Collapse of the West Antarctic Ice Sheet after local destabilization of the Amundsen Basin

Abstract

The future evolution of the Antarctic Ice Sheet represents the largest uncertainty in sea-level projections of this and upcoming centuries. Recently, satellite observations and high-resolution simulations have suggested the initiation of an ice-sheet instability in the Amundsen Sea sector of West Antarctica, caused by the last decades' enhanced basal ice-shelf melting. Whether this localized destabilization will yield a full discharge of marine ice from West Antarctica, associated with a global sea-level rise of more than 3 m, or whether the ice loss is limited by ice dynamics and topographic features, is unclear. Here we show that in the Parallel Ice Sheet Model, a local destabilization causes a complete disintegration of the marine ice in West Antarctica. In our simulations, at 5-km horizontal resolution, the region disequilibrates after 60 y of currently observed melt rates. Thereafter, the marine ice-sheet instability fully unfolds and is not halted by topographic features. In fact, the ice loss in Amundsen Sea sector shifts the catchment's ice divide toward the Filchner-Ronne and Ross ice shelves, which initiates grounding-line retreat there. Our simulations suggest that if a destabilization of Amundsen Sea sector has indeed been initiated, Antarctica will irrevocably contribute at least 3 m to global sea-level rise during the coming centuries to millennia.

Keywords: West Antarctic Ice Sheet; instability; marine ice-sheet instability; sea-level rise; tipping point.

Fig. 1.

Collapse of the WAIS in response to 60 y of currently observed sub-ice-shelf melting in the Amundsen Sea. (A) Simulated initial steady-state grounding line and calving front before perturbation (dark blue contours) and at the end of perturbation (red contours) underlaid by bed topography (blue shading). Grounding-line positions during the collapse in 1,000-y time steps (black contours). Final steady-state grounded ice body in transparent gray with gray contours of surface elevation. Cross sections for ice and bed topography of B and C (yellow lines). (B and C) Transects are displayed from the Amundsen Sea Sector (Left) into the Ross Ice Shelf (B) and Filchner–Ronne Ice Shelf (C) (Right). Colors as in A, with additional profiles from present-day observations (49) in green. (Inset) Region outside the model domain (hatched) and area of A (blue rectangle).

Fig. 2.

(A) Observed present-day (50) and (B) modeled surface speed in equilibrium for the whole model domain (region outside model domain hatched). Observed (49) and modeled grounding line and calving front, respectively (black contours), and bathymetry (light blue shading). Gray contour in A delineates present-day catchment basin of the Amundsen Sea sector (51).

Fig. 3.

Time evolution of the sea-level contribution after the beginning of the perturbation of the equilibrium ice sheet for (A) the first 250 y, covering the perturbation phase (black), and (B) the full simulation. Each colored line represents an individual simulation characterized by a different perturbation duration (end of perturbation after 0, 20, 40, 50, 60, 80, 100, 150, and 200 y, respectively, marked by vertical bars). Stable and unstable simulations in blue and red, respectively. Gray shading shows the range of cumulative sea-level contribution spanned when extrapolating rates of net ice loss from Amundsen Sea sector that were measured for five periods during the last two decades [dashed lines (26)]. Horizontal dotted gray line in B gives the calculated sea level equivalent for a complete drainage of the Amundsen catchment basins (3).

Fig. 4.

Snapshots of modeled surface speed at (A) 0, (B) 3,000, (C) 6,000, and (D) 15,000 y after onset of the 60-y perturbation (zoom into region depicted by rectangle in Fig. 2B, same region as in Fig. 1). Areas of very small ice velocities mark the ice-divide location that shifts from Amundsen into the Ross and Filchner–Ronne basins during ice-sheet retreat (compare A, B, and C). Comparison of B and C reveals grounding-line destabilization from the landward direction in the catchment basin of the Ross Ice Shelf.

Fig. 5.

Long-term sea-level contribution as a function of net ice loss from the Amundsen Sea sector during perturbation. Gray shading marks the critical threshold of ice loss between stabilization (blue) and destabilization (red) of the Amundsen Sea sector, followed by disintegration of the WAIS, corresponding to a sea-level rise contribution of almost 3 m.