Glaciers and climate change: Interpretation of 50 years of direct mass balance of Hintereisferner

Abstract

Direct mass balance data of Hintereisferner glacier annually measured for fifty years were reanalyzed and bias-corrected. The glacier area and the patterns of the spatial distribution of specific mass balance were homogenized using the measured data and the current methods of modern mass balance analysis on Hintereisferner. The homogenized mass balance shows a good agreement with the geodetic and the hydrological mass balance. The comparison with modelled mass balance and measured temperature data showed that the homogenized mass balance correlated best with TS sum (R² = 0.76) followed by the simple degree-day sum (R² = 0.60) and the mean summer temperature (R² = 0.55). From that and from the calculation of the effects of albedo changes follows that the frequency and duration of summer snowfalls play an important role in the summer ablation of the glacier.

The analysis of sub areas shows that at high elevations mass balance is dominated by the influence of winter precipitation. At low elevations, the increasingly negative mass balance was a result of the increase of the mean summer temperatures and the decrease of surface elevation. Between 1953 and 2003, the surface of the glacier tongue lowered by 160 m. This corresponds to a temperature increase of about 1 °C at the surface 2003 compared to the surface 1953. In the same period, the potential incoming solar radiation during the summer is reduced by the surface lowering. Comparing the effect of these two factors, the impact of the topographic temperature change on mass balance is much higher than the impact of increased shading. At higher elevations, the effect of topographic changes is small compared to changes in the mean surface albedo.

The separation of glacier tributaries has been decreasing the inflow of ice to the main tongue. The mass balance of the glacier parts connected to the main tongue decreases faster than the mass balance of the total area. Therefore, the retreat of Hintereisferner is governed by a more negative mass balance than measured for the total area.

Introduction

Glaciers are the most visible indicators of global change. The documentation of the changing cryosphere includes glacier inventories, records of changes in glacier length and runoff, and mass balance measurements (Lemke et al., 2007). These data allow the calculation of the past and current contributions of land ice to the sea level change (Ohmura, 2004, Cogley, 2005, Dyurgerov and Meier, 2005, Oerlemans et al., 2007). Additionally, glaciological data are used to reconstruct former climate states (Oerlemans, 2005, Kerschner and Ivy-Ochs, 2007) and complement other climate proxies for the holocene. Since the instrumental climate data are biased towards low altitudes, glacier data offer an archive of high mountain climate that is subject to strong gradients and improve our knowledge on climate to remote regions during the instrumental period. For the development of forward and inverse models, the understanding of the glacier–climate relation is essential. Thus the relationships between different types of glaciological data measured worldwide must be studied on test glaciers, where all kinds of glacier and climate data are available. Hintereisferner may be considered as one of these test glaciers: The continuous record of mass balance measurements begins in 1952/53 and thus is amongst the longest worldwide. Glacier length change has been measured annually since 1933. At the same time, the nearby climate station Vent was installed, together with a number of rain gauges in the basin and one runoff gauge (Fig. 1).

The mass balance of a glacier is determined by climate, topography and surface albedo. During long term mass balance series, the methods, the climate and the glaciers change. To provide mass balance data for climate change studies, long term mass balance series need careful reanalysis and bias correction. The long term series of Griesgletscher and Silvrettagletscher in Switzerland (Huss et al., 2009) and Glacier de Sarennes (Thibert et al., 2008) were already reanalyzed.

The first aim of this study is the correction of the mass balance series of Hintereisferner for changes in methods. That includes the homogenization of glacier area and the extrapolation of point measurements to this area with the help of spatial patterns of accumulation and ablation.

Within the last 50 years, the surface lowering caused a local decrease of the direct solar radiation, but an increase of the surface air temperature. The recession of the firn covered area and the decrease of the seasonal snow cover reduces the mean summer albedo. The second aim of this study is to quantify these effects on the mass balance. Then the homogenized mass balance is compared to (1) the volume change calculated from DEMs acquired in 1953, 1964,1967, 1969,1979, 1991, 1997 and 2006; (2) the change of glacier length; (3) the hydrological mass balance of the Rofenache basin; (3) the mass balance calculated with the TS degree-day model of Hoinkes and Steinacker, 1975a, Hoinkes and Steinacker, 1975b; and (4) the time series of meteorological data measured in Vent.

Studies of glaciers and climate often calculate glacier mass balance relating the equilibrium line altitude (ELA) to air temperature. Assuming a relationship between ELA and the vertical profiles of mass balance the annual glacier mass balance can be modelled (Oerlemans, 2001, Zemp et al., 2006). To investigate the validity of these assumptions for Hintereisferner, the shapes of the vertical profiles of the homogenized mass balance are statistically analyzed.

Several studies present numbers of the sensitivity of mass balance to temperature changes derived from measurements (Vincent, 2002) or models (Oerlemans, 2001). These numbers are compared to those derived from the meteorological data measured in Vent and the homogenized mass balance of Hintereisferner for different time periods.