Cast shadows reveal changes in glacier thickness

Abstract. Increased glacier melt rates call for accurate local estimates of glacier elevation change to predict future changes in glacier runoff and their contribution to sea level rise. Glacier elevation change is typically derived from digital elevation models (DEMs) tied to surface change analysis from satellite imagery. Yet, the rugged topography in mountain regions can cast shadows onto glacier surfaces, making it difficult to detect local glacier elevation changes in remote areas. However, most optical satellite images offer precise time-stamped meta data of the solar position and angle during the acquisition. These data are useful to simulate shadows from a given DEM. Accordingly, any differences in shadow length between simulated and mapped shadows in satellite images could indicate a change in glacier elevation relative to the acquisition date of the DEM. We tested this hypothesis at five selected glaciers with long-term monitoring programs. For each glacier, we projected cast shadows on the glacier surface from freely available DEMs and compared simulated shadows to cast shadows mapped in ~40 years of Landsat images. We validated the relative differences with in situ geodetic measurements of glacier elevation change. We find that shadow-derived glacier elevation changes are consistent with independent photogrammetric and geodetic surveys in shadowed areas. Our method shows that Baltoro Glacier (Karakoram, Pakistan) gained slightly in elevation between 1987 and 2020, while Great Aletsch Glacier (Switzerland) recorded the most negative melt rates of about 1 m per year. While hydrological mass balances are averaged over a given glacier, we provide local glacier thickness changes, a vital information to quantify variances in melt rates in the accumulation or ablation zone. Our appraisal hinges on the precision of the DEM as the geometry of ridges and peaks constrain the shadow that we cast on the glacier surface. Future generations of DEMs with higher resolution and accuracy will improve our method, enriching the toolbox for estimating glacier mass balance especially in remote glacier areas with difficult field access.