Large subglacial lakes manifest themselves as flat regions on the ice surface. ERS-1 satellite radar altimetry of the Dome C region of East Antarctica was analyzed to correlate unusually flat areas on the ice surface with known locations of subglacial lakes identified from airborne radio-echo sounding (RES) data. The mean length of subglacial lakes which have an expression in the ice-sheet surface was ~8.3 km, whilst those that did not exhibit a surface morphological manifestation had a mean length of ~3.3 km. Thus, lakes up to about 4 km in length arc unlikely to be detected from satellite radar altimetry of the ice surface. Given that the spacing of radio-echo flight tracks within the SPRI-NSF-TUD Antarctic database is 50-100 km in many areas, a number of subglacial lakes probably lie undetected beneath the ice sheet. RES information from two large, flat surface regions within Dome C, and a further flat area located at 80° S, 127° E, indicates the absence of subglacial lakes beneath the ice-surface features. However, these areas are characterised by relatively strong radio-echo returns which may indicate the presence of water-saturated basal sediments. We suggest that (1) blankets of water-saturated basal sediments may cause similar surface morphological features to those produced by subglacial lakes; and (2) misidentification of subglacial lakes from satellite altimeter observations of the ice-sheet surface is possible without the support of RES information relating to the ice-sheet base. Furthermore, our study indicates a lack of subglacial lake signals from RES data over relatively thick regions of East Antarctica such as the Adventure Subglacial Trough. We conclude that subglacial water produced in such regions may be transported by a basal hydrological system, driven by overburden pressure, to less thick regions of the ice sheet where subglacial lakes have been identified.

In calculations of the variation in the 2 m temperature along glaciers, the lapse rate is generally assumed to be constant. This implies that the ratio of changes in the 2 m temperature above a glacier to changes in the temperature outside the thermal regime of that glacier (“climate sensitivity”) is equal to 1. However, data collected during the ablation season on several mid-latitude glaciers show that this sensitivity is smaller than 1. The lowest measured value (0.3) was obtained on the tongue of the Pasterze, a glacier in Austria. The measured temperature distribution along the Pasterze cannot be described by a constant lapse rate either. However, there is almost a linear relationship between potential temperature and the distance along die glacier. This paper introduces a simple, analytical, thermodynamic glacier-wind model which can be applied to melting glaciers and which explains the observed “climate sensitivities” and temperature distributions much better than calculations based on a constant lapse rate.

This way of modelling the 2 m temperatures has implications for the sensitivity of the surface mass balance to atmospheric warming outside the thermal regime of the glacier. The magnitude of this sensitivity is computed with a surface energy-balance model applied to the Pasterze. When a constant lapse rate is used instead of the proposed glacier-wind model to compute changes in the 2 m temperature along the glacier, the negative change in mass balance due to 1°C warming is overestimated by 22%.

Previous research has documented a close association between high-resolution snow-pit profiles of hydrogen and oxygen stable-isotope ratios and multi-year Special Sensor Microwave/lmager (SSM/I) 37 GHz brightness temperature data in central Greenland. Comparison of the SSM/I data to profiles obtained during the 1989-91 field seasons indicated that δD and δ18O data from the near-surface snow at the Greenland summit are a reliable, high-resolution temperature proxy. To test this new technique further, additional stable-isotope data were obtained from a 2 m snow pit constructed during late-June 1995 near the GISP2 site.

This new profile, supported by pit stratigraphy and chemistry data, confirms the utility of comparing stable-isotope records with SSM/I brightness temperatures. The sub-annual variation of the δD record at the GISP2 site was determined using 15 match points, from approximately December 1991 through June 1995 and was guided in part by time-constrained hoar layers. The close association of these temperature proxies supports the assertion that snow accumulation occurs frequently through the year and that the isotope record initially contains temperature information from many times of the year. This is also independently confirmed by analysis of H2O2 data. The slope of the multi-year T vs δ correlation was evaluated along with the sub-annual variation in the amount, rate and timing of accumulation. These new results are consistent with those from the previous study and they also demonstrate that the snow in this area initially contains temperature and chemical records with sub-annual resolution. This encourages confident interpretation of the paleoclimatic signal variations in the GISP2 and GRIP deep cores.

Push-morainal banks at the grounding lines of tidewater termini of temperate glaciers are the source of two types of restraining forces operating at the glacier terminus. Horizontal normal forces derive from the lateral support and transport of the bank of sediment at the terminus, whereas a horizontal shear force operates along the base of a bank pushed in front of an advancing glacier. The simple model we present suggests that bank-related restraining forces are significantly larger than the restraining force derived from the hydrostatic pressure of water adjacent to the submerged terminus of a glacier. During glacier advance, restraining forces continually increase, resulting in decreasing flow rates, glacier thickening and the eventual cessation of advance. During retreat, restraining forces continually decrease, resulting in increasing flow rates, glacier thinning and the potential for unstable, rapid, sustained retreat. The normal, seasonal, oscillatory advance retreat cycle of a glacier is moderated by restraining forces associated with push moraines. Unstable retreat is likely initiated when bank-related restraining forces fall below some threshold value during the seasonal retreat cycle. Calving is not a primary cause of glacier retreat, but is more likely a short-term response to increased flow rates. Increased flow rates result in glacier thinning and an approach toward buoyancy, both of which fluctuate seasonally in accordance with bank-related restraining forces.

Glacier facies from the Greenland ice sheet and the Wrangell-St Elias Mountains, Alaska, are analyzed using multi-temporal synthetic aperture radar (SAR) data from the European Space Agency ERS-1 satellite. Distinct zones and facies are visible in multi-temporal SAR data, including the dry-snow facies, the combined percolation and wet-snow facies, the ice facies, transient melt areas and moraine. In Greenland and south-central Alaska, very similar multi-temporal signatures are evident for the same facies, although these facies are found at lower altitude in West Greenland where the equilibrium line appears to be found at sea level at 71°30?N during the year analyzed (1992-93), probably because of the cooling effect of the eruption of Mount Pinatubo. In Greenland, both the percolation and dry-snow facies are excellent distributed targets for sensor calibration, with backscatter coefficients stable to within 0.2 dB. However, the percolation facies near the top of Mount Wrangell are more complex and less easily delineated than in Greenland, and at high altitude the glacier facies have a multi-temporal signature which depends sensitively on slope orientation.

A new method for preparing thin sections of wet snow is described. Samples are flash-frozen to immobilise the liquid water content (LWC). Next, the pore space is filled with ester, and the entire sample is frozen to a solid, sliceable block, Micrographs of slices are obtained using transmitted and specular illumination. The proportion of LWC is measured on the micrographs and compared with LWC measured independently using calorimetry. Section analysis is both facilitated and complicated by small bubbles which form during sample preparation. Flash-freezing and bubble formation are discussed theoretically.

The northern slopes of Mexico’s great, active stratovolcanoes, Popocatépetl (5450 m.a.s.l.) and Pico de Orizaba (5700 m a.s.l.), are covered by glaciers. This paper analyzes the retreat of the glaciers between February 1994 and October 1995, and its geo-morphological consequences. The results discussed are a continuation of research conducted on the volcanoes up to 1994. Although the glaciers retreated a great deal during the 1980s and early 1990s, the process accelerated from 1994 to 1995. The heavy erosion that had been observed on the deglaciated walls of the volcanoes during these decades increased as the glaciers retreated. Sediments originating from material loosened from the exposed lateral walls added to the supraglacial layer. Temperature readings taken from boreholes made in the glaciers indicated that ablation had been reduced as a result of the insulating effects of the debris mantle. This allowed widespread formation of deposits that buried ice in areas where the glacier had already disappeared. These results confirmed the conclusions of earlier studies that deglaciation of stratovolcanoes has resulted in the acceleration of slope processes and the formation of buried ice deposits.

Values of the strain-rate tensor represented at a 20 m length scale are found to explain the pattern and orientation of crevasses in a 0.13 km2 reach of Worthington Glacier, Alaska, U.S.A. The flow field of the reach is constructed from surveyed displacements of 110 markers spaced 20-30 m apart. A velocity gradient method is then used to calculate values of the principal strain-rate axes at the nodes of a 20 m x 20 m orthogonal grid. Crevasses in the study reach are of two types, splaying and transverse, and are everywhere normal to the trajectories of greatest (most tensile) principal strain rate. Splaying crevasses exist where the longitudinal strain rate (x) is ≤ 0 and transverse crevasses are present under longitudinally extending flow (i.e. x > 0). The orientation of crevasses changes in the down-glacier direction, but the calculated rotation by the flow field does not account for this change in orientation. Observations suggest that individual crevasses represent local values of the regional flow field and are transient on the time-scale of 1-2 years; they are not persistent features that are translated and rotated by flow. Crevasse patterns are thus found to be a useful tool for mapping the strain-rate tensor in this reach of a temperate valley glacier.

We describe the calibration and interpretation of interferograms generated from ERS-1 synthetic aperture radar Single Look Complex (SAR.SLC) images of the Rutford Ice Stream area. Ground surveys provide over 100 tie-points with which to optimise the interferometric baselines that separate nominally repeated satellite orbits. Covariant tie-point errors are dealt with by constructing an error covariance matrix for the expected values of the unwrapped interferometric phases at the tie-points. With baseline parameters that minimise the weighted residual variance, rms tie-point residuals of less than 1 cm in slant range are obtained. These are attributed to a combination of interferometric phase noise, movement survey errors and inadequate slope information. The image set used is inadequate for isolating the influences of topography and movement, so the glaciological conclusions to be drawn are limited. Nevertheless, the interferograms confirm that the whole of the upper 50 km of Carlson Inlet flows at a speed less than a tenth of that of the neighbouring Rutford Ice Stream. Also confirmed are the entry of faster-moving ice into the lower reaches of Carlson Inlet and the position of part of the Carlson Inlet grounding line. In general, the distribution of the residuals suggests no significant differences in ice movement between 1978 and 1992. An exception is the neighbourhood of the shear margin between Rutford Ice Stream and Carlson Inlet, where inconsistencies between ground surveys over the periods 1984-86 and 1994-96 and interferograms from 1994 and 1996 suggest fluctuations in velocity of up to 10 m year−1.

Assessment of the contribution of small glaciers to sea-level rise or the reconstruction of past glacial mass balance requires knowledge of mass-balance sensitivity to climatic variations. The aim of this paper is to clarify this relation. The mass-balance fluctuations analyzed from measurements on glacier d’Argentière, Mont Blanc massif, France, between 1850 and 2700 m a.s.l. were compared with climatic variations at a nearby meteorological station. Statistical study of the data shows that: (1) the annual mass-balance fluctuations are dependent on elevation, and (2) the mass-balance sensitivity to temperature decreases with altitude and diverges from current model results. Consequences of a temperature variation of 1°C for global volume variations are significant. A simple calculation on glacier des Bossons, Mont Blanc massif, France, shows that the sensitivity from the model can lead to volume variations twice as high as results compatible with our observations.

Advanced very high-resolution radiometer (AVHRR) images and a radar-altimetry-based digital elevation model (DEM) covering part of the northeast Greenland ice stream are combined to create an improved topographic map of the area using photoclinometry. In this application of photoclinometry, a DEM is used to establish the photometric relationship for two AVHRR images of a snow surface. Slopes from the DEM are compared with AVHRR data that are filtered (i.e. blurred) to the resolution of the DEM to give an empirical photometric determination. This is then used to convert unfiltered AVHRR data into quantitative slope measurements of the surface in the along-sun direction in each image, resolving features not present (or poorly represented) in the DEM. Co-registration of the images is based on the assumption that the two slope fields from the images describe one continuous smooth surface. The combined slopes are then converted to topography. In the test case, the technique adds topographic details with spatial scales of ~3 to ~20 km. A comparison of our results with airborne laser elevation profiles demonstrates that the new technique recovers most of the topography that is missed by the DEM. The improved topographic map reveals a ten-fold increase in local surface relief over the ice-stream feature, and shows the presence of shallow troughs over the shear margins of the feature.

A simplified model of a two-dimensional ice sheet is described. It includes basal ice sliding dependent, on the basal water pressure, which itself is described by a simple theory of basal drainage. We show that this simple but sophisticated model predicts surges of the ice mass in realistic circumstances, and we describe these surges by solving the problem numerically. We also are able to describe some parts of the surge analytically. The numerical solution of the model is a delicate matter, and highlights pitfalls to be avoided if more complicated models are to be solved successfully.

Nine glaciers in Alaska and Washington, U.S.A., originally mapped as part of the International Geophysical Year (IGY) in 1957—58, were re-mapped between 1993 and 1996, eight using airborne surface elevation profiling and the ninth using ground-based kinematic global positioning system methods. Elevation, volume and terminus changes were determined for the approximately 38 year period between the IGY mapping and the profiling, All nine glaciers showed substantial thinning at lower elevations; seven of the nine thickened at higher elevations. None of the glaciers had a significant net volume increase; two had close to zero change, and the others had a decrease. For the eight glaciers for which we could obtain quantitative information, the mean thickness change was -10 m with a large scatter, 8 m standard deviation. The volume and terminus changes had no clear geographic pattern, and no simple relationship between volume change and terminus advance or retreat was identified. The largest error in the estimated volume changes is due to map errors.

Mapping of the structural glaciology of Kongsvegen, Svalbard, reveals evidence for four main deformational structures. These are stratification, longitudinal foliation, thrusts and crevasse traces. These structures are considered in terms of their contribution to debris entrainment, transport and subsequent landform development. Stratification is associated with small amounts of supraglacial debris that has been folded with flow-parallel axes; longitudinal foliation in places incorporates basal glacial sediments along folds with flow-parallel axes; and thrusts transport basal debris to the glacier surface. Crevasse traces are not significant in terms of debris entrainment. The entrainment of basal debris along longitudinal foliation is not a universally recognised process. At Kongsvegen this process is attributed to the development of a transposition foliation, in combination with incorporation of debris-rich basal ice or soft basal sediment in the fold complex. Mapping of the landforms in the proglacial area shows that debris incorporated along longitudinal foliation is released as “foliation-parallel ridges” and that transverse ridges mark debris-bearing thrusts. The role of longitudinal foliation in landform development has never been documented in this manner. Although the preservation potential of such ridges may be limited, recognition of foliation-parallel ridges in the Pleistocene landform record has important implications for the interpretation of the dynamics of former ire masses.

Digital airborne radar data were collected during the 1987-88 Antarctic field season in nine gridded blocks covering the downstream portions of Ice Stream B (6km spacing) and Ice Stream C (11 km spacing), together with a portion of ridge BC between them. An automated processing procedure was used for picking onset times of the reflected radar pulses, converting travel times to distances, interpolating missing data, converting pressure transducer readings, correcting navigational drift, performing crossover analysis, and zeroing rémanent crossover errors. Interpolation between flight-lines was carried out using the minimum curvature method.

Maps of ice thickness (estimated accuracy 20 m) and basal-reflection strength (estimated accuracy 1 dB) were produced. The ice-thickness map confirms the characteristics of previous reconnaissance maps and reveals no new features. The reflection-strength map shows pronounced contrasts between the ice streams and ridge BC and between the two ice streams themselves. We interpret the reflection strengths to mean that the bed of Ice Stream C, as well as that of Ice Stream B, is unfrozen, that the bed of ridge BC is frozen and that the boundary between the frozen bed of ridge BC and the unfrozen bed of Ice Stream C lies precisely below the former shear margin of the ice stream.

Will worldwide sea level soon rise rapidly because of a shrinkage of the West Antarctic ice sheet (WAIS)? Here I give a personal perspective of that probability. The crucial question is not whether large changes in ice mass can occur, but how likely it is that a large, rapid change, say a several-fold increase in the 20th-century rate of about 2 mm a-1, will occur in the next century or two from a West Antarctic cause.

Twenty years ago Weertman proposed that a marine ice sheet is inherently unstable. But Weertman’s analysis was based on a simple model of a marine ice sheet that did not include fast-flowing, wet-based ice streams, which are now known to dominate the grounded ice sheet. Modern analyses do not definitively determine just how ice streams affect the stability of the WAIS, but it can at least be said that there is no compelling theoretical reason to expect a rapid rise in sea level from the WAIS triggered by ice-shelf thinning.

Of the three main ice-drainage systems in the WAIS, the one that flows into Pine Island Bay might be a particularly likely site for accelerated flow since there is no ice shelf to restrain the inflowing ice streams, yet measurements show that this system is not significantly out of mass balance. If the “Ross Embayment” system, which has undergone several sudden glacial reorganizations in the last thousand years, were unstable one might expect a history of large changes in the total outflow of ice into the Ross Ice Shelf, yet the total outflow in the “Ross Embayment” has remained relatively unchanged despite the large internal perturbations, a fact that, points to a stable, not an unstable, system. Study of the third major drainage from the WAIS, into the Ronne Ice Shelf, also suggests that there is no gross discordance between the present velocity vectors and flow tracers in the ice shelf, although the evidence is limited.

In the light of the evidence for recent stability, it is difficult to see how climate warming (whether anthropogenic or natural) could trigger a collapse of the WAIS in the next century or two. Thus, I believe that a rapid rise in sea level in the next century or two from a West Antarctic cause could only occur if a natural (not induced) collapse of the WAIS were imminent. Based on a concept of pseudo-random collapse once per major glacial cycle, I estimate the chances of that to be on the order of one in a thousand.

High-resolution maps of the glacier bed are developed through a pseudo-three-dimensional migration of a dense array of radio-echo sounding profiles. Resolution of three-dimensional maps of subglacial surfaces is determined by the radio-echo sounding wavelength, data sparing in the field, and migration. Based on synthetic radio-echo sounding profile experiments, the maximum resolution of the final map cannot exceed one half-wavelength. A methodology of field and processing techniques is outlined to develop a maximum-resolution map of the glacier bed. The field and processing techniques are used to develop a map of the glacier bed below part of Worthington Glacier, a temperate valley glacier in south-central Alaska. The field techniques and the processing steps used on the glacier result in a map of 20 m x 20 m resolution.

Dielectric profiling (DEP), a capacitive method for measuring an ice core’s electrical conductivity and permittivity, has been in use for several years. We have built an improved DEP instrument with a modified guarded-electrode geometry whose capacitance can be determined from potential theory. The new instrument can measure an ice core’s conductivity and permittivity to within a few per cent error, a high enough accuracy to infer its density using well-known models.

A derivative of the Danish tipping-lower intermediate-depth ice-coring drill system has been developed for use in remote areas. The system as described is capable of drilling to depths of 250 m, although the drill has been proven to 350 m. The drill system is designed to be transported by hand and consumes 700 W maximum power from a 24 V Ac. battery bank charged by solar panels or a small generator. A field test in the St Elias Mountains, Yukon, produced good-quality core to 161.6 m depth; drilling at 6500 m a.s.l. on Far East Rongbuk Glacier, Mount Everest, produced good core to 42 m depth.

A new portable device for ice-core drilling, specially designed for thin tephra-layer sampling, was tested on the South Shetland glaciers during the 1994-95 Antarctic summer. The machine is based on a combination of the standard paleomagnetism drilling machine and a specially built drill-bit designed for ice-coring.