Glacial calving is a poorly understood process. This study tests the influence of local environmental variables on the magnitude and frequency distributions of calving behaviour at Glaciar San Rafael, Chile. Near the terminus of the glacier, surface speeds average 17 m d −1 in summer and calving is profuse and continual. The size, location and characteristics of over 7000 calving events were recorded during 32d in 1991 and 1992, together with meteorological, bathymetric and oceanographic data. Mean daily calving exceeds 400 events per day and the mean calving flux is more than 2 Mm3d1. Mean annual calving speed and calving flux are about 4500 m a −1 and 2.0 km3 a−1, respectively. This calving speed is higher than that predicted by the established empirical relationship between tide-water calving speed and water depth. This is surprising, given the low salinity of Laguna San Rafael and that fresh-water calving speeds are commonly much lower than those in ride water. Daily patterns of calving frequency and flux correlate poorly or not at all with meteorological variables, but tidal stage may have some control over the timing of large submarine calving events. Submarine calving produced the largest bergs. However, the relatively small total flux recorded from the submerged pars of the ice cliff may imply unusually rapid melt rates.

A major discontinuity in the variation of δ18O (δD) with altitude in high mountains was first seen in data from Mount Logan, YukonTerritory, Canada (Holdsworth and others, 1991). The profile of δ vs altitude revealed three well-defined regions: (1) a lower, monotonic, fractionation sequence below ∼3 km; (2) a middle layer, typically 1–2 km thick, within which δ values are nearly constant or stepped with altitude, and (3) part of another fractionation sequence in the “quasi-geostrophic flow region” above ∼5.3 km. The middle region was inferred to be a “mixed layer”, combining moisture from regions (1) and (3). This type of structure is now seen to occur on other high-altitude mountains, including Cerro Aconcagua, Argentina, where observations reach almost 7 km. The new observations confirm the general occurrence of a multi-layered atmosphere during precipitation at high-altitude glacier sites. This structure is linked to synoptic-scale polar cyclones, where the middle layer is identified as being the signature of the warm-front zone. These results have implications for the common practice of using a specific, spatially derived, isotopic thermometer in the time domain for the paleoclimatic interpretation of high-altitude ice-core δ records.

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.

Englacial temperature measurements in Arctic valley glaciers suggest in the ablation zone the existence of a basal layer of temperate ice lying below the bulk of cold ice. For such a polythermal glacier, a mathematical model is presented that calculates the temperature in the cold part and the position of the cold-temperate transition surface (CTS). The model is based on the continuum hypothesis for ice and the ice-water mixture, and on the conservation laws for moisture and energy. Temperate ice is treated as a binary mixture of ice and water at the melting point of pure ice. Boundary and transition conditions are formulated for the free surface, the base and the intraglacial cold-temperate transition surface. The model is reduced to two dimensions (plane flow) and the shallow-ice approximation is invoked. The limit of very small moisture diffusivity is analysed by using a stationary model with further reduction to one dimension (parallel-sided slab), thus providing the means of a consistent formulation of the transition conditions for moisture and heat flux through the CTS at the limit of negligibly small moisture diffusion.

The application of the model to the steady-state Laika Glacier, using present-day conditions, results in a wholly cold glacier with a cold sole, in sharp contrast to observations. The present polythermal state of this glacier is suspected to be a remnant of the varying climatic conditions and glacier geometry during the past few centuries. Steady-state solutions representing a polythermal structure can indeed be found within a range of prescribed conditions which are judged to be realistic for Laika Glacier at the last maximum extent of the glacier.

Passive microwave satellite observations are used to identify the presence of surface meltwater across Antarctica at daily intervals from July 1979 to June 2020, with a focus on ice shelves. Antarctic Peninsula ice shelves have the highest number of annual days of melt, with a maximum of 89 days. Over the entire time period, there are few significant linear trends in days of melt per year. High melt years can be split into two distinct categories, those with high melt days in Dronning Maud Land and Wilkes Land, and those with high melt days in the Antarctic Peninsula and the Bellingshausen Sea sector of West Antarctica. The first pattern coincides with significant negative correlations between melt days and spring and summer Southern Annular Mode. Both patterns also form the primary modes of spatial and annual variability in the dataset observed by Principal Component Analysis. Areas experiencing extended melt for the first time in years tend to show large decreases in subsequent winter microwave emissions due to structural changes in the firn. We use this to identify the impact of novel melt events, particularly over the austral summers of 1991/92 and 2015/16 on the Ross Ice Shelf.

Debris bands associated with supraglacial moraines and associated basal deposits have been logged and sampled for their ice and debris at three glaciers in northwest Spitsbergen, Svalbard. Physical properties, including sediment concentrations, sediment particle-size distributions, clast macro-fabrics, and oxygen isotope compositions, indicate that all transverse and some longitudinal debris bands originate from the basal zone of these glaciers. Transverse supraglacial bands are composed of extensive stratified-facies basal ice that is enriched in 18O and which contains polymodal debris with spatially consistent clast fabrics. These properties suggest initial formation as basal ice and subsequent elevation into an englacial position by thrusting rather than formation as crevasse fills. The formation of longitudinal debris bands results from laterally compressive folding in response to the convergence of multiple flow units into a narrow glacier tongue. In common with transverse debris bands, longitudinal bands appear to be composed of stratified basal ice. The bands exposed at the surface of austre Brøggerbreen comprise two subfacies, strongly suggesting that the glacier was at least partially warm-based in the past, when the basal ice formed.

A short-term series of quantitative observations of calving activity of Fuerza Aérea Glacier (Greenwich Island, the South Shetland Islands, Antarctica) was conducted in order to test new methods of monitoring calving. The volume of single calving events was quantified by combining terrestrial laser scanning (TLS) surveys with continuous video recording of the ice front. An empirical formula for area/volume scaling of the calved ice block was proposed based on the TLS measured calved ice volume and the calved ice front area obtained by manual delineation on the images acquired with the video camera. This combination of methods proves to be a valuable tool for glacier monitoring, providing both high-temporal resolution and precise quantitative measurements of the calving volume. The size distribution of calving events is best approximated by a power law and within the short period of observations (14 d) calving was found to be an intrinsic process not dependent on environmental forcings. Over the period of 21 January–04 February 2013 the ice flow velocity at the terminus of Fuerza Aérea Glacier was 0.26 ± 0.07 m d−1 and the calving rate was 0.41 ± 0.07 m d−1.

Supraglacial meltwater reaching a glacier bed can increase ice surface velocities via hydraulic jacking and enhanced basal sliding. However, the relationships between the structure of supraglacial drainage systems, sink-point distributions, glacier flow processes and the magnitude of interannual velocity variability are poorly understood. To explore the hypothesis that spatial variations in the rate and mechanisms of glacier flow are linked to variations in supraglacial drainage system structure and sink-point distribution across an ice cap, we mapped supraglacial drainage systems on Devon Ice Cap from Landsat-7 ETM+ imagery. Spatial patterns of surface velocity and interannual velocity variability were determined using gradient correlation applied to Landsat-7 ETM+ images. Velocity variability is greater in areas close to sink-point locations, presumably because hydrologically forced basal sliding and/or bed deformation are enhanced in such areas. The distribution and characteristics of supraglacial drainage systems may play an important role in determining the distribution of regions of basal sliding, highlighting the need for knowledge of the supraglacial drainage system structure and sink-point distribution to inform efforts to model the dynamic response of Arctic ice caps to future climate warming.

The cold-based termini of polythermal glaciers are usually assumed to adhere strongly to an immobile substrate and thereby supply significant resistance to the flow of warm-based ice up-glacier. This compressive environment is commonly thought to uplift basal sediment to the surface of the glacier by folding and thrust faulting. We present model and field evidence from the terminus of Storglaciären, Sweden, showing that the cold margin provides limited resistance to flow from up-glacier. Ice temperatures indicate that basal freezing occurs in this zone at 10−1 −10−2 m a−1, but model results indicate that basal motion at rates greater than 1 m a−1 must, nevertheless, persist there for surface and basal velocities to be consistent with measurements. Estimated longitudinal compressive stresses of 20–25 kPa within the terminus further indicate that basal resistance offered by the cold-based terminus is small. These results indicate that where polythermal glaciers are underlain by unlithified sediments, ice-flow trajectories and sediment transport pathways may be affected by subglacial topography and hydrology more than by the basal thermal regime.

Landsat digital imagery was used to search the island archipelagos of Franz Josef Land, Severnaya Zemlya and Novaya Zemlya, Russian High Arctic, for the presence of looped moraines characteristic of past glacier surges. The imagery provides almost complete summer-time coverage of the 60 000 km2 of ice in these islands. very few surge-type glaciers are identified: none in Franz Josef Land, three in Novaya Zemlya and two on Severnaya Zemlya. This contrasts greatly with Svalbard (ice-covered area 36 600 km2), to the west, where 36% of glaciers and ice-cap drainage basins are inferred to surge. The strong climatic gradient across the Eurasian High Arctic, with decreasing temperature and moisture eastward, may provide a gross control on this pattern through colder glacier thermal structure, limiting basal drainage on the thinner ice masses in particular.

The volume- and area-change evolution of glaciers can be obtained by employing the volume–area scaling approach during mass-balance modelling. This method usually requires information on the initial surface area and ice volume to adjust the volume–area relation to the specific ice body. However, absolute volumetric data on glaciers are very rare, so the applicability of volume–area scaling is limited. In order to use volume–area scaling on glaciers for which only limited information is available, a new method is presented to calibrate the volume–area relation without prior knowledge of this relation by using glacier extent information from different times. To validate the method and illustrate its practicability, we model the range of probable future changes in ice volume and surface area of ‘Glaciar Noroeste’, an outlet glacier of Gran Campo Nevado ice cap, southern Chilean Patagonia, during the 21st century, based on IPCC SRES scenarios B1 and A2.

Small glaciers appear to have been at equilibrium or shrinking very slightly during 1961–90, according to analysis of an essentially complete set of published measurements. Simple calculations give an average annual mass balance of –195 ± 59 mm a−1 (water equivalent) but this is too low because of systematic errors. Neglect of internal accumulation is responsible for some tens of millimeters of underestimate. Uneven spatial coverage, with fewer measurements where mass balances are less negative, accounts for about 50 mm a−1 of underestimate. This figure derives from spatial interpolation based on global data on ice extent and on an analysis of correlations between balance time series. The correlogram shows exponential decay, the scale length being about 600 km. The largest bias is due to a newly detected dependence of mass balance on glacier size. Among the 231 measured glaciers, many are small and belong to a restricted size range in which balance is negative, but much of the small-glacier extent is accounted for by larger glaciers in a size range where balance is indistinguishable from zero. Correcting for this size bias increases the average balance to –35 ± 89 mm a−1. Inspection of time series for 1940–95 (251 glaciers) shows that mass balance was least negative during the 1960s, and has varied in broad agreement with Northern Hemisphere temperature anomalies; smaller small glaciers (A < 16 km2) appear to be more sensitive than larger small glaciers to changes in thermal forcing. The small-glacier contribution to sea-level rise implied by this assessment is only 0.06–0.32 mm a−1, consistent with glaciers in general making little or no contribution to sea-level change during 1961–90.

The tongue of Triftgletscher, Switzerland, is particularly susceptible to major break-off events due to its steep slope. In order to detect precursors of such an event, we monitored the local seismic activity and detected 2426 icequakes with sources located in an area ranging between 2050 and 2350ma.s.l. Events triggered by cracks and icefalls were recorded, but no precise distinction between the two sources was possible using duration or frequency criteria. Clusters of seismic activity were located and confirmed by visual observations. We performed a surface motion analysis and found that surface motion was driven significantly by runoff changes at a timescale of 2-3 days. By means of a statistical analysis, a power-law behaviour of the released seismic energy distribution was discerned at certain times during the period investigated. Variations in power-law exponent values indicated that low- and high-energy events predominantly occurred during phases of enhanced and reduced surface motion, respectively. Substantial releases of seismic energy likely to signal the glacier recoupling were detected during phases of decreasing runoff. Clues to potential seismic precursors of break-off events are discussed.

Mass-balance analysis of the Greenland ice sheet based on surface elevation changes observed by the European Remote-sensing Satellite (ERS) (1992-2002) and Ice, Cloud and land Elevation Satellite (ICESat) (2003-07) indicates that the strongly increased mass loss at lower elevations (<2000 m) of the ice sheet, as observed during 2003-07, appears to induce interior ice thinning at higher elevations. In this paper, we perform a perturbation experiment with a three-dimensional anisotropic ice-flow model (AIF model) to investigate this upstream propagation. Observed thinning rates in the regions below 2000 m elevation are used as perturbation inputs. The model runs with perturbation for 10 years show that the extensive mass loss at the ice-sheet margins does in fact cause interior thinning on short timescales (i.e. decadal). The modeled pattern of thinning over the ice sheet agrees with the observations, which implies that the strong mass loss since the early 2000s at low elevations has had a dynamic impact on the entire ice sheet. The modeling results also suggest that even if the large mass loss at the margins stopped, the interior ice sheet would continue thinning for 300 years and would take thousands of years for full dynamic recovery.

At the time of its calving in February 2010, Mertz Glacier, East Antarctica, was characterized by a 145 km long, 35 km wide floating tongue. In this paper, we use GPS data from the Collaborative Research into Antarctic Calving and Iceberg Evolution (CRAC-ICE) 2007/08 and 2009/10 field seasons to investigate the dynamics of Mertz Glacier. Two months of data were collected at the end of the 2007/08 field season from two kinematic GPS stations situated on each side of the main rift of the glacier tongue and from rock stations located around the ice tongue during 2008/09. Using Precise Point Positioning with integer ambiguity fixing, we observe that the two GPS stations recorded vibrations of the ice tongue with several dominant periods. We compare these results with a simple elastic model of the ice tongue and find that the natural vibration frequencies are similar to those observed. This information provides a better understanding of their possible effects on rift propagation and hence on the glacier calving processes.

Over a floating glacier ice tongue or an ice shelf, the glacier motion measured by a single, repeat-pass, radar interferogram is difficult to analyze, because the long-term, steady motion of the ice is intermixed with its cyclic, downward motion induced by tidal forcing. Multiple interferograms and a quadruple-difference technique are necessary to separate the tidal signal from the long-term, steady motion of the ice. An example of application of this technique is given here using ERS-1 radar images of Petermann Gletscher, a major outlet glacier of northern Greenland. Tidal displacements are measured with < 5 mm statistical noise. The long-term ice Velocity is measured with a precision of 1 ma−1. The inferred tidal displacements agree well with model predictions from a fixed elastic beam with an elastic damping factor of 0.47 ± 0.01 km3. The hinge line is mapped with a precision of 20-80 m.

Combining the interferometric ice velocities with ice thickness data, the glacier ice discharge is calculated at and below the hinge line. At the hinge line, the ice flux is 12.1 ± 1 km3 a−1. At the ice front, calf-ice production is only 0.59 km3−1 a−1, meaning that 95% of the ice that crosses the grounding line melts before it reaches the calving front. Assuming steady-state conditions, the melt rate of the glacier tongue averages 12 ± 1 m a−1, with peak values exceeding 20 ma−1 near the hinge line. This high melt rate cannot be accommodated by surface ablation alone (only about 23 ma−1) and is attributed to pronounced basal melting of the ice tongue. Basal melting, often assumed to be negligible in Greenland, is the dominant process of mass release from the floating section of Petermann Gletscher.

The first three-dimensional properties of polar firn obtained by X-ray microtomography are used to study the microstructure of snow on a 15 m deep firn core from West Antarctica. The snow is found to undergo coarsening down to approximately 2.5 m depth before grain growth and densification become the prevalent mechanisms of microstructure change. In contrast to previous assumptions, distinct anisotropy of the ice and pore geometry is observed throughout the profile, with a maximum at 2.5 m depth. The air permeability and the degree of anisotropy vary with depth and can be linked to short-term changes in accumulation rate via the residence time for which a certain snow layer stays in the uppermost 2.5 m. Patterns of the degree of anisotropy and air permeability of buried polar firn are relative indicators of past accumulation rates.

We use an empirically validated high-resolution three-dimensional ice-sheet model to investigate the mass-balance regime, flow mechanisms and subglacial characteristics of a simulated Younger Dryas Stadial ice cap in Scotland, and compare the resulting model forecasts with geological evidence. Input data for the model are basal topography, a temperature forcing derived from GRIP δ18O fluctuations and a precipitation distribution interpolated from modern data. The model employs a positive-degree-day scheme to calculate net mass balance within a domain of 112500 km2, which, under the imposed climate, gives rise to an elongate ice cap along the axis of the western Scottish Highlands. At its maximum, the ice cap is dynamically and thermally zoned, reflecting topographic and climatic controls, respectively. In order to link these palaeoglaciological conditions to geological interpretations, we calculate the relative balance between sliding and creep within the simulated ice cap, forecast areas of the ice cap with the greatest capacity for basal erosion and predict the likely pattern of subglacial drainage. We conclude that ice flow in central areas of the ice cap is largely due to internal deformation, and is associated with geological evidence of landscape preservation. Conversely, the distribution of streamlined landforms is linked to faster-flowing ice whose velocity is predominantly the result of basal sliding. The geometry of the main ice mass focuses subglacial erosion in the mid-sections of topographic troughs, and produces glaciohydraulic gradients that favour subglacial drainage through low-order arterial routes.

The annual layers in an ice sheet become thinner by plastic deformation, and measurements of annual accumulation by the examination of cores therefore need correction. A simple correction factor is derived.

Melt layers are clear indicators of extreme summer warmth on polar ice caps. The visual identification of refrozen meltwater as clear bubble-free layers cannot be used to study some past warm periods, because, in deeper ice, bubbles are lost to clathrate formation. We present here a reliable method to detect melt events, based on the analysis of Kr/Ar and Xe/Ar ratios in ice cores, and apply it to the detection of melt in clathrate ice from the Eemian at NEEM, Greenland. Additionally, melt layers in ice cores can compromise the integrity of the gas record by dissolving soluble gases, or by altering gas transport in the firn, which affects the gas chronology. We find that the easily visible 1 mm thick bubble-free layers in the WAIS Divide ice core do not contain sufficient melt to alter the gas composition in the core, and do not cause artifacts or discontinuities in the gas chronology. The presence of these layers during winter, and the absence of anomalies in soluble gases, suggests that these layers can be formed by processes other than refreezing of meltwater. Consequently, the absence of bubbles in thin crusts is not in itself proof of a melt event.