20 years of glacier change: the homogenized glacier inventories for South Tyrol 1997-2005-2017

Abstract

This paper presents a set of three homogenized glacier inventories for the Italian region of South Tyrol for the years 1997, 2005 and 2017. Within this framework, in addition to the compilation of the 2017 inventory from newly recorded data, a complete re-evaluation of the 2005 regionwide laser scan data was performed, resulting in the creation of a new 2005 glacier inventory. Furthermore, a complete revision of the 1997 inventory was performed using modern methodology and taking into account the knowledge gained by compiling the newer inventories from high-resolution laser scan and photogrammetry data. The result is a homogeneous data set on the extent of South Tyrolean glaciers over the three recording dates which allows comparisons between the individual inventories and thus illustrates the glacier changes in the study area over two decades. The evaluation of the new 1997-inventory results in a glacier area of 121.9 km², which is about 12.2 km² (or 11%) larger than in the original inventory for 1997. This is mainly due to the inclusion of previously unrecorded glaciers, but is also owed to the improved recording of debris-covered subareas. Analyses over the 1997-2005 study period (1997-2017) show an area loss of about 18.1 km² (37.9 km²) for the entire study area. This represents a relative loss of 14.8% (31.1%). While the relative area losses in the subperiod 1997-2005 (with the exception of the Texel Group: 24.7%) are still quite similar among the individual mountain groups with 13% (Hohe Tauern, Stubai Alps) to 19% (Zillertal Alps), a clear difference between the mountain groups west and east of the Eisack-River develops in the second subperiod 2005-2017. While the 1997-2017 losses in the west of the study area range between 25% (Stubai Alps) and 29% (Otztal Alps), the values for the regions in the east range between 39% (Hohe Tauern) and 43% (Zillertal Alps). Again, the Texel Group is an exception with a loss of 45% which results from the fact that smaller, thinner or fragmented glaciers tend to show higher relative area losses than larger glaciers with thicker ice bodies. Analyses of digital terrain models of the years 1999, 2005 and 2016/17 yield a mean ice thickness loss across all studied glaciers of 8.8 m for the 1999 to 2005 subperiod, increasing to 17.6 m by 2016/2017. This corresponds to a volume loss of 1.1 (2.1) km³ or a mass loss of about 0.9 (1.8) gigatons. Here, larger glaciers that are clearly out of balance and have large areas at low elevations tend to experience greater losses in mean ice thickness than small glaciers that respond more rapidly to warming or fragmented glaciers that often persist only under favored topographic conditions. In the current study period, which was characterized by very unfavorable climatic conditions for the glaciers, topographic factors seem to be more decisive than differences in regional climatic conditions, both in terms of area and surface height changes.