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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.A high-resolution update of Antarctic bed topography using mass conservation reveals broad stabilizing ridges for glaciers flowing across the Transantarctic Mountains, and stabilizing slopes beneath Moscow University, Totten and Lambert glacier system.

Projections of Antarctica's sea‐level contribution depend on future changes in surface mass balance (SMB), yet it remains uncertain whether climate change has already impacted SMB on the East Antarctic Plateau, given diverging trends in prior studies. Using ∼3,000 km of airborne radar data from western Dronning Maud Land (DML), we reconstructed SMB over the past ∼800 years (1209–2024 C.E.) and found stable centennial averages before 1977, followed by a 21% increase in recent decades. This increase is spatially coherent despite strong small‐scale variability driven by topography and wind redistribution, which can bias upscaling of firn core records. Integrating radar and firn core data at ∼5 ×${\\times} $5 km2 scales reduces this bias. Our results show an increase in SMB in western DML over the last five decades. If sustained, it could help mitigate sea‐level rise.

Subglacial water beneath the Antarctic Ice Sheet is often funneled via subglacial channels, which inject freshwater into ice‐shelf cavities where it interacts with ocean water. The temporal variability of this system has been poorly observed, but its importance for ice dynamics is well recognized. Airborne radar data show a subglacial channel evolving within a decade near of the grounding zone of the Roi Baudouin Ice Shelf (East Antarctica), while topographic signatures on the ice shelf indicate prior inactivity for 60 years. Combining our observations with subglacial hydrological modeling, we suggest that the interplay between episodic subglacial water pulses and ocean water intrusion drive the opening and closing of the channels. Our findings illuminate the short‐term transient nature of subglacial channel activity. This impacts ice‐shelf–ocean processes, which are important for constraining increasing ocean warming onto ice‐shelf basal mass balance, but pose significant challenges for subglacial hydrological modeling at the grounding zone. Plain Language Summary This study explores how water moves under the ice in Antarctica and how it interacts at the ice–ocean boundary. Using radar, we studied a part of the Roi Baudouin Ice Shelf in East Antarctica and found that water channels under the ice have changed significantly over the last decades. The radar images from 2011 to 2019 showed that these channels open and close within a decade, likely due to changes in water supply from the inland and intrusion of ocean water. This means that the water flow under the ice is not steady, influencing melting at the underside and affecting its movement. These discoveries show that the subglacial water system under Antarctic Ice Sheet is more complicated, making it harder to project future changes in sea‐level rise and the effects of climate change on the polar regions. Key Points Subglacial channels near the grounding zone are stable in space but toggle activity within decades Ice‐shelf channel morphology identifies one water outlet as active for hundreds of years before it shut down 60 years ago Radio‐echo sounding documents the reactivation of a subglacial channel within eight years near the grounding zone

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 arcsec 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 centres 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.

The orientation of ice crystals within large ice masses has a strong influence on their mechanical properties, but cannot be directly observed from the surface. The bulk birefringence of anisotropic ice allows us to infer information about the crystal orientation fabric (COF) from polarized radar measurements. Here, we show a new approach for determining the orientation and strength of horizontal COF anisotropy from two radar reflections originating from the same physical layer in birefringent ice. We apply this method to data collected as part of a ground‐based radar survey of the North East Greenland Ice Stream. We observe a 12‐degree clockwise rotation of the fabric at the center of the ice stream, and a tendency toward a flow‐aligned COF further southeast. This asymmetry across the ice‐stream centerline adds to growing evidence for a more variable ice stream than previously assumed. Plain Language Summary The ice in glaciers and ice sheets is composed of crystals, small pieces of ice which can differ in shape and orientation. How ice crystals are arranged is closely linked to how ice sheets move and behave, which is important for predicting changes in polar ice sheets and their impact on sea level rise. Even though the arrangement of the crystals is not directly observable from the surface of ice sheets, we can use specific types of radar measurements to extract general information about how the crystals are arranged. In this study, we present a new approach for doing this. We test the method on data collected as part of a survey of the North East Greenland Ice Stream, which drains 16% of the area of the Greenland Ice Sheet. Our results show that the crystals are not aligned in the way we expected, suggesting that the flow pattern of the ice stream has changed in the past and is not as stable as previously thought. Key Points We present a novel method for deriving strength and orientation of crystal orientation fabric using double reflections in birefringent ice Method reveals a 12‐degree rotation of the crystal orientation fabric relative to flow at the center of the North East Greenland Ice Stream An asymmetry of the crystal orientation fabric across center of the ice stream indicates more variable flow than previously assumed

The dynamic mass loss of ice sheets constitutes one of the biggest uncertainties in projections of ice-sheet evolution. One central, understudied aspect of ice flow is how the bulk orientation of the crystal orientation fabric translates to the mechanical anisotropy of ice. Here we show the spatial distribution of the depth-averaged horizontal anisotropy and corresponding directional flow-enhancement factors covering a large area of the Northeast Greenland Ice Stream onset. Our results are based on airborne and ground-based radar surveys, ice-core observations, and numerical ice-flow modelling. They show a strong spatial variability of the horizontal anisotropy and a rapid crystal reorganisation on the order of hundreds of years coinciding with the ice-stream geometry. Compared to isotropic ice, parts of the ice stream are found to be more than one order of magnitude harder for along-flow extension/compression while the shear margins are potentially softened by a factor of two for horizontal-shear deformation. This study uses radio-echo sounding measurements, ice-core data and models to map the spatial variation in ice-crystal orientation in the northeast Greenland Ice Stream and shows how it potentially affects the ice-flow dynamics in this region.

Basal motion of ice sheets depends in part on the roughness and material properties of the subglacial bed and the occurrence of water. To date, basal motion represents one of the largest uncertainties in ice-flow models. It is that component of the total flow velocity that can change most rapidly and can, therefore, facilitate rapid variations in dynamic behaviour. In this study, we investigate the subglacial properties of the East Antarctic Ice Sheet by statistically analysing the roughness of the bed topography, inferred from radio-echo sounding measurements. We analyse two sets of roughness parameters, one derived in the spatial and the other in the spectral domain, with two roughness parameters each. This enables us to compare the suitability of the four roughness parameters to classify the subglacial landscapes below the ice sheet. We further investigate the relationship of the roughness parameters with observed surface flow velocity and modelled basal temperatures of the ice sheet. We find that one of the roughness parameters, the Hurst exponent derived in the spatial domain, coincides with the thermal condition at the base of the ice sheet for slow flow velocities and varies with flow velocity.

The area near Dome C, East Antarctica, is thought to be one of the most promising targets for recovering a continuous ice-core record spanning more than a million years. The European Beyond EPICA consortium has selected Little Dome C (LDC), an area ∼ 35 km southeast of Concordia Station, to attempt to recover such a record. Here, we present the results of the final ice-penetrating radar survey used to refine the exact drill site. These data were acquired during the 2019–2020 austral summer using a new, multi-channel high-resolution very high frequency (VHF) radar operating in the frequency range of 170–230 MHz. This new instrument is able to detect reflectors in the near-basal region, where previous surveys were largely unable to detect horizons. The radar stratigraphy is used to transfer the timescale of the EPICA Dome C ice core (EDC) to the area of Little Dome C, using radar isochrones dating back past 600 ka. We use these data to derive the expected depth–age relationship through the ice column at the now-chosen drill site, termed BELDC (Beyond EPICA LDC). These new data indicate that the ice at BELDC is considerably older than that at EDC at the same depth and that there is about 375 m of ice older than 600 kyr at BELDC. Stratigraphy is well preserved to 2565 m, ∼ 93 % of the ice thickness, below which there is a basal unit with unknown properties. An ice-flow model tuned to the isochrones suggests ages likely reach 1.5 Myr near 2500 m, ∼ 65 m above the basal unit and ∼ 265 m above the bed, with sufficient resolution (19 ± 2 kyr m−1) to resolve 41 kyr glacial cycles.

The Northeast Greenland Ice Stream (NEGIS) is an important dynamic component for the total mass balance of the Greenland ice sheet, as it reaches up to the central divide and drains 12% of the ice sheet. The geometric boundary conditions and in particular the nature of the subglacial bed of the NEGIS are essential to understand its ice flow dynamics. We present a record of more than 8000 km of radar survey lines of multi-channel, ultra-wideband radio echo sounding data covering an area of 24 000 km2, centered on the drill site for the East Greenland Ice-core Project (EGRIP), in the upper part of the NEGIS catchment. Our data yield a new detailed model of ice-thickness distribution and basal topography in the region. The enhanced resolution of our bed topography model shows features which we interpret to be caused by erosional activity, potentially over several glacial–interglacial cycles. Off-nadir reflections from the ice–bed interface in the center of the ice stream indicate a streamlined bed with elongated subglacial landforms. Our new bed topography model will help to improve the basal boundary conditions of NEGIS prescribed for ice flow models and thus foster an improved understanding of the ice-dynamic setting.

The increasing catalogue of high-quality ice-penetrating radar data provides a unique insight in the internal layering architecture of the Greenland ice sheet. The stratigraphy, an indicator of past deformation, highlights irregularities in ice flow and reveals large perturbations without obvious links to bedrock shape. In this work, to establish a new conceptual model for the formation process, we analysed the radar data at the onset of the Petermann Glacier, North Greenland, and created a three-dimensional model of several distinct stratigraphic layers. We demonstrate that the dominant structures are cylindrical folds sub-parallel to the ice flow. By numerical modelling, we show that these folds can be formed by lateral compression of mechanically anisotropic ice, while a general viscosity contrast between layers would not lead to folding for the same boundary conditions. We conclude that the folds primarily form by converging flow as the mechanically anisotropic ice is channelled towards the glacier. A range of mechanisms has been proposed for large-scale folding in polar ice sheets. Here, using new three-dimensional reconstructions of such folds in the onset region of the Greenland Petermann Glacier, the authors show that these formed due to flow convergence and the high mechanical anisotropy of ice.