With the use of a numerical three-dimensional (3-D) model the flow dynamics ofthe confluence area of Unteraargletscher, Bernese Alps, Switzerland, arestudied. Previous predictions, based on conceptual two-dimensional models, aboutflow characteristics at confluence areas are tested against results from thefully 3-D model. Measured winter velocities are used for model verification.Despite some consistent systematic differences, good overall agreement betweenmeasured and calculated surface velocities is obtained. The calculated verticalstrain-rate variation with depth is in good agreement with availablemeasurements from boreholes. The ice is found to be almost three times stifferthan standard estimates of rheological parameters for glacier ice would predict.The model predicts a complicated yet realistic pattern of vertical velocityvariation along the surface. The most noticeable features of the verticalvelocity distribution across the surface are listed, and their relation totopographic surface undulations and the overall dynamics of the confluencediscussed. In accordance with previous results from analytical models, astrongly localized surface trough and a concomitant negative (downwardorientation) vertical velocity anomaly develop at the junction point. Althoughdepth-integrated strain rates are positive (extension), the basal layer iscompressed vertically. The ice-cored medial moraine is formed by differentialablation. The flow mechanics of the confluence area play only an indirect role,by enabling transfer of debris-covered marginal ice towards the confluencecenter. In the absence of differential ablation, an elongated surface depressionwould be formed in the down-glacier direction from the junction point instead ofan elevated ice-cored medial moraine.

Glacier response to climate forcing can be heterogeneous and complex, depending on glacier system characteristics. This article presents the decadal evolution of the Tsarmine Glacier (Swiss Alps), a very small and heavily debris-covered cirque glacier located in the Alpine periglacial belt. Archival aerial photogrammetry and autocorrelation of orthophotos were used to compute surface elevation, volume and geodetic mass changes, as well as horizontal displacement rates for several periods between 1967 and 2012. A GPR survey allowed us to investigate glacier thickness (15 m mean) and volume (4 × 106 m3) in 2015 and to anticipate its future evolution. Different dynamics occurred in recent decades because of the heterogeneous surface characteristics. The climate-sensitive upper debris-free zone contrasts with the progressively stagnant heavily debris-covered glacier tongue. Between 1967 and 2012, the glacier lost 1/3 of its initial volume (2 × 106 m3). The average mass balance stabilised at ~−0.3 m w.e. a−1 since 1999. Compared with other local glaciers, the Tsarmine Glacier shows a particular decadal behaviour both in time (divergence of mass balance since the 2000s) and space (inverted ablation pattern). This might be explained by the combined influence of debris cover, shadow, snow redistribution and permafrost conditions on this very small glacier.

Analysis of aerial photographs, GPS mapping and comparison of digital elevation models have been used to quantify the losses in extent and volume observed (1981–2005) in the glaciers of the Maladeta massif (Spanish Pyrenees). The data are examined in relation to different climatic (temperature, precipitation) and topographic factors that control glacial retreat both at the general and local scales. The evolution observed in the massif is characterized by the remarkable surface and volumetric shrinkage registered in all the glaciers: (1) glacial ice decreased in extent 35.7%, reducing from 240.9 ha to 155.0 ha; (2) losses in total ice volume reached 0.0137 km3 (75.6 m w.e.); (3) mean altitude of the studied glaciers increased 43.5 m. These changes seem to have been forced by climatic change (reduction in the snowfall contributions and increase in the maximum temperatures) during the past few decades in this Pyrenean region. In addition, local variables such as the orientation of each glacier, their altitude and their initial size seem to have induced significant spatial differences in the magnitude of the losses.

Temperature, density and accumulation data were obtained from shallow firn cores,drilled during an overland traverse through a previously unknown part ofDronning Maud Land, East Antarctica. The traverse area is characterised by highmountains that obstruct the ice flow, resulting in a sudden transition from thepolar plateau to the coastal region. The spatial variations of potentialtemperature, near-surface firn density and accumulation suggest that katabaticwinds are active in this region. Proxy wind data derived from firn-densityprofiles confirm that annual mean wind speed is strongly related to themagnitude of the surface slope. The high elevation of the ice sheet south of themountains makes for a dry, cold climate, in which mass loss owing to sublimationis small and erosion of snow by the wind has a potentially large impact on thesurface mass balance. A simple katabatic-wind model is used to explain thevariations of accumulation along the traverse line in terms ofdivergence/convergence of the local transport of drifting snow. The resultingwind- and snowdrift patterns are closely connected to the topography of the icesheet: ridges are especially sensitive to erosion, while ice streams and otherdepressions act as collectors of drifting snow.

The Barnes Ice Cap on Baffin Island, N.W.T., Canada, was investigated during the summer of 1950. This ice cap, some 6000 sq. km. in extent, appears to have an approximately balanced budget, and yet there is no firn on its surface. It is postulated that its nourishment is by superimposed ice due to immediate refreezing of much of the melt water of summer. It is further inferred that a similar process nourishes many Arctic glaciers and ice caps where elevation, precipitation and temperature are all low. The name “Baffin Type” is proposed for these in the classification of glaciers.

A connected system of active subglacial lakes was revealed beneath Recovery Ice Stream, East Antarctica, by ICESat laser altimetry. Here we combine repeat-track analysis of ICESat (2003–09), Operation IceBridge laser altimetry and radio-echo sounding (2011 and 2012), and MODIS image differencing (2009–2011) to learn more about the lake activity history, the surface and bedrock topographic setting of the lakes and the constraints on water flow through the system. We extend the lake activity time series to 2012 for the three lower lakes and capture two major lake drainages. One lake underwent a large deflation between 2009 and 2011 while another lake, which had been continuously filling between 2003 and 2010, started to drain after 2011. Most of the active lakes are located in a ~ 1000 km long bedrock trough under the main trunk of Recovery Ice Stream, whose base is ~ 1500– 2000 m below present-day sea level. The hydrologic system beneath Recovery Ice Stream is controlled by this unusually pronounced bedrock topography, in contrast to most Antarctic systems studied to date, which are controlled by the ice surface topography. Hydrologic connections among the lakes appear to be direct and responsive, and we reproduce the lake activity using a simple subglacial water model. We discuss potential causes of non-steady hydrologic behavior in major Antarctic catchments.

The MODerate resolution Imaging Spectroradiometer (MODIS) land surface temperature (LST) data have been widely used for air temperature estimation in mountainous regions where station observations are sparse. However, the performance of MODIS LST in high-elevation glacierized areas remains unclear. This study investigates air temperature estimation in glacierized areas based on ground observations at four glaciers across the Tibetan Plateau. Before being used to estimate the air temperature, MODIS LST data are evaluated at two of the glaciers, which indicates that MODIS night-time LST is more reliable than MODIS daytime LST data. Then, linear models based on each of the individual MODIS LST products from two platforms (Terra and Aqua) and two overpasses (night-time and daytime) are built to estimate daily mean, minimum and maximum air temperatures in glacierized areas. Regional glacier surface (RGS) models (mean /-mean-square differences: 3.3, 3.0 and 4.8°C for daily mean, minimum and maximum air temperatures, respectively) show higher accuracy than local non-glacier surface models (mean root-mean-square differences: 4.2, 4.7 and 5.7°C). In addition, the RGS models based on MODIS night-time LST perform better to estimate daily mean, minimum and maximum air temperatures than using temperature lapse rate derived from local stations.

Bore-hole photography demonstrates that the glacier bed was reached by cable-tool drilling in five bore holes in Blue Glacier, Washington. Basal sliding velocities measured by bore-hole photography, and confirmed by inclinometry, range from 0.3 to 3.0 cm/d and average 1.0 cm/d, much less than half the surface velocity of 15 cm/d. Sliding directions deviate up to 30° from the surface flow direction. Marked lateral and time variations in sliding velocity occur. The glacier bed consists of bedrock overlain by a ≈ 10 cm layer of active subsole drift, which intervenes between bedrock and ice sole and is actively involved in the sliding process. It forms a mechanically and visibly distinct layer, partially to completely ice-free, beneath the zone of debris-laden ice at the base of the glacier. Internal motions in the subsole drift include rolling of clasts caught between bedrock and moving ice. The largest sliding velocities occur in places where a basal gap, of width up to a few centimeters, intervenes between ice sole and subsole drift. The gap may result from ice—bed separation due to pressurization of the bed by bore-hole water. Water levels in bore holes reaching the bed drop to the bottom when good hydraulic connection is established with sub-glacial conduits; the water pressure in the conduits is essentially atmospheric. Factors responsible for the generally low sliding velocities are high bed roughness due to subsole drift, partial support of basal shear stress by rock friction, and minimal basal cavitation because of low water pressure in subglacial conduits. The observed basal conditions do not closely correspond to those assumed in existing theories of sliding.

The evaluation of avalanche release depths constitutes a great challenge for risk assessment in mountainous areas. This study focuses on slab avalanches, which generally result from the rupture of a weak layer underlying a cohesive slab. We use the finite-element code Cast3M to build a mechanical model of the slab/weak-layer system, taking into account two key ingredients for the description of avalanche release: weak-layer heterogeneity and stress redistribution via slab elasticity. The system is loaded by increasing the slope angle until rupture. We first examine the cases of one single and two interacting weak spots in the weak layer, in order to validate the model. We then study the case of heterogeneous weak layers represented through Gaussian distributions of the cohesion with a spherical spatial covariance. Several simulations for different realizations of weak-layer heterogeneity are carried out and the influence of slab depth and heterogeneity correlation length on avalanche release angle distributions is analyzed. We show, in particular, a heterogeneity smoothing effect caused by slab elasticity. Finally, this mechanically based probabilistic model is coupled with extreme snowfall distributions. A sensitivity analysis of the predicted distributions enables us to determine the values of mechanical parameters that provide the best fit to field data.

Increased summer ice velocities on the Greenland ice sheet are driven by meltwater input to the subglacial environment. However, spatial patterns of surface input and partitioning of meltwater between different pathways to the base remain poorly understood. To further our understanding of surface drainage, we apply a supraglacial hydrology model to the Paakitsoq region, West Greenland for three contrasting melt seasons. During an average melt season, crevasses drain ~47% of surface runoff, lake hydrofracture drains ~3% during the hydrofracturing events themselves, while the subsequent surface-to-bed connections drain ~21% and moulins outside of lake basins drain ~15%. Lake hydrofracture forms the primary drainage pathway at higher elevations (above ~850 m) while crevasses drain a significant proportion of meltwater at lower elevations. During the two higher intensity melt seasons, model results show an increase (~5 and ~6% of total surface runoff) in the proportion of runoff drained above ~1300 m relative to the melt season of average intensity. The potential for interannual changes in meltwater partitioning could have implications for how the dynamics of the ice sheet respond to ongoing changes in meltwater production.

Densification of firn at the North Greenland Eemian Ice Drilling (NEEM) camp is investigated using density surrogates: dielectric permittivities ∊v and ∊h at microwave frequencies with electrical fields in the vertical and horizontal planes, respectively. Dielectric anisotropy Δ∊ (= ∊v − ∊h) is then examined as a surrogate for the anisotropic geometry of firn. Its size, fluctuations and mutual correlations are investigated in samples taken at depths from the surface to ~90 m. The initial Δ∊ of ~0.06 appears within the uppermost 0.2 m. After that, Δ∊ decreases rapidly until 21–26 m depth. Below this, Δɛ decreases slowly. Layers with more ions of fluorine, chlorine and some cations deposited between the autumn and the subsequent summer deform preferentially during all these stages. This layered deformation is explained partly by the textural effects initially formed by the seasonal variation of metamorphism, and partly by ions such as fluorine, chlorine and ammonium, which are known to modulate dislocation movement in the ice crystal lattice. Insolation-sensitive microstructure appears to be preserved all the way to the pore close-off, within layers of the summer-to-autumn metamorphism. Like previous authors, we hypothesize that calcium is not the active agent in the reported deformation– calcium correlations.

High Mountain Asia (HMA) glaciers are critical water reserves for montane regions, which are readily influenced by climate change. The glacier mass balance during 2000–2021 over HMA was estimated by comparing the elevations from ICESat-2 and the NASADEM. Radar penetration depth could be one of the intrinsic error sources in estimating glacier mass balance by using NASADEM. Therefore, we doubled elevation differences between the X-band Shuttle Radar Topography Missions (SRTMs) and NASADEM to estimate the potential error. The spatial characteristics of the altitude-dependent penetration depth can be detected in most sub-regions of HMA. Relatively deep penetrations in the Himalaya (2.3–3.7 m) and Hissar Alay (4.3 m) regions and small penetrations in the south-eastern HMA (1.0 m) were observed. The HMA region experienced a significant mass loss at a rate of −0.18 ± 0.12 m w.e. a−1, in which the Hengduan Shan exhibited the highest mass loss of −0.62 ± 0.10 m w.e. a−1, the West Kun Lun experienced a substantial mass gain of 0.23 ± 0.13 m w.e. a−1, and the Karakoram showed a more or less balance. Our results are in agreement with previous studies that assessed the mass balance of HMA glaciers from different methods.

A detailed study of the major-ion chemistry of the Dokriani glacier meltwaters, feeding the Bhagirathi river, Ganga basin, Garhwal Himalaya, India, has been carried out to assess the role of active glaciers in the higher chemical-denudation rate (CDR) in this area. The CDR of the Dokriani glacier catchment is 321.6 t km-2 a-1, higher than in other glacierized catchments of the world, indicating intense chemical erosion in the glacierized catchments of the Himalaya. The dominance of Ca2+, HCO3 - and SO4 2- in meltwaters throughout the 6 month (May–October) ablation period suggests that the chemical weathering is dominated by coupled reactions involving sulphide oxidation and carbonate dissolution. Good positive correlation between SO4 2- and suspended-sediment concentration during July and August (r 2 = 0.72 and 0.67, respectively) suggests that the southwest monsoonal rainfall enhances the weathering of supraglacial moraine and contributes significant amounts of sulphate to the high meltwater discharges. The sulphate flux, as a proportion of combined (SO4 2- + HCO3 -) flux, also increased from 45% at 2 m3 s-1 to 63% at 10 m3 s-1.

Trapridge Glacier, Yukon Territory, Canada is a subpolar surge-type glacier. It last surged in the 1940s and is now in the late stages of quiescence. Since 1969, when the glacier was first surveyed, a large wave-like bulge has formed near the glacier terminus. Our surveys from 1969–89 show the profile evolution that has accompanied the formation and downflow propagation of this feature. Ice-temperature measurements taken in 1980–81 established that the bulge was forming at the boundary between thick warm-based ice lying up-glacier from the bulge, and thin cold-based ice lying down-glacier from it. The bulge is propagating at roughly 30 m a−1 and thick ice has now completely overridden the region once covered by thin cold-based ice that we instrumented in 1980–81. In 1987, and again in 1988, the geographical positions of the 1980 measurement sites were redrilled and instrumented with new thermistor cables. Comparison of the 1980–81 data with that from 1987–88 shows that this region of the glacier has undergone a dramatic change in geometry and thermal regime. Water penetration into surface crevasses has warmed the 15-m ice temperature by roughly 2°C. The zone of transition from warm- to cold-based ice is migrating down-glacier but at a slower rate than that of the bulge feature. The transition from warm-based to cold-based ice appears to cause a discontinuity in the flow that resembles a transition from flow over a sliding boundary to flow over an adhering boundary. The discontinuity in the flow field is associated with anomalies in the temperature field and appears to be the source region for an englacial structure formed from subglacial sediment. This structure was not present in 1980–81 and is thought to have the geometry of a thrust fault or recumbent fold.

Measurements of heat balance and ablation on glaciers of the Eastern Alps carried out during a total of 45 days since the summer of 1950 indicate that in flat glaciated areas at approximately 3000 m. above sea level 81 to 84 per cent of the energy causing ablation is supplied by short wave radiation from the sun and sky. Only 16 to 19 per cent come from the air in the form of actual and latent heat. On glacier tongues at altitudes of approximately 2300 m. the percentage of ablation caused by radiation is only 58 to 65 per cent. This is primarily the result of the shortened duration of sunshine in the deeper valleys. The supply of perceptible and latent heat from the air can, at most, reach a value of 15 to 30 per cent on glacier tongues. Evaporation from the ice and heat supply by liquid precipitation are negligible during the normal ablation period (June till September).

It is to be expected therefore that the alpine glaciers will primarily react to variations of radiation and albedo during the months of June to September. The effects of changing summer temperatures are considered insufficient to cause the vast changes of the ice-cover. The variations of the duration of summer sunshine and the number of days with snowfall as a rough indication of albedo, respectively, are in perfect agreement with the behaviour of alpine glaciers during the last sixty years.

The relationship between velocity field and isochrone geometry along a steady flowline of an ice sheet is examined. The method is analytical and based upon the stream function and its vertically normalized form, the normalized stream function (NSF). We show that the slope of the isochrones is the slope of the iso-NSF lines, plus a path term which is the cumulative result of the past trajectory of the ice particles. We illustrate this path term in three different examples: varying basal melting, varying basal sliding (Weertman effect) and varying velocity profile around a divide (Raymond effect). The path term generally counteracts the slope of the iso-NSF lines. In the case of the Raymond effect, it can even lead to depressions surrounding the bumps if the transition from dome to flank velocity profile is sufficiently abrupt.

Dendroglaciological analysis of supraglacial trees represents an example of applied dendro- geomorphological methods in reconstructing glacier variations. Supraglacial trees react to glacier ice and debris movement, assuming typical shapes with modified radial growth. In this paper, based on treering analysis of Larix decidua Mill., we investigate the relationship between the distribution and growth of trees located on the most famous and representative debris-covered glacier in the Italian Alps (Miage glacier, Valle d’Aosta) and the superficial movements of ice and debris in the lower part of the tongue. Different growth anomalies (e.g. pointer years, compression wood, abrupt growth changes) were identified and dated. Three reference tree-ring chronologies based on undisturbed larches growing outside the glacier were constructed for comparison with tree-ring data from supraglacial trees. The oldest sampled trees colonized the glacier surface just before 1960. The simultaneous presence of different disturbance indicators occurred mainly between 1984 and 1990 on the southern lobe and during the period 1989–93 on the northern glacier lobe. These results fit with glaciological data documenting volume and surface-level variations in the same period.

Tidewater glaciers in Greenland experienced widespread retreat during the last century. Information on their behaviour prior to this is often poorly constrained due to lack of observations, while determining the drivers prior to instrumental records is also problematic. Here we present a record of the dynamics of Kangiata Nunaata Sermia (KNS), southwest Greenland, from its Little Ice Age maximum (LIAmax) to 1859 – the period before continuous air temperature observations began at Nuuk in 1866. Using glacial geomorphology, historical accounts, photographs and GIS analyses, we provide evidence KNS was at its LIAmax by 1761, had retreated by ~5 km by 1808 and a further 7 km by 1859. This predates retreat at Jakobshavn Isbræ by 43–113 years, demonstrating the asynchroneity of tidewater glacier terminus response following the LIA. We use a one-dimensional flowband model to determine the relative sensitivity of KNS to atmospheric and oceanic climate forcing. Results demonstrate that terminus forcing rather than surface mass balance drove the retreat. Modelled glacier sensitivity to submarine melt rates is also insufficient to explain the retreat observed. However, moderate increases in crevasse water depth, driving an increase in calving, are capable of causing terminus retreat of the observed magnitude and timing.

Using the RICE (Radio Ice Cherenkov Experiment) detector at the South Pole, we have estimated the variation in the index of refraction (n) of the firn, as a function of elevation (z) measured from the surface down to z =-150 m. Measurements were made by lowering a dipole transmitter into a dry (5 in (127 mm) caliber) borehole, originally drilled for the RICE experiment in 1998, and determining signal arrival times, as a function of transmitter depth, in the englacial RICE receiver array. We clearly confirm the expected correlation of n(z) with ice density. Our measurements are in fair agreement with previous laboratory characterizations of the dielectric properties of ice cores. These are the first such in situ measurements to be performed at the South Pole.

Optical satellite imagery was used to estimate glacier surface velocities and iceberg calving rates from Agassiz and western Grant Ice Caps, Nunavut, Canada, between 1999 and 2003. The largest mean annual surface velocities ranged from ∼400 to 700 m a−1, but velocities in the ∼100–200 m a−1 range were common. Summer velocities were up to an order of magnitude larger than annually averaged velocities. The highest velocity (∼1530 m a−1) was measured on the floating tongue of Lake Tuborg Glacier between 19 July and 19 August 2001. Calving rates from individual glaciers varied by up to a factor of two between successive years. Summer calving rates were ∼2–8 times larger than annual average rates. The average ratio of the calving flux due to terminus-volume change to that due to ice flow through the glacier terminus was ∼0.81 for the annual rates and ∼1.71 for summer rates. The estimated mean annual calving rate from Agassiz Ice Cap in the period 1999–2002 was 0.67 ± 0.15 km3 a−1, of which ∼54% emanated from Eugenie Glacier alone. This total rate is similar to a previously estimated calving rate from Devon Ice Cap.