constrained. Time-series analyses show that the tide correction systematically reduces non-physical temporal variability in SAR-derived ice velocity products while preserving the large-scale spatial structure of ice flow. Independent validation using GNSS observations on Brunt Ice Shelf shows improved agreement between tide-corrected satellite-based ice speed and in situ measurements. Our results demonstrate that tide-induced apparent motion is a source of variability in SAR-derived ice velocity products on floating ice shelves, highlighting the importance of applying a correction when tide-induced signals are not required.

Piglet Glacier, formed by ~ 20% area loss of the Pine Island Ice Shelf (2017–2020), is a compact analogue of a larger West Antarctic glacial drainage basin where ice shelf damage and retreat alter inland ice dynamics. Using Sentinel-1 feature-tracking ice velocities (Oct 2014-) and CryoSat-2 swath altimetry (Jul 2010-), we track change through May 2025. Relative to 2015–2017 mean, grounding-line ice speed rose by ~ 40%, and acceleration propagated ~ 50 km inland, with the upstream response lagging the grounding-line site by only ~ 1 month, indicating efficient transmission of reduced buttressing into grounded ice. Firn-corrected altimetry shows intensified dynamic thinning, with basin-integrated volume loss rising from ~ 2.9 to ~ 4.5 km ³ yr ^-1 (~ 60%). Downstream, post-calving acceleration focused along pre-damaged shear margins, and mélange loss in 2024–2025 coincided with further mechanical decoupling. Sustained shear-margin attrition and frontal retreat appear to be contributing to continued mass loss from Piglet Glacier and progressive fragmentation of the wider Pine Island system. These observations provide a rare benchmark showing how ice shelf retreat can propagate inland glacier and enhance dynamic mass loss, and indicate that models must include shear-margin damage, tributary detachment, and rapid stress transmission to improve near-term sea-level projections.

Synthetic Aperture Radar (SAR) offset tracking is widely used to measure ice velocity on floating ice shelves, however, vertical displacement caused by ocean tides projects into the radar-range direction, introducing apparent horizontal motion unrelated to true ice flow. Here, we develop an observation-driven tide-correction for SAR-derived ice speed measurements using differential range offset tracking (DROT) applied to Sentinel-1 SAR data. The method estimates spatially and temporally varying tidal response coefficients directly from observed radar-range displacement variability and an ocean tide model. Application of the method to SAR-derived ice velocity measurements on Antarctic Ice Shelves shows spatial variability in the magnitude of the tide-induced ice-speed correction, with an Antarctic-wide mean tide-correction range of 106 m/yr, a standard deviation of 71 m/yr and maximum values exceeding 300 m/yr. The largest tide correction range predominantly occurs on freely floating ice shelves in regions of strong tidal forcing, particularly in the Weddell Sea and Antarctic Peninsula. Smaller tide corrections are observed near grounding zones and pinned regions where tidal motion is mechanically constrained. Time-series analyses show that the tide correction systematically reduces non-physical temporal variability in SAR-derived ice velocity products while preserving the large-scale spatial structure of ice flow. Independent validation using GNSS observations on Brunt Ice Shelf shows improved agreement between tide-corrected satellite-based ice speed and in situ measurements. Our results demonstrate that tide-induced apparent motion is a source of variability in SAR-derived ice velocity products on floating ice shelves, highlighting the importance of applying a correction when tide-induced signals are not required.

Over the last few decades, ice keels have intermittently bumped against the bedrock underneath Pine Island Ice Shelf. In general, it is thought that this process, termed regrounding, could enhance buttressing and potentially slow glacier flow or arrest retreat. Here, we test this hypothesis for Pine Island Glacier. Using offset tracking applied to satellite radar data, we reveal an extensive history of regrounding on the central shelf since 2009. Combining Earth observation and ice sheet modelling, we assess the direct effects this might have had on ice dynamics and indirect effects via its influencing on calving behaviour. Our results show that regrounding caused little additional drag on the base of the ice shelf. Furthermore, it plausibly contributed to the promotion of calving between 2015 and 2020 which is, in turn, held largely responsible for the increase in flow rate of the glacier over the last decade.

Petermann Glacier drains 6% of the Greenland Ice Sheet and contains sufficient ice to raise global sea level by 0.41 m. Terminating in one of five remaining ice shelves in Greenland, it has been considered historically stable, with little evidence of a sustained response to changes in its surrounding climate. Here, we use a suite of satellite observations to document decadal-scale ice shelf thinning and acceleration, alongside the development of a major calving event. Following the rapid propagation of a rift across the ice shelf during the past two summers, the ice shelf is now poised to lose 22% of its area, leaving the calving front at its most receded position since observational records began. We show that the recent thinning of the ice shelf occurred during a period of anomalously warm upper ocean waters and substantially reduced landfast sea ice at the mouth of Petermann Fjord. Inland glacier acceleration is now detectable more than 50 km upstream of the glacier’s grounding line, indicating the emergence of a dynamic imbalance at one of Greenland’s largest marine-terminating glaciers. Our findings highlight the need to better understand the physical mechanisms that will govern Petermann’s future evolution and sea level contribution.