Measurements of the mechanical properties of snow are essential for improving our understanding and the prediction of snow failure and hence avalanche release. We performed fracture mechanical experiments in which a crack was initiated by a saw in a weak snow layer underlying cohesive snow slab layers. Using particle tracking velocimetry (PTV), the displacement field of the slab was determined and used to derive the mechanical energy of the system as a function of crack length. By fitting the estimates of mechanical energy to an analytical expression, we determined the slab effective elastic modulus and weak layer specific fracture energy for 80 different snowpack combinations, including persistent and nonpersistent weak snow layers. The effective elastic modulus of the slab ranged from 0.08 to 34 MPa and increased with mean slab density following a power-law relationship. The weak layer specific fracture energy ranged from 0.08 to 2.7 J m−2 and increased with overburden. While the values obtained for the effective elastic modulus of the slab agree with previously published low-frequency laboratory measurements over the entire density range, the values of the weak layer specific fracture energy are in some cases unrealistically high as they exceeded those of ice. We attribute this discrepancy to the fact that our linear elastic approach does not account for energy dissipation due to non-linear parts of the deformation in the slab and/or weak layer, which would undoubtedly decrease the amount of strain energy available for crack propagation.

Numerous ice-sheet reconstructions of the last glacial cycle have been proposed, however due to limited geological evidence, reconstructing older Northern Hemisphere ice sheets remains a difficult exercise. Here we focus on the penultimate glacial maximum (PGM; ~140 ka BP) over the Northern Hemisphere. While some evidence of the PGM Eurasian ice-sheet extent were found, this is not the case for the corresponding Laurentide ice sheet. To improve the glaciological reconstructions of the PGM Northern Hemisphere ice sheets, we explore the parameter space of ice-sheet model uncertainties and carry out numerous univariate ice-sheet steady-state sensitivity simulations. We use two PGM climate simulations to force the ice-sheet model, differing in the prescribed Laurentide ice topography (small and large). The simulated Northern Hemisphere ice volume ranges from 124.7 to 152 m SLE when using the climate accounting for a small Laurentide ice sheet, which is compatible with global sea-level reconstructions of this period (−92 to −150 m). Conversely, using the climate simulation with a Laurentide ice sheet comparable in size to that of the last glacial maximum results in too large ice volumes. Changes in basal drag provide the upper bound ice volume of our experiments, whereas changes in the distribution of ice streams provide the lower bound.

A multiple parameter dating technique was used to establish a depth/age scale for a 171.3 m (145.87 m w.e.) surface to bedrock ice core (Bl2003) recovered from the cold recrystallization accumulation zone of the Western Belukha Plateau (4115 m a.s.l.) in the Siberian Altai Mountains. The ice-core record presented visible layering of annual accumulation and of δ18O/δD stable isotopes, and a clear tritium reference horizon. A steady-state glacier flow model for layer thinning was calibrated and applied to establish a depth/age scale. Four radiocarbon (14C) measurements of particulate organic carbon contained in ice-core samples revealed dates for the bottom part of Bl2003 from 9075 ± 1221 cal a BC at 145.2 ± 0.1 m w.e. (0.665 m w.e. from the bedrock) to 790 ± 93 AD at 121.1 m w.e. depth. Sulfate peaks coincident with volcanic eruptions, the Tunguska meteorite event, and the 1842 dust storm were used to verify dating. Analysis of the Bl2003 ice core reveals that the modern Altai glaciers were formed during the Younger Dryas (YD) (~10 950 to ~7500 cal a BC), and that they survived the Holocene Climate Optimum (HCO) (~6500 to ~3600 cal a BC) and the Medieval Warm Period (MWP) (~640 to ~1100 AD). A decrease in air temperature at the beginning and an abrupt increase at the end of the YD were identified. Intensification of winds and dust loading related to Asian desert expansion also characterized the YD. During the YD major ion concentrations increased significantly, up to 50 times for Na+ (background), up to 45 times for Ca2+ and Mg2+, and up to 20 times for SO42− relative to the recent warm period from 1993 to 2003. A warm period lasted for about three centuries following the YD signaling onset of the HCO. A significant and prolonged decrease in air temperature from ~2000 to ~600 cal a BC was associated with a severe centennial drought (SCD). A sharp increase in air temperatures after the SCD was coincident with the MWP. After the MWP a cooling was followed gradually with further onset of the Little Ice Age. During the modern warm period (1973–2003) an increase in air temperature is noted, which nearly reaches the average of HCO and MWP air temperature values.

Two non-invasive optical Schlieren methods have been adapted to visualize brine channel development and convective processes in experimentally grown sea ice obtained when a NaCl aqueous solution is cooled from above in a quasi-two-dimensional Hele–Shaw cell. The two different visualization methods, i.e. traditional and synthetic Schlieren optical imaging, produce high spatial resolution images of transport processes during ice growth, without any external perturbation. These images allow observations of the flow dynamics simultaneously within the ice layer, around the ice/water interface, and in the liquid water layer, revealing connections between the processes occurring within the two phases. Results from these methods show that desalination of the growing ice layer occurs by two concurrent, yet independent, mechanisms: (1) boundary layer convection persisting throughout the ice growth period, with short fingers present just below the ice/water interface, and (2) gravity-driven drainage from the brine channels producing deep penetrating convective streamers, which appear after a given time from the beginning of ice growth. The improved visualization and qualitative characterization of these processes show that Schlieren optical methods have exciting potential applications for future study of convective processes during sea-ice growth.

The subglacial observatory beneath the Argentière glacier provides a rare opportunity to study the interactions between glacier sliding velocity and subglacial runoff. The sliding velocity has been monitored in this cavity almost continuously since 1997 and the resulting data indicate a decrease in annual sliding velocities over the last two decades. We found close relationships between annual surface velocity, sliding velocity and ice thickness. These relationships indicate that the ice-flow velocity changes do not depend on subglacial water runoff changes at the annual timescale. The seasonal magnitudes of sliding also show a decrease over the last two decades. At the seasonal timescale, sliding velocity increases before or simultaneously with the large runoff increase in May, indicating a distributed drainage system. Conversely, at the end of the melt season, sliding velocity continues to decrease after the runoff returns to low winter values. The simultaneous increases of runoff and sliding velocity occur mainly before the spring transition. Later, sliding velocity generally appears not to be related to water inputs coming from the surface, except for some large accelerations after midAugust that are always associated with periods of rapidly increasing water inputs to the subglacial drainage system.

The McMurdo Dry Valleys of Antarctica host the coldest and driest ecosystem on Earth, which is acutely sensitive to the availability of water coming from glacial runoff. We modeled the spatial variability in ablation and assessed climate sensitivity of the glacier ablation zones using 16 years of meteorological and surface mass-balance observations collected in Taylor Valley. Sublimation was the primary form of mass loss over much of the ablation zones, except for near the termini where melt, primarily below the surface, dominated. Microclimates in ~10 m scale topographic basins generated melt rates up to ten times higher than over smooth glacier surfaces. In contrast, the vertical terminal cliffs on the glaciers can have higher or lower melt rates than the horizontal surfaces due to differences in incoming solar radiation. The model systematically underpredicted ablation for the final 5 years studied, possibly due to an increase of windblown sediment. Surface mass-balance sensitivity to temperature was ~−0.02 m w.e. K−1, which is among the smallest magnitudes observed globally. We also identified a high sensitivity to ice albedo, with a decrease of 0.02 having similar effects as a 1 K increase in temperature, and a complex sensitivity to wind speed.

The attribution of past glacier length fluctuations to changes in climate requires characterizing glacier mass-balance variability. Observational records, which are relatively short, are consistent with random fluctuations uncorrelated in time, plus an anthropogenic trend. However, longer records of other climate variables suggest that, in fact, there is a degree of temporal persistence associated with internal (i.e. unforced) climate variability, and that it varies with location and climate. Therefore, it is likely that persistence does exist for mass balance, but records are too short to confirm its presence, or establish its magnitude, with conventional statistical tests. Extending the previous work, we explore the impact of potential climatic persistence on glacier length fluctuations. We use a numerical model and a newly developed analytical model to establish that persistence, even of a degree so small as to be effectively undetectable in the longest mass-balance records, can significantly enhance the resulting glacier length fluctuations. This has a big impact on glacier-excursion probabilities: what was an extremely unlikely event (<1%) can become virtually certain (>99%), when persistence is incorporated. Since the actual degree of climatic persistence that applies to any given glacier is hard to establish, these results complicate the attribution of past glacier changes.

Fractured systems developing at the base of temperate glaciers play a significant role in the transport of subglacial water. Their influence on dynamics and hydraulics of glaciers and the seasonal alterations they experience during the hydrological year, remain however, poorly understood, as they lay beyond the reach of most conventional glaciological techniques. In this work, we investigate the seasonal evolution of glacier seismicity in the ablation zone of Rhonegletscher (Switzerland) and focus on basal fracture processes. To do this, we use seismic measurements performed between June 2012 and July 2013 and complementary measurements of surface motion and changes in melt water input inferred from degree day modeling. Results of a spectrogram analysis highlight predominant seismic emissions until end of October 2012 and from mid April 2013 in line with measured surface velocities, and indicate a rapid adaptation of the basal drainage system at initiation and end of the winter period. The analysis of icequakes emitted from two active regions shows that the basal events, due to opening and closing of cracks within fracture networks, persist after winter initiation, and points out the year-long preservation of the associated sources. In this regard, we deduce that basal crack networks aseismically evolve during the winter period.

The isothermal, non-sliding shallow-ice approximation, combined with mass conservation, is a fundamental model for ice-sheet and glacier flow. It determines the ice extent, geometry and velocity by the solution of a free-boundary problem. In this paper, the steady-state form of this problem is solved directly, without time-stepping, thereby demonstrating a fully implicit scheme with no stability restrictions. The classical Mahaffy (1976) finite difference method is first reinterpreted as a ‘finite volume element’ scheme that has both an everywhere-defined approximate thickness function and an approximation of the conservation equation in flux integral form. From this reinterpretation an improved scheme is built by using better quadrature in the integral and upwinding on that part of the flux which is proportional to the bed gradient. The discrete equations are then solved by a parallel Newton scheme which respects the constraint that ice thickness is non-negative. The results show good accuracy on both flat-bed and bedrock-step exact solutions. The method is then applied at high resolution to model the steady-state geometry of the Greenland ice sheet, using only bedrock elevation and present-day surface mass balance as input data.

Glaciers in Alaska are currently losing mass at a rate of ~−50 Gt a−1, one of the largest ice loss rates of any regional collection of mountain glaciers on Earth. Existing projections of Alaska's future sea-level contributions tend to be divergent and are not tied directly to regional observations. Here we develop a simple, regional observation-based projection of Alaska's future sea-level contribution. We compute a time series of recent Alaska glacier mass variability using monthly GRACE gravity fields from August 2002 through December 2014. We also construct a three-parameter model of Alaska glacier mass variability based on monthly ERA-Interim snowfall and temperature fields. When these three model parameters are fitted to the GRACE time series, the model explains 94% of the variance of the GRACE data. Using these parameter values, we then apply the model to simulated fields of monthly temperature and snowfall from the Community Earth System Model, to obtain predictions of mass variations through 2100. We conclude that mass loss rates may increase between −80 and −110 Gt a−1 by 2100, with a total sea-level rise contribution of 19 ± 4 mm during the 21st century.

An extensive set of shore-parallel fractures are observed at the grounding line of Kamb Ice Stream (KIS) in West Antarctica. Seismicity measured in the grounding zone is, as elsewhere around Antarctica, tidally forced and moreover strand cracks propagate nearly exclusively on the falling tide. Measured surface deflection and a model of fracture propagation are used to conclude that bending on the falling tide favors propagation while bending on the rising tide suppresses propagation. Without the perturbation due to tidal bending, strand cracks would be rare and appear farther downstream than observed. We speculate that the very large number of cracks observed at KIS is due to the stagnant-to-floating transition at that grounding line, which allows cyclic bending of the same ice and relatively large stretching rates.

This is the first (Paper I) of three companion papers focused respectively, on the estimates of the errors in ice thickness retrieved from pulsed ground-penetrating radar (GPR) data, on how to estimate the errors at the grid points of an ice-thickness DEM, and on how the latter errors, plus the boundary delineation errors, affect the ice-volume estimates. We here present a comprehensive analysis of the various errors involved in the computation of ice thickness from pulsed GPR data, assuming they have been properly migrated. We split the ice-thickness error into independent components that can be estimated separately. We consider, among others, the effects of the errors in radio-wave velocity and timing. A novel aspect is the estimate of the error in thickness due to the uncertainty in horizontal positioning of the GPR measurements, based on the local thickness gradient. Another novel contribution is the estimate of the horizontal positioning error of the GPR measurements due to the velocity of the GPR system while profiling, and the periods of GPS refreshing and GPR triggering. Their effects are particularly important for airborne profiling. We illustrate our methodology through a case study of Werenskioldbreen, Svalbard.

We offer the first sub-seasonal view of glacial age archives from the Siple Dome-A (SDMA) ice core using the ultra-high resolution capabilities of a newly developed laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS; 121 µm sampling resolution) system capable of conducting multi-element glaciochemical analysis. Our ultra-high resolution data demonstrates that: (1) the SDMA ice core record can be annually dated based on seasonality in chemical inputs at a depth not previously possible using previous glaciochemical sampling methods, (2) winter accumulation at the SD site was greater than summer accumulation during the three late glacial periods selected (~15.3, 17.3, 21.4 Ka ago) in this study and (3) resulting annual layer thicknesses results show greater variability than the current SD ice core depth/age model (Brook and others, 2005), possibly due to depositional effects such as wind scouring and/or decadal variability in snow accumulation that is not captured by the resolution of the current depth/age model.

A 1-D ice cover model was developed to predict and constrain drivers of long-term ice thickness trends in chemically stratified lakes of Taylor Valley, Antarctica. The model is driven by surface radiative heat fluxes and heat fluxes from the underlying water column. The model successfully reproduced 16 a (between 1996 and 2012) of ice thickness changes for the west lobe of Lake Bonney (average ice thickness = 3.53 m) and Lake Fryxell (average ice thickness = 4.22 m). Long-term ice thickness trends require coupling with the thermal structure of the water column. The heat stored within the temperature maximum of lakes exceeding a liquid water column depth of 20 m can either impede or facilitate ice thickness change depending on the predominant climatic trend (cooling or warming). As such, shallow (<20 m deep water columns) perennially ice-covered lakes without deep temperature maxima are more sensitive indicators of climate change. The long-term ice thickness trends are a result of surface energy flux and heat flux from the deep temperature maximum in the water column, the latter of which results from absorbed solar radiation.

Rapid dynamic changes of ocean-terminating outlet glaciers of the Greenland ice sheet are related to atmospheric and oceanic warming but the detailed link to external forcing is not well understood. Observations show high variability in dynamic changes and are mainly limited to the past three decades with dense satellite observations. Here we compile a long-term record of dynamic changes of Eqip Sermia Glacier, West Greenland. Starting in 1912, we combine historical measurements of terminus positions, ice-surface elevation and flow velocity together with more recent in-situ and remote-sensing observations. In the 20th century, the glacier underwent small variations in terminus position and flow speed. Between 2000 and 2003, the terminus retreated substantially, but stabilized thereafter. In 2009, the northern terminus lobe started to retreat very rapidly; the southern lobe collapsed in 2013. The present terminus position, which has retreated by 4 km since 1920, is unprecedented in the historical record. Flow velocities were relatively stable until 2010. The recent acceleration reached threefold velocities in 2014 and rapidly affected the whole terminus region up to 15 km inland. Comparison with forcings from the atmosphere and the ocean over the past century shows that no dominant cause can be identified, and that local effects of bed geometry modulate the glacier response.

We use a combination of field observations and hydrological modelling to examine the mechanisms through which variability in meltwater input affects ice motion at a land-terminating Greenlandic outlet glacier. We find a close agreement between horizontal ice velocity, vertical ice velocity and modelled subglacial water pressure over the course of a melt season. On this basis, we argue that variation in horizontal and vertical ice velocity primarily reflects the displacement of basal ice during periods of cavity expansion and contraction, a process itself driven by fluctuations in basal water pressure originating in subglacial channels. This process is not captured by traditional sliding laws linking water pressure and basal velocity, which may hinder the simulation of realistic diurnal to seasonal variability in ice velocity in coupled models of glacial hydrology and dynamics.

A database of difficult-to-forecast natural persistent deep slab avalanches was analyzed to determine thresholds for parameters that contribute to their release in western Canada. The database included avalanche observations and weather station data. The avalanches were grouped based on their primary cause-of-release, either precipitation loading, wind loading, solar warming or air temperature warming using a multivariate classification tree, which first split using a solar warming parameter. The precipitation group had a median 24 h snowfall of 15 cm and 3 d snowfall of 38 cm at weather stations, mostly at or below treeline. These amounts were likely closer between 20–30 and 50–80 cm at alpine start zones. The wind loading group experienced the most wind-transported snow potential. The solar warming group had predicted solar warming of 5.2°C, 10 cm into the snowpack, on the days of release. The air temperature warming group experienced the highest median maximum air temperature (5°C) on the days of release. These thresholds may be useful to forecast the likelihood of similar avalanches with experienced-based forecasting or with decision aids, although many false alarms are possible. A companion paper, Part II, relates weather model data to avalanche occurrences.

This paper is the second (Paper II) in a set of studies concerning the errors involved in the estimate of ice thickness and ice volume. Here we present a detailed analysis of the errors involved in the generation of ice-thickness DEMs constructed, most often, from GPR data, complemented by boundary data and sometimes, additional synthetic data arising from estimates based on theoretical considerations supported by independent data. We describe a complete methodology of error analysis that, starting from the errors in the data, propagates them to the grid nodes. In turn, the interpolation error at the grid nodes is calculated using a novel procedure that also provides an estimate of the bias introduced by the interpolation process. Finally, both errors are combined at the grid nodes to produce a gridpoint-dependent error estimate, which is complemented by an overall error estimate providing an assessment of the quality of the DEM. This methodology is illustrated with the case study of Werenskioldbreen, a land-terminating polythermal glacier in Svalbard.

A short-term series of quantitative observations of calving activity of Fuerza Aérea Glacier (Greenwich Island, the South Shetland Islands, Antarctica) was conducted in order to test new methods of monitoring calving. The volume of single calving events was quantified by combining terrestrial laser scanning (TLS) surveys with continuous video recording of the ice front. An empirical formula for area/volume scaling of the calved ice block was proposed based on the TLS measured calved ice volume and the calved ice front area obtained by manual delineation on the images acquired with the video camera. This combination of methods proves to be a valuable tool for glacier monitoring, providing both high-temporal resolution and precise quantitative measurements of the calving volume. The size distribution of calving events is best approximated by a power law and within the short period of observations (14 d) calving was found to be an intrinsic process not dependent on environmental forcings. Over the period of 21 January–04 February 2013 the ice flow velocity at the terminus of Fuerza Aérea Glacier was 0.26 ± 0.07 m d−1 and the calving rate was 0.41 ± 0.07 m d−1.

Antarctic ice shelves are buttressed by numerous pinning points attaching to the otherwise freely-floating ice from below. Some of these kilometric-scale grounded features are unresolved in Antarctic-wide datasets of ice thickness and bathymetry, hampering ice flow models to fully capture dynamics at the grounding line and upstream. We investigate the role of an 8.7 km2 pinning point at the front of the Roi Baudouin Ice Shelf, East Antarctica. Using ERS interferometry and ALOS-PALSAR speckle tracking, we derive, on a 125 m grid spacing, surface velocities deviating by −5.2 ± 4.5 m a−1 from 37 on-site global navigation satellite systems-derived velocities. We find no evidence for ice flow changes on decadal time scales and we show that ice on the pinning point virtually stagnates, deviating the ice stream and causing enhanced horizontal shearing upstream. Using the BISICLES ice-flow model, we invert for basal friction and ice rigidity with three input scenarios of ice velocity and geometry. We show that inversion results are the most sensitive to the presence/absence of the pinning point in the bathymetry; surface velocities at the pinning point are of secondary importance. Undersampling of pinning points results in erroneous ice-shelf properties in models initialised by control methods. This may impact prognostic modelling for ice-sheet evolution in the case of unpinning.