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International audience; Ice losses from the Greenland and Antarctic ice sheets have accelerated since the 1990s, accounting for a significant increase in the global mean sea level. Here, we present a new 29-year record of ice sheet mass balance from 1992 to 2020 from the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE). We compare and combine 50 independent estimates of ice sheet mass balance derived from satellite observations of temporal changes in ice sheet flow, in ice sheet volume, and in Earth's gravity field. Between 1992 and 2020, the ice sheets contributed 21.0±1.9 mm to global mean sea level, with the rate of mass loss rising from 105 Gt yr−1 between 1992 and 1996 to 372 Gt yr−1 between 2016 and 2020. In Greenland, the rate of mass loss is 169±9 Gt yr−1 between 1992 and 2020, but there are large inter-annual variations in mass balance, with mass loss ranging from 86 Gt yr−1 in 2017 to 444 Gt yr−1 in 2019 due to large variability in surface mass balance. In Antarctica, ice losses continue to be dominated by mass loss from West Antarctica (82±9 Gt yr−1) and, to a lesser extent, from the Antarctic Peninsula (13±5 Gt yr−1). East Antarctica remains close to a state of balance, with a small gain of 3±15 Gt yr−1, but is the most uncertain component of Antarctica's mass balance. The dataset is publicly available at https://doi.org/10.5285/77B64C55-7166-4A06-9DEF-2E400398E452 (IMBIE Team, 2021).

Ice shelves play a key role in the stability of the Antarctic Ice Sheet due to their buttressing effect. A loss of buttressing as a result of increased basal melting or ice shelf disintegration will lead to increased ice discharge. Some ice shelves exhibit channels at the base that are not yet fully understood. In this study, we present in situ melt rates of a channel which is up to 330 m high and located in the southern Filchner Ice Shelf. Maximum observed melt rates are 2 m yr−1. Melt rates inside the channel decrease in the direction of ice flow and turn to freezing ∼55 km downstream of the grounding line. While closer to the grounding line melt rates are higher within the channel than outside, this relationship reverses further downstream. Comparing the modeled evolution of this channel under present-day climate conditions over 250 years with its present geometry reveals a mismatch. Melt rates twice as large as the present-day values are required to fit the observed geometry. In contrast, forcing the model with present-day melt rates results in a closure of the channel, which contradicts observations. The ice shelf experiences strong tidal variability in vertical strain rates at the measured site, and discrete pulses of increased melting occurred throughout the measurement period. The type of melt channel in this study diminishes in height with distance from the grounding line and is hence not a destabilizing factor for ice shelves.

Mass changes of the polar Ice Sheets and their contribution to global mean sea level is an essential climate variable, critical for adaptation planning and important to understand the earth system in a warming climate. To ensure long term continuation of ice elevation and change records ESA proposed the Copernicus Polar Ice and Snow Topography Altimeter mission (CRISTAL) to be launched in 2027. CRISTAL, for the first time, will carry a dual-frequency altimeter in Ku and Ka Band to monitor changes in the height of ice sheets and glaciers and thickness of and snow on sea ice. Therefore, with the upcoming CRISTAL mission it is absolutely necessary to focus on sensor specific characteristics and its impact on elevation change records beforehand. To tackle these questions, we determine surface elevation change rates with a focus on the last decade of laser and radar altimetry observations using ICESat2, CryoSat-2, Sentinel-3 and SARAL/Altika. We investigate ice sheet wide and regional differences of elevation change rates derived from the four missions in the period from 2019 to 2022 with focus on different sensor characteristics (Laser/Ku/Ka Band), different acquisition/processing strategies ((P)LRM, SAR, SARIn), retracking (TFMRA,ICE1,ICE2) and orbit geometry (81.5°/88°).Our findings show that trend estimates of individual missions diverge most strongly at the margins and areas of complex topography but also in the flat interior of ice sheets and are highly dependent on the choice of retracker. We show that in order to determine volume changes as consistently as possible across missions, a uniform processing is required that includes backscatter/leading edge correction to suppress the influence of time-varying radar penetration due to changes in volume scattering.

Remotely sensed point clouds provide detailed structural data of landscapes and ecosystem characteristics. Especially in the analysis of forests and topography, this data type has proven its ability to derive relevant quantitative parameters such as biomass or subsidence rates. Arctic and boreal permafrost ecosystems are severely affected by climate change and resulting vegetation shifts, environmental disturbances, and permafrost thaw which lead to rapid changes in these northern environments that can be detected and characterized with point cloud datasets. In recent decades, the amount of point clouds acquired and generated in high-latitude regions by terrestrial (TLS), mobile (MLS), unmanned aerial system (UAS) based (ULS), up to airborne-based (ALS) LiDAR (Light detection and ranging) and Structure from Motion (SfM) has steadily increased. Multi-temporal datasets are available for a wide range of observation targets.The characteristics of the point clouds such as the extent of the area covered as well as the point density and thus the level of detail differ depending on the sensor, method, and the acquisition specifications. To use point cloud data for topographic, morphological, and forestry analysis, segmentation and classification of the point cloud into specific components such as individual trees, stems, foliage, or terrain features is essential. This is a time-consuming manual process and not feasible when addressing large datasets. Several previous analyses showed the potential for machine learning-based semantic segmentation of a single point cloud type, e.g., terrestrial LiDAR (TLS) with identical acquisition mode and sensor. We aim at an automated segmentation of different point cloud types generated by i) TLS, MLS, ULS and ALS as well as ii) SfM using (multi)spectral UAS and airborne image data to enable an analysis of Arctic and boreal permafrost ecosystems. Thereby, we will focus on the following questions:1) How can we reduce the time consuming process of labeling the point clouds?2) Can we train a model for segmentation using all point clouds or does transfer learning lead to better results?3) To what level of detail can we accurately segment and classify the different point cloud types?With this automated segmentation and classification, we aim to open up the possibility of exploiting the information contained in the multitude of point cloud data for a variety of ecological research applications.

The Antarctic Ice Sheet is an important indicator of climate change and a major contributor to sea level rise. Hence, precise, long-term observations of surface elevation change are required to assess changes and their contribution to sea level rise. Satellite altimetry has been used by various missions to measure surface elevation change since 1992. It has been shown that, next to the surface slope and complex topography, one of the most challenging issues is the spatial and temporal variability of radar pulse penetration into the snowpack, especially over the vast East Antarctic plateau. This results in an inaccurate measurement of the true surface elevation and consequently affects surface elevation change (SEC) estimates.To increase the accuracy and correct the SEC, we developed a deep convolutional neural network (CNN) architecture. The CNN was trained by a simulated waveform data set containing more than 3.6 million waveforms, considering different surface slopes, topography, and attenuation. The CNN follows standard architectural design choices. The successfully trained network is finally applied as a CNN-retracker to the full time series of CryoSat-2 low resolution mode (LRM) waveforms over the Antarctic ice sheet. We will show the CNN retrieved SEC and compare it to estimates of conventional retrackers like OCOG or ICE2. Our preliminary results show reduced uncertainty and a strongly reduced time variable radar penetration, making backscatter or leading edge corrections typically applied in SEC processing obsolete. This technique provides new opportunities to utilize convolutional neural networks in altimetry, waveform retracking, and processing altimetry data, which can be applied to historical, recent, and future altimetry missions.

Reliable knowledge of ice discharge dynamics for the Greenland ice sheet via its ice streams is essential if we are to understand its stability under future climate scenarios as well as their dynamics in the past, especially when using numerical models for diagnosis and prediction. Currently active ice streams in Greenland have been well mapped using remote-sensing data while past ice-stream paths in what are now deglaciated regions can be reconstructed from the landforms they left behind. However, little is known about possible former and now defunct ice streams in areas still covered by ice. Here we use radio-echo sounding data to decipher the regional ice-flow history of the northeastern Greenland ice sheet on the basis of its internal stratigraphy. By creating a three-dimensional reconstruction of time-equivalent horizons, we map folds deep below the surface that we then attribute to the deformation caused by now-extinct ice streams. We propose that locally this ancient ice-!ow regime was much more focused and reached much farther inland than today’s and was deactivated when the main drainage system was reconfigured and relocated southwards. The insight that major ice streams in Greenland might start, shift or abruptly disappear will affect our approaches to understanding and modelling the past or future response of Earth’s ice sheets to global warming. Such behaviour has to be sufficiently reproduced by numerical models operating on the mid- to longer-term timescales to be considered adequate physical representations of the naturally occuring dynamic behaviour of ice streams.

Ice–ocean interaction is crucial for the integrity of ice shelves and thus ice sheet stability. Warm ocean currents lead to enhanced basal melting of ice shelves, which is the dominant component of mass loss for the Antarctic Ice Sheet. Knowing the current melt rates and predicting those under future climate scenarios is thus of great importance. In the course of the ­MIMO-EIS (Monitoring melt where Ice Meets Ocean) Project, we deployed a continuously measuring ApRES (Autonomous phase-sensitive Radio-Echo Sounding) device in the center of Ekström Ice Shelf, recording an hourly time series since April 2020. The continuous time series reveals a seasonal onset of enhanced melt rates, abruptly increasing from

The largest floating tongue of Greenland’s ice sheet, Nioghalvfjerdsbrae, has so far been relatively stable with respect to areal retreat. Curiously, it experienced significant less thinning and ice flow acceleration than its neighbour Zacharias Isbrae. Draining more than 6% of the ice sheet, Nioghalvfjerdsbrae might become a large contributor to sea level rise in the future. Therefore, the stability of the floating tongue is a focus of this study. We employ a suite of observational methods to detect recent changes. We found that the calving style has changed at the southern part of the eastern calving front from normal tongue-type calving to a crack evolution initiated at frontal ice rises reaching 5-7km and progressing further upstream compared to 2010. The calving front area is further weakened by a substantial increase of a zone of fragments and open water at the tongue’s southern margin, leading to the formation of a narrow ice bridge. These geometric and mechanical changes are a precursor of instability of the floating tongue. We complement our study by numerical ice flow simulations to estimate the impact of future break-up or disintegration events on the ice discharge. These idealised scenarios reveal that a loss of the south-eastern area would lead to 1% of increase of ice discharge at the grounding line, while a sudden collapse of the frontal area (46% of the floating tongue area) will enhance the ice discharge by 8.3% due to loss in buttressing.Humbert, A., Helm, V., Neckel, N., Zeising, O., Rückamp, M., Khan, S. A., Loebel, E., Gross, D., Sondershaus, R., and Müller, R.: Precursor of disintegration of Greenland's largest floating ice tongue, The Cryosphere Discuss. [preprint], https://doi.org/10.5194/tc-2022-171, in review, 2022

Ice losses from the Greenland and Antarctic Ice Sheets have accelerated since the 1990s, accounting for a significant increase in global mean sea level. Here, we present a new 29-year record of ice sheet mass balance from 1992 to 2020 from the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE). We compare and combine 50 independent estimates of ice sheet mass balance derived from satellite observations of temporal changes in ice sheet flow, in ice sheet volume and in Earth’s gravity field. Between 1992 and 2020, the ice sheets contributed 21.0 ± 1.9 mm to global mean sea-level, with the rate of mass loss rising from 105 Gt yr-1 between 1992 and 1996 to 372 Gt yr-1 between 2016 and 2020. In Greenland, the rate of mass loss is 169 ± 9 Gt yr-1 between 1992 and 2020 but there are large inter-annual variations in mass balance with mass loss ranging from 86 Gt yr-1 in 2017 to 444 Gt yr-1 in 2019 due to large variability in surface mass balance. In Antarctica, ice losses continue to be dominated by mass loss from West Antarctica (-82 ± 9 Gt yr-1) and to a lesser extent from the Antarctic Peninsula (-13 ± 5 Gt yr-1). East Antarctica remains close to a state of balance (3 ± 15 Gt yr-1), but is the most uncertain component of Antarctica’s mass balance.

Ten-year records of ice surface elevation changes derived from three GNSS stations placed on the interior of the Greenland ice sheet are used to assess the ability of CryoSat-2 radar altimetry to capture surface elevation changes during 2010-2021. We use GNSS interferometric reflectometry (GNSS-IR) to derive time series of continuous daily surface elevations. The footprint of GNSS-IR is about 1000 m2 and the accuracy is ±2cm, making it an excellent tool to validate ice surface height from satellite altimetry. We compare GNSS-IR derived ice surface elevations with CryoSat-2 derived surface elevations and find Cryosat-2 performs best at the GNSS site furthest north (GLS3) with a maximum difference of 12cm. The other GNSS sites have a higher residual range because of poorer data availability and local surface variations. The number of Cryosat-2 data points are roughly doubled from GLS1 and GLS2 to GLS3. GLS3 Is located in a very flat area of the ice sheet only moving 55m during 2011-2020. In contrast GLS1 moved 292m in the same period, clearly indicating a steeper slope to the ice sheet at this location, which we have difficulty correcting for because digital elevation models are associated with high uncertainty on the interior of the ice sheet. The strength of this assessment method lies in the continuous daily time series of surface elevation change derived from GNSS, as they clearly capture extreme short-term changes, which otherwise might have been perceived as errors in the radar altimetry measurements.

The ESA Earth Explorer CryoSat-2 was launched on 8 April 2010 to monitor the precise changes in the thickness of terrestrial ice sheets and marine floating ice. To do that, CryoSat orbits the planet at an altitude of around 720 km with a retrograde orbit inclination of 92∘ and a quasi repeat cycle of 369 d (30 d subcycle). To reach the mission goals, the CryoSat products have to meet the highest quality standards to date, achieved through continual improvements of the operational processing chains. The new CryoSat Ice Baseline-D, in operation since 27 May 2019, represents a major processor upgrade with respect to the previous Ice Baseline-C. Over land ice the new Baseline-D provides better results with respect to the previous baseline when comparing the data to a reference elevation model over the Austfonna ice cap region, improving the ascending and descending crossover statistics from 1.9 to 0.1 m. The improved processing of the star tracker measurements implemented in Baseline-D has led to a reduction in the standard deviation of the point-to-point comparison with the previous star tracker processing method implemented in Baseline-C from 3.8 to 3.7 m. Over sea ice, Baseline-D improves the quality of the retrieved heights inside and at the boundaries of the synthetic aperture radar interferometric (SARIn or SIN) acquisition mask, removing the negative freeboard pattern which is beneficial not only for freeboard retrieval but also for any application that exploits the phase information from SARIn Level 1B (L1B) products. In addition, scatter comparisons with the Beaufort Gyre Exploration Project (BGEP; https://www.whoi.edu/beaufortgyre, last access: October 2019) and Operation IceBridge (OIB; Kurtz et al., 2013) in situ measurements confirm the improvements in the Baseline-D freeboard product quality. Relative to OIB, the Baseline-D freeboard mean bias is reduced by about 8 cm, which roughly corresponds to a 60 % decrease with respect to Baseline-C. The BGEP data indicate a similar tendency with a mean draft bias lowered from 0.85 to −0.14 m. For the two in situ datasets, the root mean square deviation (RMSD) is also well reduced from 14 to 11 cm for OIB and by a factor of 2 for the BGEP. Observations over inland waters show a slight increase in the percentage of good observations in Baseline-D, generally around 5 %–10 % for most lakes. This paper provides an overview of the new Level 1 and Level 2 (L2) CryoSat Ice Baseline-D evolutions and related data quality assessment, based on results obtained from analyzing the 6-month Baseline-D test dataset released to CryoSat expert users prior to the final transfer to operations.

Ice shelves play a key role in the stability of the Antarctic Ice Sheet due to their buttressing effect. A loss of buttressing as a result of increased basal melting or ice shelf disintegration will lead to increased ice discharge. Some ice shelves exhibit channels at the base that are not yet fully understood. In this study, we present in-situ melt rates of a channel which is up to 330 m high and located at the southern Filchner Ice Shelf. Maximum observed melt rates are 2.3 m a−1. Melt rates decline inside the channel along flow and turn into freezing 55 km downstream of the grounding line. While closer to the grounding line melt rates are higher within the channel than outside, this reverses further downstream. Comparing the evolution of this channel under present-day climate conditions over 250 years with its present geometry reveals a mismatch. This mismatch indicates melt rates two times higher were necessary over the past 250 years to form today's channel geometry. In contrast, forcing the model with present-day melt rates results in a closure of the channel, which contradicts observations. Time series of melt rate measurements show strong tidally-induced variability in vertical strain-rates. We found no evidence of seasonality, but discrete pulses of increased melting occurred throughout the measurement period. The type of melt channel in this study diminishes with distance from the grounding line and are hence not a destabilizing factor for ice shelves.

Thanks to the relatively high inclination (81.5°N/S) of the ERS2, Envisat, CryoSat-2, Saral and S3 space altimeters, the Polar Regions have been observed continuously by radar altimetry since the 1990s. We thus have time series over nearly 30 years of the topography of the polar ice caps and the thickness of the ice pack. However, these measurements took a qualitative leap forward with the launch of CryoSat-2 in 2010, thanks to the advent of SAR/SARIN altimetry and a near-polar inclination of 88°N/S.SAR/SARIN altimetry has led to considerable improvements in measurement accuracy thanks to better focusing (reducing the footprint by a factor of about 100) and better resolution (by a factor of about 2). The inclination of 88°N/S provides us with almost complete coverage of the Polar Regions, enabling us to carry out 10-year assessments of polar caps and sea-ice volume variations.During this presentation, we will first show the many scientific advances made possible by polar altimetry and its various evolutions, including the high-precision lidar solution on board NASA's IceSat-2 satellite.We will then present the HPCM CRISTAL mission, the only new polar altimetry mission planned to date. We will see the technical advances proposed by this mission and its importance in monitoring the Polar Regions in the context of global warming.

In summer, melt ponds on arctic sea ice can cover up to 60% of the total sea ice area, significantly decreasing surface albedo. The latter also holds true for supraglacial lakes frequently forming in the ablation zone of the Greenland Ice Sheet. Therefore monitoring of both, melt ponds on sea ice and supraglacial lakes is of great importance. So far, detection algorithms for both phenomena have been developed seperately from each other. Here, we will use airborne optical data of supraglacial lakes acquired during a land ice campaign over north-east Greenland in 2013 and airborne images of melt ponds from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) campaign to illustrate similarities and differences in the appearance of both phenomena. As an example study, we use an open source processing chain including the Open Drone Map software as well as the AMES stereo pipeline to generate orthorectified photo mosaics. On the basis of these datasets, we will discuss typical detection methods as well as the difficulties they face in the respective environment (f.e. confusion with shadows and bare ice). Besides a modified normalized difference water index we test an adapted random forest approach that was developed for the analysis of MOSAiC melt pond data and conclude with suggestions for future algorithm development.

The Greenland ice sheet is the largest contributor to global sea-level rise. Large uncertainties remain in sea level rise projections due to limited insights in the dynamics of outlet glaciers in Greenland. Nioghalvfjerdsbræ (79°NG) is an outlet glacier of the Northeast Greenland Ice Stream (NEGIS), which holds 1.1 m sea-level equivalent of ice. While critical progress has been made in ice sheet modelling, the motion of fast-moving ice streams and their interactions with ocean tides remain poorly understood. We combine GPS observations and two-dimensional numerical modelling to show that tides alter lubrication of the glacier as far as 15 km inland. Modelling these systems is highly complex due to the need for an appropriate material model and the interaction of different components of the physical system. We associate a viscoelastic material with subglacial hydrology and get friction parameters by solving an inverse problem. Steep basal topography enhances creep by 14% locally, whereas in the majority of the fast-moving part of NEGIS the ratio of creep to sliding is below 2%. Based on the viscoelastic material model, it is possible to distinguish between elastic and viscous strains that sum up to the total strain. The elastic strain contribution in the considered cross-section is up to 34%, independent of any tidal forcing. Elastic strain contributes significantly to deformation in fast-moving outlet glaciers and appears to coincide with crevasses representing the solid nature of ice. Including sliding and elastic deformation in ice sheet models to represent recent accelerations of outlet glaciers is an important step forward in reducing uncertainties of Greenland’s contribution to future sea-level rise.

Increasing ice loss of the Antarctic and Greenland Ice Sheets (AIS, GrIS) due to global climate change affects the orientation of the Earth’s spin axis with respect to an Earth-fixed reference system (polar motion). Ice mass changes in Antarctica and Greenland are observed by the Gravity Recovery and Climate Experiment (GRACE) in terms of time variable gravity field changes and derived from surface elevation changes measured by satellite radar and laser altimeter missions such as ENVISAT, CryoSat-2 and ICESat. Beside the limited spatial resolution, the accuracy of GRACE ice mass change estimates is limited by signal noise (meridional error stripes), leakage effects and uncertainties of the glacial isostatic adjustment (GIA) models, whereas the accuracy of satellite altimetry derived ice mass changes is limited by waveform retracking, slope related relocation errors, firn compaction and the density assumption used in the volume-to-mass conversion. In this study we use different GRACE gravity field models (CSR RL06M, JPL RL06M, ITSG-Grace2018) and satellite altimetry data (from TU Dresden, University of Leeds, Alfred Wegener Institute) to assess the accuracy of the gravimetry and altimetry derived polar motion excitation functions. We show that due to the combination of individual solutions, systematic and random errors of the data processing can be reduced and the robustness of the geodetic derived AIS and GrIS polar motion excitation functions can be increased. Based on these investigations we found that AIS mass changes induce the pole position vector to drift along the 60° East meridian by 2 mas/yr during the study period 2003-2015, whereas GrIS mass changes cause the pole vector to drift along the 45° West meridian by 3 mas/yr.

The size and shape of snow grains directly impacts the reflection by a snowpack. In this article, different approaches to retrieve the optical-equivalent snow grain size (ropt) or, alternatively, the specific surface area (SSA) using satellite, airborne, and ground-based observations are compared and used to evaluate ICON-ART (ICOsahedral Non-hydrostatic – Aerosols and Reactive Trace gases) simulations. The study is focused on low Sun and partly rough surface conditions encountered during a three-week campaign conducted North of Greenland in March/April 2018 within the framework of the PAMARCMiP (Polar Airborne Measurements and Arctic Regional Climate Model Simulation Project) project. Further, we propose an adjusted airborne retrieval method by using the albedo at 1700 nm wavelength. This reduced the effect of atmospheric masking and improved the sensitivity on ropt. From this approach we achieved a significantly improved uncertainty (ropt of 15 µm within a five-day period after a snowfall event, which is small compared to previous observations under similar temperature regimes. This low growth rate is well represented by a parametrization of snow metamorphism, when the vertical temperature gradient effect is suppressed. ICON-ART captured the observed change of ropt during snow fall events, but systematically overestimated the snow grain growth by about 100 %. Satellite-based and airborne retrieval methods showed higher and more variable ropt-values over sea ice (

We derive recent surface mass balance (SMB) estimates from airborne radar observations along the iSTAR traverse (2013, 2014) at Pine Island Glacier (PIG), West Antarctica. Ground based neutron probe measurements of snow density at 22 locations allow us to derive SMB from the annual internal radar reflection layers. The 2005 layer was traced for a total distance of 2367 km allowing us to determine annual mean SMB for the period 2005–2014. Using complementary SMB estimates from the RACMO2.3p2 regional climate model and a geostatistical kriging scheme we determine a regional scale SMB distribution with the same main characteristics as that determined for the period 1985–2009 in previous studies. Local departures exist for the northern PIG slopes, where the orographic precipitation shadow effect appears to be more pronounced in our observations, and the southward interior, where the SMB gradient is more pronounced in previous studies. For the PIG basin, we derive a total mass input of 79.9 ± 19.2 Gt yr−1. This is not significantly different to the value of 78.3 ± 6.8 Gt yr−1 for the period 1985–2009. Thus there is no evidence of a secular trend in mass input to the PIG basin. We note, however, that our estimated uncertainty is more than twice the uncertainty for the 1985–2009 estimates. Our error analysis indicates that uncertainty estimates on total mass input are highly sensitive to the selected krige methodology and assumptions made on the interpolation error, which we identify as the main cause for the increased uncertainty range compared to the 1985–2009 estimates.

Flow stripes on the surface of an ice shelf indicate the presence of large channels at the base. Modelling studies have shown that where these surface expressions intersect the groundling line, they coincide with the likely outflow of subglacial water. An understanding of the initiation and the ice–ocean evolution of the basal channels is required to understand the present behaviour and future dynamics of ice sheets and ice shelves. Here, we present focused active seismic and radar surveys of a basal channel and its upstream continuation on Support Force Glacier which feeds into the Filchner Ice Shelf, West Antarctica. We map the structure of the basal channel at the ice base in the grounded and floating part and identify the subglacial material within the grounded part of the channel and also along the seafloor. Several kilometers upstream of the grounding line we identify a landform, consisting at least in part of sediments, that forms the channel at the ice base. Immediately seaward of the grounding line, the seismic profiles show a 200 m thick partly disturbed, stratified sediment sequence at the seafloor, which we interpret as grounding line deposits. We conclude that the landform hosts the subglacial transport of sediments entering Support Force Glacier at the eastern side of the basal channel. In contrast to the standard perception of a rapid change in ice shelf thickness just downstream of the grounding line, we find a very flat topography of the ice shelf base with an almost constant ice thickness gradient along flow, indicating only little basal melting, but an initial widening of the basal channel, which we ascribe to melting along its flanks. Our findings provide a detailed view of a more complex interaction of grounded landforms, ice stream shear margins and subglacial hydrology to form basal channels in ice shelves.

Grounding line/zone dynamics of floating-tongue glaciers is of major importance for changes in their contribution to sea-level rise. For floating-tongue glaciers, thermal forcing of oceanic heat and tidal forcing are the major processes acting in that zone. Here we deal with the response to tidal forcing. The 79°N Glacier, an outlet glacier of the North East Greenland Ice Stream, is the focus of the Greenland Ice Sheet Ocean Interaction project (GROCE) funded by the German Ministry of Education and Research. We present a study of this region considering the deformation of the glacier in response to ocean tidal forcing by means of observations and modeling. GPS measurements realized in 2017-2018 are analyzed for vertical and horizontal displacements of the glacier and its floating tongue. Observations on fully-grounded ice reveal a periodic horizontal displacement in response to ocean tidal forcing in a distance of more than 35 km upstream from the grounding line. In the hinge zone, i.e. the transition between grounded and floating ice, the tidal forcing leads to a measurable vertical bending of the ice and a periodic movement of the grounding line. Understanding the mechanisms of grounding line migration is important to better evaluate the contribution of grounded ice discharge to sea-level rise.In order to model the measured displacements, a viscoelastic material model is required using the observed vertical displacements at the floating ice tongue as external forcing. Geometries obtained from AWI’s new ultrawideband radar form the basis for finite-element simulations in COMSOL. With the viscoelastic Maxwell material model, the response of the ice to ocean tidal forcing can successfully be modeled. Results obtained with a nonlinear Glen-type viscosity agree very well with the observed bending near the grounding line. The expected phase shift of the horizontal displacements upstream from the grounding line is well reproduced in the model.

The 79°N Glacier (79NG) in northeast Greenland, one of the last glaciers in Greenland with a floating ice tongue, plays a crucial role for buttressing the North-East Greenland Ice Stream (NEGIS). Remote-sensing studies indicate high basal melt rates (> 50 m/a) near the grounding line but these methods are limited by the hinge zone, where the floating ice is not in hydrostatic equilibrium. As part of the Greenland Ice Sheet Ocean Interaction (GROCE) project, we have performed a dense grid of repeated measurements with a phase-sensitive radio echo sounder (pRES) accompanied with autonomous pRES (ApRES) stations to estimate basal melt rates focusing on the hinge zone of 79NG. For analysing the pRES measurements, we additionally used ice thickness information derived from AWI’s ultra-wideband radar (UWB) revealing steep channels at the base. The estimated basal melt rates downstream the hinge zone are approximately the same as satellite-derived melt rates. In the hinge zone we found by far larger basal melt rates exceeding 100 m/a next to basal channels.

Basal melting of floating ice shelves is the main process by which the Antarctic ice sheet is currently losing ice and is responsible for the accelerating Antarctic contribution to sea-level rise. Moreover, basal melting can strongly vary both spatially and temporally. Detailed observations on high spatio-temporal scales remain however challenging, not to mention accounting for them in ice-sheet models. In this study, we combine CryoSat-2 and TanDEM-X elevation changes to capture in unprecedented detail the spatial (and temporal) variability of ice-shelf basal melting in the entire region of Dronning Maud Land, East Antarctica. The high spatial resolution of TanDEM-X elevations provide us with great details on the spatial variability of the basal mass balance, whereas CryoSat-2 elevations inform us about temporal changes.We find sub-shelf melt rates that average 1 m/a for the whole of Dronning Maud Land. Those relatively low melt rates conceal however a significant spatial variability on a wide range of scales (from sub-kilometers to ice-shelf wide scales). Spatially integrated, this basal melting represent an annual basal loss ~140 Gt/a. This revised estimate corresponds to a two-fold increase compared to previous estimates, which could question the relative stability of ice shelves in this region. This study highlights different regimes in sub-shelf melting in Dronning Maud Land and sheds new light on ice-ocean interactions in a region of Antarctica that has long been considered as very stable and which is therfore regularly overlooked.

Melting at the surface of the Greenland ice sheet has significantly increased since the early 1990s and this affects the degree to which radar sensors can penetrate beyond the snow surface. Indeed, radars are sensitive to changes in the surface and subsurface properties, up to ~15 m below the snow surface for instruments using the Ku-band (13.5 GHz). When melting occurs, meltwater can percolate in the snowpack or refreeze at the surface and in turn, the degree of radar penetration is sharply reduced. Here we use measurements of near-surface density from firn cores and models and airborne radar and laser data collected during the European Space Agency of ESA’s CRYOsat Validation EXperiment (CRYOVEX) campaigns along a 675 km transect in West Central Greenland between 2006 and 2017 to examine spatial and temporal fluctuations in the near-surface properties and how this affects radar measurements. From airborne data acquired with ASIRAS at Ku-band, we identify internal layers corresponding to melt layers in the snowpack down to 15 m, in good agreement with a firn densification model. We examine the spatial and temporal distribution of these melt layers and we find that the abundance of melt layers is increasing with elevation and depicts a strong inter-annual variability and that these fluctuations are correlated with fluctuations in the degree of the radar penetration depth. For instance, in 2012, the Greenland ice sheet experienced unprecedented melting and this is seen in the radar data by a reduction of 70% of the penetration in the snowpack following this event. The 2012 melt layer is still visible in data recorded 5 years after the melt event at a depth of 5.1 m. As the frequency and extent of extreme melt events is likely to increase in the coming decades, the effects of fluctuations in Ku-band radar penetration are an important consideration for satellite radar altimetry studies. However, we show that despite large fluctuations in volume scattering, there is a good agreement between Ku-band retracked heights and coincident laser measurements of 13.9 ± 19.9 cm using a threshold retracker. Finally, we also investigate the potential of using higher-frequency KAREN Ka-band (34.5 GHz) airborne radar data to limit the impact of temporal variations in the snowpack properties on backscattered power. We show that surface scattering dominates the Ka-band radar echoes and, overall, they penetrate to significantly lower distances into the near-surface firn by comparison to those acquired at Ku-band. This suggests that Ka-band data are less sensitive to extreme melt events and that the impact of such events on Ka-band data are likely to last for a shorter period of time compared to Ku-band data.

We developed an approach for a multi-mission satellite altimetry analysis over the Antarctic Ice Sheet which comprises Seasat, Geosat, ERS-1, ERS-2, Envisat, ICESat and CryoSat-2. In a first step we apply a consistent reprocessing of the radar alitmetry data which improves the measurement precision by up to 50 %. We then perform a joint repeat altimetry analysis of all missions. We estimate inter-mission offsets by approaches adapted to the temporal overlap or non-overlap and to the similarity or dissimilarity of involved altimetry techniques. Hence, we obtain monthly grids forming a combined surface elevation change time series. Owing to the early missions Seasat and Geosat, the time series span almost four decades from 07/1978 to 12/2017 over 25 % of the ice sheet area (coastal regions of East Antarctica and the Antarctic Peninsula). Since the launch of ERS-1 79 % of the ice sheet area is covered by observations. Over this area, we obtain a negative volume trend of −34 ± 5 km3 yr−1 for the more than 25-year period (04/1992–12/2017). These volume losses have significantly accelerated to a rate of −170 ± 11 km3 yr−1 for 2010–2017. Interannual variations significantly impact decadal volume rates which highlights the importance of the long-term time series. Our time series show a high coincidence with modeled cumulated precipitation anomalies and with satellite gravimetry. This supports the interpretation with respect to snowfall anomalies or dynamic thinning. Moreover, the correlation with cumulated precipitation anomalies back to the Seasat and Geosat periods highlights that the inter-mission offsets were successfully corrected and that the early missions add valuable information.

A key objective in palaeo-climatology is the retrieval of a continuous Antarctic ice-core record dating back 1.5 Ma. The identification of a suitable Antarctic site requires sufficient knowledge of the subglacial landscape beneath the Antarctic Ice Sheet. Here, we present new ice thickness information from the Dome Fuji region, East Antarctica, based on airborne radar surveys conducted during the 2014/15 and 2016/17 southern summers. Compared to previous maps of the region, the new dataset shows a more complex landscape with networks of valleys and mountain plateaus. We use the new dataset as input in a thermokinematic model that incorporates uncertainties in geothermal heat flux values in order to improve the predictions of potential ice-core sites. Our results for obtaining an old ice core show that especially the region immediately south of Dome Fuji station persists as a good candidate site. An initial assessment of basal conditions revealed the existence several wet-based areas. Further radar data analysis shows overall high continuity of layer stratigraphy in the region. This indicates that extending the age-depth information from the Dome Fuji ice core to a new ice-core drill site is a viable option.

Sea ice thickness is one of the most sensitive variables in the Arctic climate system. In order to quantify changes in sea ice thickness, CryoSat-2 was launched in 2010 carrying a Ku-band radar altimeter (SIRAL) designed to measure sea ice freeboard with a few centimeters accuracy. The instrument uses the synthetic aperture radar technique providing signals with a resolution of about 300 m along track. In this study, airborne Ku-band radar altimeter data over different sea ice types have been analyzed. A set of parameters has been defined to characterize the differences in strength and width of the returned power waveforms. With a Bayesian-based method, it is possible to classify about 80% of the waveforms from three parameters: maximum of the returned power waveform, the trailing edge width and pulse peakiness. Furthermore, the maximum of the power waveform can be used to reduce the number of false detections of leads, compared to the widely used pulse peakiness parameter. For the pulse peakiness the false classification rate is 12.6% while for the power maximum it is reduced to 6.5%. The ability to distinguish between different ice types and leads allows us to improve the freeboard retrieval and the conversion from freeboard into sea ice thickness, where surface type dependent values for the sea ice density and snow load can be used.

Knowledge about Antarctic sea-ice volume and its changes over the past decades has been sparse due to the lack of systematic sea-ice thickness measurements in this remote area. Recently, first attempts have been made to develop a sea-ice thickness product over the Southern Ocean from space-borne radar altimetry and results look promising. Today, more than 20 years of radar altimeter data are potentially available for such products. However, data come from different sources, and the characteristics of individual sensors differ. Hence, it is important to study the consistency between single sensors in order to develop long and consistent time series over the potentially available measurement period. Here, the consistency between freeboard measurements of the Radar Altimeter 2 on-board Envisat and freeboard measurements from the Synthetic-Aperture Interferometric Radar Altimeter on-board CryoSat-2 is tested for their overlap period in 2011. Results indicate that mean and modal values are comparable over the sea-ice growth season (May–October) and partly also beyond. In general, Envisat data shows higher freeboards in the seasonal ice zone while CryoSat-2 freeboards are higher in the perennial ice zone and near the coasts. This has consequences for the agreement in individual sectors of the Southern Ocean, where one or the other ice class may dominate. Nevertheless, over the growth season, mean freeboard for the entire (regional separated) Southern Ocean differs generally by not more than 2 cm (5 cm, except for the Amundsen/Bellingshausen Sea) between Envisat and CryoSat-2, and the differences between modal freeboard lie generally within ±10 cm and often even below.

The mass export of the West Antarctic Ice Sheet (WAIS) is dominated by fast flowing ice streams. Understanding their dynamics is a key to estimate the future integrity of the WAIS and its contributions to global sea level rise. This study focuses on the Ross Ice Streams (RIS) at the Siple Coast. In this sector, observations reveal a high variability of ice stream pathways and velocities which is assumed to be driven by subglacial hydrology. We compute subglacial water pathways for the present-day ice sheet and verify this assumption by finding high correlations between areas of enhanced basal water flow and the locations of the RIS. Moreover, we reveal that the ice flow velocities of the individual ice streams are correlated with the sizes of the water catchment areas draining underneath. The future development of the subglacial hydraulic environment is estimated by applying ice surface elevation change rates observed by ICESat and CryoSat-2 to the present-day ice sheet geometry and thus assessing prognostic basal pressure conditions. Our simulations consistently indicate that a major hydraulic tributary of the Kamb and Whillans Ice Stream (KIS and WIS) will be redirected underneath the Bindschadler Ice Stream (BIS) within the next two centuries. The water catchment area feeding underneath the BIS is estimated to grow by about 50 % while the lower part of the stagnated KIS becomes increasingly separated from its upper hydraulic tributaries. We conclude, that this might be a continuation of the subglacial hydraulic processes which caused the past stagnation of the KIS. The simulated hydraulic rerouting is also capable to explain the observed deceleration of the WIS and indicates a possible future acceleration of the BIS accompanied by an increased ice drainage of the corresponding ice sheet interior.

Several studies have shown that there is considerable evidence that the Arctic sea-ice is thinning during the last decades. When combined with the observed rapid reduction of ice-covered area this leads to a decline in sea-ice volume. The only remote sensing technique capable of quantifying this ice volume decrease at global scale is satellite altimetry. In this context the CryoSat-2 satellite was launched in 2010 and is equipped with the Ku-band SAR radar altimeter SIRAL, which we use to derive sea-ice freeboard defined as the height of the ice surface above the local sea level. In the context of quantifying Arctic ice-volume decrease at global scale, the CryoSat-2 satellite was launched in 2010 and is equipped with the Ku-band SAR radar altimeter SIRAL, which we use to derive sea-ice freeboard defined as the height of the ice surface above the sea level. Accurate CryoSat-2 range measurements over open water and the ice surface in the order of centimeters are necessary to achieve the required accuracy of the freeboard to thickness conversion. Besides uncertainties of the actual sea-surface height and limited knowledge of ice and snow properties, the penetration of the radar signal into the snow cover and therefore the interpretation of radar echoes is crucial. This has consequences in the selection of retracker algorithms which are used to track the main scattering horizon and assign a range estimate to each CryoSat measurement. In this paper we apply a retracker algorithm with thresholds of 40%, 50% and 80% of the first maximum of radar echo power, spanning the range of values used in current literature. For the 40% threshold we assume that the main scattering horizon lies at a certain depth between the surface and snow-ice interface as verified through coincident CryoSat-2 and airborne laser altimetry measurements. This contrasts with the 50% and 80% thresholds where we assume the ice-snow interface as the main scattering horizon similar to other published studies. Using the selected retrackers we evaluate the uncertainties of trends in sea-ice freeboard and higher level products that arise from the choice of the retracker threshold only, independently from the uncertainties related to snow and ice properties. Our study shows that the choice of retracker thresholds does have a non-negligible impact on magnitude estimates of sea-ice freeboard, thickness and volume, but that the main trends in these parameters are less affected. Specifically we find declines of Arctic sea-ice volume of 9.7% (40% threshold), 10.9% (50% threshold) and 6.9% (80% threshold) between March 2011 and March 2013. In contrast to that we find increases in Arctic sea-ice volume of 27.88% (40% threshold), 25.71% (50% threshold) and 32.65% (80% threshold) between November 2011 and November 2013. Furthermore we obtain a significant increase of freeboard from March 2013 to November 2013 in the area for multi-seasonal sea-ice north of Greenland and the Canadian Archipelago. Since this is unlikely it gives rise to the assumption that applying different retracker thresholds depending on seasonal properties of the snow load is necessary in the future.

Sea ice thickness is one of the most sensitive variables in the Arctic climate system. In order to quantify changes in sea ice thickness, CryoSat was launched in 2010 carrying a Ku-band Radar Altimeter (SIRAL) designed to measure sea ice freeboard with a few centimeters accuracy. The instrument uses the synthetic aperture radar technique providing signals with a resolution of about 300 m along track. In this study, airborne Ku-band radar altimeter data over different sea ice types has been analyzed. A set of parameters has been defined to characterize the difference in strength and width of the returned power waveforms. With a Bayesian based method it is possible to classify about 80% of the waveforms by three parameters: maximum of the returned power echo, the trailing edge width and pulse peakiness. Furthermore, the radar power echo maximum can be used to minimize the rate of false detection of leads compared to the widely used Pulse Peakiness parameter. The possibility to distinguish between different ice types and open water allows to improve the freeboard retrieval and the conversion into sea ice thickness where surface type dependent values for the sea ice density and snow load can be used.