Abstract. Glaciers in the European Alps are typically cold above ~3500–3800 m a.s.l., which allows the formation of steep hanging glaciers. Rising air temperatures and increased surface melting are now altering their thermal regime, raising questions about when and where their bases will start to thaw which could potentially cause them to destabilize. In this study, we report unique observations of long-term temperature change (1997–2021) on the Whymper hanging glacier (4050 m a.s.l.) located on the southern side of the Grandes Jorasses (Italy). Using a state-of-the-art thermo-mechanical model and heat-flux calculations for the entire Grandes Jorasses range, we find that the observed significant temperature changes are driven by rapid surface warming since the 1990s, due to enhanced melting which has ultimately produced persistent temperate surface conditions over the past decade. Remarkably, we demonstrate that the cold basal condition will persist for several years sustained by strong negative heat flux from the colder north face of the Grandes Jorasses, though it may approach melting point temperature by 2050. However, this evolution depends on the amount of water that will percolate through ice fractures at depth, a poorly understood process that could accelerate warming. This underscores the need for continued ice-temperature monitoring in the future.

Abstract. Natural glacier ice is not a monophasic, isotropic material as commonly assumed in models based on Glen-Nye's flow law. It can contain crevasses, develop crystallographic preferred orientations, and include mixtures of debris and interstitial water in temperate glaciers. Understanding the influence of such structural heterogeneities on deformation is therefore essential for accurately modeling glacier dynamics. In this study, we investigate the multi-scale evolution of structural heterogeneities with depth in the Planpincieux Glacier (Italian Mont Blanc massif) and evaluate their respective influence on ice deformation using a borehole instrumented with an optical televiewer, a full-waveform sonic logger, a piezometer, and an inclinometer chain. Complementary GNSS and seismic data provide additional constraints on hydrological activity and surface motion. Optical and sonic logging reveal two main families of heterogeneities: open and closed crevasses in the upper 60 m, and debris-rich layers near the bedrock interface. Acoustic data show continuous but opposite trends in both P- and Stoneley-wave velocities with depth, interpreted as reflecting an increase in water content but a decrease in permeability. Tiltmeter measurements indicate that roughly one-third of the surface velocity is accommodated by internal deformation, with pronounced strain localization near the bedrock, particularly within debris-rich layers. These layers exhibit enhanced strain following hydrological drainage events, suggesting a coupling between mechanical heterogeneity, basal hydrology, and strain localization. The results highlight that glacier friction laws may be significantly influenced by such heterogeneities, including the effects of interstitial water and debris on local mechanical behavior.

Glacier internal deformation is usually described by Glen's flow law using two material parameters: the creep factor (A) and the flow law exponent (n). However, the values of these parameters and their spatial and temporal variability are rather uncertain due to the difficulty in quantifying internal strain and stress fields at natural scales. In this study, we combine 1-year-long continuous measurements of borehole inclinometry and surface velocity with three-dimensional full-Stokes ice flow modeling to infer ice rheologies and sliding velocities for the ablation zone of the Argentière Glacier, a temperate glacier in the French Alps. We demonstrate that the observed deformation rate profile has limited sensitivity to the flow law exponent (n) and instead mainly reflects an increase in the creep factor (A) with depth, with A departing from its surface value by up to a factor of 2.5 below 160 m depth. We interpret this creep factor enhancement as an effect of increasing interstitial water content with depth (from 0 % to 1.3 %), which results in an average value of A=148 MPa−3 a−1. We further observe that internal ice deformation exhibits seasonal variability similar to that concerning surface velocity, indicating that the local basal sliding velocity exhibits no significant seasonal variation. We suggest that these changes in deformation rate are due to variations in the stress field, driven by contrasting changes in subglacial hydrology conditions between the sides and center of the glacier. Our study provides further evidence that borehole inclinometry, combined with full-Stokes flow modeling, allows for the constraining of both ice rheology and basal friction at scales that cannot be inferred from surface velocity measurements alone.

Saharan dust deposits frequently turn alpine glaciers orange and darken their surface. Together with other light-absorbing particles, mineral dust reduces snow albedo, increases snow melt rate, and lowers the surface mass balance of glaciers. Since the surface mass balance drives the evolution of alpine glaciers, assessing the impact of impurities helps to understand their current and future evolution. The location of impurities within the snowpack and their effect on snow albedo can be estimated through physical modelling. In this study, we quantified the impact of dust, taking into account mineral dust and black carbon in snow, on the Argentière Glacier over the period 2019–2022. Our results show that during the three years preceding 2022, the contribution of mineral dust to the annual decrease in surface mass balance was between 0.31–0.45 m w.e., while it reached the double in 2022 with 0.63 m w.e. [0.54, 0.69] (median, [Q10–Q90]), and up to 1.2 m w.e. [0.9, 1.4] at specific locations. The impact of dust in snow was unevenly distributed over the glacier, especially in 2022. The highest simulated impacts occurred where firn layers from previous years were exposed after the total melt of the snowpack of the previous winter. The gravitational redistribution of the snow from avalanches was not taken into account, which can reduce the impact of dust at specific locations. Increasing the modelled scavenging efficiency of black carbon can double the impact of dust alone at the glacier scale. In general, the contribution of mineral dust to the melt represents between 8 % and 16 % of Argentière Glacier summer melt depending on the year. Hence, we recommend accounting for impurities to simulate the distributed surface mass balance of glaciers.

AbstractMeasuring surface mass-balance in the accumulation areas of glaciers is challenging because of the high spatial variability of snow accumulation and the difficulty of conducting annual field glaciological measurements. Here, we propose a method that can solve both these problems for many locations. Ground-penetrating radar measurements and firn cores extracted from a site in the French Alps were first used to reconstruct the topography of a buried end-of-summer snow horizon from a past year. Using these data and surface elevation observations from LiDAR and Global Navigation Satellite System instruments, we calculated the submergence velocities over the period between the buried horizon and more recent surface elevation observations. The differences between the changes in surface elevation and the submergence velocities were then used to calculate the annual surface mass-balances with an accuracy of ±0.34 m w.e. Assuming that the submergence velocities remain stable over several years, the surface mass-balance can be reconstructed for subsequent years from the differences in surface elevation alone. As opposed to the glaciological method that requires substantial fieldwork year after year to provide only point observations, this method, once submergence velocities have been calculated, requires only remote-sensing data to provide spatially distributed annual mass-balances in accumulation areas.

The Grandes Jorasses Massif culminates at 4203m at the Punta Walker summit on the border between France and Italy. The south slope of Grandes Jorasses is widely glaciated and overlies the Val Ferret, a populated and highly frequented area presenting different hamlets, the most important being Planpincieux village. Located at an altitude between 4000 and 4100 m, the Whymper Serac is a hanging glacier that undergoes periodic gravity-driven instabilities. On 1st June of 1998, 150.000m3 of ice fell, and the resulting ice avalanche reached 1750m, at a distance of about 400m from houses of the Le Pont village and the main road. The monitoring activity started in 1997: a series of boreholes had been drilled to assess the basal thermal regime of the serac and subsequently install a monitoring system for early warning signs and risk assessment In September 2020, three thermistor chains in three different boreholes were installed on Whymper Serac. Temperature profiles were measured at different periods between October and November 2020. In September 2021 another three thermistor chains were installed and their temperature profile measured in October 2021. During the same survey, temperature profiles of the 2020 thermistors could be measured again on 2 out of 3 boreholes, (one being too close to the serac front was not safe to reach) confirming data acquired on the 2020 field campaign. The outcome of basal temperature measurements of 2020 and 2021 give good spatial coverage of the serac allowing comparison with data from the 1997 measurements, despite on the fact that most of the ice mass fell in 1998. A warming trend in most of the temperature profiles is evident in comparison whith 1997 data; 5 out of 6 points of measure still show temperatures below 0° C. One point of measure shows evidence of temperate ice at the ice/bedrock interface. Whymper Serac measurements provide evidence that the glacier is still frozen to the bedrock, but one part of the serac shows the beginning of a potential transition from cold based regime to temperate based regime. If, on one hand, surface displacements of the ice mass still show low displacements (typical of a cold based glacier), on the other hand, a velocity anomaly was detected on a small portion of the serac corresponding to the temperate based sector. Further research is needed to better understand the evolution of the thermo-mechanical conditions of the Whymper Serac in the current climate change scenarios. Therefore, thermo-mechanical modeling of the Whymper Serac is underway, based on the Elmer/Ice model.

<p>Glacier deformation velocity is typically linked to ice thickness and is very sensitive to parameters of the Glen’s flow law (i.e. rate factor <em>A </em>and flow exponent <em>n</em>). Detailed observations of glacier deformation in the Greenland Ice Sheet (Ryser et al. 2014, Maier et al. 2019, 2021) reveal that the deformation profile close to the bed, where most of the deformation takes place, is not well represented by typical models and is heavily influenced by bed geometry. We present a similar study done on the ablation zone of Argentière Glacier, a temperate glacier in the french Alps. Glacier deformation was observed during 2020 with permanently installed inclinometers that record deformation at high temporal frequency. This data set is complemented with a dense network of GNSS sensors providing surface velocity. Combining the two sets of observations, we are able to reconstruct the deformation profile with depth and the sliding velocity. We show that the Shallow Ice Approximation strongly overestimates shear stress gradients at depth, such that it can lead to a wrong interpretation of deformation profiles. Instead, the use of a three-dimensional full-stokes model enables us to demonstrate that the yearly-averaged deformation profile is poorly sensitive to the flow law exponent n and instead mainly reflects an increase in the  rate factor with depth. We interpret this rate factor increase by an associated increase in water content with depth, as expected from experimental data and field observations (Duval et al., 1977, Vallon et al., 1976). We further show that internal ice deformation also exhibits seasonal variability that is concomitant with and even stronger than surface velocity changes, such that basal sliding actually has to decrease in summer (at least locally) for our observations to be consistent. These results challenge the general view that glacier seasonal variability is mainly due to seasonal variability in sliding velocity. Further work remains to be conducted in order to fully determine the cause for internal deformation seasonality, which may be explained by changes in stress distribution, in water content or to other yet unidentified factors.</p>

The hydromechanical processes by which basal water controls sliding at the glacier bed are poorly known, despite glacier basal motion being responsible for a large part of ice flux in temperate alpine glaciers. Previous studies suggest that sliding strongly relates to the quantity of water being stored at the ice‐bedrock interface. However, this water storage is difficult to quantify accurately on the basis of surface‐motion observations, given that uplift can also be affected by changes in vertical‐strain rates and sliding velocity change. Here, we use a comprehensive data set of in situ measurements performed over 2 years on the Argentière Glacier in the French Alps to investigate the relationships between horizontal and vertical velocities, basal sliding, subglacial runoff and bed separation. We observe strikingly large uplifts varying spatially between 0.20 and 0.90 m over the winter/spring seasons between January and June and with a consistent spatial pattern from 1 year to another. We show, based on observations and three dimensional ice‐flow modeling, that these large uplifts cannot be explained solely by changes in strain rates or in sliding up an inclined bed. Our results reveal that more than 80% of the observed uplift is related to enhanced bed separation through cavitation, allowing us to estimate the volume occupied by water‐filled subglacial cavities. Our interpretation of uplift being mainly caused by increased cavitation is also consistent with an associated increase in the observed surface horizontal velocity. These findings provide important observational constraints for testing subglacial hydrological models.

Abstract. The air at the surface of the high Antarctic Plateau is very cold, dry and clean. In such conditions the atmospheric moisture can significantly deviate from thermodynamic equilibrium conditions, and supersaturation with respect to ice can occur. Most conventional humidity sensors for meteorological applications cannot report supersaturation in this environment. A simple approach for measuring supersaturation using conventional instruments, one being operated in a heated airflow, is presented. Since 2018, this instrumental setup was deployed at 3 levels in the lower ~40 m above the surface at Dome C on the high Antarctic Plateau. The 3-year 2018–2020 record (Genthon et al. 2021) is presented and analyzed for features such as the frequency of supersaturation with respect to ice, diurnal and seasonal variability, and vertical distribution. As supercooled liquid water droplets are frequently observed in clouds at the temperatures met on the high Antarctic Plateau, the distribution of relative humidity with respect to liquid water at Dome C is also discussed. It is suggested that, while not strictly mimicking the conditions of the high troposphere, the surface atmosphere on the Antarctic Plateau is a convenient natural laboratory to test parametrizations of cold microphysics predominantly developed to handle the genesis of high tropospheric clouds. Data are distributed on the PANGAEA data repository at https://doi.pangaea.de/10.1594/PANGAEA.939425 (Genthon et al., 2021).

High elevation cold glaciers are experiencing important warming in response to on-going climate change. It could affect the ice/rock interface temperature and lead to avalanching glacier instabilities on steep slopes. Few of these glaciers are located in densely populated areas and could threaten inhabited areas in the future, when the ice/rock interface will become temperate. It is the case of Taconnaz hanging glacier (Mont Blanc area) with a large cold ice area located on a steep slope in the accumulation zone. This glacier is located above the valley of Chamonix. Modeling and measuring the progressive warming of this cold hanging glacier is critical to assess the evolution of its stability.The thermal regime of this glacier have been monitored over the last decades from deep boreholes observations. We found clear englacial temperatures increase. For instance, measurements performed in deep boreholes between 1994 and 2021 reveal strong changes in englacial temperature reaching 2 °C increase at a depth of 50 m close to the summit of this glacier (4300 m a.s.l.). We also found that the current englacial warming is now reaching the glacier base at most location. Our numerical simulations, using a thermo-mechanical model, are able to capture the observed warming and show that a large volume of ice could become unstable within the 21th century and could lead to a large ice avalanche. Such break-off could have catastrophic consequences for life and property in the valley.

Glacier dynamics exhibits a strong variability in response to climate forcing. To better understand the effects of this forcing, it is essential to provide continuous deformation measurements that must be long-term (over a full or several melt seasons) and high-resolution (from daily to sub-daily). GNSS monitoring represents a valuable mean to better apprehend mechanisms of basal sliding and provide high-resolution 3D constraints on physical models of glacier flow. In this study, we investigate motions and deformations of the Argentière Glacier in the French Alps at 2400 m altitude, derived from up to 12 permanent GNSS stations continuously operating since April 2019, covering two melting seasons. The Argentière glacier is particularly interesting due to (i) its long-term subglacial observatory measuring basal sliding velocity and subglacial discharge, and (ii) the wide range of complementary observations currently being acquired there, which give access to internal ice deformation thanks to tiltmeters in boreholes, and to basal stick-slip and englacial fracturing thanks to seismic observations. We present the results (i) over relatively long timescales (days to months) using the fast static positioning approach to evaluate mean variations and compare to the independent measurements mentioned above, and (ii) kinematic approach to focus on high temporal resolution velocity variations during specific short-term events that cannot be seen from the static processing. The horizontal surface velocities on daily time scales reveal spring acceleration due to meltwater followed by steadily high velocities over the summer, and significant episodic accelerations in the fall in response to the storm events. We quantify strain rates and their evolution in time that can be related to the vertical surface motions. We combine the GNSS with the englacial tiltmeters results to deduce the basal speed variations. The GNSS confrontation with other independent observations also allows analyzing the surface motions that combine horizontal speed-ups with uplift due to bed separation of the ice sheet. We will further search for evidence for surface motions that might occur in daily cycles in summer, as hinted at by the basal sliding measurements. But before analyzing daily cycles of glacier motions, it is critical to remove positioning artefacts due to multipath effects with a repeat period close to 24 hours. These effects are enhanced on the Argentière Glacier by the limited number of visible satellites in the narrow valley. Moreover, it evolves with the dynamically changing environment (snow accumulation and snowmelt that create variations in ground reflectivity properties). A multi-GNSS analysis combining GPS and GLONASS data helps overcome the lack of satellite data and increase the time resolution on a sub-daily scale. If daily cycles are resolvable from the improved GNSS analysis, their phase offsets with respect to meteorological, hydrological and seismic observations can give us indices of eventual mechanisms of sliding at the bedrock interface.

Mass balance observations are very useful to assess climate change in different regions of the world. As opposed to glacier-wide mass balances which are influenced by the dynamic response of each glacier, point mass balances provide a direct climatic signal that depends on surface accumulation and ablation only. Unfortunately, major efforts are required to conduct in situ measurements on glaciers. Here, we propose a new approach that determines point surface mass balances from remote sensing observations. We call this balance the geodetic point surface mass balance. From observations and modelling performed on the Argentière and Mer de Glace glaciers over the last decade, we show that the vertical ice flow velocity changes are small in areas of low bedrock slope. Therefore, assuming constant vertical velocities in time for such areas and provided that the vertical velocities have been measured for at least 1 year in the past, our method can be used to reconstruct annual point surface mass balances from surface elevations and horizontal velocities alone. We demonstrate that the annual point surface mass balances can be reconstructed with an accuracy of about 0.3 m of water equivalent per year (m w.e. a−1) using the vertical velocities observed over the previous years and data from unmanned aerial vehicle images. Given the recent improvements of satellite sensors, it should be possible to apply this method to high-spatial-resolution satellite images as well.

Basal sliding speed is a main component of glacier flow. However, acquiring direct observations of the velocity at the base of a glacier is a challenging task due to limited accessibility. One option consists in indirectly measuring basal speed by subtracting the internal deformation velocity from the velocity observed at the surface. Internal deformation has been mostly studied through annual surveys of borehole inclinometry that provide a snapshot of the internal velocity field of the glacier, while more recent efforts have installed continuously recording sensors at different depths. The former method provides a good resolution in depth, while the latter provides a good resolution in time, but few studies have provided both.In this study we quantify basal speed variations at both short and long timescales through the combined analysis of one year of continuous half-hour sampled borehole tilt measurements and high resolution GNSS positioning. The instrumentation campaign has been done in the framework of the SAUSSURE project, in which we drilled five boreholes in the ablation area of Argentière Glacier, a temperate mountain glacier in the French Alps. The boreholes were positioned along the center flow line, and each one was equipped with an array of ~18 sensors that recorded the tilt and azimuth at different depths as well as water pressure at the bottom and middle depth. With this dataset we are able to investigate how melt season impacts the internal dynamics of the glacier, or how the sudden accelerations of the glacier after heavy storms events are shared between changes in internal deformation and basal speed ups. We find that the yearly averaged internal deformation profile can be well described using a two dimensional Glen flow law with exponent n ~ 3.4. We observe as well that deformational velocities can represent up to 60% of the total velocity, more than previously considered for Argentière Glacier. Our findings suggest that weekly accelerations, usually observed along raises in water pressure, are due to the increase of basal speed paired with a decrease in deformation, which suggests stress reconfiguration. We don’t observe journal cycles of deformation velocity, which would indicate that journal variations of glacier velocity are due only to changes of basal speed. In contrast, glacier acceleration during melt season at monthly timescales is accommodated by deformation velocity and not by sliding.

Abstract Spatio-temporal variability of the winter surface mass balance is a major uncertainty in the modelling of annual surface mass balance. Moreover, its measurement at high spatio-temporal resolution (sub-200 m) is very useful to force, calibrate or validate models. This study presents the results of year-round field campaigns to study the evolution of the surface mass balance in a ~2 km2 portion of the accumulation zone of the Mer de Glace (France). It is based on repeated LiDAR acquisitions, submergence-velocity measurements and meteorological records. The two methods used to quantify submergence velocities show good agreement. They present a linear temporal evolution without significant seasonal changes but display significant spatial variability. We conclude that a dense network of submergence velocity measurements is required to reduce the uncertainties when computing winter and annual surface mass balance from digital elevation model differencing. Finally, a hight spatio-temporal variability of the winter surface mass balance is highlighted (e.g., a std dev. of 0.92 m in April) even though the topography is homogeneous (std dev. of 25 m). Attempts to relate this variability to different morpho-topographic variables and wind-related indexes show the need for studies conducted at the snowfall event scale to obtain a better understanding of the variability in mass balance at the glacier scale.

Five continuous GNSS stations monitor the Argentière glacier surface motion on a longitudinal profile at 2400 m altitude over a full melt season, from April to November 2019. High precision data analysis is enabled by a close-by reference station on the bedrock. This GNSS survey is part of the SAUSSURE project 2019-2022 that aims at increasing our knowledge on the physics of glacier basal sliding, by improving friction laws and validating them in a natural environment. The Argentière glacier is particularly interesting due to its long-term subglacial observatory measuring basal sliding velocity and subglacial discharge. The SAUSSURE project furthermore includes seismic, tiltmeter and piezometer measurements. The bedrock topography is obtained from a Ground Penetrating Radar. The dense GNSS station setup permits to validate individual antenna movements. We then retrieve horizontal and vertical surface velocities on daily and sub-daily time scales. We can deduce strain rates in between the stations and their evolution in time, and relate this observable with the vertical surface motions. The confrontation of the GNSS data with independent observations allows analyzing the surface motions searching for glacier surges that combine horizontal speed-ups combined with uplift due to bed separation of the ice sheet. These events could give indications about cavity growth in spring. We will also try to investigate sub-daily motions that seem to occur in daily cycles in summer, as hinted at by the basal sliding measurements. These daily cycles are usually also seen in the seismic activity. The phase of the different features varies with respect to the daily cycles of temperature and sub-glacial water pressure. These phase offsets can give us indices on eventual mechanisms of sliding at the bedrock interface. The GNSS measurements represent a rare in situ data set that can contribute to better apprehend mechanisms of basal sliding and to provide high-resolution 3D constraints on physical models of glacier flow.

Abstract. Water flowing below glaciers exerts a major control on glacier basal sliding. However, our knowledge of the physics of subglacial hydrology and its link with sliding is limited because of lacking observations. Here we use a 2-year-long dataset made of on-ice-measured seismic and in situ-measured glacier basal sliding speed on Glacier d'Argentière (French Alps) to investigate the physics of subglacial channels and its potential link with glacier basal sliding. Using dedicated theory and concomitant measurements of water discharge, we quantify temporal changes in channels' hydraulic radius and hydraulic pressure gradient. At seasonal timescales we find that hydraulic radius and hydraulic pressure gradient respectively exhibit a 2- and 6-fold increase from spring to summer, followed by comparable decrease towards autumn. At low discharge during the early and late melt season channels respond to changes in discharge mainly through changes in hydraulic radius, a regime that is consistent with predictions of channels' behaviour at equilibrium. In contrast, at high discharge and high short-term water-supply variability (summertime), channels undergo strong changes in hydraulic pressure gradient, a behaviour that is consistent with channels behaving out of equilibrium. This out-of-equilibrium regime is further supported by observations at the diurnal scale, which prove that channels pressurize in the morning and depressurize in the afternoon. During summer we also observe high and sustained basal sliding speed, which supports that the widespread inefficient drainage system (cavities) is likely pressurized concomitantly with the channel system. We propose that pressurized channels help sustain high pressure in cavities (and therefore high glacier sliding speed) through an efficient hydraulic connection between the two systems. The present findings provide an essential basis for testing the physics represented in subglacial hydrology and glacier sliding models.

This dataset belongs to the study of Nanni et al., 2020 "Quantification of seasonal and diurnal dynamics of subglacial channels using seismic observations on an Alpine Glacier." accepted for publication in The Cryosphere on March 9th 2020. You can find additional information on the "README_data_NANNI_2020_glacier"

Five continuous GNSS stations monitor the Argentière glacier surface motion on a longitudinal profile at 2400 m altitude over a full melt season, from April to November 2019. High precision data analysis is enabled by a close-by reference station on the bedrock. This GNSS survey is part of the SAUSSURE project 2019-2022 that aims at increasing our knowledge on the physics of glacier basal sliding, by improving friction laws and validating them in a natural environment. The Argentière glacier is particularly interesting due to its long-term subglacial observatory measuring basal sliding velocity and subglacial discharge. The SAUSSURE project furthermore includes seismic, tiltmeter and piezometer measurements. The bedrock topography is obtained from a Ground Penetrating Radar. The dense GNSS station setup permits to validate individual antenna movements. We then retrieve horizontal and vertical surface velocities on daily and sub-daily time scales. We can deduce strain rates in between the stations and their evolution in time, and relate this observable with the vertical surface motions. The confrontation of the GNSS data with independent observations allows analyzing the surface motions searching for glacier surges that combine horizontal speed-ups combined with uplift due to bed separation of the ice sheet. These events could give indications about cavity growth in spring. We will also try to investigate sub-daily motions that seem to occur in daily cycles in summer, as hinted at by the basal sliding measurements. These daily cycles are usually also seen in the seismic activity. The phase of the different features varies with respect to the daily cycles of temperature and sub-glacial water pressure. These phase offsets can give us indices on eventual mechanisms of sliding at the bedrock interface. The GNSS measurements represent a rare in situ data set that can contribute to better apprehend mechanisms of basal sliding and to provide high-resolution 3D constraints on physical models of glacier flow.

The response of very-high-elevation glaciated areas on Mont Blanc to climate change has been analysed using observations and numerical modelling over the last 2 decades. Unlike the changes at low elevations, we observe very low glacier thickness changes, of about −2.6 m on average since 1993. The slight changes in horizontal ice flow velocities and submergence velocities suggest a decrease of about 10 % in ice flux and surface mass balance. This is due to less snow accumulation and is consistent with the precipitation decrease observed in meteorological data. Conversely, measurements performed in deep boreholes since 1994 reveal strong changes in englacial temperature reaching a 1.5 ∘C increase at a depth of 50 m. We conclude that at such very high elevations, current changes in climate do not lead to visible changes in glacier thickness but cause invisible changes within the glacier in terms of englacial temperatures. Our analysis from numerical modelling shows that glacier near-surface temperature warming is enhanced by increasing melt frequency at high elevations although the impact on surface mass balance is low. This results in a non-linear response of englacial temperature to currently rising air temperatures. In addition, borehole temperature inversion including a new dataset confirms previous findings of similar air temperature changes at high and low elevations in the Alps.