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The largest uncertainty in future sea-level rise is loss of ice from the Greenland and Antarctic Ice Sheets. Ice shelves, freely floating platforms of ice that fringe the ice sheets, play a crucial role in restraining discharge of grounded ice into the ocean through buttressing. However, since the 1990s, several ice shelves have thinned, retreated, and collapsed. If this pattern continues, it could expose thick cliffs that become structurally unstable and collapse in a process called marine ice cliff instability (MICI). However, the feedbacks between calving, retreat, and other forcings are not well understood. Here we review observed modes of calving from ice shelves and marine-terminating glaciers, and their relation to environmental forces. We show that the primary driver of calving is long-term internal glaciological stress, but as ice shelves thin they may become more vulnerable to environmental forcing. This vulnerability—and the potential for MICI—comes from a combination of the distribution of preexisting flaws within the ice and regions where the stress is large enough to initiate fracture. Although significant progress has been made modeling these processes, theories must now be tested against a wide range of environmental and glaciological conditions in both modern and paleo conditions. ▪ Ice shelves, floating platforms of ice fed by ice sheets, shed mass in a near-instantaneous fashion through iceberg calving. ▪ Most ice shelves exhibit a stable cycle of calving front advance and retreat that is insensitive to small changes in environmental conditions. ▪ Some ice shelves have retreated or collapsed completely, and in the future this could expose thick cliffs that could become structurally unstable called ice cliff instability. ▪ The potential for ice shelf and ice cliff instability is controlled by the presence and evolution of flaws or fractures within the ice. 

Abstract Rift propagation, rather than basal melt, drives the destabilization and disintegration of the Thwaites Eastern Ice Shelf. Since 2016, rifts have episodically advanced throughout the central ice-shelf area, with rapid propagation events occurring during austral spring. The ice shelf's speed has increased by ~70% during this period, transitioning from a rate of 1.65 m d⁻¹in 2019 to 2.85 m d⁻¹by early 2023 in the central area. The increase in longitudinal strain rates near the grounding zone has led to full-thickness rifts and melange-filled gaps since 2020. A recent sea-ice break out has accelerated retreat at the western calving front, effectively separating the ice shelf from what remained of its northwestern pinning point. Meanwhile, a distributed set of phase-sensitive radar measurements indicates that the basal melting rate is generally small, likely due to a widespread robust ocean stratification beneath the ice–ocean interface that suppresses basal melt despite the presence of substantial oceanic heat at depth. These observations in combination with damage modeling show that, while ocean forcing is responsible for triggering the current West Antarctic ice retreat, the Thwaites Eastern Ice Shelf is experiencing dynamic feedbacks over decadal timescales that are driving ice-shelf disintegration, now independent of basal melt. 

Abstract. The discovery of Antarctica's deepest subglacial troughbeneath the Denman Glacier, combined with high rates of basal melt at thegrounding line, has caused significant concern over its vulnerability toretreat. Recent attention has therefore been focusing on understanding thecontrols driving Denman Glacier's dynamic evolution. Here we consider theShackleton system, comprised of the Shackleton Ice Shelf, Denman Glacier,and the adjacent Scott, Northcliff, Roscoe and Apfel glaciers, about whichalmost nothing is known. We widen the context of previously observed dynamicchanges in the Denman Glacier to the wider region of the Shackleton system,with a multi-decadal time frame and an improved biannual temporal frequencyof observations in the last 7 years (2015–2022). We integrate newsatellite observations of ice structure and airborne radar data with changesin ice front position and ice flow velocities to investigate changes in thesystem. Over the 60-year period of observation we find significant riftpropagation on the Shackleton Ice Shelf and Scott Glacier and notablestructural changes in the floating shear margins between the ice shelf andthe outlet glaciers, as well as features indicative of ice with elevatedsalt concentration and brine infiltration in regions of the system. Over theperiod 2017–2022 we observe a significant increase in ice flow speed (up to50 %) on the floating part of Scott Glacier, coincident with small-scalecalving and rift propagation close to the ice front. We do not observe anyseasonal variation or significant change in ice flow speed across the restof the Shackleton system. Given the potential vulnerability of the system toaccelerating retreat into the overdeepened, potentially sediment-filledbedrock trough, an improved understanding of the glaciological,oceanographic and geological conditions in the Shackleton system arerequired to improve the certainty of numerical model predictions, and weidentify a number of priorities for future research. With access to theseremote coastal regions a major challenge, coordinated internationallycollaborative efforts are required to quantify how much the Shackletonregion is likely to contribute to sea level rise in the coming centuries. 

Abstract. The Thwaites Eastern Ice Shelf (TEIS) buttresses the eastern grounded portion of Thwaites Glacier through contact with a pinning point at itsseaward limit. Loss of this ice shelf will promote further acceleration of Thwaites Glacier. Understanding the dynamic controls and structuralintegrity of the TEIS is therefore important to estimating Thwaites' future sea-level contribution. We present a ∼ 20-year record of change onthe TEIS that reveals the dynamic controls governing the ice shelf's past behaviour and ongoing evolution. We derived ice velocities from MODIS andSentinel-1 image data using feature tracking and speckle tracking, respectively, and we combined these records with ITS_LIVE and GOLIVE velocityproducts from Landsat-7 and Landsat-8. In addition, we estimated surface lowering and basal melt rates using the Reference Elevation Model of Antarctica (REMA) DEM in comparison to ICESat andICESat-2 altimetry. Early in the record, TEIS flow dynamics were strongly controlled by the neighbouring Thwaites Western Ice Tongue (TWIT). Flowpatterns on the TEIS changed following the disintegration of the TWIT around 2008, with a new divergence in ice flow developing around thepinning point at its seaward limit. Simultaneously, the TEIS developed new rifting that extends from the shear zone upstream of the ice rise andincreased strain concentration within this shear zone. As these horizontal changes occurred, sustained thinning driven by basal melt reduced icethickness, particularly near the grounding line and in the shear zone area upstream of the pinning point. This evidence of weakening at a rapid pacesuggests that the TEIS is likely to fully destabilize in the next few decades, leading to further acceleration of Thwaites Glacier.