Global warming has caused substantial retreat and downwasting of mountain glaciers worldwide, particularly in the European Alps. Glacier shrinkage has been accelerating in recent decades, posing challenges to water supply, civil security, and tourism. Alpine glaciers act as "water towers," influencing seasonal runoff and contributing significantly to late-summer flows of major rivers like the Rhone and Po. Changes in seasonal runoff affect renewable energy production and hydropower strategies. Summer tourism also depends on the glacierized landscape, which is impacted by shrinking glaciers and increased natural hazards. To better predict future water availability and the impacts on various sectors, accurate and consistent data on past glacier changes are crucial. However, most previous studies either focus on large-scale catchments or use limited in situ measurements, lacking comprehensive, cross-border analysis of glacier changes across the entire Alps with a consistent observation interval. This research addresses this gap by providing glacier-specific area and elevation measurements and regional mass changes for all Alpine regions, using a combination of digital elevation models (DEMs) from the Shuttle Radar Topography Mission (SRTM) and TanDEM-X, along with optical imagery from the Landsat program.

Previous research has documented accelerated glacier shrinkage in the European Alps since the Little Ice Age (~1850). Studies have reported mass-change rates close to 1 m.w.e. a⁻¹ during the early 2000s. Many studies focused on specific regions or catchments, using either in situ measurements or large-scale DEM differencing. However, a comprehensive analysis covering the entire Alps with consistent methodology and temporal resolution has been lacking. Existing studies on glacier mass change in the Alps often rely on large-scale catchments or limited in situ measurements, potentially lacking representativeness. Some studies have integrated geodetic and glaciological measurements to improve accuracy, but comprehensive, methodologically consistent, cross-border analyses remain limited. This study aims to address this gap by providing a detailed, region-wide assessment of glacier changes in the early 21st century.

This study uses a combination of remote sensing data to measure glacier area and elevation changes across the European Alps from 2000 to 2014. Synthetic aperture radar (SAR) data from SRTM (2000) and TanDEM-X (2011-2012, 2013-2014) were used to create DEMs. Optical imagery from the Landsat program (1999-2001, 2011, 2013-2015) was used to delineate glacier outlines. The study used approximately 270 TanDEM-X DEMs and 185 Landsat scenes. Elevation changes were calculated by differencing the SRTM DEM and the TanDEM-X DEMs. An altitude-dependent correction was applied to account for potential SAR signal penetration into the winter glacier surface. Area changes were calculated from the Landsat imagery using band ratios to create binary raster masks. Glacier outlines were refined manually to correct for misclassifications. Mass changes were calculated using a combination of area and elevation change data, along with a density conversion factor of 850 ± 60 kg m⁻³. The study employs the International Standardized Mountain Subdivision of the Alps (IMSA) to define 10 subregions within the Alps, plus Western and Eastern Alps divisions. Uncertainty analysis accounts for errors in DEM differencing, glacier area delineation, density conversion, and SAR signal penetration. The results were compared with glaciological measurements from 25 glaciers with continuous records and previous geodetic studies. A detailed uncertainty analysis was performed considering errors from DEM differencing, glacier area measurements, density conversion, and SAR signal penetration. The study also compared the findings with existing glaciological and geodetic observations to validate the results.

The study revealed a significant and widespread decline of glaciers across the European Alps between 2000 and 2014. The overall glacier area decreased by approximately 39 ± 9 km² a⁻¹, representing an area loss rate of roughly 1.8% per year. The strongest downwasting (-0.5 to -0.9 m a⁻¹) occurred in the Swiss Glarus and Lepontine Alps, with specific mass change rates reaching up to -1.03 m.w.e. a⁻¹. The total mass loss for the entire Alps during this period was estimated to be 1.3 ± 0.2 Gt a⁻¹. Regional variations in mass loss were observed, with the highest losses concentrated in the Western Alps (Bernese, Pennine Alps). The study found a strong correlation between elevation changes and glacier size, with highly negative values at lower altitudes and less pronounced changes above 3500 m a.s.l. The analysis of altitudinal distribution of elevation changes indicated that most areas experience negative rates, showing loss of accumulation areas and thinning across the entire glacier, particularly pronounced below 2000 m a.s.l. The highest absolute area reductions were found in the Bernese, Pennine, and Graian Alps, which have the largest glacier areas. Comparison with glaciological measurements showed that geodetic measurements are often less negative than glaciological ones, although the regional geodetic mass-change rates are close to the area-weighted glaciological average in regions with a larger number of measurements. Extrapolation of current mass-change rates suggests that lower Alpine regions may be almost ice-free by the end of the century, with the total Alpine glacier volume reduced to approximately one-third of its 2000 value.

The findings confirm the significant impact of climate change on Alpine glaciers, revealing widespread surface thinning even at higher elevations. The magnitude of mass loss observed is consistent with previous studies, although regional variations exist. The comparison with glaciological data helps validate the geodetic measurements, although some discrepancies remain that might be due to factors like small glacier size, radar acquisition biases, or local climatic conditions. The extrapolated volume losses highlight the vulnerability of Alpine glaciers and the potential for near-complete deglaciation in lower-elevation areas within this century. These results underscore the urgency for mitigation strategies to address climate change and its impacts on water resources, risk assessment, and tourism in the Alpine region.

This study provides the first comprehensive, Alpine-wide assessment of glacier mass change during the early 21st century, showing substantial and widespread ice loss. The findings confirm the strong influence of climate change on Alpine glaciers and highlight the urgent need for mitigation efforts. Future research could focus on improving the accuracy of glacier mass balance estimates, particularly at higher elevations, and exploring the socio-economic consequences of ongoing glacier loss in greater detail. Improved regional climate models and more detailed, high-resolution elevation measurements could improve the accuracy of future predictions.

The study’s reliance on remote sensing data introduces potential limitations related to DEM accuracy, SAR signal penetration, and the accuracy of glacier outline delineation. The use of a constant density conversion factor might introduce slight biases in mass change estimates. The extrapolation of current mass loss rates to predict future glacier volume ignores potential changes in climate and glacier dynamics, making the long-term predictions uncertain. The spatial resolution of data used might limit the assessment of very small glaciers.