Multi-decadal ice shelf retreat driven by ocean wave erosion in the absence of sea-ice

In recent years, a number of studies have focused on the mechanical impacts of sea-ice loss on Antarctic ice shelves. These impacts arise either through the potential buttressing provided by multi-year landfast sea ice or through increased ocean swell as pack ice diminishes. Increasing periods of sea-ice-free conditions near ice shelves also modify thermal forcing, as sea-surface temperatures seasonally increase. The number of sea-ice-free days has increased by around 50% at the eastern Getz Ice Shelf since the 1970s, to the point where it is virtually sea-ice-free throughout December and January each year, when solar insolation is at its highest. With the exception of the Ross Ice Shelf, no other major ice shelf experiences comparable summer sea-ice-free conditions. We explore the calving processes along the eastern Getz Ice Shelf, with the underlying hypothesis that these processes will become increasingly relevant across Antarctica as sea ice continues to diminish.

The calving fronts of the eastern outlets of the Getz Ice Shelf have been retreating since the earliest satellite observations in the 1970s. This retreat is persistent and is characterised by advance during the winter months and retreat during the summer, with frontal ablation rates of around 650 m a⁻¹. This retreat has occurred despite no detectable changes in ice-shelf damage over the past 50 years, the absence of landfast sea ice, limited changes in ice velocity seaward of the grounding zone, and no recorded thinning in the outlet experiencing the most significant retreat. Surface profiles of the ice shelf reveal widespread evidence of rampart–moat structures, which are highly indicative of buoyancy-driven calving. Sea-ice-free conditions allow the ocean surface to heat up; this heat is sufficient to drive undercutting at the ice front, resulting in cliff retreat and the formation of an underwater foot, which in turn promotes buoyancy-driven calving, termed ‘footloose’ calving. In the case of the easternmost outlet of the Getz Ice Shelf, retreat is already progressing into its embayment; in the coming years, this will result in a loss of buttressing, acceleration, and a change in the dynamic state of the ice shelf.

Nearly all other Antarctic ice shelves remain encased by sea ice during the summer. Many of these ice shelves, particularly those in regions such as Dronning Maud Land, flow at only ~200 m a⁻¹, meaning that a similar  frontal ablation rate of 650 m a⁻¹ would be highly significant. As sea ice diminishes and this mechanism becomes increasingly important, we cannot rule out widespread retreat of Antarctica’s ice shelves driven by a process not currently incorporated into ice-sheet models.