Many marine-terminating glaciers draining the Greenland ice sheet have retreated over the past decade, yet the extent and magnitude of retreat relative to past variability is unknown. We measure changes in front positions of 210 marine-terminating glaciers using Landsat imagery spanning nearly four decades and compare decadal-scale rates of change with earlier observations. We find that 90% of the observed glaciers retreated between 2000 and 2010, approaching 100% in the northwest, with rapid retreat observed in all sectors of the ice sheet. The current retreat is accelerating and likely began between 1992 and 2000, coincident with the onset of warming, following glacier stability and minor advance during a mid-century cooling period. While it is clear an extensive retreat occurred in the early 20th century, a period of increasing air temperatures, a comparison of our results with historical observations provides evidence that the current retreat is more widespread. The onset of rapid retreat with warming relative to the slow and lagged advance with cooling suggests an asymmetry in the response of marine fronts to external forcing.

We use optical (ASTER and Landsat) and radar (ERS-1 and ERS-2) satellite imagery to document changes in the Prince Gustav Ice Shelf, Antarctic Peninsula, and its tributary glaciers before and after its January 1995 collapse. The satellite image record captures the transition from an ice-shelf glacier system to a tidewater glacial system and the subsequent rapid retreat and inferred ‘fatal’ negative mass balances that occur as lower glacier elevations lead to higher ablation and tidewater-style calving collapse. Pre-1995 images show that the central ice shelf was fed primarily by Sjögren Glacier flowing from the Antarctic Peninsula and by Röhss Glacier flowing from James Ross Island. Numerous structural discontinuities (rifts and crevasses) and melt ponds were present on the ice shelf before the collapse. After the ice shelf collapsed, Röhss Glacier retreated rapidly, becoming a tidewater glacier in 2002 and receding a total of ∼15 km between January 2001 and March 2009, losing >70% of its area. Topographic profiles of Röhss Glacier from ASTER-derived digital elevation models show a thinning of up to ∼150 m, and surface speeds increased up to ninefold (0.1–0.9 m d−1) over the same period. The rates of speed increase and elevation loss, however, are not monotonic; both rates slowed between late 2002 and 2005, accelerated in 2006 and slowed again in 2008–09. We conclude that tributary glaciers react to ice-shelf removal by rapid (if discontinuous) recession, and that the response of tidewater glaciers on the Antarctic Peninsula to ice-shelf removal occurs over timescales ranging from sub-annual to decadal.

We present a study of the mechanical (in)stability of the ephemeral waterfall ice structures that form from the freezing of liquid water seeping on steep rock. Three vertical structures were studied, two near Glacier d’Argentière, France, and one in the Valsavarenche valley, northern Italy. The generation of internal stresses in the ice structure in relation to air- and ice-temperature conditions is analyzed from pressure sensor records. Their role in the mechanical instability of the structures is discussed from a photographic survey of these structures. The main result is that dramatic air cooling (several °Ch−1 over several hours) and low temperatures (<−10°C), generating tensile stresses and brittleness, can trigger a spontaneous or climber-induced mechanical collapse, leading to unfavorable climbing conditions. Ice internal pressure fluctuations are also associated with episodes of marked diurnal air-temperature cycle, with mild days (few above 0) and cool nights (few below 0), through the occurrence of water ↔ ice phase transitions within the structure. These ice internal stress fluctuations seem, however, to have a local influence, are associated with warm (near 0), wet and therefore particularly soft ice and do not trigger a collapse of the structure.

Repeated measurements of density profiles and surface elevation along a 515 km section of the Greenland ice sheet have been used to determine elevation change rates and the error in determining mass balance from these rates which arises from short-term fluctuations in mass input, compaction and surface density. Over the 28 months from spring 2004 to summer 2006 the average error over 100 km sections of the traverse ranged from −0.006 to 0.100 ma−1. The lowest values, comparable with the system accuracy of the CryoSat radar altimeter (0.033 m a−1), were found below 3000 m. The surface density required to translate the elevation change into mass change decreased from 0.40 g cm−3 at an elevation of 2348 m to 0.33 g cm−3 at an elevation of 3264 m. From the density profiles the equivalent values for a time period of 10 years were found to be 0.48 and 0.38 g cm−3, respectively.

A mass-balance model using upper-air meteorological data for input was calibrated with surface mass balance measured mainly during 1977–78 at 67 sites on Columbia Glacier, Alaska, between 135 and 2645 m a.s.l. Root-mean-square error, model vs measured, is 1.0 m w.e. a−1, with r2 = 0.88. A remarkable result of the analysis was that both precipitation and the factor in the positive degree-day model used to estimate surface ablation were constant with altitude. The model was applied to reconstruct glacier-wide components of surface mass balance over 1948–2007. Surface ablation, 4 km3 ice eq. a−1 (ice equivalent), has changed little throughout the period. From 1948 until about 1981, when drastic retreat began, the surface mass balance was positive but changes in glacier geometry were small, so the positive balance was offset by calving, ∼0.9 km3 ice eq. a−1 . During retreat, volume loss of the glacier accounted for 92% of the iceberg production. Calving increased to ∼4.3 km3 ice eq. a−1 from 1982 to 1995, and after that until 2007 to ∼8.0 km3 ice eq. a−1, which was about twice the loss by surface ablation, whereas prior to retreat it was only about a quarter as much. Calving is calculated as the difference between glacier-wide surface mass balance and geodetically determined volume change.

Buried surface hoar and near-surface faceted crystals are known to lead to deadly avalanches. Over the course of three winter seasons a field investigation detailing the environmental conditions leading to the formation of these crystals was performed. Weather stations on north- and south-facing aspects were established. The weather data were accompanied by detailed daily observations and grain-scale photographs of the snow surface. During the three seasons, 35 surface hoar and 47 near-surface facets events were recorded. The mean weather conditions for the entire dataset (all three seasons and both stations) were compared to the nights when surface hoar formed. The comparison yielded five parameters that were statistically linked to the formation of surface hoar: incoming longwave radiation, snow surface temperature, wind velocity, relative humidity and the air/snow temperature difference. A similar comparison between the daytime mean values for all days with near-surface facet events revealed three parameters with statistically significant differences. Thus, these parameters (short- and longwave radiation and relative humidity) could be statistically linked to facet formation. This research also suggests that environmental conditions in the daytime hours before and after surface hoar formation are statistically similar to the conditions causing near-surface facet formation.

A hanging glacier at the east face of Weisshorn, Switzerland, broke off in 2005. We were able to monitor and measure surface motion and icequake activity for 25 days up to 3 days prior to the break-off. The analysis of seismic waves generated by the glacier during the rupture maturation process revealed four types of precursory signals of the imminent catastrophic rupture: (1) an increase in seismic activity within the glacier; (2) a change in the size–frequency distribution of icequake energy; (3) a modification in the structure of the waiting-time distributions between two successive icequakes; and (4) a correlation between the seismic activity and the log-periodic oscillations of the surface velocities superimposed on the global acceleration of the glacier during the rupture maturation. Analysis of the seismic activity led us to identify two regimes: a stable phase with diffuse damage and an unstable and dangerous phase characterized by a hierarchical cascade of rupture instabilities where large icequakes are triggered.

There are many advantages to formulating an ice-sheet model in terms of a variational principle. In particular, this applies to the specification of boundary conditions, which might otherwise be problematic to implement. Here we focus primarily on the frictional basal sliding boundary condition in a non-Newtonian Stokes model. This type of boundary condition is particularly difficult because it is heterogeneous, requiring both a Dirichlet (no-penetration) condition normal to the bed, and a Neumann (frictional sliding) condition tangential to the bed. In general, Neumann conditions correspond to natural boundary conditions in a variational principle; that is, they arise naturally in the variational formulation and thus need not be explicitly specified. While the same is not necessarily true of Dirichlet conditions, it is possible to enforce a no-penetration condition using Lagrange multipliers within the variational principle so that the Dirichlet condition becomes a natural boundary condition. Thus, in the case of ice sheets, all relevant boundary conditions may be incorporated in the variational functional, making them particularly easy to discretize. For the Stokes model, the resulting basal boundary condition is valid for arbitrary topographic slopes. Here we apply the same methodology to the Blatter– Pattyn higher-order approximate model, which is ordinarily limited to small basal slopes by the smallaspect-ratio approximation. We introduce a modification that improves on the accuracy of the standard Blatter–Pattyn model for all values of the basal slope, as we demonstrate in the slow sliding regime for which analytical results are available. The remaining error is due to the effects of the small-aspect-ratio approximation in the Blatter–Pattyn model.

Rain-on-snow events trigger immediate and delayed avalanches as liquid water penetrates the snowpack. We present results from an extreme rain-on-snow event that triggered a glide avalanche near Snoqualmie Pass, Washington, USA. Snoqualmie Pass recorded 463 cm of snowfall from 13 December 2008 to 6 January 2009. This period of snowfall was followed by a strong southwesterly tropical flow that resulted in an extreme rain-on-snow event. Sensors at Snoqualmie Pass recorded 285 mm of precipitation over a 52 hour period. Flooding, slush flows, landslides and avalanches resulted from the influx of precipitation. Snow heights decreased rapidly over the period, with settlement rates approaching 80 mm h−1. Liquid water infiltrated and flowed through the snowpack within a few hours of the arrival of rain, yet many of the major avalanches occurred 12–30 or more hours after the onset of rain and water outflow. A glide avalanche occurred ∼30 hours after the onset of rain and the establishment of drainage through the snowpack. Increasing glide rates correlate with periods of rapid snow settlement. Here glide rates approached 670 mm h−1. Although glide and settlement rates increased during periods of intense precipitation, glide failure occurred 8 hours after peak precipitation and outflow.

We report on satellite and ground-based observations that link glacier motion with subglacial hydrology beneath Skeiðarárjökull, an outlet glacier of Vatnajökull, Iceland. We have developed a technique that uses interferometric synthetic aperture radar (InSAR) data, from the European Remote-sensing Satellite (ERS-1/-2) tandem mission (1995–2000), to detect localized anomalies in vertical ice motion. Applying this technique we identify an area of the glacier where these anomalies are frequent: above the subglacial course of the river Skeiðará, where we observed uplift of 0.15–0.20 m d−1 during a rainstorm and a jökulhlaup, and subsidence at a slower rate subsequent to rainstorms. A similar pattern of motion is apparent from continuous GPS measurements obtained at this location in 2006/07. We argue that transient uplift of the ice surface is caused by water accumulating at the glacier base upstream of an adverse bed slope where the overburden pressure decreases significantly over a short distance. Most of the frictional energy of the flowing water is therefore needed to maintain water temperature at the pressure-melting point. Hence, little energy is available to enlarge water channels sufficiently by melting to accommodate sudden influxes of water to the base. This causes water pressure to exceed the overburden pressure, enabling uplift to occur.

Ice-shelf thickness is an important boundary condition for ice-sheet and sub-ice-shelf cavity modelling. It is required near the grounding line to calculate the ice flux used to determine ice-sheet mass balance by comparison with the upstream accumulation. In this mass budget approach, the accuracy of the ice thickness is one of the limiting factors in the calculation. We present a satellite retrieval of the ice thickness for all Antarctic ice shelves using satellite radar altimeter data from the geodetic phases of the European Remote-sensing Satellite (ERS-1) during 1994–95 supplemented by ICESat data for regions south of the ERS-1 latitudinal limit. Surface elevations derived from these instruments are interpolated on to regular grids using kriging, and converted to ice thicknesses using a modelled firn-density correction. The availability of a new spatial variable firn-density correction significantly reduces the error in ice thickness as this was previously the dominant error source. Comparison to airborne data shows good agreement, particularly when compared to SOAR CASERTZ data on the largest ice shelves. Biases range from −13.0 m for areas where the assumption of hydrostatic equilibrium breaks down, to 53.4 m in regions where marine ice may be present.

We investigated the morphological evolution of laboratory snow under isothermal conditions at −3,−9 and −19°C, using X-ray tomography. We employed a two-point density correlation function to measure spatial fluctuations of the density of the bicontinuous ice/vapor system at different length scales. Length scales were derived from the correlation function to distinguish between interfacial coarsening due to the minimization of surface energy on the smallest scales and anisotropic structural rearrangements due to gravity on larger scales. On the smallest scales our data suggest a crossover between T = −9 and −19°C from evaporation/condensation to surface diffusion as the dominant transport mechanism. Anomalous growth was found for the slope of the correlation function at the origin, and it was similar to those reported for the coarsening of fractal clusters. This is consistent with the observed persistence of dendritic structures throughout an entire year. The dynamics of large-scale morphology was characterized by the first zero-crossing of the correlation function which displays a nonmonotonic evolution with a pronounced anisotropy between the direction of gravity and horizontal directions. Since the correlation function naturally emerges in problems of scattering of radiation in snow, our results appear to be important for optical and remote-sensing methods.

Dust particles in an ice core from East Rongbuk Glacier on the northern slope of Qomolangma (Mount Everest; 28°01′ N, 86°58′ E; 6518 m a.s.l.), central Himalaya, have been identified as mica using a combination of scanning electron microscope-based techniques and energy-dispersive X-ray spectroscopy to identify the elements present, and electron backscatter diffraction to identify the crystal type. This technique for identifying individual crystalline dust particles in samples of glacial ice could be especially useful in the future for identifying water-soluble crystals in ice, for studying the strain history (glaciotectonics) of basal ice or in studies of ice–mica composites used as analogs of quartz-mica rocks.

Recent surface elevation changes of Kaskawulsh Glacier, Yukon, Canada, are quantified by comparing an air-photo derived DEM from 1977 and airborne lidar measurements from 1995, 2000 and 2007. Surface-area changes are assessed using historical aerial photography from 1956 and satellite imagery from 1977 to 2007. Combined, these measurements provide some of the first detailed records of volume change of a large Yukon glacier. Between 1977 and 2007, Kaskawulsh Glacier underwent a decrease in area of 1.53% and a decrease in volume of 3.27–5.94 km3 w.e.). The terminus also retreated by 655 m over the period 1956–2007. There was relatively minor volume change over the period 1977–95 (<+0.01 km3 w.e.a−1), while over the periods 1995–2000 and 2000–07 volume losses occurred at a relatively constant rate of −0.51 and −0.50 km3 a−1 w.e., respectively. Since 1995, thinning has been prominent throughout the ablation zone, while relative stability and even slight thickening has occurred in the accumulation zone. These findings are similar to those recently observed at other nearby Alaskan glaciers.

SnowDrift3D, a high-resolution, atmospheric snow-transport model, is presented for the first time. In contrast to most state-of-the-art snowdrift models, atmospheric particle transport, i.e. saltation and suspension, is accounted for by one passive transport equation. The model uses unsteady wind fields (spatial resolution of up to 2 m) computed with an atmospheric computational fluid dynamics model that is directly connected to the numerical weather prediction model ALADIN. Sensitivity runs show that (1) the saltation mass flux is a function of cubic shear velocity, , (2) the model is marginally sensitive to the grid spacing at high resolutions (up to 2 m), (3) the model computes the redistribution of snow at high resolution in real time on dual core personal computers and (4) the changing topography of the snow cover should be included in cases of local erosion or deposition of a large amount of snow. Finally, we present a comparison of modeled and measured snow distributions obtained by terrestrial laser scanning showing area-wide linear correlation up to R = 0.33.

Glacier outlines are mapped for the upper Bhagirathi and Saraswati/Alaknanda basins of the Garhwal Himalaya using Corona and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite images acquired in 1968 and 2006, respectively. A subset of glaciers was also mapped using Landsat TM images acquired in 1990. Glacier area decreased from 599.9 ± 15.6 km2 (1968) to 572.5 ± 18.0 km2 (2006), a loss of 4.6 ± 2.8%. Glaciers in the Saraswati/Alaknanda basin and upper Bhagirathi basin lost 18.4 ± 9.0 km2 (5.7 ± 2.7%) and 9.0 ± 7.7 km2 (3.3 ± 2.8%), respectively, from 1968 to 2006. Garhwal Himalayan glacier retreat rates are lower than previously reported. More recently (1990–2006), recession rates have increased. The number of glaciers in the study region increased from 82 in 1968 to 88 in 2006 due to fragmentation of glaciers. Smaller glaciers (<1 km2) lost 19.4 ± 2.5% (0.51 ± 0.07% a−1) of their ice, significantly more than for larger glaciers (>50 km2) which lost 2.8 ± 2.7% (0.074 ± 0.071 % a−1). From 1968 to 2006, the debris-covered glacier area increased by 17.8 ± 3.1% (0.46 ± 0.08% a−1) in the Saraswati/Alaknanda basin and 11.8 ± 3.0% (0.31 ± 0.08% a−1) in the upper Bhagirathi basin. Climate records from Mukhim (∼1900 m a.s.l.) and Bhojbasa (∼3780 m a.s.l.) meteorological stations were used to analyze climate conditions and trends, but the data are too limited to make firm conclusions regarding glacier–climate interactions.

Snow slab avalanches primarily release by propagation of shear fractures within thin weak layers under much thicker slabs. In some cases, the weak layer is on the order of 1 mm thick and such fractures may be considered to be a mode II shear fracture at initiation. In the cases analysed in this paper, the weak layer has finite thickness, and slope-normal effects may be present. Field data from >500 snow shear fracture tests are analysed and applied to the problem of weak-layer fracture. The paper contains a detailed analysis using a simple analytical model to estimate the critical length prior to an unstable shear fracture. The model contains the assumption of a finite fracture process zone which may be a significant fraction of the slab depth, D, or the critical length, L, for weak-layer shear fracture. The field results show that the L/D ratio varies from ∼0.1 to just over 2, and the model results are close to the same range. The analysis also shows that both the field and model results for L/D follow a Gumbel probability density function. Since the experimental field data contain rate-dependent (viscoelastic) and slope-normal effects, it is imperative to account for these in the model and for snow slab instability evaluation when using test data. Detailed evaluations considering both these effects are given. The applicability of the test data to avalanche release is discussed.

Glaciers respond to long-term climate changes and also to the year-to-year fluctuations inherent in a constant climate. Differentiating between these factors is critical for the correct interpretation of past glacier fluctuations and for the correct attribution of current changes. Previous work has established that century-scale, kilometre-scale fluctuations can occur in a constant climate. This study asks two further questions of practical significance: how likely is an excursion of a given magnitude in a given amount of time, and how large a trend in length is statistically significant? A linear model permits analytical answers wherein the dependencies on glacier geometry and climate setting can be clearly understood. The expressions are validated with a flowline glacier model. The likelihood of glacier excursions is well characterized by extreme-value statistics, although probabilities are acutely sensitive to some poorly known glacier properties. Conventional statistical tests can be used for establishing the significance of an observed glacier trend. However, it is important to determine the independent information in the observations which can be effectively estimated from the glacier geometry. Finally, the retreat of glaciers around Mount Baker, Washington State, USA, is consistent with, but not independent proof of, the regional climate warming that is established from the instrumental record.