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Abstract. The Thwaites Eastern Ice Shelf buttresses a significant portion of Thwaites Glacier through contact with a pinning point 40 km offshore of the present grounding line. Predicting future rates of Thwaites Glacier’s contribution to sea-level rise depends on the evolution of this pinning point and the resultant change in the ice-shelf stress field since the breakup of the Thwaites Western Glacier Tongue in 2009. Here we use Landsat-8 feature tracking of ice velocity in combination with ice-sheet model perturbation experiments to show how past changes in flow velocity have been governed in large part by changes in lateral shear and pinning point interactions with the Thwaites Western Glacier Tongue. We then use recent satellite altimetry data from ICESat-2 to show that Thwaites Glacier’s grounding line has continued to retreat rapidly; in particular, the grounded area of the pinning point is greatly reduced from earlier mappings in 2014, and grounded ice elevations are continuing to decrease. This loss has created two pinned areas with ice flow now funneled between them. If current rates of surface lowering persist, the Thwaites Eastern Ice Shelf will unpin from the seafloor in less than a decade, despite our finding from airborne radar data that the seafloor underneath the pinning point is about 200 m shallower than previously reported. Advection of relatively thin and mechanically damaged ice onto the remaining portions of the pinning point and feedback mechanisms involving basal melting may further accelerate the unpinning. As a result, ice discharge will likely increase up to 10 % along a 45 km stretch of the grounding line that is currently buttressed by the Thwaites Eastern Ice Shelf.
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Two decades of dynamic change and progressive destabilization on the Thwaites Eastern Ice Shelf
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.
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- Award ID(s):
- 1929991
- PAR ID:
- 10327375
- Date Published:
- Journal Name:
- The Cryosphere
- Volume:
- 15
- Issue:
- 11
- ISSN:
- 1994-0424
- Page Range / eLocation ID:
- 5187 to 5203
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Thwaites Glacier is one of the fastest-changing ice–ocean systems in Antarctica 1–3 . Much of the ice sheet within the catchment of Thwaites Glacier is grounded below sea level on bedrock that deepens inland 4 , making it susceptible to rapid and irreversible ice loss that could raise the global sea level by more than half a metre 2,3,5 . The rate and extent of ice loss, and whether it proceeds irreversibly, are set by the ocean conditions and basal melting within the grounding-zone region where Thwaites Glacier first goes afloat 3,6 , both of which are largely unknown. Here we show—using observations from a hot-water-drilled access hole—that the grounding zone of Thwaites Eastern Ice Shelf (TEIS) is characterized by a warm and highly stable water column with temperatures substantially higher than the in situ freezing point. Despite these warm conditions, low current speeds and strong density stratification in the ice–ocean boundary layer actively restrict the vertical mixing of heat towards the ice base 7,8 , resulting in strongly suppressed basal melting. Our results demonstrate that the canonical model of ice-shelf basal melting used to generate sea-level projections cannot reproduce observed melt rates beneath this critically important glacier, and that rapid and possibly unstable grounding-line retreat may be associated with relatively modest basal melt rates.more » « less
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Abstract Thwaites Glacier is one of the fastest‐changing ice‐ocean systems in Antarctica. Basal melting beneath Thwaites' floating ice shelf, especially around pinning points and at the grounding line, sets the rate of ice loss and Thwaites' contribution to global sea‐level rise. The rate of basal melting is controlled by the transport of heat into and through the ice–ocean boundary layer toward the ice base. Here we present the first turbulence observations from the grounding line of Thwaites Eastern Ice Shelf. We demonstrate that contrary to expectations, the turbulence‐driven vertical flux of heat into the ice–ocean boundary layer is insufficient to sustain the basal melt rate. Instead, most of the heat required must be delivered by lateral fluxes driven by the large‐scale advective circulation. Lateral processes likely dominate beneath the most unstable warm‐cavity ice shelves, and thus must be fully incorporated into parameterizations of ice shelf basal melting.more » « less
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Rift propagation signals the last act of the Thwaites Eastern Ice Shelf despite low basal melt ratesAbstract 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.more » « less
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Abstract Thwaites Glacier represents 15% of the ice discharge from the West Antarctic Ice Sheet and influences a wider catchment 1–3 . Because it is grounded below sea level 4,5 , Thwaites Glacier is thought to be susceptible to runaway retreat triggered at the grounding line (GL) at which the glacier reaches the ocean 6,7 . Recent ice-flow acceleration 2,8 and retreat of the ice front 8–10 and GL 11,12 indicate that ice loss will continue. The relative impacts of mechanisms underlying recent retreat are however uncertain. Here we show sustained GL retreat from at least 2011 to 2020 and resolve mechanisms of ice-shelf melt at the submetre scale. Our conclusions are based on observations of the Thwaites Eastern Ice Shelf (TEIS) from an underwater vehicle, extending from the GL to 3 km oceanward and from the ice–ocean interface to the sea floor. These observations show a rough ice base above a sea floor sloping upward towards the GL and an ocean cavity in which the warmest water exceeds 2 °C above freezing. Data closest to the ice base show that enhanced melting occurs along sloped surfaces that initiate near the GL and evolve into steep-sided terraces. This pronounced melting along steep ice faces, including in crevasses, produces stratification that suppresses melt along flat interfaces. These data imply that slope-dependent melting sculpts the ice base and acts as an important response to ocean warming.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/tc-15-5187-2021
