Understanding Black Glaciers

Glaciers are often covered by supraglacial debris in their ablation zone, in the Alps these "black glaciers", the geomorphologic term would be Debris Covered Glaciers (DCG), are relatively rare. In contrast such DCG are common in high mountain ranges like the Andes or the Himalaya, probably as consequence of the high sun-radiation levels in these regions.
Supraglacial debris is composed of material transported onto the glacier by rockfalls, avalanches, rubble of moraines and rocks melting out from the ice.

Fig.1. Huge erratic boulder on the Swiss glacier of Vorderaar after a picture published in a travel account of C. Wolf and M. Descourtis in 1785. By providing shadow and shelter to the underlying ice a pillar balancing the rock formed.

The thermal properties of this debris are profoundly different to blank ice and can significantly influence the glacier mass balance. Isolated rocks or a sparse cover, darker then the surrounding ice and snow, tend to absorb radiation of the sun and heat up, melting ice and increasing the loss of a glacier. A sufficient thick cover of rubble however isolates the underlying ice, and prevents or reduces melting and is so decreasing the loss of a glacier.
Research on the Swiss glaciers of Unteraar and Lauteraar have shown that a cover of debris ranging between a thickness of 5 to 20 centimetres shows higher temperatures on the superficial layer than the surrounding environment, however the thermal isolation is sufficient to reduces the ablation of the underlying ice by 32%.
The important influence of the debris cover on glaciers worldwide is however still poorly understand, hard to study and to quantify (variables like what kind of rock with connected physical properties is covering the surface and the highly variable thickness of the debris must be considered) and therefore neglected in many models to calculate future trends .

A new publication by SCHERLER et al. has analyzed 286 glaciers in the Himalaya using satellite images recorded from 2000 to 2008. The research confirmed that more than 65% of the glaciers are retreating with an average of 10 meters per year; however glaciers without debris cover are much faster in this general retreat then glacier with cover.

Fig.2. The glacierized Himalayan border region of Bhutan (bottom) and Tibet (top) seen in a satellite image. From the crest of the mountain range clean glaciers flow northwards onto the Tibetan Plateau, while DCG flow south into densely forested valleys. The debris cover on the glaciers depends of the topography of the surrounding mountains; steep slopes provide much more debris in form of rockfalls than gentle slopes (ASTER-image by NASA, 08 June 2006).

Many of the debris covered glaciers appear to be stable but the underlying ice shows no movement, it has become dead ice - former glacier ice that is not longer connected to the active glacier.

The research shows that to predict the behaviour of single glaciers the debris must be considered, also the comparison between well studied areas, like the Alps, to poorly covered high mountain ranges is not sufficient or even possible, more fieldwork, research and data on the mysterious DCG is essential.

Bibliography:

SCHERLER, D.; BOOKHAGEN, B. & STRECKER, M.R: (2011): Spatially variable response of Himalayan glaciers to climate change affected by debris cover. Nature Geoscience. doi:10.1038/ngeo1068