Abstract
Abstract. In the European Alps, almost half the glacier volume has disappeared over the past 150 years. The loss is reflected in glacier retreat and ice surface lowering even at high altitude. In steep glacial cirques, surface lowering exposes rock to atmospheric conditions probably for the very first time in several millennia. Instability of rockwalls has long been identified as one of the direct consequences of deglaciation, but so far cirque-wide quantification of rockfall at high resolution is missing. Based on terrestrial lidar, a rockfall inventory for the permafrost-affected rockwalls of two rapidly deglaciating cirques in the Central Alps of Austria (Kitzsteinhorn) is established. Over 6 years (2011–2017), 78 rockwall scans were acquired to generate data of high spatial and temporal resolution. Overall, 632 rockfalls were registered, ranging from 0.003 to 879.4 m3, mainly originating from pre-existing structural rock weaknesses. A total of 60 % of the rockfall volume detached from less than 10 vertical metres above the glacier surface, indicating enhanced rockfall activity over tens of years following deglaciation. Debuttressing seems to play a minor effect only. Rather, preconditioning is assumed to start inside the randkluft (void between cirque wall and glacier) where measured sustained freezing and ample supply of liquid water likely cause enhanced physical weathering and high quarrying stresses. Following deglaciation, pronounced thermomechanical strain is induced and an active layer penetrates into the formerly perennially frozen bedrock. These factors likely cause the observed paraglacial rockfall increase close to the glacier surface. This paper, the first of two companion pieces, presents the most extensive dataset of high-alpine rockfall to date and the first systematic documentation of a cirque-wide erosion response of glaciated rockwalls to recent climate warming.
Highlights
High-alpine, glacial environments are severely affected by recent climate warming (WGMS, 2017)
Using data from a 6-year terrestrial lidar monitoring campaign (2011–2017), we present a rockfall inventory from the Central Alps of Austria that is unique for high-alpine study areas in spatial and temporal extent, and level of detail
The minimum usable volume of 0.1 m3 derived here is higher than values specified in lidarbased change detection surveys using shorter object distances and higher point densities (e.g. Rosser et al, 2007; Williams et al, 2018) but is in good agreement with similar monitoring campaigns carried out in high-alpine settings (e.g. Strunden et al, 2015)
Summary
High-alpine, glacial environments are severely affected by recent climate warming (WGMS, 2017) This is especially true for the European Alps, where the mean temperature increase over the last 150 years more than doubled the global mean (Böhm, 2012), and over this period approximately 50 % of the glacier volume has disappeared (Haeberli et al, 2007). Glacier retreat rates have increased since the 1980s and have been exceeding historical precedents in the early 21st century (Zemp et al, 2015). The consequences of these changes are most visible in lower-lying glacierized cirques where ice surface lowering in the ablation area is apparent (Kaser et al, 2006; Pelto, 2010) and exposes cirque walls to the atmosphere probably for the first time in several millennia. Hartmeyer et al.: Current glacier recession causes significant rockfall increase
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