Comment on tc-2021-265

Abstract

The discovery of the deepest subglacial trough beneath the Denman Glacier, combined with high rates of basal melt at the grounding line, have caused significant concern over its vulnerability to retreat. Recent attention has therefore been focusing on understanding the governing dynamic controls, although knowledge of the wider regional context and timescales over which the future responses may occur remains poor. Here we consider the whole Shackleton system, comprising of the Shackleton ice shelf, Denman Glacier and adjacent Scott, Northcliffe, Roscoe and Apfel glaciers, about which almost nothing is known. We widen the context of previously observed dynamic changes in the Denman Glacier into the wider region of the Queen Mary and Knox coasts; with a multi-decadal timeframe and an improved biannual temporal frequency of observations in the last seven years (2014–21). We integrate new satellite observations of ice structure, changes in ice front position and ice-flow velocities to investigate changes in the system. We furthermore use the BISICLES ice sheet model to assess the sensitivity and simulate the response times of the Queen Mary and Knox coasts to hypothetical disintegration of its floating ice areas, in response to coupled ocean and atmospheric forcing. Over the 60-year period of observation, the Queen Mary and Knox coasts do not appear to have changed significantly and higher frequency observations have not revealed any significant annual or sub-annual variations in ice flow. A previously observed increase in the ice flow speed of the Denman Glacier has not continued beyond 2008, and we cannot identify any related change in the surface structure of the system since then. We do, however, observe more significant change in the Scott Glacier, with an acceleration in ice flow associated with calving and progressing from the ice front along the floating tongue since early 2020. No changes in surface structure or ice flow speed are observed closer to the grounded ice. Our upper limit numerical simulations for a 400-year period are consistent with noticeable grounding line retreat in the Denman Glacier in the next two centuries if all floating ice were lost, before stabilising again in the third century from now. This equates to around 6 cm of sea level rise, a small contribution when compared to other areas of East Antarctica expected to change over the same time frame. It is clear that current knowledge is insufficient to explain the observed spatial and temporal changes in the dynamic behaviour of the grounded and floating sections in the Shackleton system. Given the potential vulnerability of the system to accelerating retreat better data recording the glaciological, oceanographic, and geological conditions in the Queen Mary and Knox coasts are required to improve the certainty of numerical model predictions. With access to these remote coastal regions a major challenge, coordinated internationally collaborative efforts are required to quantify how much the Queen Mary and Knox coastal region is likely contribute to sea level rise in the coming centuries.