It is likely that climate change will have a significant impact on the mass balance of the Greenland ice sheet, contributing to future sea-level rise. Here we present the implementation of the full Stokes model Elmer/Ice for the Greenland ice sheet, which includes a mesh refinement technique in order to resolve fast-flowing ice streams and outlet glaciers. We discuss simulations 100 years into the future, forced by scenarios defined by the SeaRISE (Sea-level Response to Ice Sheet Evolution) community effort. For comparison, the same experiments are also run with the shallow-ice model SICOPOLIS (SImulation COde for POLythermal Ice Sheets). We find that Elmer/Ice is ~43% more sensitive (exhibits a larger loss of ice-sheet volume relative to the control run) than SICOPOLIS for the ice-dynamic scenario (doubled basal sliding), but ~61 % less sensitive for the direct global warming scenario (based on the A1 B moderate-emission scenario for greenhouse gases). The scenario with combined A1B global warming and doubled basal sliding forcing produces a Greenland contribution to sea-level rise of ~15cm for Elmer/Ice and ~12cm for SICOPOLIS over the next 100 years.

Polythermal conditions are ubiquitous among glaciers, from small valley glaciers to ice sheets. Conventional temperature-based ‘cold-ice’ models of such ice masses cannot account for that portion of the internal energy which is latent heat of liquid water within temperate ice, so such schemes are not energy-conserving when temperate ice is present. Temperature and liquid water fraction are, however, functions of a single enthalpy variable: a small enthalpy change in cold ice is a change in temperature, while a small enthalpy change in temperate ice is a change in liquid water fraction. The unified enthalpy formulation described here models the mass and energy balance for the threedimensional ice fluid, for the surface runoff layer and for the subglacial hydrology layer, together in a single energy-conserving theoretical framework. It is implemented in the Parallel Ice Sheet Model. Results for the Greenland ice sheet are compared with those from a cold-ice scheme. This paper is intended to be an accessible foundation for enthalpy formulations in glaciology.

The major outlet glaciers that drain the eastern sector of the Amundsen Sea Embayment (Smith, Haynes, Thwaites and Pine Island) are among the largest, fastest-flowing and fastest-thinning glaciers in West Antarctica. Their recent ice-flow acceleration is linked to ocean-induced ice-shelf thinning, but may also arise from additional losses of ice-shelf buttressing that are not well understood. Here we present a comprehensive history of coastal change in the eastern Amundsen Sea Embayment between 1972 and 2011 derived mostly from Landsat imagery. The termini of all four major outlet glaciers have retreated, but retreat is most rapid along the ice-shelf margins, where progressive rifting has occurred. This pattern of retreat coincides with the recent acceleration of grounded ice and contributed to loss of ice-shelf buttressing. The observed pattern of margin-led gradual ice-shelf disintegration appears to be common in accelerating ocean-terminating outlet glaciers. We hypothesize that this pattern is part of a positive feedback between glacier acceleration and rift growth that could drive further buttressing loss in the eastern Amundsen Sea Embayment.

A 6 year application of an enhanced temperature-index mass-balance model to Careser and La Mare glaciers, Eastern Italian Alps, is presented. The two glaciers exhibit very different characteristics, and a comprehensive dataset of distributed mass-balance measurements was used to test the model performance. The model was run using meteorological data acquired outside the glaciers. The work was focused on two main aspects: (1) the development of a morphological redistribution procedure for snow, and (2) the comparison of three different melt algorithms proposed in the literature. The results show that the simple method proposed for snow redistribution can greatly improve simulation of winter balance, and further improvements would be achievable by collecting data on inaccessible and high-altitude areas. All three melt formulations displayed a good skill level and very similar results in modelling the mass-balance distribution over glacier areas, with slightly better results from a multiplicative algorithm in capturing the vertical balance gradient. The simulation errors are related to aspect and elevation, and tend to be spatially aggregated. Some assumptions concerning the spatial and temporal distribution of air temperature and incoming solar radiation, although reasonable and widely used in the literature, may be responsible for this aggregation. Hence, there is a need to further investigate the processes that regulate the distribution of melt energy, and that appear to control the current deglaciation phase in this area.

The nature and behaviour of sediment beneath glaciers influences how they flow and respond to changing environmental conditions. The difficulty of accessing the bed of current glaciers is a key constraint to studying the processes involved. This paper explores an alternative approach by relating sediments under the beds of former mid-latitude ice sheets to changing ice behaviour during a glacial cycle. The paper focuses on the partly marine-based Pleistocene British-Irish ice sheet in the Clyde basin, Scotland. A three-dimensional computation of subsurface glacial sediment distribution is derived from 1260 borehole logs. Sediment distribution is linked to an empirically based reconstruction of ice-sheet evolution, permitting identification of distinctive phases of sedimentation. Maximum sediment mobilization and till deposition (~0.04ma_1) occurred during ice advance into the basin from adjacent uplands. Transport distances were generally short. Subglacial processes were influenced locally by the relative stiffness of pre-existing sediments, the permeability of the sub-till lithology, and topography; the resulting mean till thickness is 7.7 m with a high standard deviation of 7.0 m. In places, focused till deposition sealed pre-existing permeable substrates, promoting lower effective pressures. Sediment remobilization by meltwater was a key process as ice margins retreated through the, basin.

By subtracting surface topographies from 1979, 1994, 2000 and 2008, we measured icethinning rates increasing from 1 ma-1 (1979-94) to >4 ma-1 (2000-08) on the tongue of Mer de Glace, French Alps. The relative contributions of changes in surface mass balance and ice fluxes to this acceleration in the thinning are estimated using field and remote-sensing measurements. Between 1979-94 and 2000-08, surface mass balance diminished by 1.2mw.e.a-1, mainly because of atmospheric warming. Mass-balance changes induced by the growing debris-covered area and the evolving glacier hypsometry compensated each other. Meanwhile, Mer de Glace slowed down and the ice fluxes through two cross sections at 2200 and 2050ma.s.l. decreased by 60%. Between 1979-94 and 2000-08, two-thirds of the increase in the thinning rates was caused by reduced ice fluxes and one- third by rising surface ablation. However, these numbers need to be interpreted cautiously given our inability to respect mass conservation below our upper cross section. An important implication is that large errors would occur if one term of the continuity equation (e.g. surface mass balance) were deduced from the two others (e.g. elevation and ice-flux changes).

The tongue of Triftgletscher, Switzerland, is particularly susceptible to major break-off events due to its steep slope. In order to detect precursors of such an event, we monitored the local seismic activity and detected 2426 icequakes with sources located in an area ranging between 2050 and 2350ma.s.l. Events triggered by cracks and icefalls were recorded, but no precise distinction between the two sources was possible using duration or frequency criteria. Clusters of seismic activity were located and confirmed by visual observations. We performed a surface motion analysis and found that surface motion was driven significantly by runoff changes at a timescale of 2-3 days. By means of a statistical analysis, a power-law behaviour of the released seismic energy distribution was discerned at certain times during the period investigated. Variations in power-law exponent values indicated that low- and high-energy events predominantly occurred during phases of enhanced and reduced surface motion, respectively. Substantial releases of seismic energy likely to signal the glacier recoupling were detected during phases of decreasing runoff. Clues to potential seismic precursors of break-off events are discussed.

During the Japanese-Swedish Antarctic traverse expedition of 2007/08, we measured the surface snow density at 46 locations along the 2800 km long route from Syowa station to Wasa station in East Antarctica. The mean snow density for the upper 1 (or 0.5) m layer varied from 333 to 439 kg m-3 over a region spanning an elevation range of 365-3800 ma.s.l. The density variations were associated with the elevation of the sampling sites; the density decreased as the elevation increased, moving from the coastal region inland. However, the density was relatively insensitive to the change in elevation along the ridge on the Antarctic plateau between Dome F and Kohnen stations. Because surface wind is weak in this region, irrespective of elevation, the wind speed was suggested to play a key role in the near-surface densification. The results of multiple regression performed on the density using meteorological variables were significantly improved by the inclusion of wind speed as a predictor. The regression analysis yielded a linear dependence between the density and the wind speed, with a coefficient of 13.5 kg m-3 (m s-1)-1. This relationship is nearly three times stronger than a value previously computed from a dataset available in Antarctica. Our data indicate that the wind speed is more important to estimates of the surface snow density in Antarctica than has been previously assumed.

A possible cause for accelerated thinning and break-up of floating marine ice shelves is warming of the water in the cavity below the ice shelf. Accurate bathymetry beneath large ice shelves is crucial for developing models of the ocean circulation in the sub-ice cavities. A grid of free-air gravity data over the floating Larsen C ice shelf collected during the IceBridge 2009 Antarctic campaign was utilized to develop the first bathymetry model of the underlying continental shelf. Independent control on the continental shelf geologic structures from marine surveys was used to constrain the inversion. Depths on the continental shelf beneath the ice shelf estimated from the inversion generally range from about 350 to 650 m, but vary from <300 to >1000 m. Localized overdeepenings, 20-30 km long and 900-1000 m deep, are located in inlets just seaward of the grounding line. Submarine valleys extending seaward from the overdeepenings coalesce into two broad troughs that extend to the seaward limit of the ice shelf and appear to extend to the edge of the continental shelf. The troughs are generally at a depth of 550-700 m although the southernmost mapped trough deepens to over 1000 m near the edge of the ice shelf just south of 68° S. The combination of the newly determined bathymetry with published ice-draft determinations based on laser altimetry and radar data defines the geometry of the water-filled cavity. These newly imaged troughs provide a conduit for water to traverse the continental shelf and interact with the overlying Larsen C ice shelf and the grounding lines of the outlet glaciers.

Snow slab avalanches primarily release by propagation of shear fractures within thin weak layers under thicker slabs. The weak layer is typically on the order of 1 mm thick and fails in mode II. In some cases, the weak layer is thicker and there may be a need to consider slope-normal deformation as part of the energy condition prior to rapid propagation. In this paper, field measurements from shear fracture initiation and high-speed films are combined to consider the effects of slope-normal deformation on bending of the slab prior to propagation and its relation to the propagation condition. Slab bending is modelled using two limiting cases: (1) a uniformly loaded beam supported by a deforming weak layer, analogous to a Winkler foundation, and (2) a uniformly loaded unsupported cantilevered beam. The experimental and analytical results suggest that slab bending prior to fracture initiation is small or negligible. Two previous approaches to modelling slab avalanche initiation involving slab bending are discussed. Both models proposed strong slab-bending effects prior to initiation, which conflicts with our results. Our field observations and modelling both show that strong bending is a dynamic effect following slope-parallel weak-layer fracture initiation.

We have adapted inexpensive ultrasonic rangefinders to measure ablation rates on the surface of a glacier. While ultrasonic rangers are commercially available for this purpose, our goal was to utilize rangefinders typically used in hobby robotics without significantly compromising performance. To correct for environmental factors that affect the speed of sound we use two ultrasonic rangefinders, one focused on a fixed target. Measurements of ablation correlate well with manual measurements with an uncertainty of about ±3 cm, suggesting an accuracy comparable with other non-manual methods of recording ablation. The limitations of our rangefinder include those inherent in commercially available units as well as having less acoustical power, which results in a reduced effective range of the sensor (~2 m) and difficulties in detecting surfaces lying below low-density snow. Our sensor design provides a cost-effective means of increasing the spatial coverage of ice ablation measurements.

Despite global warming and unlike their Himalayan neighbours, glaciers in the Karakoram mountains do not show signs of significant retreat. Here we report high velocity variations of Biafo Glacier, central Karakoram, which occurred between 2001 and 2009 and which indicate considerable dynamics in its flow behaviour. We have generated a dense time series of glacier surface velocities, based on cross-correlation of optical satellite images, which clearly shows seasonal and interannual velocity variations, reaching 50% in some places. The interannual velocity variations resemble the passing of a broad wave of high velocities, with peak velocities during 2005 and some diffusion down-glacier over a period of at least 4 years. High interannual velocity variations are also observed at other glaciers in the vicinity, suggesting a common cause, although these appear to partly comprise longer acceleration phases. Analysis of weather station data provides some indications of meteorological conditions that could have promoted sustained sliding events during this period, but this does not explain the wave-like nature of the acceleration at Biafo Glacier, and the regular, protracted velocity changes.

In this paper we document changes of Langfjordjøkelen, a small ice cap in northern Norway. Surface mass-balance measurements have been carried out on an east-facing part (3.2 km2) of the ice cap since 1989. Measurements reveal a strong thinning; the balance year 2008/09 was the 13th successive year with significant negative annual balance (≤-0.30 m w.e.). The average annual deficit was 0.9m w.e. over 1989-2009. The recent thinning of Langfjordjøkelen is stronger than observed for any other glacier in mainland Norway. Maps from 1966, 1994 and 2008 show that the whole ice cap is shrinking. The total volume loss over 1966-2008 was 0.264 km3. The east-facing part has been greatly reduced in volume (46%), area (38%) and length (20%). For this part over 1994-2008, the cumulative direct mass balance (-14.5 m w.e.) is less negative than the geodetic mass balance (-17.7 m w.e.). A surface mass-balance model using upper-air meteorological data was used to reconstruct annual balances back to 1948 and to reconstruct unmeasured years 1994 and 1995. Sensitivity of annual balance to 1°C warming is -0.76 m w.e. and to 10% increase in precipitation is +0.20 m w.e.

Under turbulent conditions ice growth in sea water often occurs as tiny suspended frazil ice crystals. When the turbulence is insufficient to keep the crystals in suspension, they may accumulate in a surface grease layer of pure ice and sea water. Here we give an account of this grease of low solid fraction and high salinity prior to its freeze-up into a solid ice cover. We provide equations for determining the bulk salinity, Sg, and solid ice volume fraction, ϕs, of the grease layer by indirect and direct methods, review previous observations, and present new data. For the evolution of the solid fraction of accumulating grease ice we find (I) an early mode 0.08 < ϕs < 0.12 and (II) a long-term packing limit of 0.28 < ϕs < 0.31, which for sea water of salinity 34 corresponds to 29.5 < Sg <31.5 and 24 < Sg < 26, respectively. We associate (I) with a mechanical limit related to initial random packing of frazil crystals and (II) with a thermodynamic limit, beyond which the grease will freeze-up into a solid ice cover. By comparing the results with a simple model of random close packing of anisotropic particles, we find that the results are consistent with frazil flocs having an aspect ratio of ˜10, much smaller than values assumed in most model applications that include sea-water frazil ice processes.

Radar data (center frequency 150 MHz) collected on the Antarctic plateau near the EPICA deep-drilling site in Dronning Maud Land vary systematically in backscattered power, depending on the azimuth antenna orientation. Backscatter extrema are aligned with the principal directions of surface strain rates and change with depth. In the upper 900m, backscatter is strongest when the antenna polarization is aligned in the direction of maximal compression, while below 900 m the maxima shift by 90° pointing towards the lateral flow dilatation. We investigate the backscatter from elongated air bubbles and a vertically varying crystal-orientation fabric (COF) using different scattering models in combination with ice-core data. We hypothesize that short-scale variations in COF are the primary mechanism for the observed anisotropy, and the 900 m boundary between the two regimes is caused by ice with varying impurity content. Observations of this kind allow the deduction of COF variations with depth and are potentially also suited to map the transition between Holocene and glacial ice.