Global differences in the energy balance and melt rates of debris-covered glacier surfaces

Supraglacial debris covers 4% of mountain glacier area globally and generally reduces glacier surface melt. Studies have identified enhanced energy absorption at ice cliffs and supraglacial ponds scattered across the debris surface. Although these features generally cover a small portion of glacier surface area (5-10%) they contribute disproportionately to mass loss at the local glacier scales (20-40%). While past studies have identified their melt-enhancing role in High Mountain Asia, Alaska, and the Alps, it is not clear to what degree they enhance mass loss in other areas of the globe.

We model the surface energy balance for debris-covered ice, ice cliffs, and supraglacial ponds using meteorological records (4 radiative fluxes, wind speed, air temperature, humidity) from a set of on-glacier automated weather stations representing the global prevalence of debris covered glaciers. We generate 5000 random sets of values for physical parameters using probability distributions derived from literature. We also model the hypothetical energy balance of a debris-free glacier surface at each site, which we use to investigate the melt rates of distinct surface types relative to that of a clean ice glacier. This approach allows us to isolate the melt responses of debris, cliffs and ponds to the site specific meteorological forcing.

For each site we determine an Østrem curve for sub-debris melt as a function of debris thickness and a probabilistic understanding of surface energy absorption for ice cliffs, supraglacial ponds, and debris-covered ice. While debris leads to strong reductions in melt at all sites, we find an order-of-magnitude spread in sub-debris melt rates due solely to climatic differences between sites. The melt enhancement of ice cliffs relative to debris-covered ice is starkly apparent at all sites, and ice cliffs melt rates are generally 1.5-2.5 times the ablation rate for a clean ice surface. The supraglacial pond energy balance varies regionally, and is sensitive to wind speed and relative humidity, leading to energy absorption 0.4-1.2 times that of clean ice, but 5-10 times higher than debris-covered ice. Our results support the few past assessments of melt rates for cliffs and ponds, and indicate sub-regional coherence in the energy balance response of these features to climate.