Heterogeneous glacier thinning patterns over the last 40 years in Langtang Himal

Himalayan glaciers are losing mass at rates similar to glaciers elsewhere, but heavily debris-covered glaciers are receding less than debris-free glaciers or even have stable fronts. There is a need for multi-temporal mass balance data to determine if glacier wastage of debris-covered glaciers is accelerating. Here, we present glacier volume and mass changes of seven glaciers (5 partially debris-covered, 2 debris-free) in the upper Langtang catchment in Nepal of 28 different periods between 1974 and 2015 based on 8 digital elevation models (DEMs) derived from high-resolution stereo satellite imagery. We show that glacier volume decreased during all periods between 2006 and 2015 (2006–2015: −0.60 ± 0.34 m a−1) and at higher rates than between 1974 and 2006 (−0.28 ± 0.42 m a−1). However, the behavior of glaciers in the study area was highly heterogeneous, and the presence of debris itself does not seem to be a good predictor for mass balance trends. Debris-covered tongues have highly non-linear thinning profiles, and we show that local accelerations in thinning correlate with complex thinning patterns characteristic of areas with a high concentration of supraglacial cliffs and lakes. At stagnating glacier area near the glacier front, on the other hand, thinning rates may even decrease over time. We conclude that trends of glacier mass loss rates in this part of the Himalaya cannot be generalized, neither for debris-covered nor for debris-free glaciers.