Abstract
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.
Highlights
The Amundsen Sea coast of West Antarctica has been the setting of drastic glacier retreat, ice-shelf thinning and associated speedup since the beginning of the satellite measurement era (Scambos et al, 2017)
Our results show continued grounding-line retreat along a 45 km stretch of Thwaites Glacier’s grounding line, with sporadically accelerating retreat rates
Our perturbation experiments confirm our hypothesis that weakening of the lateral shear margin in the years following breakup of the Western Glacier Tongue is the mechanism behind the observed counterclockwise rotation of ice flow
Summary
The Amundsen Sea coast of West Antarctica has been the setting of drastic glacier retreat, ice-shelf thinning and associated speedup since the beginning of the satellite measurement era (Scambos et al, 2017). Wherever ice shelves reground on elevated seafloor such as offshore ridges and islands, significant resistance against the flow of floating ice controls the overall ice-shelf buttressing, which, in turn, stabilizes the location of the grounding line (e.g., Alley et al, 2015). This is supported by model experiments showing that reduced ice-shelf buttressing is linked to retreat of grounding lines even far upstream of the location where the change in buttressing occurred, leading to an overall net loss of ice volume above flotation (Dupont and Alley, 2005). Ungrounding of ice shelves from these pinning points, has the potential to initiate rapid changes on a regional scale (Goldberg et al, 2009; Favier et al, 2012; Favier and Pattyn, 2015; Favier et al, 2016; Reese et al, 2018).
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