Ice plug prevents irreversible discharge from East Antarctica

Changes in ice discharge fromAntarctica constitute the largest uncertainty in future sea-level projections, mainly because of the unknown response of its marine basins1. Most of West Antarctica’s marine ice sheet lies on an inland-sloping bed2 and is thereby prone to a marine ice sheet instability3–5. A similar topographic configuration is found in large parts of East Antarctica, which holds marine ice equivalent to 19m of global sea-level rise6, that is, more than five times that of West Antarctica. Within East Antarctica, the Wilkes Basin holds the largest volume of marine ice that is fully connected by subglacial troughs. This ice body was significantly reduced during the Pliocene epoch7. Strong melting underneath adjacent ice shelves with similar bathymetry8 indicates the ice sheet’s sensitivity to climatic perturbations. The stability of the Wilkes marine ice sheet has not been the subject of any comprehensive assessment of future sea level. Using recently improved topographic data6 in combination with ice-dynamic simulations,we showhere that the removal of a specific coastal ice volume equivalent to less than 80mm of global sea-level rise at the margin of theWilkes Basin destabilizes the regional ice flow and leads to a self-sustained discharge of the entire basinandaglobal sea-level riseof3–4m.Our results are robust with respect to variation in ice parameters, forcing details and model resolution as well as increased surface mass balance, indicating that East Antarcticamay become a large contributor to future sea-level rise on timescales beyond a century. Sea-level rise is a major consequence of climatic warming and impacts coastal areas through increased risk of floodingworldwide9. Improved sea-level projections are required for global and regional adaptation strategies10. Most recent work on Antarctica’s sea-level contribution concentrated on West Antarctica’s Amundsen sector where the grounding line is retreating11,12 and large regions of ice are grounded below sea level on an inland-sloping bed. This topographic situation was shown to be potentially unstable4, even when stabilizing effects of marginal stresses and bottom topography are accounted for13,14. As the East Antarctic ice sheet holds a multiple volume of marinebased ice as compared with West Antarctica6, the understanding of East Antarctica’s marine ice sheet dynamics is key to better determine Antarctica’s future contribution to sea-level changes. The vast Wilkes subglacial basin is located west of the Transantarctic Mountains and is drained through the Ninnis and Cook ice streams15 at George V Coast (Fig. 1). As revealed by recently improved bed topography data6, the Wilkes ice sheet rests on two deep troughs that are the remnants of larger palaeo-streams. The troughs’ shape (Fig. 2b) implies a deeper-lying grounding line if the ice recedes from its present position, with the potential of an instability with increased ice flux4. Further west at the neighbouring Wilkes coast strong thermal forcing is observed under the Totten and Moscow University ice shelves and has been suggested to cause the imbalance of Totten Glacier with thinning rates up to 1.9m yr−1 observed at present16. Although there is no evidence for a present large-scale imbalance of the Cook and Ninnis glaciers—reported thinning rates range from ∼ 0.025 m yr−1 (ref. 17) to ∼ 0.15 m yr−1 (ref. 18)—the similar regional bathymetry in principle allows for warm modified Circumpolar DeepWater to access the Ninnis and Cook grounding lines. The observed low melting rate and low melting-to-calving ratio8 indicates that heat transport to the grounding line is not excessive although a downstream freshening off the Adélie coast may be attributed to Cook shelf melting19. During the midto late Pliocene (4.8–3.5 million years ago) massive ice discharge occurred from the unstable margins of Adélie and Wilkes Land due to ice-stream surges that were linked to rapid grounding-line retreat during a warming climate20. Marine sediment cores from offshore Adélie land include continental bedrock from the interior of the Wilkes subglacial basin7. As active erosion zones lie close to the margin of the ice sheet, the Pliocene, featuring temperature and CO2 levels similar to end-of-this-century projections, may have had a significantly smaller ice sheet in East Antarctica’s Wilkes Basin, posing the question of the stability of its present marine-based ice. Here we explore this stability using regional simulations with the Parallel Ice Sheet Model (PISM; Methods) and appropriate boundary conditions. PISMapplies the superposition of two shallow approximations to consistently simulate slowly moving grounded ice and the fast-flowing ice of outlet glaciers, ice streams and ice shelves21. The transition zone between sheet and shelf is not parametrized and can fully adapt to changes in ice flow and geometry. The local interpolation of grounding-line position and basal friction and a specific driving stress scheme at the grounding line make PISM capable of modelling reversible grounding-line motion in the Mismip3d experiments22 comparable to the results obtained from full-Stokes flow23. We start from an ensemble of stable equilibrium states, created through the perturbation of ice-flow and basal-friction parameters under time-constant present-day atmosphere and ocean boundary conditions24,25 (Supplementary Table 1). Model domain boundaries lie at present ice divides surrounding the drainage basin15 and are far from the main ice-dynamical changes induced by the forcing (Fig. 1). The coastal ice margin at George V Coast is free to evolve in the Pacific sector of the Southern Ocean. We force the ice sheet’s equilibrium states with warm water pulses by raising the ocean temperature beneath the ice shelves to 1–2.5 C above the values applied during equilibrium (Supplementary Table 2). The pulses have a length between 200