Description: Greenland's bed topography is a primary control on ice flow, grounding line migration, calving dynamics, and subglacial drainage. Moreover, fjord bathymetry regulates the penetration of warm Atlantic water (AW) that rapidly melts and undercuts Greenland's marine-terminating glaciers. Here we present a new compilation of Greenland bed topography that assimilates seafloor bathymetry and ice thickness data through a mass conservation approach. A new 150 m horizontal resolution bed topography/bathymetric map of Greenland is constructed with seamless transitions at the ice/ocean interface, yielding major improvements over previous data sets, particularly in the marine-terminating sectors of northwest and southeast Greenland. Our map reveals that the total sea level potential of the Greenland ice sheet is 7.42 ± 0.05 m, which is 7 cm greater than previous estimates. Furthermore, it explains recent calving front response of numerous outlet glaciers and reveals new pathways by which AW can access glaciers with marine-based basins, thereby highlighting sectors of Greenland that are most vulnerable to future oceanic forcing.

Description: We report the discovery of a large impact crater beneath Hiawatha Glacier in northwest Greenland. From airborne radar surveys, we identify a 31-kilometer-wide, circular bedrock depression beneath up to a kilometer of ice. This depression has an elevated rim that cross-cuts tributary subglacial channels and a subdued central uplift that appears to be actively eroding. From ground investigations of the deglaciated foreland, we identify overprinted structures within Precambrian bedrock along the ice margin that strike tangent to the subglacial rim. Glaciofluvial sediment from the largest river draining the crater contains shocked quartz and other impact-related grains. Geochemical analysis of this sediment indicates that the impactor was a fractionated iron asteroid, which must have been more than a kilometer wide to produce the identified crater. Radiostratigraphy of the ice in the crater shows that the Holocene ice is continuous and conformable, but all deeper and older ice appears to be debris rich or heavily disturbed. The age of this impact crater is presently unknown, but from our geological and geophysical evidence, we conclude that it is unlikely to predate the Pleistocene inception of the Greenland Ice Sheet.

Description: The Greenland ice sheet is one of the largest contributors to global mean sea-level rise today and is expected to continue to lose mass as the Arctic continues to warm. The two predominant mass loss mechanisms are increased surface meltwater run-off and mass loss associated with the retreat of marine-terminating outlet glaciers. In this paper we use a large ensemble of Greenland ice sheet models forced by output from a representative subset of the Coupled Model Intercomparison Project (CMIP5) global climate models to project ice sheet changes and sea-level rise contributions over the 21st century. The simulations are part of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). We estimate the sea-level contribution together with uncertainties due to future climate forcing, ice sheet model formulations and ocean forcing for the two greenhouse gas concentration scenarios RCP8.5 and RCP2.6. The results indicate that the Greenland ice sheet will continue to lose mass in both scenarios until 2100, with contributions of 90±50 and 32±17 mm to sea-level rise for RCP8.5 and RCP2.6, respectively. The largest mass loss is expected from the south-west of Greenland, which is governed by surface mass balance changes, continuing what is already observed today. Because the contributions are calculated against an unforced control experiment, these numbers do not include any committed mass loss, i.e. mass loss that would occur over the coming century if the climate forcing remained constant. Under RCP8.5 forcing, ice sheet model uncertainty explains an ensemble spread of 40 mm, while climate model uncertainty and ocean forcing uncertainty account for a spread of 36 and 19 mm, respectively. Apart from those formally derived uncertainty ranges, the largest gap in our knowledge is about the physical understanding and implementation of the calving process, i.e. the interaction of the ice sheet with the ocean.

Description: Predictions of marine ice-sheet behaviour require models able to simulate grounding-line migration. We present results of an intercomparison experiment for plan-view marine ice-sheet models. Verification is effected by comparison with approximate analytical solutions for flux across the grounding line using simplified geometrical configurations (no lateral variations, no buttressing effects from lateral drag). Perturbation experiments specifying spatial variation in basal sliding parameters permitted the evolution of curved grounding lines, generating buttressing effects. The experiments showed regions of compression and extensional flow across the grounding line, thereby invalidating the boundary layer theory. Steady-state grounding-line positions were found to be dependent on the level of physical model approximation. Resolving grounding lines requires inclusion of membrane stresses, a sufficiently small grid size (〈500m), or subgrid interpolation of the grounding line. The latter still requires nominal grid sizes of 〈5 km. For larger grid spacings, appropriate parameterizations for ice flux may be imposed at the grounding line, but the short-time transient behaviour is then incorrect and different from models that do not incorporate grounding-line parameterizations. The numerical error associated with predicting grounding-line motion can be reduced significantly below the errors associated with parameter ignorance and uncertainties in future scenarios.

Description: On behalf of the journal, AGU, and the scientific community, the editors would like to sincerely thank those who reviewed the manuscripts for Geophysical Research Letters in 2020. The hours reading and commenting on manuscripts not only improve the manuscripts but also increase the scientific rigor of future research in the field. We particularly appreciate the timely reviews in light of the demands imposed by the rapid review process at Geophysical Research Letters. The COVID pandemic imposed additional stresses on the review process, as many reviewers had to juggle increased family commitments, hours of online meetings, remote work and instruction, lack of physical access to library resources, and other hardships to maintain the quality and timeliness of their reviews. Although we witnessed an increase in the number of submissions, the average number of days to complete a review increased by less than one day!! That says a lot about the diligence of our reviewers. We deeply appreciate their contributions in these challenging times. With the advent of AGU's data policy, many reviewers have also helped immensely to evaluate the accessibility and availability of data, and many have provided insightful comments that helped to improve the data presentation and quality. We greatly appreciate the assistance of the reviewers in advancing open science, which is a key objective of AGU's data policy. Individuals in italics provided three or more reviews for Geophysical Research Letters during the year. In total, 5,177 referees contributed to 8,786 individual reviews. Thank you again. We look forward to the coming year of exciting advances in the field and communicating those advances to our community and to the broader public.

Description: Accurately projecting mass loss from ice sheets is critical to help societies best prepare for the change in sea level. Despite tremendous improvements, several recent studies show that the agreement between models and the observational record remains poor. The inability of numerical models to reproduce observations raises concerns about their ability to provide accurate projections. Data assimilation approaches are great tools to infer unknown parameters by minimizing the misfit between model and observations. Inversions have been used in glaciology since the 1990s, but only for a given point in time. These “snapshot inversions” are routinely used to infer unknown parameters, such as basal friction, but they do not take advantage of time series of observations to which we have access today. The advent of Automatic Differentiation and its recent integration in the Ice-sheet and Sea-level System Model and STREAMICE makes it possible to assimilate almost any type of data using time dependent models. Here we apply transient calibration to the Amundsen Sea Embayment between 2004 and 2022 using surface velocities from MEaSUREs and ITS_LIVE, and surface altimetry data from Cryosat and ICESat-2. We assess the performance of transient compared to snapshot calibrations in terms of capturing past and current trends in speed change, thinning, grounding line retreat and mass change. We then compare future projections over the next 100 years. This exercise paves the way to future modeling work that makes use of more dense time series to constrain critical model parameters and reduce uncertainty in future sea level rise.

Description: On behalf of the journal, AGU, and the scientific community, the editors of Geophysical Research Letters would like to sincerely thank those who reviewed manuscripts for us in 2022. The hours reading and commenting on manuscripts not only improve the manuscripts, but also increase the scientific rigor of future research in the field. With the advent of AGU's data policy, many reviewers have also helped immensely to evaluate the accessibility and availability of data, and many have provided insightful comments that helped to improve the data presentation and quality. We greatly appreciate the assistance of the reviewers in advancing open science, which is a key objective of AGU's data policy. We particularly appreciate the timely reviews in light of the demands imposed by the rapid review process at Geophysical Research Letters. We received 6,687 submissions in 2022 and 5,247 reviewers contributed to their evaluation by providing 8,720 reviews in total. We deeply appreciate their contributions in these challenging times.

Description: Determining the multidecadal evolution of ice sheets and the associated sea-level change informs major decisions for coastal hazard mitigation and societal development at the global scale. Such systems involve physical processes occurring within and between ice sheets, terrestrial water, oceans, and the solid Earth. Important feedback loops have been identified in marine-terminating ice sheets between thickness change near the grounding line and the bedrock response. We present a new framework for the Ice Sheet and Sea-level system Model that aims at coupling ice dynamics, hydrology, sea level, and solid-earth deformation. This framework features dynamic interactions between processes with support for multiscale resolution. For example, we are able to capture grounding line dynamics in West Antarctica at a kilometric and biweekly resolution, while global sea level is simultaneously computed on yearly timescales. This is achieved by combining spatial partitioning of physical processes, anisotropic meshing, subelemental geometry tracking, and an asynchronous mass transport approach. Our solid-Earth model is based on high-degree (~10〈sup〉4〈/sup〉) love numbers and is able to account for viscoelastic response in the lithosphere and mantle to surface loading and rotational feedback. Rheology models supported include the Hookean, Maxwell, Burgers, and Extended Burgers models. Our framework is fully parallelized and optimized to support ensemble modeling for uncertainty quantification purposes. Intended applications include the modeling of ice sheet retreat, sea-level projections, high-resolution coastline migration, Glacial Isostatic Adjustment, and hydrology fingerprints. These will affect the interpretation of numerous geodetic datasets, such as GRACE-FO, GPS, NISAR, ICESat-2, and SWOT. © 2023 California Institute of Technology. Government sponsorship acknowledged.