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 km(2) tongue of Pasterze Glacier (47 degrees 05'N, 12 degrees 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.