Refining the history of changes in glacier geometry since the LIA at selected sites across the Austrian Alps

Information on past glacier states is fundamental for improving projections of future glacier evolution under ongoing climate change. In this study, we present a 170-years reconstruction of glacier area and volume changes of five glaciers providing comprehensive regional representation of the Austrian Alps, covering the period from the Little Ice Age (LIA) maximum (~1850) to the present. LIA glacier extents are derived from the national moraine-based LIA glacier inventory and complemented by historical topographic maps, terrestrial oblique photographs, historical orthophotos, and modern digital elevation models (DEMs) as references. 

We evaluate glacier geometry and its temporal changes at specific, data-constrained time points between the LIA maximum and the first comprehensive Austrian glacier inventory in 1969 by using monoplotting and DEM differencing techniques. Monoplotting intersects terrestrial photographs with DEMs to extract georeferenced glacier outlines from oblique photographs, for example. This approach allows us to derive glacier front variations or even approximate glacier areas at times where observational records are lacking. Historical DEMs were generated from scanned topographic maps, co-registered with modern elevation data, and used to compute volumetric changes. These multi-source datasets enable the reconstruction of glacier extents and ice-surface topography for several dates between the LIA and the first Austrian glacier inventory in 1969.  

Results show that, across all glaciers studied, area decline rates increased around the 1940s relative to the period between end of LIA (~1850) to the 1940s, coinciding with periods of positive summer air-temperature anomalies, particularly during the 1940s. Between approximately 1975 and 1990, decline rates decreased for most glaciers, reflecting the cooler period of the 1970s and 1980s. Whereas most glaciers stabilized or even temporary advanced around that period, Niederjochferner exhibited strong glacier area loss, consistent with front position measurements of the Austrian Alpine Club (Österreichischer Alpenverein, ÖAV). Relative area decline rates are generally larger for smaller glaciers. While the area of Gepatschferner, one of Austria’s largest glaciers, has decreased by around 30% since 1850, the medium-sized glaciers (< 6 km² at LIA) have lost at least 45% of their LIA area. Despite its northward orientation, the Niederjochferner shows the strongest area reduction with over 70% since LIA. In contrast, the Mullwitzkees shows the lowest relative area retreat (≈45%) since LIA, even though it is the only southward facing glacier in this study. 

Ongoing work uses aerial imagery of high quality for its time, acquired by the US Army in 1945 covering the entire Austrian Alpine region, to reconstruct glacier extents and volumes. By generating orthophotos and DEMs, we aim to create a new national glacier inventory that can be used in various research fields. The extended time series of glacier volume and area change can provide valuable calibration and validation data for glacier models, such as the Integrated Glacier Model (IGM), enabling three-dimensional surface reconstruction and forward simulations under various climate scenarios.