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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Weakening of the pinning point buttressing Thwaites Glacier, West Antarctica
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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- Award ID(s):
- 1929991
- NSF-PAR ID:
- 10327373
- Date Published:
- Journal Name:
- The Cryosphere
- Volume:
- 16
- Issue:
- 2
- ISSN:
- 1994-0424
- Page Range / eLocation ID:
- 397 to 417
- 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 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
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Abstract Thwaites Ice Shelf (TWIS), the floating extension of Thwaites Glacier, West Antarctica, is changing rapidly and may completely disintegrate in the near future. Any buttressing that the ice shelf provides to the upstream grounded Thwaites glacier will then be lost. Previously, it has been argued that this could lead to onset of dynamical instability and the rapid demise of the entire glacier. Here we provide the first systematic quantitative assessment of how strongly the upstream ice is buttressed by TWIS and how its collapse affects future projections. By modeling the stresses acting along the current grounding line, we show that they deviate insignificantly from the stresses after ice shelf collapse. Using three ice‐flow models, we furthermore model the transient evolution of Thwaites Glacier and find that a complete disintegration of the ice shelf will not substantially impact future mass loss over the next 50 years.
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null (Ed.)Abstract. The geometry of the sea floor immediately beyondAntarctica's marine-terminating glaciers is a fundamental control onwarm-water routing, but it also describes former topographic pinning pointsthat have been important for ice-shelf buttressing. Unfortunately, thisinformation is often lacking due to the inaccessibility of these areas forsurvey, leading to modelled or interpolated bathymetries being used asboundary conditions in numerical modelling simulations. At Thwaites Glacier(TG) this critical data gap was addressed in 2019 during the first cruise ofthe International Thwaites Glacier Collaboration (ITGC) project. We present more than 2000 km2 of new multibeamecho-sounder (MBES) data acquired in exceptional sea-ice conditionsimmediately offshore TG, and we update existing bathymetric compilations.The cross-sectional areas of sea-floor troughs are under-predicted by up to40 % or are not resolved at all where MBES data are missing, suggesting thatcalculations of trough capacity, and thus oceanic heat flux, may besignificantly underestimated. Spatial variations in the morphology oftopographic highs, known to be former pinning points for the floating iceshelf of TG, indicate differences in bed composition that are supported bylandform evidence. We discuss links to ice dynamics for an overriding icemass including a potential positive feedback mechanism where erosion ofsoft erodible highs may lead to ice-shelf ungrounding even with littleor no ice thinning. Analyses of bed roughnesses and basal drag contributionsshow that the sea-floor bathymetry in front of TG is an analogue for extantbed areas. Ice flow over the sea-floor troughs and ridges would have beenaffected by similarly high basal drag to that acting at the grounding zonetoday. We conclude that more can certainly be gleaned from these 3Dbathymetric datasets regarding the likely spatial variability of bedroughness and bed composition types underneath TG. This work also addressesthe requirements of recent numerical ice-sheet and ocean modelling studiesthat have recognised the need for accurate and high-resolution bathymetry todetermine warm-water routing to the grounding zone and, ultimately, forpredicting glacier retreat behaviour.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/tc-16-397-2022
