Nearly 100 days of hourly glacier motion, hydrology and hydrochemistry measurements on Kennicott Glacier, Alaska, USA, demonstrate the complicated relationship between water and motion at the glacier bed. Our observations capture the transient glacier response to seasonal and daily melt cycles, and to a jökulhlaup that prompts a sixfold increase in glacier speed. Sliding is promoted whenever the water inputs to the glacier exceed the capacity of the subglacial hydrologic system to transmit the water. Sensitivity of sliding to daily meltwater inputs varies strongly through the season, implying that the state of the hydrologic system governs the sensitivity of basal sliding. A numerical model constructed to explore these relationships reveals: the roles of the effective pressure; the exponent to which this is taken in the ‘sliding law’ (0.1 <γ< 0.6); glacier macroporosity (φ < 2%); and the ‘cavity-generating capacity’ of the glacier bed, which encapsulates the sizes and spacing of bed roughness elements. Temporal changes in the effective pressure associated with evolution of both water inputs and subglacial water transmission capacity can explain the varying strength of diurnal velocity fluctuations of Kennicott Glacier. Spatial patterns of glacier macroporosity and of basal roughness govern variation in sensitivity of sliding to water inputs.

In a two-dimensional (plane strain) glacier domain, gravity-driven ice flow is balanced by basal drag and the resistance associated with longitudinal stress gradients. The plane strain Stokes model accommodates both these resistances, whereas several simpler models only account for basal drag. Solving the Stokes equations is numerically challenging and computationally expensive, but simpler models may lead to unrealistic dynamical behaviour. Here, we propose a factor which can be introduced in shear-deformational flow models to yield results comparable to those from the plane strain Stokes model. As this factor adapts simpler models to capture the effects of missing dynamics, i.e. longitudinal stress gradients, we refer to it as the longitudinal stress (L-)factor. We assess the usefulness of this factor for idealized domains with complex basal topography and evolving geometry. We apply the model to Haig Glacier, Canadian Rockies, in order to present an illustration of how simulations of glacier response to climate forcing can be improved through the introduction of the L-factor in a shear-deformational flow model.

Air-hydrate crystals store most of the ancient air contained in deep ice sheets. We carried out microscopic observations of air-hydrate crystals below 2000 m depth within the ice core from Dome Fuji, Antarctica, to obtain their number and size distributions. We found that the number density continuously decreased with depth, whereas the average size increased, in contrast to findings from shallower depths. In addition, the characteristic perturbations in both number density and average size distribution with climatic changes almost disappeared, although they are clearly observed in shallow cores. These results indicate that the air-hydrate crystals grow considerably in deeper parts of the ice sheet, and this growth is accompanied by the diffusion of air molecules in the ice. The permeation coefficient of the air molecules in the ice sheet was estimated from the geometric parameters of the air-hydrate distributions. This is the first practical evidence comparable to the previous model estimations. It allows us to evaluate the impacts of the air-molecule migration in the ice sheet on the paleoclimatic information recorded in the deep ice cores.

This study clarifies changes in the chemical forms of microparticles during Termination I, the period of drastic climate change between the Last Glacial Maximum (LGM) and the Holocene. We determine the chemical forms of individual water-soluble microparticles through micro-Raman spectroscopy and compare the relative frequencies of different types with the ion concentrations in melted ice. Micro-Raman spectroscopy shows that Na2SO4·10H2O and MgSO4·11H2O are abundant in Holocene ice, while CaSO4·2H2O and other salts are abundant in LGM ice. Further, the number of CaSO4·2H2O particles is strongly correlated with the concentration of Ca2+ during Termination I. Taken together, the evidence strongly suggests that most of the Ca2+ exists as CaSO4·2H2O. The different compositions of microparticles from the Holocene and LGM can be explained by ion balance arguments.

For more than a century Alpine glaciers have been retreating dramatically, and they are expected to shrink even more quickly over the coming decades. This study addresses the future evolution of Grosser Aletschgletscher, Switzerland, the largest glacier in the European Alps. A three-dimensional combined surface mass-balance and glacier dynamics model was applied. The ice flow was described with the full Stokes equations. The glacier surface evolution was obtained by solving a transport equation for the volume of fluid. Daily surface melt and accumulation were calculated on the basis of climate data. The combined model was validated against several types of measurements made throughout the 20th century. For future climate change, scenarios based on regional climate models in the ENSEMBLES project were used. According to the median climatic evolution, Aletschgletscher was expected to lose 90% of its ice volume by the end of 2100. Even when the model was driven using current climate conditions (the past two decades) the glacier tongue experienced a considerable retreat of 6 km, indicating its strong disequilibrium with the present climate. By including a model for the evolution of supraglacial debris and its effect in reducing glacier melt, we show that this factor can significantly slow future glacier retreat.

An in-depth analysis of seven samples from the Siple Dome (Antarctica) ice core, using optical microscopy and electron backscatter diffraction, illustrates rotational recrystallization or polygonization in the fine-grained region of the core between 700 and 800 m. Between 640 and 700 m, the microstructure is characterized by a bimodal grain-size distribution and a broken girdle fabric with evidence of polygonization. From 727 to 770 m, mean grain size decreases and a single-maximum fabric is found, and, by 790 m, mean grain size has again increased and a multiple-maxima fabric manifests itself. We compare grain-size distribution, c- and a-axis orientation, and misorientation between adjacent grains. We found that misorientations between adjacent grains in the 727–770 m region were predominantly low-angle and typically around a common a-axis, suggesting polygonization. This conclusion is supported by radar evidence of a physical disturbance at 757 m, which may be correlated with higher than usual strain in the 700–800 m range. Below 770 m, larger less regular misorientations and textural evidence show that migration recrystallization is the primary recrystallization mechanism.

Three positive feedback mechanisms that accelerate ice-sheet melting are assessed in a warming climate, using a numerical ice model driven by atmospheric climate models. The Greenland ice sheet (GrIS) is the modeling test-bed under accelerated melting conditions. The first feedback is the interaction of sea water with ice. It is positive because fresh water melts ice faster than salty water, owing primarily to the reduction in water heat capacity by solutes. It is shown to be limited for the GrIS, which has only a small ocean interface, and the grounding line of some fast glaciers becomes land-terminating during the 21st century. The second positive feedback, strain heating, is positive because it produces further ice heating inside the ice sheet. The third positive feedback, granular basal sliding, applies to all ice sheets and becomes the dominant feedback during the 21st century. A numerical simulation of Jakobshavn Isbræ over the 21st century reveals that all three feedback processes are active for this glacier. Compared with the year 2000 level, annual ice discharge into the ocean could increase by ∼1.4 km3 a−1 (∼5% of the present annual rate) by 2100. Granular basal sliding contributes ∼40% of this increase.

To investigate the subsurface hydrological characteristics of an overdeepened cirque glacier, nine boreholes were drilled to the bed of West Washmawapta Glacier, British Columbia, Canada, in summer 2007. All holes were surveyed with a video camera, and four were subsequently instrumented with a combination of pressure transducers, thermistors and conductivity sensors. Diurnal pressure and temperature records indicate the presence of a hydraulically connected subglacial drainage system towards the northern glacier margin. Hydraulic jacking in the overdeepening, controlled by changing water volume in the marginal zone, potentially impacts basal ice flow and erosion. The presence of a sediment layer underlying the glacier also likely impacts hydrology and ice dynamics. Influx of warm groundwater into the basal system raises subglacial water temperatures above the pressure-melting point (pmp) and induces diurnal water temperature fluctuations of as much as 0.8°C; water temperatures above the pmp could affect basal melt rates and the development of subglacial drainage systems. These observations suggest that the characteristics of the subglacial drainage system substantially affect patterns of flow and erosion by this small cirque glacier.

Historical records are valuable for assessing glacier change in the Canadian High Arctic. Ommanney’s (1969) detailed inventory of Axel Heiberg Island glaciers, based on photography from 1958–59, has been revisited, converted into digital format and compared to glacier extents mapped from 1999–2000 satellite imagery. Our results show that the island-wide ice coverage decreased by 15.92 km2 in the 42 year period, a loss of <1%. However, two trends are apparent: one of advance or minor retreat from basins hosting outlet glaciers from Müller and Steacie Ice Caps, and one of significant retreat, on the order of 50–80%, for independent ice masses, which include valley glaciers, mountain glaciers, glacierets, and ice caps smaller than 25 km2. If the contributions to ice advance of only three surging glaciers are removed, then the island-wide ice loss approaches 60 km2. Furthermore, it is notable that 90% of ice masses smaller than 0.2 km2 disappeared entirely during the 42 year study period, an observation confirmed by field studies. Successful predictions from the original inventory are highlighted and the likely mechanisms driving the observed advances and retreats are discussed.

Ice cores from small ice caps provide valuable climatic information, additional to that of Greenland and Antarctica. However, their integrity is usually compromised by summer meltwater percolation. To determine to what extent this can affect such ice cores, we performed high-resolution tritium measurements on samples from two ice cores from Spitsbergen covering the period AD1955–75. The very sharp and distinct peaks in the tritium precipitation record are subject to several post-depositional processes. We developed a model that uses the precipitation record as input and incorporates the three most important processes (radioactive decay, isotope diffusion and meltwater percolation). Results are compared with measured tritium and density profiles. Both ice-core records contain sharp bomb peaks in the pre-1963 period. It is shown that these peaks would be much smoother in the absence of melt. In this case the main effect of melt and the refreezing of percolation water is the formation of ice layers that form barriers for firn diffusion; thus melt paradoxically results in better preservation of the annual isotope signals. Conversely, for the period after 1963 the main effect of melt is a stronger smoothing of the tritium profiles.

Subglacial lakes beneath Antarctica’s fast-moving ice streams are known to undergo ∼1 km3 volume changes on annual timescales. Focusing on the MacAyeal Ice Stream (MacIS) lake system, we create a simple model for the response of subglacial water distribution to lake discharge events through assimilation of lake volume changes estimated from Ice, Cloud and land Elevation Satellite (ICESat) laser altimetry. We construct a steady-state water transport model in which known subglacial lakes are treated as either sinks or sources depending on the ICESat-derived filling or draining rates. The modeled volume change rates of five large subglacial lakes in the downstream portion of MacIS are shown to be consistent with observed filling rates if the dynamics of all upstream lakes are considered. However, the variable filling rate of the northernmost lake suggests the presence of an undetected lake of similar size upstream. Overall, we show that, for this fast-flowing ice stream, most subglacial lakes receive >90% of their water from distant distributed sources throughout the catchment, and we confirm that water is transported from regions of net basal melt to regions of net basal freezing. Our study provides a geophysically based means of validating subglacial water models in Antarctica and is a potential way to parameterize subglacial lake discharge events in large-scale ice-sheet models where adequate data are available.

We describe a compact lightweight impulse radar for radio-echo sounding of subsurface structures designed specifically for glaciological applications. The radar operates at frequencies between 10 and 75 MHz. Its main advantages are that it has a high signal-to-noise ratio and a corresponding wide dynamic range of 132 dB due mainly to its ability to perform real-time stacking (up to 4096 traces) as well as to the high transmitted power (peak voltage 2800 V). The maximum recording time window, 40 μs at 100 MHz sampling frequency, results in possible radar returns from as deep as 3300 m. It is a versatile radar, suitable for different geophysical measurements (common-offset profiling, common midpoint, transillumination, etc.) and for different profiling set-ups, such as a snowmobile and sledge convoy or carried in a backpack and operated by a single person. Its low power consumption (6.6 W for the transmitter and 7.5 W for the receiver) allows the system to operate under battery power for >7 hours with a total weight of <9 kg for all equipment, antennas and batteries.

Radar studies of firn on the ice sheets have revealed complex folds on its internal layering that form from the interplay of snow accumulation and ice flow. A mathematical theory for these fold structures is presented, for the case where the radar cross section lies along the ice-flow direction and where the accumulation rate and ice-flow velocity are time-invariant. Our model, which accounts for firn densification, shows how ‘information’ (the depth and slope of isochrones) propagates on the radargram to govern its layer undulations. This leads us to discover universal rules behind the pattern of layer slopes on a distance–age domain and understand why the loci of layer-fold hinges curve, emerge and combine on the radargram to form closed loops that delineate areas of rising and plunging isochrones. We also develop a way of retrieving the accumulation rate distribution and layer ages from steady isochrone patterns. Analysis of a radargram from the onset zone of Bindschadler Ice Stream, West Antarctica, indicates that ice flow and accumulation rates have been steady there for the past ∼400 years, and that spatial anomalies in the latter are coupled to surface topography induced by ice flow over the undulating ice-stream bed. The theory provides new concepts for the morphological interpretation of radargrams.

A new ring-shear device allows basal slip and related processes to be studied in laboratory experiments for the cases of hard or soft beds. The device rotates a confined ring of ice (0.9 m outside diameter) across a horizontal bed at a constant velocity or drag, while a vertical stress is applied and basal water pressure is controlled. A bath with circulating fluid regulated to ∼0.01°C surrounds the ice chamber and keeps the ice at its pressure-melting temperature. In a first experiment with a stepped rigid bed and zero basal water pressure, steady lengths of step cavities depended upon slip velocity raised to a power of 0.59, in general agreement with the square-root dependence of some models of sliding and linked-cavity hydraulics. Transient cavity growth after slip velocity increases was not monotonic, with damped volume oscillations that converged to a steady value. Once ice separated from lee surfaces, drag on the bed was constant and independent of slip velocity and cavity size, consistent with a shear-stress upper bound like that indicated by sliding models. Shear strains near the bed exceeded 30 and ice developed multiple-cluster c-axis fabrics similar to those of sheared ice in temperate glaciers.

Debris cover is widely present in glacier ablation areas of the Tibetan Plateau, and its spatial distribution greatly affects glacier melt rates. High-resolution in situ measurements of debris thickness on Hailuogou glacier, Mount Gongga, southeastern Tibetan Plateau, show pronounced inhomogeneous debris distribution. An analysis of transverse and longitudinal profiles indicates that the ground-surveyed debris thicknesses and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)-derived thermal resistances of debris layers correlate strongly over the entire ablation area. Across- and along-glacier patterns of ASTER-derived thermal resistance correspond well with spatial patterns of debris thickness, which may reflect large-scale variations in the extent and thickness of the debris cover. The ice melt rate variability over the ablation area simulated by a surface energy-balance model that considered thermal resistance of the debris layer indicates clearly the crucial role of debris and its spatial continuity in modifying the spatial characteristics of melt rates. Because of the inhomogeneous distribution of debris thickness, about 67% of the ablation area on Hailuogou glacier has undergone accelerated melting, whereas about 19% of the ablation area has experienced inhibited melting, and the sub-debris melt rate equals the bare-ice melt rate in only 14% of the ablation area.

We revisit the results of the ISMIP-HEINO benchmark by first analyzing the differences in various model outputs using a wavelet-based spectral technique. Second, the ISMIP-HEINO benchmark experiments are recomputed with a novel numerical ice-sheet model based on the SIA-I algorithm that enables both the shallow-ice and a higher-order approximation of the ice-flow equations to be performed. To assess the significance of the higher-order approximation in the ISMIP-HEINO experiment, a numerical sensitivity study for the shallow-ice approximation (SIA) simulations is also carried out. A high sensitivity of the SIA model response to surface temperature perturbations is found. We conclude that the variations in ISMIP-HEINO results are due to the differences in (1) simulated basal temperatures and (2) numerical treatment of the basal sliding condition.