No abstract

Glacier-wide mass balance has been measured for more than sixty years and is widely used as an indicator of climate change and to assess the glacier contribution to runoff and sea level rise. Until recently, comprehensive uncertainty assessments have rarely been carried out and mass balance data have often been applied using rough error estimation or without consideration of errors. In this study, we propose a framework for reanalysing glacier mass balance series that includes conceptual and statistical toolsets for assessment of random and systematic errors, as well as for validation and calibration (if necessary) of the glaciological with the geodetic balance results. We demonstrate the usefulness and limitations of the proposed scheme, drawing on an analysis that comprises over 50 recording periods for a dozen glaciers, and we make recommendations to investigators and users of glacier mass balance data. Reanalysing glacier mass balance series needs to become a standard procedure for every monitoring programme to improve data quality, including reliable uncertainty estimates

Understanding the flow of water through the body of a glacier is important, because the spatial distribution of water and the rate of infiltration to the glacier bottom is one control on water storage and pressure, glacier sliding and surging, and the release of glacial outburst floods. According to the prevailing hypothesis, this water flow takes place in a network of tubular conduits. Here we analyse video images from 48 boreholes drilled into the small Swedish glacier Storglaciären, showing that the glacier's hydrological system is instead dominated by fractures that convey water at slow speeds. We detected hydraulically connected fractures at all depths, including near the glacier bottom. Our observations indicate that fractures provide the main pathways for surface water to reach deep within the glacier, whereas tubular conduits probably form only in special circumstances. A network of hydraulically linked fractures offers a simple explanation for the origin and evolution of the englacial water flow system and its seasonal regeneration. Such a fracture network also explains radar observations that reveal a complex pattern of echoes rather than a system of conduits. Our findings may be important in understanding the catastrophic collapse of ice shelves and rapid hydraulic connection between the surface and bed of an ice sheet.

[1] A characteristic feature of ground penetrating radar (GPR) surveys on polythermal glaciers is an internal reflection presumably caused by the cold temperate transition surface (CTS), hence providing a possible tool for mapping thermal structure with high accuracy. Comparison of detailed temperature measurements in bore holes and GPR profiles at 345 MHz and 800 MHz center frequencies on Storglaciären, Sweden, show that the CTS can be detected and mapped with an accuracy of about ±1 m at both frequencies. A comparison between comprehensive GPR surveys of the cold surface layer, separated by 12 years (1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001), shows a substantial and complex thinning of the cold layer. An overall decrease of 8.3 m (22% of average thickness) of the CTS depth is much larger than uncertainties in CTS depth determinations. The stability of the cold surface layer depends on the net ice ablation at the surface and the downward migration of CTS. There is no evidence of substantial increased net ablation between the survey dates that could explain the observed thinning. However, small increase in average winter air temperature, a limiting factor for the temperature gradient through the cold surface layer, may provide a partial explanation. The weaker temperature gradient reduces the transport of latent heat from the CTS, thus slowing down its downward migration.

Subhourly measurements of bed deformation, bed shear strength, subglacial water pressure, and surface speed at Storglaciären, a glacier in northern Sweden, showed that the shear-strain rates of the bed decrease during periods of high water pressure and surface speed. High water pressures appear to be accompanied by a reduction in the coupling of ice with the bed that is sufficient to reduce or eliminate shearing. The instability of large ice masses may result from similar decoupling rather than from pervasive bed deformation, as has been commonly thought.

Abstract. Storglaciären, located in the Kebnekaise massif in northern Sweden, has a long history of glaciological research. Early photo documentations date back to the late 19th century. Measurements of front position variations and distributed mass balance have been carried out since 1910 and 1945/46, respectively. In addition to these in-situ measurements, aerial photographs have been taken at decadal intervals since the beginning of the mass balance monitoring program and were used to produce topographic glacier maps. Inaccuracies in the maps were a challenge to early attempts to derive glacier volume changes and resulted in major differences when compared to the direct glaciological mass balances. In this study, we reanalyzed dia-positives of the original aerial photographs of 1959, -69, -80, -90 and -99 based on consistent photogrammetric processing. From the resulting digital elevation models and orthophotos, changes in length, area, and volume of Storglaciären were computed between the survey years, including an assessment of related errors. Between 1959 and 1999, Storglaciären lost an ice volume of 19×10 6 m 3 , which corresponds to a cumulative ice thickness loss of 5.69 m and a mean annual loss of 0.14 m. This ice loss resulted largely from a strong volume loss during the period 1959-80 and was partly compensated during the period 1980-99. As a consequence, the glacier shows a strong retreat in the 1960s, a slowing in the 1970s, and pseudo-stationary conditions in the 1980s and 1990s.

[1] The volume fraction of liquid water in temperate glacier ice is important not only for the flow of glaciers and the analysis and processing of ground penetrating radar data from glaciers but also for the stability of the thermal layering in polythermal glaciers. However, little is known about the spatial variations of water content in glaciers. We use relative backscatter strength of ground-penetrating radar signals to estimate the spatial distribution of water content close to the cold-temperate transition on Storglaciären, northern Sweden, in an area close to the equilibrium line. The values of relative backscatter strength are calibrated using determinations of absolute water content from temperature measurements across the cold-temperate transition and the thermodynamic boundary condition at the freezing front. The results show a water content of 0.80%, 0.75%, and 0.58% at three calibration points and a mean water content of 0.8% with a standard deviation of ±0.26% for the extrapolated water content. The extrapolated water content shows a distinct pattern, with lower water content on one side of the glacier center line and higher water content on the other side, with higher water content on the northern side. We hypothesize that the different water contents result from the fact that the ice on either side of the center line originates from different cirques, thus implying spatial variations in the entrapment of water in the firn-ice transition process in the different cirques.

The discussion on global change has led to increased interest in glacier mass balance since glaciers can be used as climatic indicators. To meet the need for high-quality mass-balance data requires critical examination of traditional mass-balance methods and their possible errors. One issue regarding mass-balance measurements that has received little attention is internal accumulation. Our study shows that internal accumulation in the firn layer of Storglaciären, Sweden, significantly affects the mass balance of the glacier. This occurs because the winter cold wave penetrates below the previous year’s summer surface and into underlying firn. We estimated internal accumulation from measurements of temperature and water content in firn. The depth of the 0°C isotherm correlated with snow depth and air temperature, so that low snow depth and low air temperature separately cause a deeper 0°C isotherm. We determined irreducible gravimetric water content in firn to 2–3%, which corresponds to an irreducible water saturation of 6–8%. Our value for firn is relatively high compared with that for snow, probably due to trapped water in isolated firn pores. Refreezing of percolating meltwater in spring accounted for ~30% of annual internal accumulation. The remaining 70% was due to re-freezing of retained capillary water in firn pores during winter. Disregarding internal accumulation would lead to underestimation of annual net mass balance by 0.04–0.06 m w.e., corresponding to 3–5% of annual accumulation of the entire glacier in an average year. Hence, internal accumulation potentially becomes a source for systematic error if not accounted in mass-balance measurements.