West Antarctic ice streams show pronounced flow variability in their downstream reaches, with changes stranding formerly fast-flowing ice and redirecting discharge. A simple model, in which the temperature gradient in basal ice provides control of fast sliding in the downstream reach, can explain this behavior. Downstream thinning steepens the temperature gradient near the bed, increasing upward heat flow and the tendency toward basal freezing. The basal temperature gradient is steepest and the tendency toward basal freezing the strongest in ice that has experienced the most rapid downstream thinning, that is, the fastest-flowing ice. The most ‘successful’ rapid outflows are regions where basal water from elsewhere is likely to be consumed. Freezing here leads to episodic slow-downs and redirections of flow, the history of which appears in satellite imagery as ice rises, distorted streaklines, and margin jumps created when discharge migrates to areas with more favorable basal conditions. One compelling consequence of this process is that it makes catastrophic collapse less likely; if discharge currents are forced to slow when they become too fast (thin), then there may be an upper bound on the retreat rate and discharge flux of the West Antarctic ice sheet (WAIS) ice-stream system under the present climate.

A level-set method is proposed for modeling the evolution of a glacier surface subject to a prescribed mass balance. This leads to a simple and versatile approach for computing the evolution of glaciers: the description of vertical fronts and overriding phenomena presents no difficulties, topological changes are handled naturally and steady-state solutions can be calculated without integration over time. A numerical algorithm is put forth as a means of solving the proposed model of glacier surface evolution. It is evaluated by comparing different numerical solutions of the model with analytical and published numerical solutions. The level-set method appears to be a reliable approach for dealing with different glaciological problems.

The floating terminal of Jakobshavn Isbræ, the fastest Greenland ice stream, has disintegrated since 2002, resulting in a doubling of ice velocity and rapidly lowering inland ice elevations. Conditions prior to disintegration were modeled using control theory in a plane-stress solution, and the Missoula model of ice-shelf flow. Both approaches pointed to a mechanism that inhibits ice flow and that is not captured by either approach. Jamming of flow, an inherent property of granular materials passing through a constriction (Jakobshavn Isfjord), is postulated as the mechanism. Rapid disintegration of heavily crevassed floating ice accompanies break-up of the ice jam.

A photogrammetric survey in 1998 formed the basis for compiling a map of the Tuyuksu glacier region in the northern Tien Shan, Central Asia. Comparison with a map from 1958 enables calculation of the change in glacier volume over 40 years. The results are compared with direct annual measurements carried out continuously since 1956. Central Tuyuksu glacier shows a mass balance of –12.6 mw.e. for the geodetic method and –16.8 m for glaciological measurements. In view of the high accuracy of the maps, the discrepancy can be explained mainly by deficiencies in the glaciological measurements. Application of the precipitation–runoff model HBV-ETH represents the hydrological method of mass-balance determination and delivers specific mass-balance losses almost twice those indicated by the geodetic observations. This discrepancy is suspected to be caused by erroneous runoff measurements. The volume balance derived by the geodetic method is used to calibrate both the glaciological and the hydrological method.

As part of a project to investigate the flow of ice at low effective stress, two independent strain-gauge systems were used to measure vertical strain rate as a function of depth and time at Siple Dome, Antarctica. The measurements were made from January 1998 until January 2002 at the ice divide and a site 7km to the northeast on the flank. The strain-rate profiles place constraints on the rheology of ice at low stress, show the expected differences between divide and flank flow (with some structure due to firn compaction and probably ice stratigraphy), and suggest that the flow of the ice sheet has not changed much in the last 8.6 kyr. The strain rates show an unexpected time dependence on scales ranging from several months to hours, including discrete summer events at the divide. Time dependence in strain rate, water pressure, seismicity, velocity and possibly basal motion has been seen previously on the Siple Coast ice streams, but it is especially surprising on Siple Dome because the bed is cold.

Using the RICE (Radio Ice Cherenkov Experiment) detector at the South Pole, we have estimated the variation in the index of refraction (n) of the firn, as a function of elevation (z) measured from the surface down to z =-150 m. Measurements were made by lowering a dipole transmitter into a dry (5 in (127 mm) caliber) borehole, originally drilled for the RICE experiment in 1998, and determining signal arrival times, as a function of transmitter depth, in the englacial RICE receiver array. We clearly confirm the expected correlation of n(z) with ice density. Our measurements are in fair agreement with previous laboratory characterizations of the dielectric properties of ice cores. These are the first such in situ measurements to be performed at the South Pole.

On 20 September 2002, an enormous rock/ice slide and subsequent mud-flow occurred on the northern slope of the Kazbek massif, Northern Ossetia, Russian Caucasus. It started on the north- northeast wall of Dzhimarai-Khokh (4780 ma.s.l.) and seriously affected the valley of Genaldon/ Karmadon. Immediate governmental actions, available scientific information, first reconstructions, hazard assessments and monitoring activities as well as initial expert judgments/recommendations are documented in order to enable more detailed analyses and modelling of the event by the wider scientific community. Among the most remarkable aspects related to this event are (1) the relation between the recent event and somewhat smaller but quite similar events that occurred earlier in historical times (1835, 1902), (2) the interactions between unstable local geological structures and complex geothermal and hydraulic conditions in the starting zone with permafrost, cold to polythermal hanging glaciers and volcanic effects (hot springs) in close contact with each other, (3) the erosion and incorporation of a debris-covered valley glacier largely enhancing the sliding volume of rocks, ice, firn, snow, water and probably air to a total of about 100 × 106m3, and (4) the astonishingly high flow velocities (up to 300 km h-1) and enormous length of travel path (18 km plus 15 km of debris/mud-flow). This extraordinary case illustrates that large catastrophic events in high mountain regions typically involve a multitude of factors and require integrated consideration of complex chains of processes, a task which must be undertaken by qualified groups of experts.

Hessbreen is a 5km2, polythermal glacier in southern Spitsbergen. It is believed to have surged at the end of the 19th century, and another surge started in 1969, not in 1972 as earlier studies suggested. The surge was probably triggered in the middle section of the glacier and affected the upper part approximately 4 years before including the whole glacier. Present velocities are low, increasing from 0.3m a-1 at the snout to 4.8 m a-1 above the equilibrium line. A distinct velocity reduction occurs where the glacier narrows. This suggests the velocities here are affected by the cold marginal and frontal ice. Large seasonal variations, with summer velocities of more than twice the winter velocities in the middle part of the glacier, indicate a seasonal change in the drainage system in this area. In the lower part there is no apparent sliding. The balance-flux estimate indicates that the actual ice flux is only about 50% of that required for steady state, so Hessbreen appears to be building up in the accumulation area. The surge duration is now estimated to be 8 years.

Changes in the extent of the polar alpine glaciers within Taylor Valley, Antarctica, are important for understanding past climates and past changes in ice-dammed lakes. Comparison of ground-based photographs, taken over a 20 year period, shows glacier advances of 2-100 m. Over the past ~103 years the climate has warmed. We hypothesize that an increase in average air temperature alone can explain the observed glacier advance through ice softening. We test this hypothesis by using a flowband model that includes a temperature-dependent softness term. Results show that, for a 2˚C warming, a small glacier (50 km2) advances ~25m and the ablation zone thins, consistent with observations. A doubling of snow accumulation would also explain the glacial advance, but predicts ablation-zone thickening, rather than thinning as observed. Problems encountered in modeling glacier flow lead to two intriguing but unresolved issues. First, the current form of the shape factor, which distributes the stress in simple flow models, may need to be revised for polar glaciers. Second, the measured mass-balance gradient in Taylor Valley may be anomalously low, compared to past times, and a larger gradient is required to develop the glacier profiles observed today.

We use data from four automatic weather stations (AWSs) in Dronning Maud Land, East Antarctica, to study the surface mass balance and its components. Distinct differences were found between the moisture climates of the high plateau, the katabatic wind zone and the coastal ice shelves: significant undersaturation occurs year-round in the katabatic wind zone, while on the high plateau and on the coastal ice shelf the air is usually close to saturation. In summer, absorption of shortwave radiation at the snow surface enhances surface sublimation at all sites, removing 3-9% of the annual solid precipitation. Significant summer melting is an equally important ablation term near the coast, but vanishes inland. Vertically integrated column drifting-snow sublimation was estimated using two different methods. This process appears to be similar to or greater in magnitude than surface sublimation. Because intervals between significant precipitation events may last as long as several months, sublimation and melt cause extended periods of surface ablation in summer. In summer, all ablation processes together remove 15-56% of the solid precipitation, or 6-27% on an annual basis.

We applied a newly developed three-dimensional, time-dependent, thermomechanical icesheet model including higher-order stress gradients, to simulate the ice flow across subglacial Vostok lake, East Antarctica. Simulations of both ‘lake’ and ‘no lake’ conditions (by treating the ice/lake interface as a stress-free surface, similar to an ice shelf) demonstrate the effects of the subglacial lake, such as pronounced surface flattening and ice-flow turning, on the overall ice dynamics in the vicinity, although subglacial lake dynamics are not treated explicitly. When buoyancy forces and hydrostatic equilibrium of the ice above the lake are taken into account, the along-lake surface slope is preserved and the ice-flow pattern is in accord with sparse observations. Model experiments point to a local ice speed-up in the northern part of the lake, which can be associated with the onset of an enhanced ice-flow feature, more precisely the onset of the Totten Glacier catchment.

A scheme for katabatic turbulent heat transfer proposed by Oerlemans and Grisogono (2002), here referred to as the OG parameterization, is compared with bulk heat-transfer estimates on Peyto Glacier, Alberta, Canada. Automatic weather stations (AWSs) provide off-glacier data to drive the parameterization and glacier data for bulk estimates. Micrometeorological datasets are used to assess two schemes that employ the Monin-Obukhov stability parameter, z/L, to modify logarithmic, or neutral, bulk heat-transfer equations to allow for stability. Both schemes fail at >1 m above the surface, where the AWS sensors are located, unless a modified approach is used in which the stability correction is constant for z/L ≥1/3. Then the bulk sensible-heat-flux density falls to ≈0.93 of its neutral estimate at all measurement levels, thus providing a basis for comparison with the parameterization. The results of the comparison are very good, indicating that a one-to-one relationship between bulk and parameterized values can be achieved by optimizing the fit with a background exchange coefficient and, because there is only one off-glacier AWS, using a sinusoidal function to model the diurnal variation of the potential temperature lapse rate.

Global positioning system (GPS) data are now routinely used for many glaciological applications. In some common cases, systematic errors are unmodelled at the data-processing stage, although they are often presumed insignificant. In this paper, I investigate these assumptions for three different scenarios: (1) measurements on a moving glacier; (2) measurements on a floating ice shelf; and (3) precise height determination over large elevation ranges, such as for aircraft positioning in lidar/laser altimeter missions. In each case, systematic errors are shown to be present in the coordinate solutions that have a far greater magnitude than the formal error estimates produced by the GPS processing software, under certain conditions. If these coordinate biases go undetected, short- and long-term measurements of horizontal ice velocity or rates of ice-thickness change may be erroneous and the coordinates could not be expected to match rigorously processed data or results from different processing techniques. More rigorous processing strategies are discussed that allow for bias-free parameter estimation.

A heavy down-hole hammer actuated from the surface by a light composition rope was used to place instrumented probes into the active, 7m thick, clast-rich till underlying a site on Black Rapids Glacier, Alaska, USA, where the ice is 500m thick. A till penetration of about 2.5m was obtained, and greater depths seem possible. The probes measured pore-water pressure and two axes of tilt, which they broadcasted, without wires, to a receiver just above the ice–till interface.