Description: The mechanisms causing widespread flow acceleration of Jakobshavn Isbræ, West Greenland, remain unclear despite an abundance of observations and modeling studies. Here we simulate the glacier's evolution from 1985 to 2016 using a three-dimensional thermomechanical ice flow model. The model captures the timing and 90% of the observed changes by forcing the calving front. Basal drag in the trough is low, and lateral drag balances the ice stream's driving stress. The calving front position is the dominant control on changes of Jakobshavn Isbræ since the ice viscosity in the shear margins instantaneously drops in response to the stress perturbation caused by calving front retreat, which allows for widespread flow acceleration. Gradual shear margin warming contributes 5 to 10% to the total acceleration. Our simulations suggest that the glacier will contribute to eustatic sea level rise at a rate comparable to or higher than at present.

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 (MC) 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 datasets, particularly in the marine-terminating sectors of northwest and southeast Greenland. Our map reveals 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: The hypothesis that an impact crater underlies Hiawatha Glacier in northwest Greenland was motivated by serendipitous NASA airborne radar sounding, but geologic field mapping and a focused survey using a new ultrawideband (UWB; 150&ndasg 520 MHz) radar system onboard the Alfred Wegener Institute’s Polar 6 Basler DC-3T aircraft were essential to confirm this hypothesis. Here we describe the multiple anomalous subsurface features observed by this survey and discuss the broader potential value of UWB airborne radar sounding of ice sheets. In the near-surface of Hiawatha Glacier’s ablation zone, discrete cross-bedded units up to tens of meters thick are likely superimposed ice associated with advected former supraglacial lakes. Elsewhere, numerous emerging reflections intersect the surface where satellite-observed outcrops of units with known visual characteristics from the Holocene epoch and the Last Glacial Period (LGP), and radar-reflectivity and surface-stratigraphy patterns are self-consistent. The Holocene–LGP interface is conforming above the northeast half of the crater but undulates dramatically at sub-kilometer scales above its southwestern half, likely recording a significant change in ice flow of unknown origin. Within the basal ice above the crater, numerous bed-originating reflectors emanate mostly from the central uplift, and point scatterers are common, which we interpret as evidence of basal freeze-on over a bedrock obstacle and vigorous subglacial erosion of the crater, including quarrying of boulder-sized clasts. Finally, a remarkably flat reflection typically 15 m below the ice–bed interface is sometimes detected at the downstream end of the crater, which likely represents the first detection of a subglacial groundwater table below presumably well drained impact breccia. As with previous generational advances in airborne ice-penetrating systems, we should expect that UWB surveys of ice sheets will detect previously unobserved subsurface features and identify new directions in radioglaciology. While some of the reported features may be unique to Hiawatha Glacier, our observations demonstrate the value of this new UWB airborne radar system to advance our understanding of processes at ice-sheet margins, which have historically been a challenging radioglaciological target.

Description: The Antarctic ice sheet has been losing mass over past decades through the accelerated flow of its glaciers, conditioned by ocean temperature and bed topography. Glaciers retreating along retrograde slopes (that is, the bed elevation drops in the inland direction) are potentially unstable, while subglacial ridges slow down the glacial retreat. Despite major advances in the mapping of subglacial bed topography, significant sectors of Antarctica remain poorly resolved and critical spatial details are missing. Here we present a novel, high-resolution and physically based description of Antarctic bed topography using mass conservation. Our results reveal previously unknown basal features with major implications for glacier response to climate change. For example, glaciers flowing across the Transantarctic Mountains are protected by broad, stabilizing ridges. Conversely, in the marine basin of Wilkes Land, East Antarctica, we find retrograde slopes along Ninnis and Denman glaciers, with stabilizing slopes beneath Moscow University, Totten and Lambert glacier system, despite corrections in bed elevation of up to 1 km for the latter. This transformative description of bed topography redefines the high- and lower-risk sectors for rapid sea level rise from Antarctica; it will also significantly impact model projections of sea level rise from Antarctica in the coming centuries.

Description: The thermal state of an ice sheet is an important control on its past and future evolution. Some parts of the ice sheet may be polythermal, leading to discontinuous properties at the cold–temperate transition surface (CTS). These discontinuities require a careful treatment in ice sheet models (ISMs). Additionally, the highly anisotropic geometry of the 3D elements in ice sheet modelling poses a problem for stabilization approaches in advection-dominated problems. Here, we present extended enthalpy formulations within the finite-element Ice-Sheet and Sea-Level System model (ISSM) that show a better performance than earlier implementations. In a first polythermal-slab experiment, we found that the treatment of the discontinuous conductivities at the CTS with a geometric mean produces more accurate results compared to the arithmetic or harmonic mean. This improvement is particularly efficient when applied to coarse vertical resolutions. In a second ice dome experiment, we find that the numerical solution is sensitive to the choice of stabilization parameters in the well-established streamline upwind Petrov–Galerkin (SUPG) method. As standard literature values for the SUPG stabilization parameter do not account for the highly anisotropic geometry of the 3D elements in ice sheet modelling, we propose a novel anisotropic SUPG (ASUPG) formulation. This formulation circumvents the problem of high aspect ratio by treating the horizontal and vertical directions separately in the stabilization coefficients. The ASUPG method provides accurate results for the thermodynamic equation on geometries with very small aspect ratios like ice sheets.

Description: Jakobshavn Isbræ, one of Greenland’s major outlet glaciers, displayed rapid changes since the mid-1990s. Its floating ice tongue broke up around 1997, followed by a rapid calving front retreat over tens of kilometres, possibly linked to a prior warming of the ocean waters adjacent to the fjord. Parallel to this major retreat, a quasi-simultaneous process of acceleration and thinning of the glacier has been observed, currently making it a major contributor to eustatic sea-level rise. However, the causal interplay between the various factors involved has not yet been fully understood. Numerical studies of Jakobshavn Isbræ so far are either 2-D plan view ice flow models with a fixed calving front or 2-D flowline models, which have to parameterize lateral stresses. Hence the interaction between changes in calving front position and ice dynamics could not be studied consistently. To overcome this limitation, we implemented an implicit boundary tracking method in the Ice Sheet System Model (ISSM). This tool allows us to freely evolve the calving front by prescribing a calving rate, the ice front velocity being therefore the sum of ice velocity and calving rate. A suite of sensitivity experiments perturbing an initial steady-state calving rate has been performed to study its impact on the dynamics of the glacier. Our numerical results suggest a high sensitivity of the glacier dynamics to the applied calving rate. Changes in calving rate quickly affect upstream areas of the ice stream through a combination of changes in calving front position, ice velocity, thickness, grounding and ungrounding, and surface gradient change. Consequently, acceleration triggered at the calving front quickly affects the entire drainage basin. Moreover, the model results suggest that the ice stream does not recover from a short duration (about a year) calving rate perturbation over timescales on the order of a century. We present selected results of the sensitivity experiments to support the discussion clarifying the causes of the current changes occurring at this dynamic ice stream.

Description: Recent observations highlight the influence of the calving front position on tidewater glaciers. Here, we present the theoretical and technical framework for a level-set method (LSM), which is now implemented into the Ice Sheet System Model. The LSM proves to be a robust tool to implicitly represent a dynamically evolving calving front in an Eulerian approach. We apply this method to a 3D thermodynamically coupled model of Jakobshavn Isbræ, West Greenland.

Description: Glacier-front dynamics is an important control on Greenland's ice mass balance. Warmer ocean waters trigger ice-front retreats of marine-terminating glaciers, and the corresponding loss in resistive stress leads to glacier acceleration and thinning. Here we present an approach to quantify the sensitivity and vulnerability of marine-terminating glaciers to ocean-induced melt. We develop a plan view model of Store Gletscher that includes a level set-based moving boundary capability, a parameterized ocean-induced melt, and a calving law with complete and precise land and fjord topographies to model the response of the glacier to increased melt. We find that the glacier is stabilized by a sill at its terminus. The glacier is dislodged from the sill when ocean-induced melt quadruples, at which point the glacier retreats irreversibly for 27 km into a reverse bed. The model suggests that ice-ocean interactions are the triggering mechanism of glacier retreat, but the bed controls its magnitude.

Description: The ocean plays an important role in modulating the mass balance of the polar ice sheets by interacting with the ice shelves in Antarctica and with the marine-terminating outlet glaciers in Greenland. Given that the flux of warm water onto the continental shelf and into the sub-ice cavities is steered by complex bathymetry, a detailed topography data set is an essential ingredient for models that address ice-ocean interaction. We followed the spirit of the global RTopo-1 data set and compiled consistent maps of global ocean bathymetry, upper and lower ice surface topographies and global surface height on a spherical grid with now 30-arc-seconds grid spacing. For this new data set, called RTopo-2, we used the General Bathymetric Chart of the Oceans (GEBCO_2014) as the backbone and added the International Bathymetric Chart of the Arctic Ocean version 3 (IBCAOv3) and the International Bathymetric Chart of the Southern Ocean (IBCSO) version 1. While RTopo-1 primarily aimed at a good and consistent representation of the Antarctic ice sheet, ice shelves and sub-ice cavities, RTopo-2 now also contains ice topographies of the Greenland ice sheet and outlet glaciers. In particular, we aimed at a good representation of the fjord and shelf bathymetry surrounding the Greenland continent. We modified data from earlier gridded products in the areas of Petermann Glacier, Hagen Bræ and Sermilik Fjord assuming that sub-ice and fjord bathymetries roughly follow plausible Last Glacial Maximum ice flow patterns. For the continental shelf off northeast Greenland and the floating ice tongue of Nioghalvfjerdsfjorden Glacier at about 79°N, we incorporated a high-resolution digital bathymetry model considering original multibeam survey data for the region. Radar data for surface topographies of the floating ice tongues of Nioghalvfjerdsfjorden Glacier and Zachariæ Isstrøm have been obtained from the data centers of Technical University of Denmark (DTU), Operation Icebridge (NASA/NSF) and Alfred Wegener Institute (AWI). For the Antarctic ice sheet/ice shelves, RTopo-2 largely relies on the Bedmap-2 product but applies corrections for the geometry of Getz, Abbot and Fimbul ice shelf cavities. The data set is available in full and in regional subsets in NetCDF format from the PANGAEA database at doi:10.1594/PANGAEA.856844.