Ablation of debris-mantled glaciers in Nepal has resulted in the formation of several potentially unstable moraine-dammed lakes, some of which constitute serious hazards. Ngozumpa Glacier, Khumbu Himal, has undergone significant downwasting in recent decades, and is believed to lie close to the threshold for moraine-dammed lake formation. The debris-mantled ablation area of the glacier is studded with numerous supraglacial lakes, the majority of which occupy closed basins with no perennial connections to the englacial drainage system (“perched lakes”). Perched lakes can undergo rapid growth by subaerial and water-line melting of exposed ice faces, and calving. Subaerial and subaqueous melting beneath thick (>1 m) debris mantles is comparatively insignificant. Although lake expansion can contribute substantially to ablation of the glacier, perched lakes cannot continue to grow indefinitely, but are subject to rapid drainage once a connection is made to englacial conduits. The level of one of the lakes on the Ngozumpa, however, is controlled by the altitude of a spillway through the lateral moraine of the glacier. This lake underwent only limited growth in the period 1998–2000, but is likely to experience monotonic growth ifglacier mass balance continues to be negative.

We use a self-gravitating viscoelastic model of the Earth and a dynamically consistent marine ice-sheet model to study the relationships between marine ice-sheet dynamics, relative sea level, basal topography and bedrock dynamics. Our main conclusion is that sea-level change and lithospheric coupling are likely to have played limited roles in the postglacial retreat of marine ice sheets. The postglacial rise in sea level would only have caused at the most around 100 km of grounding-line retreat for an ice sheet of similar dimensions to the West Antarctic ice sheet, compared with the several hundred km of retreat which has occurred in the Ross Sea. There is no evidence that reverse slopes lead to instability. Incorporating coupling with lithospheric dynamics does not produce markedly different effects. The implication of these studies is that marine ice-sheet retreat is the result of physical mechanisms other than lithospheric coupling and sea-level rise.

The drift trajectory of giant iceberg C-7 traversing the Weddell Sea, Antarctica, for > 2 years was successfully simulated. Application of the “classical” driving forces like wind and ocean currents resulted in a significant discrepancy between modeled and observed iceberg velocities in the western Weddell Sea. The most realistic drift pattern in space and time was achieved by adding a sea-ice force which represents the ability of a dense sea-ice cover (≥90%) to lock in icebergs and collect the momentum of the wind over an area much larger than the area of the iceberg proper. This process was parameterized using a sea-ice strength P which depends on sea-ice concentration and thickness, both having highest values in winter and in the western Weddell Sea which is covered with multi-year sea ice. As a consequence of the sensitivity to sea ice, the timing of the iceberg drift becomes important, revealing the region off Brunt Ice Shelf (eastern Weddell Sea) as a location where bergs either continue westward with the coastal current or follow a southern branch onto the shallow continental shelf.

In mountainous regions, snow transport due to wind significantly influences snow distribution and, as a result, avalanche danger. A physically based numerical two-layer model is developed to simulate blowing and drifting snow in Alpine terrain. One layer describes the driving-wind field and the transport in suspension. The description is based on the atmospheric boundary-layer equations, using an e − ∊ model for the turbulent closure. The second layer describes the transport due to saltation, including erosion and deposition of snow. Here, conservation equations for mass and momentum are formulated for the mixture of snow and air. Particle trajectory calculations are used to parameterize quantities characterizing the saltation layer. Both layers are mutually coupled by boundary conditions. A two-way coupling between particles and airflow is taken into account. Comparisons between simulation results and field measurements around an Alpine crest show encouraging results.

Flood and debris-flow hazards at Grubengletscher near Saas Balen in the Saas Valley, Valais, Swiss Alps, result from the formation and growth of several lakes at the glacier margin and within the surrounding permafrost. In order to prevent damage related to such hazards, systematic investigations were carried out and practical measures taken. The evolution of the polythermal glacier, the creeping permafrost within the large adjacent rock glacier and the development of the various periglacial lakes were monitored and documented for the last 25 years by photogrammetric analysis of annually flown high-resolution aerial photographs. Seismic refraction, d.c. resistivity and gravimetry soundings were performed together with hydrological tracer experiments to determine the structure and stability of a moraine dam at a proglacial lake. The results indicate a maximum moraine thickness of > 100 m; extremely high porosity and even ground caverns near the surface may have resulted from degradation of sub- and periglacial permafrost following 19th/20th-century retreat of the partially cold glacier tongue. The safety and retention capacity of the proglacial lake were enhanced by deepening and reinforcing the outlet structure on top of the moraine complex. The water level of an ice-dammed lake was lowered and a thermokarst lake artficially drained.

Marine ice sheets with mechanics described by the shallow-ice approximation by definition do not couple mechanically with the shelf. Such ice sheets are known to have neutral equilibria. We consider the implications of this for their dynamics and in particular for mechanisms which promote marine ice-sheet retreat. The removal of ice-shelf buttressing leading to enhanced flow in grounded ice is discounted as a significant influence on mechanical grounds. Sea-level rise leading to reduced effective pressures under ice streams is shown to be a feasible mechanism for producing postglacial West Antarctic ice-sheet retreat but is inconsistent with borehole evidence. Warming thins the ice sheet by reducing the average viscosity but does not lead to grounding-line retreat. Internal oscillations either specified or generated via a MacAyeal–Payne thermal mechanism promote migration. This is a noise-induced drift phenomenon stemming from the neutral equilibrium property of marine ice sheets. This migration occurs at quite slow rates, but these are sufficiently large to have possibly played a role in the dynamics of the West Antarctic ice sheet after the glacial maximum. Numerical experiments suggest that it is generally true that while significant changes in thickness can be caused by spatially uniform changes, spatial variability coupled with dynamical variability is needed to cause margin movement.

Lack of agreement between the deep portions of the Greenland Icecore Project (GRIP) and Greenland Ice Sheet Project II (GISP2) ice cores from central Greenland suggests that folds may disrupt annual layering, even near ice divides. We use a simple kinematic flow model to delineate regions where slope disturbances (“wrinkles”) introduced into the layering could overturn into recumbent folds, and where they would flatten, leaving the stratigraphic record intact. Wrinkles are likely to originate from flow disturbances caused internally by inhomogeneities and anisotropy in the ice rheological properties, rather than from residual surface structures (sastrugi), or from open folds associated with transient flow over bed topography. If wrinkles are preferentially created in anisotropic ice under divides, where the resolved shear stress in the easy-glide direction can be weak and variable, then the deep intact climate record at Dye 3 may result from its greater distance from the divide. Alternatively, the larger simple shear at Dye 3 may rapidly overturn wrinkles, so that they are not recognizable as folds. The ice-core record from Siple Dome may be intact over a greater fraction of its depth compared to the central Greenland records if its flat bedrock precludes fluctuations in the stress orientation near the divide.

When a mass balance is computed using an outdated map, that computation does not reveal the actual mass change. But older maps often must be used for practical reasons. We present a method by which, with a few additional measurements each year, a mass balance computed with an outdated map can be transformed into an actual mass change. This is done by taking into account the influence of changes in areal extent and changes in the surface elevation of the glacier since the map was made. This method is applied to South Cascade Glacier, Washington, U.S.A., as an example. The computed cumulative mass balance from 1970 to 1997 would have been 16% too negative if the 1970 map had not been updated. While the actual volume change of a glacier is relevant to hydrological studies, the change that would have occurred on a constant (or static) surface is more relevant to certain glacier dynamics problems and most climate problems. We term this the reference-surface balance and propose that such a balance, which deliberately omits the influence of changes in area and surface elevation, is better correlated to climatic variations than the conventional one, which incorporates those influences.

The region where two active tributaries feed into the now stagnant Ice Stream C (ISC), West Antarctica, is thickening. In this region, we observe a correlation between faster ice flow (the tributaries) and elevated topography. We conclude that stagnation of ISC resulted in compression and thickening along the tributaries, eventually forming a “bulge” on the ice-sheet surface. Modern hydraulic potential gradients would divert basal meltwater from ISC to Ice Stream B (ISB). These gradients are primarily controlled by the bulge topography, and so likely formed subsequent to trunk stagnation. As such, we argue against “water piracy” as being the cause for ISC’s stagnation. Kinematic-wave theory suggests that thickness perturbations propagate downstream over time, but that kinematic-wave speed decreases near the stagnant trunk. This and modest diffusion rates combine to trap most of the tributary-fed ice in the bulge region. Using interfero-metric synthetic aperture radar velocity measurements, we observe that half of the ice within ISC’s southern tributary flows into ISB. That flow pattern and other observations of non-steady flow in the region likely result from stagnation-induced thickening along upper ISC combined with a longer period of thinning on upper ISB. If current trends in thickness change continue, more ice from upper ISC will be diverted to ISB.

The dynamics of glacier motion are governed to a large extent by the properties of the basal interface. In this paper we address the interaction of a glacier with a layer of till at its bed in an attempt to test whether our physical understanding of till is sufficient to explain general features of the observed flow field and changes in geometry of Black Rapids Glacier, Alaska, U.S.A. We also investigate whether or not a till layer has a clear surface-observable signature in the dynamics of the glacier. Towards this end we use a finite-element ice-flow model with a Coulomb failure criterion within the basal till layer. We find that simple “till physics” can be used to describe decadal, seasonal and short-term (hours to days) velocity variations, and possibly uplift events. Mechanisms for each of these variations involve an increase in the extent of till at failure, a transfer of shear stress across the bed, and a consequent increase in ice deformation. “Effective shape factors” are calculated that permit a simple incorporation of this boundary condition into glacier response models. Our analyses, however, have not resulted in the identification of a clear and unique signature of a till layer in the surface dynamics of a glacier.

The sediment in icebergs offers an opportunity to sample the otherwise inaccessible beds of modern tidewater glaciers. This paper presents a discussion of the benefits and problems of using iceberg debris to make inferences about the nature of the beds of modern tidewater glaciers. As an example, we present data obtained from icebergs calved from fast-flowing tidewater glaciers in Kongsfjorden, Svalbard. Sediment samples obtained from 12 icebergs show that clast-rich muddy diamicton is the dominant facies, although muddy gravel and mud are also present in some debris layers. Sediment texture and clast-shape analyses indicate that most debris in the icebergs is of basal origin. Micro-morphological analyses of thin sections of the diamicton also indicate pervasive deformation of the sediment. Although individual icebergs only represent small samples of the bed of a tidewater glacier, and problems remain in linking iceberg sediments to their precalving transport history beneath the glacier, this method provides a potentially fruitful way of linking sediments to glacier dynamics.

High-resolution crystallographic, salinity and isotopic analyses of a 45 m ice core reveal the presence of a thick layer of marine ice near the grounding line of the Nansen Ice Shelf, Antarctica. The anomalous formation of marine ice in a zone assumed to be the site of active basal melting leads us to propose the hypothesis of large basal crevasses as a favorable environment for important marine-ice accretion. This hitherto unexplored possibility is supported by the overall field configuration and by the discrepancy in some ice properties between this core and the marine-ice sections of previous drilling projects. These findings could have important implications for the general stability of ice shelves and their disintegration processes. The specific properties of this core reveal that marine ice is post-genetically deformed.

Glaciar Nef, a 164 km2 eastern outlet of Hielo Patagónico Norte (the northern Patagonia icefield), terminates in a proglacial lake that has formed in conjunction with 20th-century glacier retreat. The terminus is inferred to be transiently afloat. A hinge-calving mechanism is proposed in which buoyant forces impose a torque on the glacier tongue, resulting in the release of coherent sections of the glacier tongue as “tabular” icebergs. A simple model shows how torque and tensile stress reach a maximum at the up-glacier limit of the buoyant zone, and that glacier thinning causes this point to migrate up-glacier. Empirical evidence supporting this model includes elevated thermo-erosional notches ≤6.5 m above lake level, and the ubiquitous presence since 1975 of “tabular” icebergs with surface areas ≤0.3 km2. Flow speeds of 1.2–1.3 m d−1 were measured near the terminus in February 1998. Extrapolations from these short-term data yield a calving rate of 785–835 m a−1 and a calving flux of 232 × 106 m3 a−1 or 0.2 km3 a−1. The calculated mean water depth at the terminus is 190 m. This calving rate is higher than at grounded temperate glaciers calving in fresh water, but is nevertheless almost an order of magnitude less than calving rates at both grounded and floating tidewater glaciers.

Glacier response to climate can be characterized by a single time-scale when the glacier changes sufficiently slowly. Then the derivative of volume with respect to area defines a thickness scale similar to that of Jóhannesson and others, and the time-scale follows from it. Our version of the time-scale is different from theirs because it explicitly includes the effect of surface elevation on mass-balance rate, which can cause a major increase in the time-scale or even lead to unstable response. The time constant has a dual role, controlling both the rate and magnitude of response to a given climate change. Data from South Cascade Glacier, Washington, U.S.A., illustrate the ideas, some of the difficulty in obtaining accurate values for the thickness and time-scales, and the susceptibility of all response models to potentially large errors.

Stratigraphic variations of oxygen isotopes in the snow which accumulates during the winter at the Norwegian glacier Austre Okstindbreen are not entirely eliminated after 1–2 months of ablation in the following summer. The relationship between regional temperature changes and δ18O values in the snowpack is affected by many natural factors, but 1989/90 winter air temperatures were reflected in the snow which remained on Austre Okstindbreen at 1350 m a.s.l. in July 1990. There were many variations of δ18O values in the 4.1m of snow above the 1989 summer surface, but variations in the underlying firn were relatively small. Meltwater percolation modifies the initial variations of δ18O values in the snowpack. At a site below the mean equilibrium-line altitude on Austre Okstindbreen, increased isotopic homogenization within a 10 day period in July accompanied an increase of the mean δ18O value. Although the isotopic record at a temperate glacier is likely to be influenced by more factors than is that at polar glaciers, it can provide an estimate of the approximate trend of local temperature variations.

Measurements of the surface and internal layer geometry from ice-penetrating radar and global positioning system surveys on three inter-ice-stream ridges in West Antarctica (Siple Dome, ridge DE and ridge BC) are examined with ice-flow models to infer (1) the history of the divide position at each site and (2) the spatial pattern of accumulation across the ridges. We find that the divide position is most steady at Siple Dome, somewhat steady at ridge DE and highly variable at ridge BC. Data from Siple Dome and ridge DE show evidence for steady northward motion of the ice divide for the past few thousand years. The layers beneath ridge BC suggest a 5 km northward shift of the divide position within the past several hundred years. Assuming the divide shifts are all due to changing elevation of the bounding ice streams, we infer the relative elevation history for segments of Ice Streams B–E. The northward displacement of the divide for all ridges implies a progressive relative thinning of the ice streams from E to B, with most dramatic recent thinning (100 m in <103 years) of Ice Stream B relative to Ice Stream C. Analysis of the internal layer pattern across the ridges indicates a south–north accumulation gradient with higher accumulation rates on the northern flanks of the ridges in all three cases. The inferred accumulation distribution is nearly uniform on the northern flanks, decreases sharply within a few ice thicknesses across the divides, and then decreases gradually farther to the south. The north/south decrease is strongest for ridge DE and weakest for ridge BC. This spatial pattern and the reduction in gradient strength with distance from the Amundsen Sea is consistent with the hypothesis that storms from the Amundsen Sea carry moisture first south then west over West Antarctica and deposit more snow on the windward side of ridges due to orographic lifting. This pattern has been stable for at least the past several thousand years.

During the snowmelt period in 1998, air-temperature data were acquired at 1 min intervals using different measurement systems as part of a field campaign in the Kärkevagge, Swedish Lapland. A comparison reveals that temperatures from naturally ventilated sensors exceed temperatures from aspirated sensors by as much as 6.2 K. Errors in temperature are closely connected to high values of upwelling shortwave radiation and are larger in periods of low wind speed. Measurement errors result from the instantaneous radiation conditions and propagate over the next measurements due to slow response time of the naturally ventilated sensor. A physically based method is developed for correcting temperature data influenced by radiation errors, which requires additional measurements of wind speed and upwelling shortwave radiation. Coefficients of the correction formula are automatically determined from the erroneous temperature data, so the method is independent of accurate air-temperature measurements. The high quality of the correction method could be validated by accurate psychrometer measurements. One of the most important applications is the computation of sensible-heat fluxes from snow-covered surfaces during the snowmelt period using the bulk-aerodynamic method, which is greatly improved by the new correction method.

A large number of mass-balance measurements have been carried out on the Greenland ice sheet, but few of the series obtained are well suited for statistical studies. Rather than looking at the global mass-balance value, this paper deals with the spatial and temporal mass-balance variability on the ice sheet. Two sorts of analysis are possible: direct comparisons of the series measured at a given site, or a broader approach involving comparisons between different sites using variance analysis. For glaciers in the southwest ablation area, a significant interannual variability (around 1.0 m w.e.) is found. This variability is spatially consistent. In the accumulation area, the results are more complex. For example, there is consistent evidence of year-to-year variations on the west–east Expéditions Glaciologiques Internationales au Groenland (EGIG) profile, but other closer sites are weakly correlated. These results emphasize the need for a better coverage of measurements over the entire ice sheet, as well as longer and more continuous measurement series.