冰川冰储量计算方法及发展趋势

冰川冰储量不仅是冰川的重要属性,而且是核算冰川水资源及预测冰川变化的基础数据,因此准确计算冰川冰储量及其变化具有重要的理论与现实意义。目前冰川储量估算的主要方法有经验公式法、冰厚模型估算法、探地雷达法;冰川储量相对变化计算方法有实地测量法和遥感监测法。通过系统分析和讨论各计算方法的原理、现状及存在的问题,以期为冰川储量估算提供方法参考。研究表明：对于冰川冰储量计算而言,经验公式法适用于区域性或全球性的冰川储量估算;模型估算法适用于个体或小范围冰川储量估算;探地雷达法适用于人类易到达区域冰川储量的估算。对于冰川冰储量相对变化计算,实地测量法适用于对精度要求高且满足实地测量条件的单条或中小型冰川,遥感监测法适用于全球性冰储量变化估算,但需改进算法和提高数据空间分辨率。目前,随着无人机技术的逐步应用,以及冰川流速等理论模型的提出,为冰川冰储量估算方法的发展提供了新契机。     

The drainage pattern of Hansbreen and Werenskioldbreen, two polythermal glaciers in Svalbard

To improve our understanding of Svalbard-type polythermal glacier drainage, hydraulic geometry models of the subglacial hydrology of two contrasting glaciers in Svalbard have been constructed. The models are tested against a uniquely long and rich set of field observations spanning 45 years. Digital elevation models (DEMs) were constructed from bedrock data measured with ground penetrating radar and surface data of two medium-sized polythermal glaciers, Hansbreen and Werenskioldbreen, in south-west Spitsbergen. Hansbreen has a low angle bed with over-deepenings and a calving front, while Werenskioldbreen has steeper bed and terminates on land. Together they are representative of many Svalbard glaciers. The DEMs were used to derive maps of hydraulic potential and subglacial drainage networks. Validation of the models was done using field observations including location mapping and speleological exploration of active moulins, positions of main river outflows, dyetracing and water chemistry studies, and observations of water pressure inside moulins. Results suggest that the water pressure is generally close to ice overburden pressure but varies greatly depending on local conditions such as bed location, the thickness of cold ice layer, the thickness of the glacier and seasonal changes in meltwater input.     

CHANGES IN ICE-MARGIN PROCESSES AND SEDIMENT ROUTING DURING ICE-SHEET ADVANCE ACROSS A MARGINAL MORAINE

Advance of part of the margin of the Greenland ice sheet across a proglacial moraine ridge between 1968 and 2002 caused progressive changes in moraine morphology, basal ice formation, debris release, ice‐marginal sediment storage, and sediment transfer to the distal proglacial zone. When the ice margin is behind the moraine, most of the sediment released from the glacier is stored close to the ice margin. As the margin advances across the moraine the potential for ice‐proximal sediment storage decreases and distal sediment flux is augmented by reactivation of moraine sediment. For six stages of advance associated with distinctive glacial and sedimentary processes we describe the ice margin, the debris‐rich basal ice, debris release from the glacier, sediment routing into the proglacial zone, and geomorphic processes on the moraine. The overtopping of a moraine ridge is a significant glaciological, geomorphological and sedimentological threshold in glacier advance, likely to cause a distinctive pulse in distal sediment accumulation rates that should be taken into account when glacial sediments are interpreted to reconstruct glacier fluctuations.     

A Century of Climate Variability and Climate Gradients from Coast to Ice Sheet in West Greenland

This work describes the temperature trends of the Nuuk fjord area during the last century and analyses the spatial variability of temperature and precipitation along the coast‐inland gradient of the region. A better understanding of the climate of the coastal region of Greenland is important for ice sheet studies since significant thinning has recently been detected on local outlet glaciers with driving factors still to be identified. The glaciological reconstruction of the ice sheet mass balance has often made use of climate datasets from the coastal stations to substitute missing data from the inland areas. The information on the coast‐inland gradient of temperature and precipitation hereafter provided is of relevance in other areas around the ice sheet as it represents a mean for increasing the accuracy of historical glacier‐climate interactions. We therefore discuss climatological information from the automatic station that operated at the glacier throughout the 1980s as well as from other stations located along the fjord and with a longer coverage.     

斯瓦尔巴群岛冰川学研究进展与我国北极冰川监测系统建设

北极斯瓦尔巴群岛冰川大多数属于亚极地型(sub-polar)或多热型(polythermal)。Austre Br(?)ggerbreen和Midre Lovénbreen冰川(＜10km~2)长时间系列物质平衡研究显示,自小冰期结束以来几乎所有的观测年中夏季消融比冬季积累更大,导致冰体稳定地减小;而面积更大、海拔高度更高的冰川如Kongsvegen冰川(105km~2)则更加接近稳定态的平衡。斯瓦尔巴群岛冰川流动速率一般较低,但跃动相当频繁,控制跃动型冰川空间分布的因素包括冰川长度、基底岩性和多热场。可通过冰川水文特征、钻孔温度测量和无线电回波探测获取斯瓦尔巴群岛冰川热场的信息。斯瓦尔巴群岛冰川的低流速和多热性结构对冰川上的排水系统相当重要,整个群岛淡水径流的四个主要来源分别是冰川消融、雪融化、夏季降雨和冰崩解,经验回归方法和模式方法用于计算淡水径流量。因夏季融水渗浸作用、采样分辨率低和化学成分分析有限,早期斯瓦尔巴群岛冰芯的准确定年受到严重影响,但最近的研究显示,来自斯瓦尔巴群岛冰帽的冰芯数据仍然能够提供重要的气候和环境信息。通过我国北极黄河站2005年度科学考察,我们已初步建立了Austre Lovénbreen冰川和Pedersenbreen冰川监测系统,并计划在Austre Lovénbreen冰川进行钻孔温度测量、冰川气象要素观测、冰川前缘水文观测以及冰川厚度和内部结构测量,重点开展斯瓦尔巴群岛冰川基本特征和发育条件、冰川表面能量和物质平衡、冰川波动与气候变化关系、淡水径流年际和季节性变化和气/雪/冰界面过程等方面的研究。     

The Origin and Significance of Debris-charged Ridges at the Surface of Storglaciären, Northern Sweden

Storglaciären is a 3.2 km long polythermal valley glacier in northern Sweden. Since 1994 a number of small (1–2 m high) transverse debris‐charged ridges have emerged at the ice surface in the terminal zone of the glacier. This paper presents the results of a combined structural glaciological, isotopic, sedimentological and ground‐penetrating radar (GPR) study of the terminal area of the glacier with the aim of understanding the evolution of these debris‐charged ridges, features which are typical of many polythermal glaciers. The ridges originate from steeply dipping (50–70°) curvilinear fractures on the glacier surface. Here, the fractures contain bands of sediment‐rich ice between 0.2 and 0.4 m thick composed of sandy gravel and diamicton, interpreted as glaciofluvial and basal glacial material, respectively. Structural mapping of the glacier from aerial photography demonstrates that the curvilinear fractures cannot be traced up‐glacier into pre‐existing structures visible at the glacier surface such as crevasses or crevasse traces. These curvilinear fractures are therefore interpreted as new features formed near the glacier snout. Ice adjacent to these fractures shows complex folding, partly defined by variations in ice facies, and partly by disseminated sediment. The isotopic composition (δ¹⁸O) of both coarse‐clear and coarse‐bubbly glacier ice facies is similar to the isotopic composition of the interstitial ice in debris layers that forms the debris‐charged ridges, implying that none of these facies have undergone any significant isotopic fractionation by the incomplete freezing of available water. The GPR survey shows strong internal reflections within the ice beneath the debris‐charged ridges, interpreted as debris layers within the glacier. Overall, the morphology and distribution of the fractures indicate an origin by compressional glaciotectonics near the snout, either at the thermal boundary, where active temperate glacier ice is being thrust over cold stagnant ice near the snout, or as a result of large‐scale recumbent folding in the glacier. Further work is required to elucidate the precise role of each of these mechanisms in elevating the basal glacial and glaciofluvial material to the ice surface.     

Inferring Glacier Mass Balance Using Radarsat: Results From Peyto Glacier, Canada

The capability of RADARSAT synthetic aperture radar (SAR) for the purpose of snow-line/accumulation area mapping for a temperate alpine glacier is examined. In agreement with other orbital C-band SAR studies, RADARSAT can discriminate between firn and bare ice facies. Limited observations are reported with respect to the electromagnetic variability of the ice facies in the ablation area, but they are inconclusive. Operational considerations are discussed with respect to reconciling the uncertainties of late-summer weather and their possible impact on the dielectric and scattering properties of the glacier surface. Vagaries associated with other glacier settings, mass balance states and their associated facies configurations are discussed including the difficulty of using the transient snow-line to define the equilibrium line and the lower extent of the accumulation area for glaciers where superimposed ice may form.
The radar remote-sensing reconnaissance of equilibrium line altitude (ELA) and accumulation area ratio (AAR) for estimating glacier mass balance requires serious consideration in those instances where traditional ground measurements used in the direct glaciological method are absent. However, with respect to the ELA, such estimates can vary depending on the accuracy of the reference digital elevation information. Moreover, for many glacier configurations, where mass balance variations due to altitude are influenced or in some cases completely masked by local balance variations, defining the ELA may be an irreconcilable problem. Using the AAR may be more robust in this regard. It is further determined that the total error inherent in the reconnaissance method would have serious implications for the confident estimation of mass balance normals and climate-related trends if the method were to be utilized over the longer term.     

Surface Heat Balance and Spatially Distributed Ablation Modelling at Koryto Glacier, Kamchatka Peninsula, Russia

To investigate the characteristics of ablation at Koryto Glacier, a mountain glacier under maritime climate in Kamchatka Peninsula, Russia, we made field observations from August to early September 2000. At a site near the equilibrium line, the 31‐day average net radiation, sensible heat flux, and latent heat flux were 43, 59 and 31 W^?2, respectively. We developed a new distributed ablation model, which only needs measurements of air temperature and global radiation at one site. Hourly ablation rates at this site obtained by the energy balance method are related to measured air temperature and global radiation by linear multiple regression. A different set of multiple regression coefficients is fitted for snow and ice surfaces. Better estimates of ablation rate can be obtained by this approach than by other temperature index models. These equations are then applied to each grid cell of a digital elevation model to estimate spatially distributed hourly melt. Air temperature is extrapolated using a constant temperature lapse rate and global radiation is distributed considering topographic effects. The model enables us to calculate the hourly spatial distribution of ablation rates within the glacier area and could well provide a realistic simulation of ablation over the whole glacier.     

GLACIOLOGICAL STUDIES AT RAE GLACIER,CANADIAN ROCKY MOUNTAINS

Rae Glacier is a small cirque glacier located in the front ranges of the Canadian Rocky Mountains. In 1990 and 1991 field research was completed to describe the physical glaciology of Rae Glacier and to characterize historical glaciological trends at the site. Ablation and surface movement rates were measured using a network of stakes drilled into the glacier and radio-echo sounding was used to describe local ice depths.

Rae Glacier has experienced a significant loss in size and mass during the historical period, owing to a lengthy interval of negative mass-balance conditions. The glacier has decreased in surface area by over 50% and now contains less than 24% of the ice it did at the end of the last century.

Surface-ice velocity varied between 1.4 and 5.4 m from 1990 to 1991. Rates of ice ablation proved to be highly variable, with steeper areas showing up to 50% more ablation. Combined with data on the emergent flow component of the glacier, the ablation data suggest that the glacier presently is unable to replenish the amount of ice annually being lost to ablation. The glacier has a lag time of 5 to 10 years, which confirms that it is sensitive to climatic fluctuations and responds to changes in mass balance within a very short time. This observation is supported by an estimated response time of 42 years. [Key words: glaciology, Rae Glacier, Canadian Rocky Mountains.]     

Measurements and Models of the Mass Balance of Hintereisferner

This paper summarizes the methods applied to determine the mass balance of Hintereisferner and several other glaciers in the Tyrolean Alps since 1952. On an annual basis the direct glaciological method was applied with fixed date measurements on 10–15 accumulation pits and 30–90 ablation stakes on 9 km².
Indirect mass balance determination from equilibrium line altitude, accumulation area ratios or representative stakes, yield fair results and some exceptions could be related to anomalous meteorological conditions.
Monthly or more frequent stake readings supplied time series of ablation at various altitudes and slope aspects that served as basis for the calibration of energy and mass balance models. Of various models developed, two are presented in this paper. Both are based on degree days, one using daily values from a valley station to predict the mean annual balance of the entire glacier, while the other calculates day-to-day changes at 50-m grid points on the glacier.
The geodetic method has been applied for longer periods and yields results consistent with those of the glaciological method. The balance velocity calculated from recent ice thickness soundings and accumulation measurements is significantly less than observed velocity.     

THE RESPONSE OF PARTIALLY DEBRIS‐COVERED VALLEY GLACIERS TO CLIMATE CHANGE: THE EXAMPLE OF THE PASTERZE GLACIER (AUSTRIA) IN THE PERIOD 1964 TO 2006

Long‐term observations of partly debris‐covered glaciers have allowed us to assess the impact of supra‐glacial debris on volumetric changes. In this paper, the behaviour of the partially debris‐covered, 3.6 km² tongue of Pasterze Glacier (47°05′N, 12°44′E) was studied in the context of ongoing climate changes. The right part of the glacier tongue is covered by a continuous supra‐glacial debris mantle with variable thicknesses (a few centimetres to about 1 m). For the period 1964–2000 three digital elevation models (1964, 1981, 2000) and related debris‐cover distributions were analysed. These datasets were compared with long‐term series of glaciological field data (displacement, elevation change, glacier terminus behaviour) from the 1960s to 2006. Differences between the debriscovered and the clean ice parts were emphasised. Results show that volumetric losses increased by 2.3 times between the periods 1964–1981 and 1981–2000 with significant regional variations at the glacier tongue. Such variations are controlled by the glacier emergence velocity pattern, existence and thickness of supra‐glacial debris, direct solar radiation, counter‐radiation from the valley sides and their changes over time. The downward‐increasing debris thickness is counteracting to a compensational stage against the common decrease of ablation with elevation. A continuous debris cover not less than 15 cm in thickness reduces ablation rates by 30–35%. No relationship exists between glacier retreat rates and summer air temperatures. Substantial and varying differences of the two different terminus parts occurred. Our findings clearly underline the importance of supra‐glacial debris on mass balance and glacier tongue morphology.     

东帕米尔高原昆盖山跃动冰川遥感监测研究

冰川跃动是冰川周期性地快速运动,给下游生命财产安全带来巨大威胁。对已经发现的跃动冰川进行监测不仅有助于提高对冰川跃动机理的认识,而且对冰川跃动灾害预警预报和风险评估都具有重要的意义。在中国第二次冰川编目中发现,1963-2009年东帕米尔高原昆盖山的5Y663L0023冰川末端发生大幅前进。本文利用Landsat影像、ASTER立体像对等数据对该冰川前进过程进行监测研究。结果表明：该冰川于1990-1992年和2007-2013年分别前进81±30 m和811±30 m,其中2007-2013年的前进属于跃动引发的前进。其中跃动最高峰在2007年8月21日-2008年10月26日,期间32.7×10⁶ m³的冰体发生卸载,导致末端前进了704±30 m,面积扩张了0.34 km²。针对东帕米尔地区跃动冰川周期研究的空白,本文认为该冰川跃动周期中跃动期为4 a,平静期最短为15 a左右。该冰川属于多温型冰川,跃动受热力学机制影响的可能性较大,但液态降水、冰雪融水的增加也是影响因素。     

The Verification and Significance of Three Approaches to Longitudinal Stresses in High–resolution Models of Glacier Flow

With the purpose of improving the ice physics underpinning time–dependent glacier flowline models, three independent approaches for solving longitudinal stresses in glaciers are discussed and verified by application to Haut Glacier d'Arolla. To highlight any shortcomings, the reduced and much utilised driving stress approximation is also applied and compared. Modelled velocity patterns using the three full stress schemes exhibit consistency with one another and good coincidence with observed velocities for the 1991 summer melt season. Furthermore, these stress patterns indicate that longitudinal stresses are significant and of a similar order of magnitude as the basal shear stress components. However, the driving stress approximation yields erratic fluctuations in the stress and velocity fields which are neither realistic in terms of mass continuity nor agree with observations. Basal decoupling experiments indicate a complex relationship between basal velocity and englacial stresses with considerable dampening of any basal perturbation occurring as it is dissipated towards the surface and transferred throughout the ice mass. The driving stress approximation fails to account at all for any such coupling. Experiments to identify the length scale over which longitudinal effects operate indicate that they are significant even up to 10 ice thicknesses. The implication here is that longitudinal stresses play a significant role in determining glacier dynamics on length scales up to at least 2 km and that the predictive power of models of glacier flow based purely on the driving stress approximation is therefore subject to significant limitations. Inclusion of longitudinal stresses overcomes one of the main limitations imposed on such models and, given the potential ease of incorporation of the schemes described here, this deficiency may readily be resolved.     

Snow accumulation, melt, mass loss, and the near-surface ice temperature structure of Irenebreen, Svalbard

This study examines the mass balance, accumulation, melt, and near-surface ice thermal structure of Irenebreen, a 4.1 km² glacier located in northwest Spitsbergen, Svalbard. Traditional glaciological mass balance measurements by stake readings and snow surveying have been conducted annually at the glacier since 2002, yielding a mean annual net mass balance of −65 cm w.e. for the period 2002–2009. In 2009, the annual mass balance of Irenebreen was −63 cm w.e. despite above-average snow accumulation in winter. The near-surface ice temperature in the accumulation area was investigated with automatic borehole thermistors. The mean annual surface ice temperatures (September–August) of the accumulation area were −3.7 °C at 1 m depth and −3.3 °C at 10 m depth. Irenebreen is potentially polythermal, with cold ice and a temperate surface layer during summer. This temperate surface layer is influenced by seasonal changes in temperature. In winter, the temperature of all the ice is below the melting point and temperate layers are probably present in basal sections of the glacier. This supposition is supported by the presence of icings in the forefield of Irenebreen.     

ICE‐MARGINAL SEDIMENT DELIVERY TO THE SURFACE OF A HIGH‐ARCTIC GLACIER: AUSTRE BRØGGERBREEN,SVALBARD

Enhanced delivery of water‐saturated, ice‐marginal sediments to the glacier surface is a response to glacier thinning that has the potential to increase both levels of sediment transfer through the glacier hydrological system and total basin sediment yields. Preliminary observations made during summer 2007 at Austre Brøggerbreen, Svalbard, confirm that ice‐marginal debris flows in the upper reaches of the glacier are actively delivering sediments to the glacier surface, which may then be flushed into the glacier's hydrological system. During a four‐day observation period, several stochastic pulses in water turbidity were observed at a single portal where solely supra‐ and englacial drainage emerge at the glacier margin. The erratic suspended sediment fluxes were hypothesized to originate from ice‐marginal sources. Quantitative analysis of continuous turbidity and discharge data confirm that discharge is not driving these turbidity pulses and, combined with observational data, that the most likely origin is the delivery of water‐saturated sediments to the glacier surface from ice‐marginal, debris flows with subsequent transfer to the portal via the glacial drainage system. These observations illustrate the potential importance of the paraglacial component to the overall sediment cascade of deglaciating basins and highlight the need for careful interpretation of turbidity records, where stochastic pulses in turbidity may be attributed to sources and processes other than ice‐marginal sediment inputs.     

Decadal‐Scale Changes of the Ödenwinkelkees,Central Austria,Suggest Increasing Control of Topography and Evolution Towards Steady State

Small mountain glaciers have short mass balance response times to climate change and are consequently very important for short‐term contributions to sea level. However, a distinct research and knowledge gap exists between (1) wider regional studies that produce overview patterns and trends in glacier changes, and (2) in situ local scale studies that emphasise spatial heterogeneity and complexity in glacier responses to climate. This study of a small glacier in central Austria presents a spatiotemporally detailed analysis of changes in glacier geometry and changes in glaciological behaviour. It integrates geomorphological surveys, historical maps, aerial photographs, airborne LiDAR data, ground‐based differential global positioning surveys and Ground Penetrating Radar surveys to produce three‐dimensional glacier geometry at 13 time increments spanning from 1850 to 2013. Glacier length, area and volume parameters all generally showed reductions with time. The glacier equilibrium line altitude increased by 90 m between 1850 and 2008. Calculations of the mean bed shear stress rapidly approaching less than 100 kPA, of the volume–area ratio fast approaching 1.458, and comparison of the geometric reconstructions with a 1D theoretical model could together be interpreted to suggest evolution of the glacier geometry towards steady state. If the present linear trend in declining ice volume continues, then the Ödenwinkelkees will disappear by the year 2040, but we conceptualise that non‐linear effects of bed overdeepenings on ice dynamics, of supraglacial debris cover on the surface energy balance, and of local topographically driven controls, namely wind‐redistributed snow deposition, avalanching and solar shading, will become proportionally more important factors in the glacier net balance.     

Study of annual mass balance(2011–2013) of Rikha Samba Glacier,Hidden Valley,Mustang, Nepal

Although Himalayan glaciers are of particular interest in terms of future water supplies, regional climate changes, and sea-level rises, little is known about them due to lack of reliable and consistent data. There is a need for monitoring these glaciers to bridge this knowledge gap and to provide field measurements necessary to calibrate and validate the results from different remote sensing operations. Therefore, glaciological observations have been carried out by the Cryosphere Monitoring Project(CMP) since September 2011 on Rikha Samba Glacier in Hidden valley, Mustang district in western Nepal in order to study its annual mass balance. This paper presents the first results of that study. There are 10 glaciers in Hidden Valley, named G1, G2, G3, up to G10. Of these, G5 is the Rikha Samba Glacier, which has the largest area(5.37 km2) in this valley and the highest and lowest altitudes(6,476 and 5,392 m a.s.l., respectively). The glacier mass balance discussed in this paper was calculated using the glaciological method and the equilibrium line altitude(ELA). The glacier showed a negative annual point mass balance along the longitudinal profile of its lower part from September 10, 2011 to October 3, 2012. Stake measurements from October 4, 2012 to September 30, 2013 indicated a negative areal average of annual mass balance-0.088±0.019 m w.e. for the whole glacier. Based on these observations, the ELA of the Rikha Samba Glacier is estimated at 5,800 m a.s.l. in 2013. This negative balance may be due to rising air temperatures in the region, which have been incrementally rising since 1980 accompanied by little or no significant increase in precipitation in that period. The negative mass balance confirms the general shrinking trend of the glacier.     

Subglacial topography of Jutulstraumen outlet glacier,East Antarctica,mapped from ground-penetrating radar,optical and interferometric synthetic aperture radar satellite data

The bed topography of Jutulstraumen is of interest in connection with glaciological modelling for climatic and geological studies. During the Norwegian Antarctic Research Expedition 1996/97 (NARE 96/97) 971 km of bed topography profiles were mapped using ground-penetrating radar (GPR) and a global positioning system (GPS). As additional information to the GPR data for the map compilation, surface features connected to the bed topography were identified in optical satellite images, and relative velocities derived from interferometric synthetic aperture radar (INSAR) data were interpreted using principles of ice flow related to the bed topography. Grids (250 x 250 m) covering the area 73° to 71°S, 0° to 4°E of bed topography and ice thickness were developed. Several observations are confirmed by the independent data sets. The Jutulstraumen outlet glacier shows extreme topography with subglacial plateaus and deep valleys due to faulting. A previously unmapped valley in connection with the fault system is identified. The data sets provide evidence of a grounding line position 6-20 km further inland than previously expected, and a trough occurs 60 km up-glacier from this position.     

IDENTIFICATION OF CLIMATE CONTROLS ON THE DYNAMIC BEHAVIOUR OF THE SUBARCTIC GLACIER SALAJEKNA, NORTHERN SCANDINAVIA

ABSTRACT. In this paper we describe the dynamic behaviour of Salajekna, a valley glacier, over the last 200 years using terrestrial observations, in situ measurements, remote sensing observations and glacier reconstructions. The response time of the glacier was calculated using analytical models and field measurements. We were subsequently able to attribute specific dynamic responses to climate trends in the available climate record. The glacier's historical maximum extension was reached around 1880–1910 and was the result of a more continental climate with multi-modal airflows in the late 18th and early 19th centuries. A transition to more maritime conditions in the mid-19th century resulted in a near-continuous 20th century retreat before the glacier adjusted to a near-steady state.     

Observations of the Drainage of an Ice-Dammed Lake — Strupvatnet,Troms, Norway

The behaviour of the ice-dammed lake, Strupvatnet, Troms County, Norway, is described. Past observations are noted and related to observations in 1959 and to other ice-dammed lakes. Mechanisms for opening and maintaining water flow during a ‘hlaup’ are discussed. It is considered that Liestol's melt widening process operates after initiation by a pressure gradient across the dam. The lake and the internal drainage system of the glacier are thus linked. There is no evidence of lifting of the ice dam at Strupbreen.