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Understanding glaciers evolution is of major concern to evaluate their contribution to sea level rise in the context of global warming. Among the various processes involved in glacier dynamics, fractures like the calving of ice at the front of marine terminating glacier and crevasse formation affect the stress state, potentially modifying the glacier’s velocity. Crevasses also impact the melting rate. The fractures alter the roughness of the ground, increasing the amount of absorbed radiation, and open new networks in which the meltwater is likely to penetrate deeper toward the glacier bed.In this work we propose to model fractures in glacier based on finite strain elastoplasticity, using the Material Point Method (Wolper et al. 2021): we solve the classical governing equations for ice deformation in an Eulerian-Lagrangian framework and we use a strain softening Drucker-Prager constitutive model to simulate plasticity. Thanks to the Lagrangian part of the model, the fractures appear explicitly where high levels of total plastic deformation are reached. The behaviour of a glacier flowing over a step in the bedrock is investigated. The simulations show that crevasse patterns appear with regular spacing between the fractures. We perform a parametric study to determine which parameters affect the length of these patterns and potential dimensionless numbers are outlined.

Outlet glaciers and ice streams transport ice from the ice sheets to the ocean, where the glaciers lose mass by iceberg calving. Sermeq Kujalleq (SKK, Jakobshavn Isbræ) is one of the largest and most dynamic ice streams of the Greenland Ice Sheet with ice flow velocities up to 40 m/day. With extensive fieldwork and detailed repeated UAV surveys we aim at understanding the complex processes occuring at the ice stream margins and at the calving front of SKK. Such processes are often neclected in numerical models inducing uncertainties in projections of the ice sheet evolution.Within the framework of the COEBELI project we conducted drone photogrammetry surveys in July 2022 at SKK along other field measurements including in-situ GPS, GPRI, seismometers, and time-lapse imagery. Despite challenging weather conditions and constraints due to flying restrictions, we acquired 17 repeated flight surveys over the calving front of SKK during two weeks. As a result, we produced a large imagery data set, which was processed to infer high-resolution ortho-images and digital elevation models (DEM). Comparing the different products enables us to estimate changes in surface topography and ice dynamics. During the observation period several large calving events occurred allowing us to investigate the interaction between frontal processes and ice flow dynamics. With the very detailed data we can study crevasse opening, acceleration at the front, weaknesses in the ice and their origin, and the reaction of the glacier to large calving events.

Outlet glaciers and ice streams are the main channels through which ice sheets transport their mass towards the ocean. One of Greenland’s largest outlet glaciers Sermeq Kujalleq in Kangia (Jakobshavn Isbrae) has been broadly researched after experiencing a rapid retreat of the terminus and accompanying speedup to up to 40 m/day in the early 2000’s. However, such short-term ice dynamic variations remain poorly understood making numerical models difficult to constrain and predictions on future sea-level rise uncertain.The short-term ice dynamics of Sermeq Kujalleq consists in transient states and can only be captured by in-situ measurements of high spatial and temporal resolution. Glacier seismology has proven to be a valuable tool to study these dynamics, it provides data with a high temporal resolution and can provide information on processes happening below the ice surface. Within the COEBELI project we combine passive glacier seismology with global navigation satellite system (GNSS) receivers, long-range drones, time-lapse cameras and terrestrial radar interferometry to capture processes such as calving and basal sliding at their respective timescales.Here, we present results from a multi-array seismic deployment at Sermeq Kujalleq in Summer 2022. From May until September two arrays were deployed in the upstream part of the fast-flowing ice stream (>22 km from calving front) and one array on slower moving ice North of the main trunk. For a 3-week period in July, four more arrays were deployed on the fast-flowing ice stream closer to the calving front (50 Hz) icequakes. By studying these different signals, we are able to better constrain the processes and forces that control fluctuating ice-flow velocity and calving events.

Sermeq Kujalleq in Kangia (Jakobshavn Isbræ), Greenland is one of the most studied glaciers in the world mainly due to its recent retreat associated with extremely fast ice stream flow and high solid ice discharge. However, large limitations remain in the understanding of its short-term ice dynamics as the study of sub-daily variations, generally undetectable in spaceborne observations, requires high-rate field measurements that are challenging to acquire. Here, we present glacier surface velocities determined in Post-Processed Kinematic (PPK) mode from eight autonomous Global Navigation Satellite System (GNSS) stations deployed in July 2022 along the ice stream at a distance of 4 to 30 kilometers from the calving front. During this field campaign, we identified an 8-hour-long glacier speedup which was recorded at all GNSS stations and reached up to 11% of the pre-event velocity, followed by a 12-hour-long slowdown of similar magnitude. We further found the peak velocity was first measured at a GNSS station 16 kilometers away from the calving front, then recorded consecutively at each of the three other downstream GNSS stations with a positive time lag corresponding to a ~3 km/h wave propagation speed. At the station closest to the calving front, the timing of peak velocity corresponded to the occurrence of large-scale calving events. We further present line-of-sight glacier surface velocities measured along three shear margin transects with a terrestrial radar interferometer deployed simultaneously with the GNSS array. Across all profiles, we observed a widespread and simultaneous response of fast- and slow-moving ice suggesting a strong coupling between the main trunk and the shear margins of the ice stream.

Here, we present a compilation of 85 ice temperature profiles from 79 boreholes from the Greenland Ice Sheet and peripheral ice caps, as well as local ice caps in the Canadian Arctic. Only 25 profiles (32 %) were previously available in open-access data repositories. The remaining 54 profiles (68 %) are being made digitally available here for the first time. These newly available profiles, which are associated with pre-2010 boreholes, have been submitted by community members or digitized from published graphics and/or data tables. All 85 profiles are now made available in both absolute (meters) and normalized (0 to 1 ice thickness) depth scales, and are accompanied by extensive metadata. This metadata includes a transparent description of data provenance. The ice temperature profiles span 70 years, with the earliest profile being from 1950 at Camp VI, West Greenland. To highlight the value of this database in evaluating ice flow simulations, we compare the ice temperature profiles from the Greenland Ice Sheet with an ice flow simulation by the Parallel Ice Sheet Model (PISM). We find a cold bias in modeled near-surface ice temperatures within the ablation area, a warm bias in modeled basal ice temperatures at inland cold-bedded sites, and an apparent underestimation of deformational heating in high-strain settings. These biases provide process-level insight on simulated ice temperatures., The Cryosphere Discussions, ISSN:1994-0432, ISSN:1994-0440

Sermeq Kujalleq in Kangia (Jakobshavn Isbræ), Greenland has been extensively investigated over the past decades due to its recent retreat associated with extremely fast ice stream flow and high solid ice discharge. However, its short-term dynamics still remain poorly understood as they consist in transient states that can only be captured by high spatial and temporal in situ measurements. In the new COEBELI project, we aim at combining high resolution field data sets from seismic arrays, global navigation satellite system (GNSS) receivers, long-range uncrewed aerial vehicles and terrestrial radar interferometers (TRI) to achieve a comprehensive and detailed study of the short-term ice stream dynamics. Here, we present TRI and GNSS retrievals of surface velocity and elevation acquired during the first, exploratory field campaign of the COEBELI project in summer 2021. Seven kilometers away from the calving front, we specifically identified a slowdown of 1.12 m d-1 within a single day in the main trunk of the ice stream. While the absolute slowdown is larger in the main trunk than in the outer area of the shear margin (1.12 m d-1 versus 0.75 m d-1), it corresponds to a larger fraction of the pre-slowdown velocity in the latter zone (-4.48% versus -7.94%). We further discuss the challenges associated with the acquisition, processing and analysis of high-resolution data sets for the study of such complex and dynamic environments.

International audience; The complexity of karst groundwater flow modelling is reflected by the amount of simulation approaches. The goal of the Karst Modelling Challenge (KMC) is comparing different approaches on one single system using the same data set. Thirteen teams with different computational models for simulating discharge variations at karst springs have applied their respective models on one single data set coming from the Milandre Karst Hydrogeological System (MKHS). The approaches include neural networks, reservoir models, semi-distributed models and fully distributed groundwater models. Four and a half years of hourly or daily meteorological input and hourly discharge data were provided for model calibration. The validation comprised forecasting one year of discharge, without the observed discharge data. The model performance was evaluated using the volume conservation, Nash-Sutcliffe efficiency (NSE) and the Kling-Gupta efficiency (KGE) applied on the total discharge and individual flow components. As a result, the comparison of model performances is a challenging task due to the differences in the model architecture but also required time steps: some of the models require aggregated daily steps while others could be run using hourly data, which provided some interesting differences depending on how the data was transformed. The use of instantaneous data (e.g. value at noon) produces less bias that averaging hourly data over one day. The transformation of hourly into daily data produces a decrease of Nash and KGE of 0.05 to 0.08 (i.e. from 1 to ~0.93). The resulting simulations (forecasted values for year 2016) produced KGEs ranging between 0.83 and 0.37 (0.83 to −0.24 for NSE). Although the simulations matched the monitored flows reasonably well, most models struggled to simulate baseflow conditions accurately. In general, the models that performed the best for this exercise were the global ones (Gardenia and Varkarst), with a limited number of parameters, which can be calibrated using automatic calibration procedures. The neural network models also showed a fair potential, with one providing reasonable results despite the relatively short dataset available for warming-up (4.5 years). Semi-and fully distributed models also suggested that with some more effort they could perform well. The accuracy of model predictions does not seem to increase by using models with more than 9–12 calibration parameters. An evaluation of the relative errors between the forecasted and the observed values revealed that for most models, 50% of the forecasted values contained more than 50% of difference against the observed discharge rate, with 25% having a difference larger than 100%. A significant part of the poorly forecasted values corresponded to base-flow which was surprising given that as base-flow is generally much easier to predict than peak flow. Hence, this shows that modelling approaches and criteria for the calibration are too oriented towards peak-flow sections of the hydrographs, and that improvements could be gained by more focus on the base-flow.

Glaciers calving icebergs into the ocean significantly contribute to sea-level rise and can trigger tsunamis, posing severe hazards for coastal regions. Computational modeling of such multiphase processes is a great challenge involving complex solidfluid interactions. Here, a new continuum damage Material Point Method has been developed to model dynamic glacier fracture under the combined effects of gravity and buoyancy, as well as the subsequent propagation of tsunami-like waves induced by released icebergs. We reproduce the main features of tsunamis obtained in laboratory experiments as well as calving characteristics, the iceberg size, tsunami amplitude and wave speed measured at Eqip Sermia, an ocean-terminating outlet glacier of the Greenland ice sheet. Our hybrid approach constitutes important progress towards the modeling of solidfluid interactions, and has the potential to contribute to refining empirical calving laws used in large-scale earth-system models as well as to improve hazard assessments and mitigation measures in coastal regions, which is essential in the context of climate change., Communications Earth & Environment, 2, ISSN:2662-4435

Glacier calving plays a key role in the recently observed dynamic massloss of the Greenland ice sheet. Calving waves, generated by thesudden detachment of ice from the glacier terminus, can reach tens ofmeters of height and have devastating effects upon impact onsurrounding shores. In this study, we describe a new method for thedetection of source location and timing of calving waves, and theanalysis of their magnitude and spreading properties using aterrestrial radar interferometer (TRI). This method was applied to11,500 minute-interval TRI acquisitions from Eqip Sermia, Greenland.More than 2,000 calving waves were detected within sevendays. Quantitative assessment with a Wave Power Index (WPI) showedspatially distinctive patterns: the sector of the calving front endingin deep water shows a higher wave activity (+49%) with highercumulative WPI (+34%) than the shallow sector. In combination witha detection of meltwater plume locations, we highlighted a 2.3 timeshigher occurrence of visible meltwater plumes in the deep sector than theshallow one. We found both the cumulated WPI and the number of wavesto increase by more than 80% in the presence of a meltwater plumein the deep sector while only by 30% in the shallow sector. Wetherefore explain the higher calving activity in the deep sector to be strongly related to a combination of higher occurrence of meltwater plumes and more efficient calving enhancement linked to better connections to deep warm waters.

The recent rapid retreat of many Arctic outlet glaciers has been attributed to increased oceanic melt, but the relationship between oceanic melt and iceberg calving remains poorly understood. Here, we employ a transient finite‐element model that simulates oceanic melt and ice break‐off at the terminus. The response of an idealized tidewater glacier to various submarine melt rates and seasonal variations is investigated. Our modeling shows that for zero to low oceanic melt, the rate of volume loss at the front is similar or higher than for intermediate oceanic melt rates. Only very high melt rates lead to increasing volume losses. These results highlight the complex interplay between oceanic melt and calving and question the general assumption that increased submarine melt leads to higher calving fluxes and enhanced retreat. Models for tidewater glacier evolution should therefore consider calving and oceanic melt as tightly coupled processes rather than as simple, additive parametrizations.

We observed two outlet glaciers in West- and Nordwest-Greenland with a terrestrial radar interferometer (TRI), pressure sensors and time-lapse cameras over six and two years, respectively. The resulting detailed dataset provides us with insights on the calving process and the changes in front geometry over the last years. Since the two glaciers are characterised by different geometries and velocity fields, the influence of those parameters on the calving process can be investigated. The combination of the three different observation methods enable us to overcome their individual disadvantages. With the time-lapse camera taking pictures of the glacier front every 10 seconds, we detect all calving events of different sizes and styles but cannot quantify the volume. We used the TRI to quantify the volumes of aerial calving events by DEM differentiation. Further, calving waves measured with pressure sensors are used to distinguish between different calving types. We develop a relationship between calving volumes and wave heights and use this as an additional indirect method to estimate calving volumes. We find that the calving style and size as well as the front geometry is mainly controlled by the bed topography and the presence of a subglacial discharge plume. The location of the plume is observed to migrate from year to year, which leads also to changes in the calving pattern. Calving style and pattern as well as glacier velocity fields and geometry changes are additionally compared with environmental conditions such as the air temperature and the presence of ice-mélange in the proglacial fjord. In years with an early spring we find different front characteristics and calving patterns than for years with colder conditions.

Calving is a crucial process for the recently observed dynamic mass loss changes of the Greenland ice sheet. Despite its importance for global sea level change, major limitations in understanding the process of calving remain. This study presents high-resolution calving event data and statistics recorded with a terrestrial radar interferometer at the front of Eqip Sermia, a marine-terminating outlet glacier in Greenland. The derived digital elevation models with a spatial resolution of several metres recorded at 1 min intervals were processed to provide source areas and volumes of 906 individual calving events during a 6 d period. The calving front can be divided into sectors ending in shallow and deep water with different calving statistics and styles. For the shallow sector, characterized by an inclined and very high front, calving events are more frequent and larger than for the vertical ice cliff of the deep sector. We suggest that the calving volume deficiency of 90 % relative to the estimated ice flux in our observations of the deep sector is removed by oceanic melt, subaquatic calving, and small aerial calving events. Assuming a similar ice thickness for both sectors implies that subaqueous mass loss must be substantial for this sector with a contribution of up to 65 % to the frontal mass loss. The size distribution of the shallow sector is represented by a log-normal model, while for the deep sector the log-normal and power-law model fit well, but none of them are significantly better. Variations in calving activity and style between the sectors seem to be controlled by the bed topography and the front geometry. Within the short observation period no simple relationship between environmental forcings and calving frequency or event volume could be detected., The Cryosphere, 14 (3), ISSN:1994-0416, ISSN:1994-0424

Large, plume-like internal ice-layer-structures have been observed in radar images from both Antarctica and Greenland, rising from the ice-sheet base to up to half of the ice thickness. Their origins are not yet understood. Here, we simulate their genesis by basal freeze-on using numerical ice-flow modelling and analyse the transient evolution of the emerging ice-plume and the surrounding ice-layer structure as a function of both freeze-on rate and ice flux. We find good agreement between radar observations, modelled ice-plume geometry and internal layer structure, and further show that plume height relates primarily to ice-flux and only secondarily to freeze-on. An in-depth analysis, performed for Northern Greenland of observed spatial plume distribution related to ice flow, basal topography and water availability supports our findings regarding ice flux and suggests freeze-on is controlled by ascending subglacial water flow. Our results imply that widespread basal freeze-on strongly affects ice stratigraphy and consequently ice-core interpretations., Subsurface ice-sheet radar images reveal large plume-shaped bodies rising from the base, with their origin not yet understood. Here, the authors show that freeze-on of water at the ice-sheet base combined with ice-flux explains the vertical extent, shape and structure of the observed plumes.

The impact of summer surface melt on the dynamics of the Greenland Ice Sheet is modulated by the state of the subglacial hydrologic system. Studies of ice motion indicate that efficiency of the subglacial system increases over the melt season, decreasing the sensitivity of ice motion to surface melt. However, these inferences are based on limited indirect observations of the subglacial hydrologic system that leave many factors poorly constrained, particularly the presence and stability of subglacial channels. Here we use observations from 11 GPS stations, from which we derive ice velocity, longitudinal strain rates, and basal uplift, alongside observations of surface ablation and supraglacial lake drainage events, to explore the coevolution of ice motion and the subglacial hydrologic system in the Pakitsoq region of western Greenland during the 2011 melt season. We observe ice acceleration after the onset of local surface melting, followed by gradual ice deceleration, consistent with the pattern expected from increased subglacial drainage efficiency. Supraglacial lake drainages appear to precipitate ice deceleration and increased basal traction, suggesting that lake drainages effectively reorganize the local subglacial hydrologic system into a more efficient state that persists through the remainder of the melt season. At high elevations, ice velocity and inferred basal uplift suggest that continued cavity growth or sediment behavior, not subglacial channelization, drive the apparent increase in subglacial efficiency. Our results further indicate that these transient perturbations are critical in the seasonal evolution of ice motion.

Ocean terminating glaciers in Arctic regions have undergone rapid dynamic changes in recent years, which have been related to a dramatic increase in calving rates. Iceberg calving is a dynamical process strongly influenced by the geometry at the terminus of tidewater glaciers. We investigate the effect of varying water level, calving front slope and basal sliding on the stress state and flow regime for an idealized grounded ocean-terminating glacier and scale these results with ice thickness and velocity. Results show that water depth and calving front slope strongly affect the stress state while the effect from variations in basal sliding is much smaller. An increased relative water level or a reclining calving front slope strongly decrease the stresses and velocities in the vicinity of the terminus and hence have a stabilizing effect on the calving front. We find that surface stress magnitude and distribution are determined by solely the water depth relative to ice thickness for simple geometries. Based on this scaled relationship for the stress peak at the surface, and assuming a critical stress for damage initiation, we propose a simple and new parametrization for calving rates for grounded tidewater glaciers that is in good agreement with observations.

Moulins permit access of surface meltwater to the glacier bed, causing basal lubrication and ice speedup in the ablation zone of western Greenland during summer. Despite the substantial impact of moulins on ice dynamics, the conditions under which they form are poorly understood. We assimilate a time-series of ice surface velocity from a network of eleven Global Positioning System receivers into an ice sheet model to estimate ice sheet stresses during winter, spring, and summer in an approx. 30 x 10 km region. Surface-parallel von Mises stress increases slightly during spring speedup and early summer, sufficient to allow formation of 16% of moulins mapped in the study area. In contrast, 63% of moulins experience stresses over the tensile strength of ice during a short (hours) supraglacial lake drainage event. Lake drainages appear to control moulin density, which is itself a control on subglacial drainage efficiency and summer ice velocities.

Measuring the ice flow motion accurately is essential to better understand the time evolution of glaciers and ice sheets, and therefore to better anticipate the future consequence of climate change in terms of sea-level rise. Although there exist a variety of remote sensing methods to fill this task, in-situ measurements are always needed for validation or to capture high temporal resolution movements. Yet glaciers are in general hostile environments where the installation of instruments might be tedious and risky when not impossible. Here we report the first-ever in-situ measurements of ice flow motion using a remotely controlled Unmanned Aerial Vehicle (UAV). We used a multicopter UAV to land on a highly crevassed area of Eqip Sermia Glacier, West Greenland, to measure the displacement of the glacial surface with the aid of an on-board differential GNSS receiver. Despite the unfortunate loss of the UAV, we measured approximately 70 cm of displacement over 4.36 hours without setting foot onto the glacier – a result validated by applying UAV photogrammetry and template matching techniques. Our study demonstrates that UAVs are promising instruments for in-situ monitoring, and have a great potential for capturing short-term ice flow variations in inaccessible glaciers – a task that remote sensing techniques can hardly achieve.

In previous studies, the incorporation of polychlorinated biphenyls (PCBs) has been quantified in the accumulation areas of Alpine glaciers. Here, we introduce a model framework that quantifies mass fluxes of PCBs in glaciers and apply it to the Silvretta glacier (Switzerland). The models include PCB incorporation into the entire surface of the glacier, downhill transport with the flow of the glacier ice, and chemical fate in the glacial lake. The models are run for the years 1900-2100 and validated by comparing modeled and measured PCB concentrations in an ice core, a lake sediment core, and the glacial streamwater. The incorporation and release fluxes, as well as the storage of PCBs in the glacier increase until the 1980s and decrease thereafter. After a temporary increase in the 2000s, the future PCB release and the PCB concentrations in the glacial stream are estimated to be small but persistent throughout the 21st century. This study quantifies all relevant PCB fluxes in and from a temperate Alpine glacier over two centuries, and concludes that Alpine glaciers are a small secondary source of PCBs, but that the aftermath of environmental pollution by persistent and toxic chemicals can endure for decades.

Our ability to measure, quantify and assimilate hydrological properties and processes of snow in operational models is disproportionally poor compared to the significance of seasonal snowmelt as a global water resource and major risk factor in flood and avalanche forecasting. We show here that strong electrical self-potential fields are generated in melting in situ snowpacks at Rhone Glacier and Jungfraujoch Glacier, Switzerland. In agreement with theory, the diurnal evolution of self-potential magnitudes ( ∼ 60–250mV) relates to those of bulk meltwater fluxes (0–1.2 × 10−6m3s−1) principally through the permeability and the content, electrical conductivity and pH of liquid water. Previous work revealed that when fresh snow melts, ions are eluted in sequence and electrical conductivity, pH and self-potential data change diagnostically. Our snowpacks had experienced earlier stages of melt, and complementary snow pit measurements revealed that electrical conductivity ( ∼ 1–5 × 10−6Sm−1) and pH ( ∼ 6.5–6.7) as well as permeabilities (respectively ∼ 9.7 × 10−5 and ∼ 4.3 × 10−5m2 at Rhone Glacier and Jungfraujoch Glacier) were invariant. This implies, first, that preferential elution of ions was complete and, second, that our self-potential measurements reflect daily changes in liquid water contents. These were calculated to increase within the pendular regime from ∼ 1 to 5 and ∼ 3 to 5.5% respectively at Rhone Glacier and Jungfraujoch Glacier, as confirmed by ground truth measurements. We conclude that the electrical self-potential method is a promising snow and firn hydrology sensor owing to its suitability for (1) sensing lateral and vertical liquid water flows directly and minimally invasively, (2) complementing established observational programs through multidimensional spatial mapping of meltwater fluxes or liquid water content and (3) monitoring autonomously at a low cost. Future work should focus on the development of self-potential sensor arrays compatible with existing weather and snow monitoring technology and observational programs, and the integration of self-potential data into analytical frameworks., The Cryosphere, 10 (1), ISSN:1994-0416, ISSN:1994-0424

Following the general warming trend in Greenland, an increase in calving rates, retreat and ice flow has been observed at ocean-terminating outlet glaciers. These changes contribute substantially to the current mass loss of the Greenland Ice Sheet. In order to constrain models of ice dynamics as well as estimates of mass change, detailed knowledge of geometry and ice-flow are needed, in particular on the rapidly changing tongues of ocean-terminating outlet glaciers. In this study, we validate velocity estimates and spatial patterns close to the calving terminus of such an outlet derived from an iterative offset tracking method based on SAR intensity data with a collection of three independent reference measurements of glacier flow. These reference data sets are comprised of measurements from differential GPS, a Terrestrial Radar Interferometer (TRI) and repeated UAV surveys. Our approach for the SAR-velocity processing aims achieving at high spatial and temporal resolution in order to best resolve the steep velocity gradients in the terminus area and to exploit the 12 day repeat interval of the single-satellite Sentinel-1A sensor. Results from images of the medium-sized ocean terminating outlet glacier Eqip Sermia acquired by Sentinel-1A and RADARSAT-2 exhibit a mean difference of 8.7 % when compared to the corresponding GPS measurements. An areal comparison of our SAR velocity-fields with independently generated velocity maps from TRI and UAV showed a good agreement in magnitude and spatial patterns, with mean differences smaller than 0.7 md−1. In comparison with existing operational velocity products, our SAR-derived velocities showed a strongly improved spatial velocity pattern near the margins and calving front. There 10 % to 20 % higher surface ice velocities are produced, which has substantial implications on ice fluxes and on mass budget estimates of ice sheets. Further, we showed that offset tracking from SAR intensity data at a high spatio-temporal resolution is a valid method to derive glacier flow fields for fast-flowing glacier termini of outlet glaciers and, given the repeat interval of 12 days of the Sentinel-1A sensor (6 days with Sentinel-1B), has the potential to be applied operationally in a quasi-continuous mode.

The rapid drainage of supraglacial lakes introduces large pulses of meltwater to the subglacial environment and creates moulins, surface-to-bed conduits for future melt. Introduction of water to the subglacial system has been shown to affect ice flow, and modeling suggests that variability in water supply and delivery to the subsurface play an important role in the development of the subglacial hydrologic system and its ability to enhance or mitigate ice flow. We developed a fully automated method for tracking meltwater and rapid drainages in large (> 0.125 km2) perennial lakes and applied it to a 10 yr time series of ETM+ and MODIS imagery of an outlet glacier flow band in West Greenland. Results indicate interannual variability in maximum coverage and spatial evolution of total lake area. We identify 238 rapid drainage events, occurring most often at low (< 900 m) and middle (900–1200 m) elevations during periods of net filling or peak lake coverage. We observe a general progression of both lake filling and draining from lower to higher elevations but note that the timing of filling onset, peak coverage, and dissipation are also variable. Lake coverage is sensitive to air temperature, and warm years exhibit greater variability in both coverage evolution and rapid drainage. Mid-elevation drainages in 2011 coincide with large surface velocity increases at nearby GPS sites, though the relationships between ice-shed-scale dynamics and meltwater input are still unclear.

Ice temperature profiles from the Greenland Ice Sheet contain information on the deformation history, past climates and recent warming. We present full-depth temperature profiles from two drill sites on a flow line passing through Swiss Camp, West Greenland. Numerical modeling reveals that ice temperatures are considerably higher than would be expected from heat diffusion and dissipation alone. The possible causes for this extra heat are evaluated using a Lagrangian heat flow model. The model results reveal that the observations can be explained with a combination of different processes: enhanced dissipation (strain heating) in ice-age ice, temperate paleo-firn, and cryo-hydrologic warming in deep crevasses.

Penetration of surface meltwater to the bed of the Greenland Ice Sheet each summer causes an initial increase in ice speed due to elevated basal water pressure, followed by slowdown in late summer that continues into fall and winter. While this seasonal pattern is commonly explained by an evolution of the subglacial drainage system from an inefficient distributed to efficient channelized configuration, mounting evidence indicates that subglacial channels are unable to explain important aspects of hydrodynamic coupling in late summer and fall. Here we use numerical models of subglacial drainage and ice flow to show that limited, gradual leakage of water and lowering of water pressure in weakly connected regions of the bed can explain the dominant features in late and post melt season ice dynamics. These results suggest that a third weakly connected drainage component should be included in the conceptual model of subglacial hydrology., Surface meltwater draining to the bed of the Greenland Ice Sheet each summer causes ice flow changes inconsistent with the prevailing theory of channelizing subglacial drainage. Here, the authors show this is caused by limited, gradual leakage of water from previously ignored weakly connected regions of the bed.

Current understanding of ice dynamics predicts that increasing availability and variability of meltwater will have an impact on basal motion and therefore on the evolution and future behavior of the Greenland ice sheet. We present measurements of ice deformation, subglacial water pressure, and surface velocity that show periodic and episodic variations on several time scales (seasonal, multiday, and diurnal). These variations, observed with GPS and sensors at different depths throughout the ice column, are not synchronous but show delayed responses of ice deformation with increasing depth and basal water pressure in antiphase with surface velocity. With the help of a Full-Stokes ice flow model, these observations are explained as ice motion in a caterpillar-like fashion. Caused by patches of different basal slipperiness, horizontal stress transfer through the stiff central part of the ice body leads to spatially varying surface velocities and ice deformation patterns. Variation of this basal slipperiness induces characteristic patterns of ice deformation variability that explain the observed behavior. Ice flow in the ablation zone of the Greenland ice sheet is therefore controlled by activation of basal patches by varying slipperiness in the course of a melt season, leading to caterpillar-like ice motion superposed on the classical shear deformation.

Simultaneous observations of moulins and boreholes in western Greenland show that water delivery to the base of the ice sheet via moulins affects short-term ice velocity fluctuations, but not late-season ice velocity decelerations. Increased meltwater delivery to the base of the Greenland Ice Sheet will increase the ice sheet velocity, accelerating its inevitable rush towards the ocean and subsequent sea level rise. Or will it? Debate on this topic is at the forefront of cryospheric research, but has been hampered by the lack of simultaneous observations of hydraulic head in moulins, vertical shafts that deliver water to the base of the ice sheet, and boreholes, which monitor basal water pressure. Lauren Andrews and colleagues now provide these observations from a small region in western Greenland and reveal that water delivery by moulins into channelized basal flow is indeed linked to short-term fluctuations in ice velocity. Late season decelerations in ice velocity, however, seem to be controlled by changes in unchannelized flow, rather than any shifts in the moulin system. Seasonal acceleration of the Greenland Ice Sheet is influenced by the dynamic response of the subglacial hydrologic system to variability in meltwater delivery to the bed1,2 via crevasses and moulins (vertical conduits connecting supraglacial water to the bed of the ice sheet). As the melt season progresses, the subglacial hydrologic system drains supraglacial meltwater more efficiently1,2,3,4, decreasing basal water pressure4 and moderating the ice velocity response to surface melting1,2. However, limited direct observations of subglacial water pressure4,5,6,7 mean that the spatiotemporal evolution of the subglacial hydrologic system remains poorly understood. Here we show that ice velocity is well correlated with moulin hydraulic head but is out of phase with that of nearby (0.3–2 kilometres away) boreholes, indicating that moulins connect to an efficient, channelized component of the subglacial hydrologic system, which exerts the primary control on diurnal and multi-day changes in ice velocity. Our simultaneous measurements of moulin and borehole hydraulic head and ice velocity in the Paakitsoq region of western Greenland show that decreasing trends in ice velocity during the latter part of the melt season cannot be explained by changes in the ability of moulin-connected channels to convey supraglacial melt. Instead, these observations suggest that decreasing late-season ice velocity may be caused by changes in connectivity in unchannelized regions of the subglacial hydrologic system. Understanding this spatiotemporal variability in subglacial pressures is increasingly important because melt-season dynamics affect ice velocity beyond the conclusion of the melt season8,9,10.

The geological nature of glacier beds plays a key role in ice sheet dynamics. Whereas little is known about Greenland’s subglacial geology, the presence of basal sediments is a necessary condition for fast-flowing Antarctic ice streams. Such sediments sustain subglacial till layers, which if water saturated and under high pore-water pressure provide little resistance to ice flow. Using receiver function modeling of teleseismic P-waves, we report a thick (at least tens of meters) sediment layer beneath a site in Greenland’s ablation zone, ∼15 km away from the western ice sheet margin. Although we do not discuss the origin or detailed nature of these subglacial sediments, we suggest that they are capable of sustaining a till layer. Due to the prevalence of an inefficient, pressurized subglacial drainage system, this till layer would typically be under high pore pressures and fail at the shear stresses transmitted from the overlaying ice. Ice flow resistance is thus focused on regions where till is consolidated or absent, or where form drag over obstacles takes place. In contrast to Antarctica, Greenland’s surface melt now affects practically all latitudes, and efficient hydraulic connections between ice sheet surface and bed are pervasive throughout the ablation zone. This implies that rapid, melt-induced changes in subglacial water pressure are possible. The time required for basal till strength to react to these changes depends on the till’s properties. We estimate that formation and destruction of flow resistance can occur on time scales of less than a few years. This could lead to changes in ice flow that are currently difficult to predict.

Mountain glaciers sample a combination of climate fields – temperature, precipitation and radiation – by accumulation and melting of ice. Flow dynamics acts as a transfer function that maps volume changes to a length response of the glacier terminus. Long histories of terminus positions have been assembled for several glaciers in the Alps. Here I analyze terminus position histories from an ensemble of seven glaciers in the Alps with a macroscopic model of glacier dynamics to derive a history of glacier equilibrium line altitude (ELA) for the time span 400–2010 C.E. The resulting climatic reconstruction depends only on records of glacier variations. The reconstructed ELA history is similar to recent reconstructions of Alpine summer temperature and Atlantic Multidecadal Oscillation (AMO) index, but bears little resemblance to reconstructed precipitation variations. Most reconstructed low-ELA periods coincide with large explosive volcano eruptions, hinting at a direct effect of volcanic radiative cooling on mass balance. The glacier advances during the LIA, and the retreat after 1860, can thus be mainly attributed to temperature and volcanic radiative cooling., The Cryosphere, 8 (2), ISSN:1994-0416, ISSN:1994-0424

Dynamic changes on Greenland outlet glaciers are a primary driver for increases in ice-sheet mass loss and its contribution to sea-level rise. One dramatic example of such change has been observed at Jakobshavn Isbræ, which has thinned, retreated and doubled in speed since the early 2000s. Complementary to large changes on decadal scales, we observe the glacier response on shorter timescales, driven by tidal forcing and calving events. During a 14 day period in August 2009, we documented changes in geometry and speed near the terminus. On this timescale, ice flow responds to forcing at the front from iceberg calving and ocean tides. We observe a step-increase in velocity near the terminus during a large calving event, with transient deceleration in the days following the event. A simple calving-response model explains 94–99% of variations in detrended positions at the four sites considered. During each day, variability due to tidal forcing covers 10–90% of the variability that remains after removing effects accounted for by the calving-response model. The influence of the tidal forcing on flow decays upstream with a characteristic length scale of 2 km, comparable with about two ice thicknesses.

During summer 2011, seismic activity in the ablation zone of the western Greenland ice sheet (GrIS) was monitored using a network of three-component seismometers. The seismic record includes a large variety of icequakes and seismic tremors that demonstrate a clear correlation with subglacial water flow. We verified the existence of well-known shallow icequakes (related to surface crevasse formation), deep icequakes (located at 100–160 m depth) and narrow-banded short-term seismic tremors (tens of seconds in duration). In addition, we present previously unreported long-term tremors lasting several hours. Using attenuation of the measured tremor amplitude, we locate the epicentre of this long-term tremor to a large moulin within our study area. Between 3 and 11 Hz, our continuous seismic record is dominated by this ‘moulin tremor’ and shows strong correlation with the water level of the generating moulin. We argue that monitoring of icequake and glacial tremor sources bears high potential for investigating glacier hydraulics and dynamics, and is thus an ideal supplement to traditional glaciological measurements.

[1] Cold ice within a polythermal ice body controls its flow dynamics through the temperature dependence of viscosity, and affects glacier hydrology by blocking water flow paths. Lakes on the surface, linked by persistent, deeply incised meltwater streams, are hallmark features of cold ice in the ablation zone of a glacier or ice sheet. Ice radar is a convenient method to map scattering from internal water bodies present in ice at the pressure melting temperature (PMT). Consequently, lack of internal scatters is indicative of cold ice. We use a helicopter-borne 30 MHz ice radar to delineate the extent of cold ice within Grenzgletscher (Zermatt, Swiss Alps). The inferred thermal structure is validated with temperature measurements in 15 deep boreholes, showing excellent agreement. The cold ice occupies 80–90 % of the total ice thickness in a 400 m wide flow band along the central flow line. Quantitative interpretation of ice radar scattering power indicates a decrease of ice water content between PMT and 0.5 K below PMT, as predicted by theory, and observed in the laboratory. The cold ice which emerges at the surface in the lower ablation zone is impermeable to water, and is thus devoid of moulins if not crevassed. The surface water from melt and rain is routed through deeply incised, persistent streams and lakes, and cryoconite holes are frequent, in stark contrast to the adjacent temperate ice from other tributaries. The cold ice thus has a strong control on glacier hydrology, but is likely to change due to continued warming.

We use time-lapse photography, MODIS satellite imagery, ocean wave measurements and regional broadband seismic data to demonstrate that icebergs that calve from Jakobshavn Isbræ, Greenland, can generate ocean waves that are detectable over 150 km from their source. The waves, which are recorded seismically, have distinct spectral peaks, are not dispersive and persist for several hours. On the basis of these observations, we suggest that calving events at Jakobshavn Isbræ can stimulate seiches, or basin eigenmodes, in both Ilulissat Icefjord and Disko Bay. Our observations furthermore indicate that coastal, land-based seismometers located near calving termini (e.g. as part of the new Greenland Ice Sheet Monitoring Network (GLISN)) can aid investigations into the largely unexplored, oceanographic consequences of iceberg calving.

A range of englacial temperature measurements was acquired in the Monte Rosa area at the border of Switzerland and Italy in the years 1982, 1991, 1994, 1995, 1999, 2000, 2003, 2007 and 2008. Englacial temperatures revealed no evidence of warming at the firn saddle of Colle Gnifetti at 4452 m a.s.l. between 1982 and 1991, the 1991 to 2000 period showed an increase of 0.05 °C per year at a depth of 20 m. From 2000 to 2008 a further increase of 1.3 °C or 0.16 °C per year was observed, indicating that the amount of infiltrating and refreezing meltwater at Colle Gnifetti has probably increased since 2000. The measured temperatures give clear evidence of firn warming since 1991. This is confirmed by five existing boreholes with measured temperature down to bedrock, which were drilled in 1982, 1995, 2003 and 2005. All the observed temperature profiles show a slight bending to warmer temperatures in their uppermost part indicating a warming of the firn, which can be related to the observed atmospheric warming in the 20th century. However, the drilling sites on Colle Gnifetti are still located in the recrystallisation-infiltration zone. A much stronger warming of 6.8 °C or 0.4 °C per year was found at locations beneath Colle Gnifetti on Grenzgletscher from 1991 to 2008. This warming is one order of magnitude greater than the atmospheric warming and can be explained only by a strong increase in the latent heat input by infiltrating and refreezing meltwater. The observations indicate that since 1991, an important firn area beneath Colle Gnifetti has already undergone a firn facies change from the recrystallisation-infiltration to the cold infiltration zone due to an increasing supply of surface melt energy., The Cryosphere, 5 (1), ISSN:1994-0416, ISSN:1994-0424

The length change record of 91 glaciers in the Swiss Alps was analyzed with a novel macroscopic glacier model (LV-model). Based on a history of equilibrium line variations, synthetic length change data were calculated. From the LV-models matching best the measured length changes, characteristic parameters were obtained. The volume time scale thus determined ranges from 5 to 170 years for glaciers of different slope and length. The analysis shows that the observed glacier length changes cannot be reproduced with an equilibrium line variation based on temperature and precipitation alone. The equilibrium line has to be lowered by 100 to 200 meters during several phases of the Little Ice Age (in the time span 1650 to 1850) to obtain observed glacier responses. Such an effect might be attributable to either higher winter precipitation in the Alps, or to radiation forcing.

Recently, increasing concentrations of persistent organic pollutants (POPs) have been observed in the sediment of glacier-fed Lake Oberaar, Switzerland. Melting glaciers have been suggested as a secondary source of POPs released to Alpine lakes. Here we further investigate whether climate warming may accelerate the release of POPs previously deposited to Alpine glaciers ("glacier hypothesis"). To test this hypothesis, a dynamic multimedia mass balance model is developed for the catchment area of Lake Oberaar and is applied to polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), and dichlorodiphenyl trichloroethane (DDT). This lake model is combined with two other models. The first is a dynamic multimedia mass balance model parametrized for the Swiss lowlands that is used to calculate (on the basis of historical emission data) the atmospheric concentrations that are an advective input into the model of the lake catchment. The second is a flow model of Oberaar Glacier that determines the residence time of persistent chemicals in the glacier after their deposition to the glacier surface. According to results from these three models in combination, the release of POPs by the glacier is currently increasing and accounts for the observed increase in concentrations in the lake sediment. The models indicate that approximately half of the amount of PCBs, PCDD/Fs, and DDT initially incorporated into the glacier ice is still stored in the glacier. Under the assumption that the climate is warming, accelerated release of POPs is to be expected for the future; in a model run where no climate warming is assumed, the period of time required for release of the same amount of chemicals is longer by several decades than in the scenario with a changing climate.

The transient response of glaciers to climate variations is investigated with a novel low-order model of an idealized glacier resting on uniformly inclined bedrock. The model consists of two volumes representing the accumulation and ablation areas, which are joined by a flux gate controlling mass flow according to the shallow-ice approximation. Under the assumption of a constant vertical mass-balance gradient, a volume–length scaling relation is derived which depends explicitly on mass-balance gradient and bedrock slope. Analytic expressions for the volume and area timescales are given, which are inversely proportional to the mass-balance gradient and a geometric factor which is the ratio between the vertical extent of the ablation area and the ice thickness at the equilibrium line. From the low-order model, a dynamical system in length and volume is obtained. Results from this system are in good agreement with solutions obtained from a transient finite-element model solving the full force-balance and mass-conservation equations. Under periodic forcing there are significant deviations of the length response, which show that the usual relaxation-type parameterization of length change is not well suited for short-term reactions. Switching on and off periodic climate forcing, the model glaciers show surprisingly large initial and final transient responses that have not been investigated before. These results are of significance for the interpretation of length and thickness changes observed on glaciers.

The puzzling fact that Jakobshavn Isbræ, West Greenland, is flowing very fast but without any significant seasonal velocity changes, despite big amounts of surface-derived meltwater entering the ice stream (Echelmeyer and Harrison, 1990), has been explained by a combination of different types of measurements. It is now well established from seismic measurements and radio-echo sounding that Jakobshavn Isbræ flows through a deeply eroded subglacial trench that, even 50 km inland of the grounding line, extends as far as 1500 m below sea level (Clarke and Echelmeyer, 1996; Legarsky and Huang, 2006). Temperature measurements in boreholes down to 65% of the 2500 m thick ice stream at site B, some 50 km upstream of the calving front (Fig. 1b; Iken and others, 1993), were used to infer the presence of a substantial layer of temperate ice, the thickness of which was estimated to be at least 300 m by modeling and matching internal layering structures (Funk and others, 1994; Lüthi and others, 2002). The presence of a thick layer of temperate ice under very high driving stress allows for high ice-deformation rates, which contribute substantially to the observed fast flow velocities. Basal motion, while certainly important, seems to be barely influenced by the seasonal meltwater input (Echelmeyer and Harrison, 1990).

We present the thermal distribution in the confluence area of Gorner- and Grenzgletscher, Valais, Switzerland. The area was mapped by ice-penetrating radar at 1–5 and 40 MHz. The higher-frequency data reveal a thick surface layer of low backscatter in the center of the Grenzgletscher branch. Based on datasets of borehole-temperature measurements and flow velocity, we interpret this as a thick layer of cold ice, advected from the accumulation region of Grenzgletscher. Along seven profiles the base of the low-backscatter zone can be found at a maximum depths between approximately 100 and 200 m. Laterally, the layer extends some 400 m, ∼1/3 of the width of the Grenzgletscher branch. The lower boundary of the low-backscatter zone is systematically higher than the cold–temperate transition surface found in the boreholes. This discrepancy is attributed to the direct sensitivity of radar backscatter to liquid-water inclusions, rather than to the temperature distributions as observed in boreholes. We present the current state of the cold layer and discuss its influence on other glacier characteristics.

Penetration of surface meltwater to the bed of the Greenland Ice Sheet each summer causes an initial increase in ice speed due to elevated basal water pressure, followed by slowdown in late summer that continues into fall and winter. While this seasonal pattern is commonly explained by an evolution of the subglacial drainage system from an inefficient distributed to efficient channelized configuration, mounting evidence indicates that subglacial channels are unable to explain important aspects of hydrodynamic coupling in late summer and fall. Here we use numerical models of subglacial drainage and ice flow to show that limited, gradual leakage of water and lowering of water pressure in weakly connected regions of the bed can explain the dominant features in late and post melt season ice dynamics. These results suggest that a third weakly connected drainage component should be included in the conceptual model of subglacial hydrology.