At Taylor Glacier, a cold-based outlet glacier of the East Antarctic ice sheet, observed surface speeds in the terminus region are 20 times greater than those predicted using Glen’s flow law for cold (–17°C), thin (100 m) ice. Rheological properties of the clean meteoric glacier ice and the underlying deformable debris-rich basal ice can be inferred from surface-velocity and ablation-rate profiles using inverse theory. Here, with limited data, we use a two-layer flowband model to examine two end-member assumptions about the basal-ice properties: (1) uniform softness with spatially variable thickness and (2) uniform thickness with spatially variable softness. We find that the basal ice contributes 85–98% to the observed surface velocity in the terminus region. We also find that the basal-ice layer must be 10–15 m thick and 20–40 times softer than clean Holocene-age glacier ice in order to match the observations. Because significant deformation occurs in the basal ice, our inverse problem is not sensitive to variations in the softness of the meteoric ice. Our results suggest that despite low temperatures, highly deformable basal ice may dominate flow of cold-based glaciers and rheologically distinct layers should be incorporated in models of polar-glacier flow.

Ice, Cloud and land Elevation Satellite (ICESat) repeat-track laser altimetry has identified 17 sites within the Byrd Glacier catchment, East Antarctica, where rapid ice-surface height changes have occurred, which have been interpreted as evidence for ‘active’ subglacial lakes. Here we present evidence from a new radio-echo sounding (RES) survey at 11 of these locations to understand the bed conditions associated with the proposed hydrological activity. At none of the sites examined did we find evidence in support of substantial pooled basal water. In the majority of cases, along-track RES bed reflection amplitudes either side of the locations of surface height change are indistinguishable from those within the features. These results indicate that, in most cases, hypothesized ‘active’ lakes are not discrete radar targets and are therefore much smaller than the areas of surface height change. In addition, we have identified three new relatively large subglacial lakes upstream of the region where most ‘active’ subglacial lakes are found, in an area where the hydraulic gradient is significantly lower. Our results suggest that substantial and long-lasting basal water storage in the Byrd Glacier catchment occurs only under low hydraulic gradients, while coast-proximal sites of hydraulic activity likely involve small or temporary accumulations of basal water.

We use a 3-D higher-order glacier flow model for Vadret da Morteratsch, Engadin, Switzerland, to simulate its strong retreat since the end of the Little Ice Age (LIA) and to project its future disintegration under a warming climate. The flow model, coupled to a 2-D energy-balance model, is initialized with the known maximum glacier extent during the LIA and subsequently forced with mean monthly precipitation and temperature records. To correctly reproduce the observed retreat of the glacier front for the period 1864–2010, additional mass-balance perturbations are required to account for uncertainties in the initial state, the mass-balance model and climate variations not captured by the ambient meteorological records. Changes in glacier volume and area are in good agreement with additional information from historical topographic maps. Under constant 2001–10 climate conditions, a strong retreat and mass loss continue and Vadret da Morteratsch disconnects from its main tributary, Vadret Pers, before 2020. The future glacier evolution is analysed in detail to understand the timing and rate of retreat, and to assess the role of ice dynamics. Assuming a linearly increasing warming of >3°C by 2100, only isolated and largely stagnant ice patches remain at high elevation.

Penitents are broad snow spikes and ridges that range in height between centimeters and meters. Two key features of penitents remain unexplained: (1) they generally form at low latitudes and (2) their ridges and troughs have an east–west orientation. Here we show that surface-to-surface exchange of shortwave radiation and the local geometry of the sun’s daily arc across the sky are the key processes that, in the absence of other effects, determine the geographic extent of where surface roughness features can grow or dissipate on snowfields and ice surfaces. As an application of our analysis, we examine the question of the geographic extent of snow penitents on horizontal surfaces. The results show that surface-to-surface radiative transfer can explain why penitents do not form on horizontal surfaces more than ~55˚ off the equator. We further show on the basis of the sun’s path across the sky that penitents that are corrugated, i.e. occur in long ridges or rows, must align their ridge axis within 308 of the east–west transect.

A pulse is a type of unstable glacier flow intermediate between normal flow and surging. Using Landsat MSS, TM and ETM+ imagery and feature-tracking software, a time series of mostly annual velocity maps from 1973 to 2012 was produced that reveals five pulses of Ruth Glacier, Alaska. Peaks in ice velocity were found in 1981, 1989, 1997, 2003 and 2010, approximately every 7 years. During these peak years the ice velocity increased 300%, from approximately 40 m a–1 to 160 m a–1. Based on the spatio-temporal behavior of Ruth Glacier during the pulse cycles, we suggest the pulses are due to enhanced basal motion via deformation of a subglacial till. The cyclical nature of the pulses is interpreted to be due to a thin till, with low permeability, that causes incomplete drainage of the till between the pulses, followed by eventual recharge and dilation of the till. These findings suggest care is needed when attempting to correlate changes in regional climate with decadal-scale changes in velocity, because in some instances basal conditions may have a greater influence on ice dynamics than climate.

We have used tide-gauge data from near Helheim Glacier, East Greenland, and GPS data acquired on the glacier to investigate the spectra of tidal forcing and flow response. For both the tide-gauge and GPS time series, we calculated amplitudes and phases for a harmonic expansion using a limited set of harmonic constituents. We find that the semidiurnal constituents of the glacier flow are well modeled using a single admittance and lag with respect to the tide-gauge data. However, diurnal variations in the glacier flow cannot be simply described using this model. We find an additional signal in glacier position, in phase with the S1 solar tide, with some modulations at other frequencies. These non-tidal variations account for a peak-to-peak variation in glacier flow speed at a site close to the terminus of ~0.7 m d-1, compared with a mean flow rate at this location of ~22 m d-1. The speed variations reach their daily maximum value ~6 hours after local noon. We hypothesize that these additional diurnal variations are associated with peaks in lubrication of the glacier bed due to surface melting driven by diurnal solar heating.

Substantial ice loss has occurred in the Russian High Arctic during the past decade, predominantly on Novaya Zemlya, yet the region has been studied relatively little. Consequently, the factors forcing mass loss and the relative contribution of ice dynamics versus surface melt are poorly understood. Here we evaluate the influence of atmospheric/oceanic forcing and variations in fjord width on the behaviour of 38 glaciers on the northern ice cap, Novaya Zemlya. We compare retreat rates on land- versus marine-terminating outlets and on the Kara versus Barents Sea coasts. Between 1992 and 2010, 90% of the study glaciers retreated and retreat rates were an order of magnitude higher for marine-terminating outlets (52.1 ma-1) than for land-terminating glaciers (4.8ma-1). We identify a post-2000 acceleration in marine-terminating glacier retreat, which corresponded closely to changes in sea-ice concentrations. Retreat rates were higher on the Barents Sea coast, which we partly attribute to lower sea-ice concentrations, but varied dramatically between individual glaciers. We use empirical data to categorize changes in along-flow fjord width, and demonstrate a significant relationship between fjord width variability and retreat rate. Results suggest that variations in fjord width exert a major influence on glacier retreat.

Recovery Ice Stream has multiple branches reaching far into the East Antarctic ice sheet. We use new airborne and ground-based geophysics to give the first comprehensive overview of the upper catchment and, by constraining the physical setting, to advance our understanding of the controlling mechanisms for the onset of fast flow. The 400 km wide ice stream extends towards the Recovery Subglacial Lakes, a region characterized by a crustal boundary, a change in bed roughness, a bedrock topographic step and four topographic basins (A–D), three of which (A–C) contain subglacial water. All these characteristics are considered potential causal mechanisms that contribute to the onset of fast flow. In Lakes B and C the subglacial water is located in basins with sharp downstream ridges, in contrast to the gently sloping ridge on the downstream margin of Lake A. The fastest-flowing branch of the ice stream emanates from Lake A. The presence of multiple causal mechanisms along the four Recovery Lakes allows us to identify basal water as a dominant factor for the onset of fast flow, but only if it is stored in a shallow-sided basin where it can lubricate the flow downstream. Relatively minor topographic barriers appear to inhibit streaming.

Marine ice is an important component of ice shelves in Antarctica. It accretes in substantial amounts at weak points and below ice shelves. It is likely to exhibit peculiar rheological properties, which are crucial to understanding its potential role in stabilizing ice-shelf flow. Due to its location and consolidation processes, marine ice can present a variety of textures which are likely to influence its rheological properties. We present a new dataset of unconfined uniaxial compression experiments on folded marine ice samples that have been cut at various angles to the folds. Texture and fabric analyses are described ‘before’ and ‘after’ the deformation experiment. It is shown that, in the given stress configuration, the geometry of the anisotropy controls the rheological behaviour of the marine ice. During secondary creep, folded marine ice is harder to deform than weakly textured ice when compressed parallel or perpendicular to the folds' hinges, while the reverse is true for ice compressed at 45°. The observed range of values for the n exponent in Glen's flow law is between 2.1 and 4.1. Surprisingly, we see that tertiary creep tends to develop at a higher total strain than for randomly oriented impurity-free meteoric ice.

Making confident statements about the evolution of an ice-sheet–shelf system with a numerical model requires the capability to reproduce the migration of the grounding line. Here we show that the shallow-ice approximation/shallow-shelf approximation hybrid-type Parallel Ice Sheet Model (PISM), with its recent improvements, is capable of modeling the grounding line motion in a perturbed ice-sheet–shelf system. The model is set up according to the three-dimensional Marine Ice-Sheet Model Intercomparison Project (MISMIP3d), and simulations are carried out across a broad range of spatial resolutions. Using (1) a linear interpolation of the grounding line with locally interpolated basal friction and (2) an improved driving-stress computation across the grounding line, the reversibility of the grounding line (i.e. its retreat after an advance forced by a local perturbation of basal resistance) is captured by the model even at medium and low resolutions (∆x > 10 km). The transient model response is qualitatively similar to that of higher-order models but reveals a higher initial sensitivity to perturbations on very short timescales. Our findings support the application of PISM to the Antarctic ice sheet from regional up to continental scales and on relatively low spatial resolutions.

The orientations of individual crystals within a polycrystalline aggregate subjected to stress have a strong influence on its bulk strain rate and flow behavior. The ability to include the effect of crystal fabric and recrystallization processes in an ice flow law, especially at the bottom of glaciers and ice sheets where temperature is close to the pressure-melting point, is important because the stratigraphy of the ice body may be affected and the paleoclimate reconstruction hampered. We present herein three newly developed deformation apparatuses offering the possibility, from single experiments, of investigating different finite strain stages and their corresponding c-axis fabric and grain texture patterns in various confined, shear flow configurations (simple shear, pure shear and compression/extension bending). The technical set-ups and major advantages compared to classical methods are explained, and results from experiments are discussed in order to illustrate the functioning and purposes of the methods. In all experiments, significant variations in the microstructural development have been observed that reflect the varying orientations of the anisotropy and its relationship to the stress pattern. In monocrystalline ice-bending experiments, the pre-existing c-axis fabric is shown to have a profound influence on the response to stress and possibly to the type of slip system activated.

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.

Model simulations of the Greenland ice sheet contribution to 21st-century sea-level rise are performed with a state-of-the-art ice-sheet model (Parallel Ice Sheet Model (PISM)). The climate-forcing fields are obtained from the European Union’s Seventh Framework Programme project ice2sea, in which three regional climate models are used to dynamically downscale two scenarios (A1B and E1) from two general circulation models (ECHAM5 and HadCM3). To assess the sensitivity of the projections to the model initial state, four initialization methods are applied. In these experiments, the simulated contribution to sea-level rise by 2100 ranges from an equivalent of 0.2 to 6.8 cm. The largest uncertainties arise from different formulations of the regional climate models (0.8–3.9 cm) and applied scenarios (0.65–1.9 cm), but an important source of uncertainty is the initialization method (0.1–0.8 cm). These model simulations do not account for the recently observed acceleration of ice streams and consequent thinning rates, the changing ice discharge that may result from the spatial and temporal variability of ocean forcing, or the feedback occurring between ice-sheet elevation changes and climate forcing. Thus the results should be considered the lower limit of Greenland ice sheet contributions to sea-level rise, until such processes have been integrated into large-scale ice-sheet models.

The WAIS (West Antarctic Ice Sheet) Divide deep ice core was recently completed to a total depth of 3405 m, ending 50 m above the bed. Investigation of the visual stratigraphy and grain characteristics indicates that the ice column at the drilling location is undisturbed by any large-scale overturning or discontinuity. The climate record developed from this core is therefore likely to be continuous and robust. Measured grain-growth rates, recrystallization characteristics, and grain-size response at climate transitions fit within current understanding. Significant impurity control on grain size is indicated from correlation analysis between impurity loading and grain size. Bubble-number densities and bubble sizes and shapes are presented through the full extent of the bubbly ice. Where bubble elongation is observed, the direction of elongation is preferentially parallel to the trace of the basal (0001) plane. Preferred crystallographic orientation of grains is present in the shallowest samples measured, and increases with depth, progressing to a vertical-girdle pattern that tightens to a vertical single-maximum fabric. This single-maximum fabric switches into multiple maxima as the grain size increases rapidly in the deepest, warmest ice. A strong dependence of the fabric on the impurity-mediated grain size is apparent in the deepest samples.

Distributed energy-balance melt models have rarely been applied to glaciers with extensive supraglacial debris cover. This paper describes the development of a distributed melt model and its application to the debris-covered Miage glacier, western Italian Alps, over two summer seasons. Sub-debris melt rates are calculated using an existing debris energy-balance model (DEB-Model), and melt rates for clean ice, snow and partially debris-covered ice are calculated using standard energy-balance equations. Simulated sub-debris melt rates compare well to ablation stake observations. Melt rates are highest, and most sensitive to air temperature, on areas of dirty, crevassed ice on the middle glacier. Here melt rates are highly spatially variable because the debris thickness and surface type varies markedly. Melt rates are lowest, and least sensitive to air temperature, beneath the thickest debris on the lower glacier. Debris delays and attenuates the melt signal compared to clean ice, with peak melt occurring later in the day with increasing debris thickness. The continuously debris-covered zone consistently provides 30% of total melt throughout the ablation season, with the proportion increasing during cold weather. Sensitivity experiments show that an increase in debris thickness of 0.035 m would offset 18C of atmospheric warming.

The McMurdo Dry Valleys, southern Victoria Land, East Antarctica, are a polar desert, and melt from glacial ice is the primary source of water to streams, lakes and associated ecosystems. Previous work found that to adequately model glacier ablation and subsurface ice temperatures with a surface energy-balance model required including the transmission of solar radiation into the ice. Here we investigate the contribution of subsurface melt to the mass balance of (and runoff from) Dry Valley glaciers by including a drainage process in the model and applying the model to three glacier sites using 13 years of hourly meteorological data. Model results for the smooth glacier surfaces common to many glaciers in the Dry Valleys showed that sublimation was typically the largest component of surface lowering, with rare episodes of surface melting, consistent with anecdotal field observations. Results also showed extensive internal melting 5–15 cm below the ice surface, the drainage of which accounted for ~50% of summer ablation. This is consistent with field observations of subsurface streams and formation of a weathering crust. We identify an annual cycle of weathering crust formation in summer and its removal during the 10 months of winter sublimation.

Here we present ground-penetrating radar measurements of Mittivakkat Gletscher, southeast Greenland, to estimate an empirical volume–area scaling relationship at an individual glacier. Between a previous direct volume survey in 1994 and one in 2012, the glacier volume has decreased from 2.02 km3 to 1.44 km3 while the study area has decreased from 17.6 km2 to 15.8 km2. These results are in accordance with the cumulative mass loss observed by long-term mass-balance measurements (1995/96–2011/12) at Mittivakkat Gletscher and confirm that the glacier is in severe climatic disequilibrium (AAR = 0.17). The observed scaling exponent γ = 3.14 and coefficient c = 0.00025 km3–2γ are outside the range of global scaling parameters but sensitive to small uncertainties. Nevertheless, existing global volume-area scaling relationships estimate the volume of Mittivakkat Gletscher within 31% and 15% of the measured 1994 and 2011 volumes, respectively.

We summarize new observations of the deceleration and stick–slip motion of Whillans Ice Stream (WIS), Antarctica. We refine the location of the large sticky spots that resist motion between slip events, the locations of which are controlled by the patterns of subglacial water flow. Our examination of the long-term velocity time series for the ice stream reveals that the decadal-scale deceleration is not occurring at a steady rate, but varies at the sub-decadal timescale. This unsteady deceleration modulates the temporal evolution of a broad (~50 km across) surface-elevation bulge forming at the junction between the relatively narrow upstream portion of the ice stream and broad ice plain that constitutes the downstream end of WIS. Comparison of observations from April 2003 and November 2010 reveals significant changes in the tidally modulated stick–slip cycle that regulates motion on the ice plain. We observe that the timing of slip events has become less regular in response to decreased flow speed in the upstream portions of the ice stream. The decreased regularity of slip events has reduced the release of stored elastic strain during slip events, increasing the rate of deceleration.

Accumulation is a key parameter governing the mass balance of the Greenland ice sheet. Several studies have documented the spatial variability of accumulation over wide spatial scales, primarily using point data, remote sensing or modeling. Direct measurements of spatially extensive, detailed profiles of accumulation in Greenland, however, are rare. We used 400 MHz ground-penetrating radar along the 1009 km route of the Greenland Inland Traverse from Thule to Summit during April and May of 2011, to image continuous internal reflecting horizons. We dated these horizons using ice-core chemistry at each end of the traverse. Using density profiles measured along the traverse, we determined the depth to the horizons and the corresponding water-equivalent accumulation rates. The measured accumulation rates vary from ~0.1 m w.e. a–1 in the interior to ~0.7 m w.e. a–1 near the coast, and correspond broadly with existing published model results, though there are some excursions. Comparison of our recent accumulation rates with those collected along a similar route in the 1950s shows a ~10% increase in accumulation rates over the past 52 years along most of the traverse route. This implies that the increased water vapor capacity of warmer air is increasing accumulation in the interior of Greenland.