Comment on tc-2021-232

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.