Velocity response of Petermann Glacier, northwest Greenland, to past and future calving events

Abstract

Abstract. Dynamic ice discharge from outlet glaciers across the Greenland Ice Sheet has increased since the beginning of the 21st century. Calving from floating ice tongues that buttress these outlets can accelerate ice flow and discharge of grounded ice. However, little is known about the dynamic impact of ice tongue loss in Greenland compared to ice shelf collapse in Antarctica. The rapidly flowing (∼1000 m a−1) Petermann Glacier in northwest Greenland has one of the ice sheet's last remaining ice tongues, but it lost ∼50 %–60 % (∼40 km in length) of this tongue via two large calving events in 2010 and 2012. The glacier showed a limited velocity response to these calving events, but it is unclear how sensitive it is to future ice tongue loss. Here, we use an ice flow model (Úa) to assess the instantaneous velocity response of Petermann Glacier to past and future calving events. Our results confirm that the glacier was dynamically insensitive to large calving events in 2010 and 2012 (<10 % annual acceleration). We then simulate the future loss of similarly sized sections to the 2012 calving event (∼8 km long) of the ice tongue back to the grounding line. We conclude that thin, soft sections of the ice tongue >12 km away from the grounding line provide little frontal buttressing, and removing them is unlikely to significantly increase ice velocity or discharge. However, once calving removes ice within 12 km of the grounding line, loss of these thicker and stiffer sections of ice tongue could perturb stresses at the grounding line enough to substantially increase inland flow speeds (∼900 m a−1), grounded ice discharge, and Petermann Glacier's contribution to global sea level rise.

Twenty-first century response of Petermann Glacier, northwest Greenland to ice shelf loss

“…The stress exponents in the Weertman sliding law ( m ) and Glen's flow law ( n ) were both set to 3, as commonly used in glaciological studies. This same inverse methodology and model has now been used in a number of previous studies (Hill and others, 2018 b ; Reese and others, 2018 b ; Gudmundsson and others, 2019). Inversion was done by minimizing the cost function of a misfit and regularization term.…”

“…The model has been used to understand grounding line dynamics (Gudmundsson and others, 2012; Pattyn and others, 2012) and the impact of ice shelf buttressing and collapse on outlet glacier dynamics in both Antarctica (De Rydt and others, 2015; Reese and others, 2018 a ) and Greenland (e.g. Hill and others, 2018 b ).…”

“…Along the inland catchment boundary we used a fixed (no-slip in normal or tangential directions) zero velocity condition to conserve mass within our model domain. Velocities were also fixed to zero along the lateral ice shelf margins (excluding along the fronts of glaciers on the eastern side of the fjord) as this optimally replicates lateral stresses and ice flow along the PGIS (see Hill and others, 2018 b ).…”

“…However, one-horizontal dimensional (1HD) models do not account for lateral stresses and buttressing in both horizontal directions which limits the accuracy of sea level rise projections (Gudmundsson, 2013; Bondzio and others, 2017). More recently, Hill and others (2018 b ) used a two-horizontal dimensional (2HD) ice flow model Úa (Gudmundsson and others, 2012), to examine the time-independent response of PG to large calving events. While this showed ice shelf collapse could cause a 96% instantaneous speed-up, it did not examine the transient response of PG to a loss of ice-shelf buttressing.…”

Instantaneous Antarctic ice sheet mass loss driven by thinning ice shelves

“…We invert for both A and C simultaneously using Tikhonov regularization on both the amplitudes and the slopes of A and C. The regularization parameters were determined from an L-curve analysis. The same methodology using the same numerical model has been implemented in several recent publications (Hill et al, 2018;Rosier & Gudmundsson, 2018). The optimized model that closely reproduced observed velocities over the AIS (Figure S1) provided our "reference" model of the ice sheet flow.…”