Resolving GIA in response to modern and future ice loss at marine grounding lines in West Antarctica

Abstract

Abstract. Accurate glacial isostatic adjustment (GIA) modeling in the cryosphere is required for interpreting satellite, geophysical and geological records and to assess the feedbacks of Earth deformation and sea level change on marine ice-sheet grounding lines. Assessing GIA in areas of active ice loss in West Antarctica is particularly challenging because the ice is underlain by laterally varying mantle viscosities that are up to several orders of magnitude lower than the global average, leading to a faster and more localized response of the solid Earth to ongoing and future ice sheet retreat and necessitating GIA models that incorporate 3-D viscoelastic Earth structure. Improvements to GIA models allow for computation of the viscoelastic response of the Earth to surface ice loading at sub-kilometre resolution and ice-sheet models and observational products now provide the inputs to GIA models at comparably unprecedented detail. However, the resolution required to capture GIA in models remains poorly understood, and high-resolution calculations come at heavy computational expense. We adopt a 3-D GIA model with a range of Earth structure models based on recent seismic tomography and geodetic data to perform a comprehensive analysis of the influence of grid resolution on predictions of GIA in the Amundsen Sea Embayment (ASE) in West Antarctica. Through idealized sensitivity testing down to sub-kilometre resolution with spatially isolated ice loading changes, we find that a grid resolution of ~3 times the radius of the load is required to accurately capture the elastic response of the Earth. However, when we consider more realistic, spatially coherent ice loss scenarios based on modern observational records and future ice sheet model projections and adopt a viscoelastic Earth, we find that errors of less than 5 % along the grounding line can be achieved with a 7.5 km grid, and less than 2 % with a 3.75 km grid, even when the input ice model is on a 1 km grid. Furthermore, we show that low mantle viscosities beneath the ASE lead to viscous deformation that contributes to the instrumental record on decadal timescales and equals or dominates over elastic effects by the end of the 21st century. Our findings suggest that for the range of resolutions of 1.9–15 km that we considered, the error due to adopting a coarser grid in this region is negligible compared to the effect of neglecting viscous effects and the uncertainty in the adopted mantle viscosity structure.

The Influence of the Solid Earth on the Contribution of Marine Sections of the Antarctic Ice Sheet to Future Sea‐Level Change

“…Two 3-D mantle viscosity structures, which were first constructed in Wan et al (2022), are built upon two different 1-D reference viscosity profiles:-the first model we referred to as EM3D_L, is characterized by a uniform viscosity of 𝐴𝐴 10 20 Pa s in the upper mantle (from the bottom of the lithosphere to the depth of 670 km), and a viscosity value of 𝐴𝐴 5 × 10 21 Pa s within the lower mantle (from 670 km depth to the core-mantle boundary);-the second model, EM3D_M, adopts the same reference viscosity within the lower mantle, but has higher background viscosity of 𝐴𝐴 5 × 10 20 Pa s in the upper mantle (Figure S1b in Supporting Information S1). The variations in viscosity relative to the assumed reference model are derived by first combining the ANT-20 regional shear-wave seismic velocity model of the upper mantle beneath Antarctica (Lloyd et al, 2020) with the S362ANI global seismic tomography model (Kustowski et al, 2008).…”

The robustness of geodetically derived 1-D Antarctic viscosity models in the presence of complex 3-D viscoelastic Earth structure

“…That is, we find the best-fitting linear trend through the last 5 yr of the 1-D viscoelastic-minus-elastic crustal uplift time-series. We performed a set of test calculations using a harmonic truncation level of 1024, and found that while moving to higher resolution did impact the elastic component of the uplift in some areas (see Wan et al 2021 for an in-depth discussion of the resolution dependence of viscoelastic deformation predictions in West Antarctica), our predictions of viscous uplift were negligibly impacted by this change (Fig. S1).…”