Slowdown in Antarctic mass loss from solid Earth and sea-level feedbacks

INTRODUCTION Geodetic investigations of crustal motions in the Amundsen Sea sector of West Antarctica and models of ice-sheet evolution in the past 10,000 years have recently highlighted the stabilizing role of solid-Earth uplift on polar ice sheets. One critical aspect, however, that has not been assessed is the impact of short-wavelength uplift generated by the solid-Earth response to unloading over short time scales close to ice-sheet grounding lines (areas where the ice becomes afloat). Here, we present a new global simulation of Antarctic evolution at high spatiotemporal resolution that captures solid-Earth processes stabilizing and destabilizing ice sheets. These include interactions with global eustatic sea-level rise (SLR), self-attraction and loading (SAL) of the oceans, Earth’s rotational feedback to SAL, and elastic uplift of the solid Earth. RATIONALE In Antarctica, dynamic thinning and retreat of ice streams has been the main driver of mass loss over the past decades, largely controlled by how grounding lines migrate and interact with bedrock pinning points. Studies have shown that the physical representation of grounding-line dynamics (GLD) can only be captured through simulations with a horizontal resolution higher than 1 km. Current models of SLR that incorporate solid-Earth processes with viscoelastic response tend to involve GLD that is resolved at much coarser resolutions (25 to 100 km) and involve time steps on the order of decades. Such resolution bounds are incompatible with capturing the complex bed topography of the ice streams of West Antarctica that are vulnerable to rapid retreat. In addition, a resolution of 25 to 100 km is inherently too coarse to capture short-wavelength elastic uplift generated by fast GLD. Elastic uplift generated by a 2-km grounding-line retreat, modeled as loss of 100-m-thick ice from a disk of 2-km radius, can reach 52 mm near the grounding line (centroid of the equivalent disk). At coarser resolutions (say, 16 km), the same model generates uplift one order of magnitude lower. This implies that uplift generated in simulations at coarse resolutions might underestimate how much uplift is generated during ungrounding of active areas of Antarctica such as Thwaites Glacier (TG), where highly complex grounding-line geometries and associated retreat are observed over short time scales on the order of years. Our goal was therefore to carry out a sensitivity study of sea level– and ice flow–related processes that incorporate kilometer-scale resolutions and global processes involving solid-Earth dynamics. RESULTS Our sensitivity study spans 500 years and demonstrates how in Antarctica, TG is particularly prone to negative feedback from SAL and elastic uplift. At year 2350, including these feedbacks leads to a ~20-year delay in dynamic mass loss of TG, corresponding to a 26.8% reduction in sea-level contribution, along with a reduction in grounding-line retreat of 38% and elastic uplift rates reaching ~0.25 m/year. At year 2100, though, this negative feedback is considerably lower, with a 1.34% reduction in sea-level contribution only. Not including a kilometer-scale resolution representation of such processes will lead to projections of SLR that will significantly overestimate Antarctic Ice Sheet contribution over several centuries. CONCLUSION For 21st-century projections, the effects we have modeled here remain negligible. However, for the period starting 2250 and after, SLR projections that would not account for such dynamic geodetic effects run the risk of consistently overestimating (by 20 to 40%) relative sea-level estimates. Our approach shows that significant stabilization in grounding-line migration occurs when uplift rates start approaching 10 cm/year. This has strong implications for late Quaternary reconstructions of SLR, for example, in which the inclusion of these solid-Earth processes will allow SLR modelers to gain a better grasp of the time scales involved in reaching maximum coastal inundation levels during extended warm periods.