Abstract

Rocky debris covers 30% of glacier ablation areas in High Mountain Asia and generally suppresses melt. However, remote sensing observations have shown no statistical difference in glacier thinning rates between areas with and without debris cover; the ‘debris cover anomaly’. This pattern is apparent at subregional and regional scales, even after controlling for the elevation differences between debris-covered and clean ice. 

Two primary hypotheses to explain this behaviour have interpreted the thinning patterns in terms of melt or ice supply differences. First, rapid melt at supraglacial ponds and ice cliffs could enhance ablation in debris-covered areas, and therefore thinning as well. These features cannot entirely compensate for the melt reduction under debris, so a second hypothesis interprets the anomaly to indicate differences in emergence velocity between debris-covered and clean ice. However, complete understanding of the problem is challenged by a scale gap: the prior process studies have focused on single glaciers, whereas the anomaly has been identified for subregional- to regional spatial scales. Furthermore, these hypotheses neglect numerous other differences between debris-covered and clean glaciers (e.g. topo-climatic situation, accumulation mechanisms), which could bias this comparison.

We overcome these limitations and investigate which, if any hypothesis, explains the observed behaviour through a direct assessment leveraging diverse large datasets and modelling.