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1. Realistic ice-shelf/ocean state estimates (RISE) of Antarctic basal melting and drivers

   https://doi.org/10.5194/egusphere-2024-4047  

   Galton-Fenzi, Benjamin Keith; Porter-Smith, Richard; Cook, Sue; Cougnon, Eva; Gwyther, David E; Huneke, Wilma_G C; Rosevear, Madelaine G; Asay-Davis, Xylar; Boeira_Dias, Fabio; Dinniman, Michael S; et al (February 2025, EGUsphere)

   Abstract. Societal adaptation to rising sea levels requires robust projections of the Antarctic Ice Sheet’s retreat, particularly due to ocean-driven basal melting of its fringing ice shelves. Recent advances in ocean models that simulate ice-shelf melting offer an opportunity to reduce uncertainties in ice–ocean interactions. Here, we compare several community-contributed, circum-Antarctic ocean simulations to highlight inter-model differences, evaluate agreement with satellite-derived melt rates, and examine underlying physical processes. All but one simulation use a melting formulation depending on both thermal driving (T ⋆) and friction velocity (u⋆), which together represent the thermal and ocean current forcings at the ice–ocean interface. Simulated melt rates range from 650 to 1277 Gt year−1 (m = 0.45 − 0.91 m year−1), driven by variations in model resolution, parameterisations, and sub-ice shelf circulation. Freeze-to-melt ratios span 0.30 to 30.12 %, indicating large differences in how refreezing is represented. The multi-model mean (MMM) produces an averaged melt rate of 0.60 m year−1 from a net mass loss of 842.99 Gt year−1 (876.03 Gt year−1 melting and 33.05 Gt year−1 refreezing), yielding a freeze-to-melt ratio of 3.92 %. We define a thermo-kinematic melt sensitivity, ζ = m/(T ⋆ u⋆) = 4.82 × 10−5 °C−1 for the MMM, with individual models spanning 2.85 × 10−5 to 19.4 × 10−5 °C−1. Higher melt rates typically occur near grounding zones where both T ⋆ and u⋆ exert roughly equal influence. Because friction velocity is critical for turbulent heat exchange, ice-shelf melting must be characterised by both ocean energetics and thermal forcing. Further work to standardise model setups and evaluation of results against in situ observations and satellite data will be essential for increasing model accuracy, reducing uncertainties, to improve our understanding of ice-shelf–ocean interactions and refine sea-level rise predictions. 

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2. Thwaites Glacier thins and retreats fastest where ice-shelf channels intersect its grounding zone

   https://doi.org/10.5194/tc-18-4971-2024  

   Chartrand, Allison M; Howat, Ian M; Joughin, Ian R; Smith, Benjamin E (November 2024, The Cryosphere)

   Abstract. Antarctic ice shelves buttress the flow of the ice sheet but are vulnerable to increased basal melting from contact with a warming ocean and increased mass loss from calving due to changing flow patterns. Channels and similar features at the bases of ice shelves have been linked to enhanced basal melting and observed to intersect the grounding zone, where the greatest melt rates are often observed. The ice shelf of Thwaites Glacier is especially vulnerable to basal melt and grounding zone retreat because the glacier has a retrograde bed leading to a deep trough below the grounded ice sheet. We use digital surface models from 2010–2022 to investigate the evolution of its ice-shelf channels, grounding zone position, and the interactions between them. We find that the highest sustained rates of grounding zone retreat (up to 0.7 km yr−1) are associated with high basal melt rates (up to ∼250 m yr−1) and are found where ice-shelf channels intersect the grounding zone, especially atop steep local retrograde slopes where subglacial channel discharge is expected. We find no areas with sustained grounding zone advance, although some secular retreat was distal from ice-shelf channels. Pinpointing other locations with similar risk factors could focus assessments of vulnerability to grounding zone retreat. 

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3. Rift propagation signals the last act of the Thwaites Eastern Ice Shelf despite low basal melt rates

   https://doi.org/10.1017/jog.2024.64  

   Wild, Christian T; Kachuck, Samuel B; Luckman, Adrian; Alley, Karen E; Sharp, Meghan A; Smith, Haylee; Tyler, Scott W; Kratt, Christopher; Dotto, Tiago S; Price, Daniel; et al (January 2024, Journal of Glaciology)

   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. 

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4. Multidecadal Basal Melt Rates and Structure of the Ross Ice Shelf, Antarctica, Using Airborne Ice Penetrating Radar

   https://doi.org/10.1029/2019JF005241  

   Das, Indrani; Padman, Laurie; Bell, Robin E.; Fricker, Helen A.; Tinto, Kirsty J.; Hulbe, Christina L.; Siddoway, Christine S.; Dhakal, Tejendra; Frearson, Nicholas P.; Mosbeux, Cyrille; et al (March 2020, Journal of Geophysical Research: Earth Surface)

   Abstract Basal melting of ice shelves is a major source of mass loss from the Antarctic Ice Sheet. In situ measurements of ice shelf basal melt rates are sparse, while the more extensive estimates from satellite altimetry require precise information about firn density and characteristics of near‐surface layers. We describe a novel method for estimating multidecadal basal melt rates using airborne ice penetrating radar data acquired during a 3‐year survey of the Ross Ice Shelf. These data revealed an ice column with distinct upper and lower units whose thicknesses change as ice flows from the grounding line toward the ice front. We interpret the lower unit as continental meteoric ice that has flowed across the grounding line and the upper unit as ice formed from snowfall onto the relatively flat ice shelf. We used the ice thickness difference and strain‐induced thickness change of the lower unit between the survey lines, combined with ice velocities, to derive basal melt rates averaged over one to six decades. Our results are similar to satellite laser altimetry estimates for the period 2003–2009, suggesting that the Ross Ice Shelf melt rates have been fairly stable for several decades. We identify five sites of elevated basal melt rates, in the range 0.5–2 m a⁻¹, near the ice shelf front. These hot spots indicate pathways into the sub‐ice‐shelf ocean cavity for warm seawater, likely a combination of summer‐warmed Antarctic Surface Water and modified Circumpolar Deep Water, and are potential areas of ice shelf weakening if the ocean warms. 

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5. Ice shelf basal melt rates from a high-resolution DEM record for Pine Island Glacier, Antarctica

   https://doi.org/10.5194/tc-2018-209  

   Shean, David E.; Joughin, Ian R.; Dutrieux, Pierre; Smith, Benjamin E.; Berthier, Etienne (October 2018, The Cryosphere Discussions)

   Abstract. Ocean-induced basal melting is directly and indirectly responsible for much of the Amundsen Sea Embayment ice loss in recent decades, but the total magnitude and spatiotemporal evolution of this melt is poorly constrained. To address this problem, we generated a record of high-resolution Digital Elevation Models (DEMs) for Pine Island Glacier (PIG) using commercial sub-meter satellite stereo imagery and integrated additional 2002–2015 DEM/altimetry data. We implemented a Lagrangian elevation change (Dh/Dt) framework to estimate ice shelf basal melt rates at 32–256-m resolution. We describe this methodology and consider basal melt rates and elevation change over the PIG shelf and lower catchment from 2008–2015. We document the evolution of Eulerian elevation change (dh/dt) and upstream propagation of thinning signals following the end of rapid grounding line retreat around 2010. Mean full-shelf basal melt rates for the 2008–2015 period were ~82–93 Gt/yr, with ~ 200–250 m/yr basal melt rates within large channels near the grounding line, ~ 10–30 m/yr over the main shelf, and ~ 0–10 m/yr over the North and South shelves, with the notable exception of a small area with rates of ~ 50–100 m/yr near the grounding line of a fast-flowing tributary on the South shelf. The observed basal melt rates show excellent agreement with, and provide context for, in situ basal melt rate observations. We also document the relative melt rates for km-scale basal channels and keels at different locations on the shelf and consider implications for ocean circulation and heat content. These methods and results offer new indirect observations of ice-ocean interaction and constraints on the processes driving sub-shelf melting beneath vulnerable ice shelves in West Antarctica. 

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