Slowdown in Antarctic mass loss from solid Earth and sea-level feedbacks
E. Larour1,*, H. Seroussi1, S. Adhikari1, E. Ivins1, L. Caron1, M. Morlighem2, N. Schlegel1
Science 25 Apr 2019:
DOI: 10.1126/science.aav7908
https://science.sciencemag.org/content/early/2019/04/24/science.aav7908?rss=1"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 all solid Earth processes impacting ice sheets and show a projected negative feedback in grounding line migration of 38% for Thwaites Glacier 350 years in the future, or 26.8% reduction in corresponding sea-level contribution.
The specific processes involved in this negative feedback have previously been extensively investigated, such as self-attraction and loading (SAL) (1), rotational feedback (2) and redistribution of mass in the Earth due to Glacial Isostatic Adjustment (GIA) (3, 4). Some of these processes, SAL and GIA in particular, have been shown to stabilize grounding line retreat of ice sheets resting on retrograde slope (5–7). Geodetic investigations of crustal uplift in the Amundsen Sea Sector (ASS) (
and models of grounding-line retreat followed by re-advance in the Ross Sea Sector during the past 10,000 years (9) have recently confirmed the stabilizing role of solid-Earth deformation. Our focus here is however on how short-wavelength uplift generated by the unloading of the Earth crust over short time scales in the immediate vicinity of grounding lines further impacts the dynamics of ice-sheets grounding-line migration. This particular aspect has not previously been investigated. Indeed, grounding lines in Antarctica are geographically refined features that need to be spatially resolved at resolutions below 1 km (10), and that migrate over short time scales (weeks to month) (11), which triggers the question of how to avoid underestimating the resulting uplift upon migration. Our global simulation of Antarctic evolution presented here is carried out at the necessary high temporal resolution (14 d and 365 d for the ice and solid Earth, respectively) and spatial resolution (1-50 km) required to capture the processes stabilizing and destabilizing ice sheets. These include interactions that involve global eustatic sea-level rise (SLR), SAL, elastic rebound of the solid Earth and rotational feedback.
In Antarctica, dynamic retreat of ice streams has been the main driver of mass loss (12). These ice streams are largely controlled by how their grounding lines migrate (13) and interact with pinning points (14). Recent research efforts have focused on the complex interactions between intrusion of warm circumpolar water near the grounding line (11), ungrounding of pinning points (14, 15) and reduction in buttressing through loss of friction (16). A key aspect of understanding grounding-line dynamics (GLD) is to understand the relationship between the evolving sea level relative to the exact position of pinning points. As shown through NASA’s Operation IceBridge topographic mapping as well as decades long efforts to map grounding-line migration, highly resolved pinning points are present in critical areas of WAIS that can only be captured at kilometer scale resolutions (17, 18). In parallel, studies have shown that the physical representation of grounding line migration can only be modeled through meshes that attain 1 km resolution (19). There has been recent interest in developing future projections of SLR that incorporate solid-Earth processes with Maxwellian viscoelastic response (5, 6, 20) and SAL (5, 21). However, these tend to involve GLD that is resolved at much coarser resolutions (25-100 km) and involve time steps on the order of years or even decades. Such resolution bounds are incompatible with capturing the complex geometry of WAIS ice streams that are vulnerable to rapid retreat. For example, Pine Island Glacier (PIG) is 20-30 km wide at the grounding line, with complex grounding-line geometry that can only be resolved spatially at the 1-2 km level (10). In addition, a resolution of 25-100 km is inherently too coarse to capture short wavelength elastic uplift generated by fast grounding-line retreat and associated mass loss. As shown in Fig. 1, 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 magnitude lower. This implies that uplift generated in simulations such as (5, 6, 20) might underestimate how much uplift is generated during ungrounding of active areas of Antarctica such as TG or PIG, where highly complex grounding line geometries and associated retreat are observed over short time scales on the order of years. Some models such as (22) have attained resolutions down to 6 km, however in such cases GLD has not been considered interactively but prescribed offline, which precluded extensive negative feedback from manifesting themselves during the simulations. Our goal here is to carry out a sensitivity study of sea-level and ice-flow related processes by incorporating kilometer scale resolutions and global processes that involve solid-Earth dynamics. The ice-flow model robustly captures grounding line dynamics at high resolution (1 km) and over very short time scales (2 week"
