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  1. SUMMARY

    Mass loss from polar ice sheets is becoming the dominant contributor to current sea level changes, as well as one of the largest sources of uncertainty in sea level projections. The spatial pattern of sea level change is sensitive to the geometry of ice sheet mass changes, and local sea level changes can deviate from the global mean sea level change due to gravitational, Earth rotational and deformational (GRD) effects. The pattern of GRD sea level change associated with the melting of an ice sheet is often considered to remain relatively constant in time outside the vicinity of the ice sheet. For example, in the sea level projections from the most recent IPCC sixth assessment report (AR6), the geometry of ice sheet mass loss was treated as constant during the 21st century. However, ice sheet simulations predict that the geometry of ice mass changes across a given ice sheet and the relative mass loss from each ice sheet will vary during the coming century, producing patters of global sea level changes that are spatiotemporally variable. We adopt a sea level model that includes GRD effects and shoreline migration to calculate time-varying sea level patterns associated with projections of the Greenland and Antarctic Ice Sheets during the coming century. We find that in some cases, sea level changes can be substantially amplified above the global mean early in the century, with this amplification diminishing by 2100. We explain these differences by calculating the contributions of Earth rotation as well as gravitational and deformational effects to the projected sea level changes separately. We find in one case, for example, that ice gain on the Antarctic Peninsula can cause an amplification of up to 2.9 times the global mean sea level equivalent along South American coastlines due to positive interference of GRD effects. To explore the uncertainty introduced by differences in predicted ice mass geometry, we predict the sea level changes following end-member mass loss scenarios for various regions of the Antarctic Ice Sheet from the ISMIP6 model ensemblely, and find that sea level amplification above the global mean sea level equivalent differ by up to 1.9 times between different ice mass projections along global coastlines outside of Greenland and Antarctica. This work suggests that assessments of future sea level hazard should consider not only the integrated mass changes of ice sheets, but also temporal variations in the geometry of the ice mass changes across the ice sheets. As well, this study highlights the importance of constraining the relative timing of ice mass changes between the Greenland and Antarctic Ice Sheets.

     
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  2. Abstract. Retreat and advance of ice sheets perturb the gravitational field, solidsurface and rotation of the Earth, leading to spatially variable sea-levelchanges over a range of timescales O(100−6 years), which in turn feedback onto ice-sheet dynamics. Coupled ice-sheet–sea-level models havebeen developed to capture the interactive processes between ice sheets, sealevel and the solid Earth, but it is computationally challenging to captureshort-term interactions O(100−2 years) precisely within longer O(103−6 years) simulations. The standard forward sea-level modelling algorithmassigns a uniform temporal resolution in the sea-level model, causing aquadratic increase in total CPU time with the total number of input icehistory steps, which increases with either the length or temporal resolutionof the simulation. In this study, we introduce a new “time window”algorithm for 1D pseudo-spectral sea-level models based on the normal modemethod that enables users to define the temporal resolution at which the iceloading history is captured during different time intervals before thecurrent simulation time. Utilizing the time window, we assign a finetemporal resolution O(100−2 years) for the period of ongoing andrecent history of surface ice and ocean loading changes and a coarsertemporal resolution O(103−6 years) for earlier periods in thesimulation. This reduces the total CPU time and memory required per modeltime step while maintaining the precision of the model results. We explorethe sensitivity of sea-level model results to the model temporal resolutionand show how this sensitivity feeds back onto ice-sheet dynamics in coupledmodelling. We apply the new algorithm to simulate sea-level changes inresponse to global ice-sheet evolution over two glacial cycles and the rapidcollapse of marine sectors of the West Antarctic Ice Sheet in the comingcenturies and provide appropriate time window profiles for each application.The time window algorithm reduces the total CPU time by ∼ 50 % in each of these examples and changes the trend of the total CPU timeincrease from quadratic to linear. This improvement would increase withlonger simulations than those considered here. Our algorithm also allows for couplingtime intervals of annual temporal scale for coupled ice-sheet–sea-levelmodelling of regions such as West Antarctica that are characterized byrapid solid Earth response to ice changes due to the thin lithosphere andlow mantle viscosities. 
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  3. Abstract. Accurate glacial isostatic adjustment (GIA) modelling in the cryosphere is required for interpreting satellite, geophysical and geological recordsand for assessing the feedbacks of Earth deformation and sea-level change on marine ice-sheet grounding lines. GIA modelling in areas of active ice lossin West Antarctica is particularly challenging because the ice is underlain by laterally varying mantle viscosities that are up to several orders ofmagnitude lower than the global average, leading to a faster and more localised response of the solid Earth to ongoing and future ice-sheet retreatand necessitating GIA models that incorporate 3-D viscoelastic Earth structure. Improvements to GIA models allow for computation of the viscoelasticresponse of the Earth to surface ice loading at sub-kilometre resolution, and ice-sheet models and observational products now provide the inputs toGIA models at comparably unprecedented detail. However, the resolution required to accurately capture GIA in models remains poorly understood, andhigh-resolution calculations come at heavy computational expense. We adopt a 3-D GIA model with a range of Earth structure models based on recentseismic tomography and geodetic data to perform a comprehensive analysis of the influence of grid resolution on predictions of GIA in the AmundsenSea Embayment (ASE) in West Antarctica. Through idealised sensitivity testing down to sub-kilometre resolution with spatially isolated ice loadingchanges, we find that a grid resolution of ∼ 13 of the radius of the load or higher is required to accurately capture the elasticresponse of the Earth. However, when we consider more realistic, spatially coherent ice loss scenarios based on modern observational records andfuture ice-sheet model projections and adopt a viscoelastic Earth, we find that predicted deformation and sea-level change along the grounding lineconverge to within 5 % with grid resolutions of 7.5 km or higher, and to within 2 % for grid resolutions of 3.75 km andhigher, even when the input ice model is on a 1 km grid. Furthermore, we show that low mantle viscosities beneath the ASE lead to viscousdeformation that contributes to the instrumental record on decadal timescales and equals or dominates over elastic effects by the end of the 21stcentury. Our findings suggest that for the range of resolutions of 1.9–15 km that we considered, the error due to adopting a coarser gridin this region is negligible compared to the effect of neglecting viscous effects and the uncertainty in the adopted mantle viscosity structure. 
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  4. Geodetic, seismic, and geological evidence indicates that West Antarctica is underlain by low-viscosity shallow mantle. Thus, as marine-based sectors of the West Antarctic Ice Sheet (WAIS) retreated during past interglacials, or will retreat in the future, exposed bedrock will rebound rapidly and flux meltwater out into the open ocean. Previous studies have suggested that this contribution to global mean sea level (GMSL) rise is small and occurs slowly. We challenge this notion using sea level predictions that incorporate both the outflux mechanism and complex three-dimensional viscoelastic mantle structure. In the case of the last interglacial, where the GMSL contribution from WAIS collapse is often cited as ~3 to 4 meters, the outflux mechanism contributes ~1 meter of additional GMSL change within ~1 thousand years of the collapse. Using a projection of future WAIS collapse, we also demonstrate that the outflux can substantially amplify GMSL rise estimates over the next century. 
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  6. Abstract

    Retreat or advance of an ice sheet perturbs the Earth's solid surface, rotational vector, and the gravitational field, which in turn feeds back onto the evolution of the ice sheet over a range of timescales. Throughout the last glacial cycle, ice sheets over the Northern Hemisphere have gone through multiple growth and retreat phases, but the dynamics during these phases are not well understood. In this study, we apply a coupled ice sheet‐glacial isostatic adjustment model to simulate the Northern Hemisphere Ice Sheets over the last glacial cycle. We focus on understanding the influence of solid Earth deformation and gravitational field perturbations associated with surface (ice and water) loading changes on the dynamics of terrestrial and marine‐based ice sheets during different phases of the glacial cycle. Our results show that solid Earth deformation enhances glaciation during growth phases and melting during retreat phases in terrestrial regions through ice‐elevation feedback, and gravitational field perturbations have a stabilizing influence on marine‐based ice sheets in regions such as Hudson Bay in North America and Barents and Kara Seas in Eurasia during retreat phases through sea‐level feedback. Our results also indicate that solid Earth deformation influences the relative sensitivity of the North American and Eurasian ice sheets to climate and thus the timing and magnitude of their fluctuations throughout the last glacial cycle.

     
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