Black glaciers: what they are and why they're bad news

We imagine them as white, pure, shiny, and immaculate. But, in reality, they increasingly appear dark and gray. This is why they are called black glaciers , or debris-covered glaciers in technical terms: expanses of ice covered with debris and sediment, which are mostly concentrated in the lower portion, the so-called ablation zone. These deposits modify the way the ice sheet interacts with sunlight, altering the albedo, or the reflectivity of the surface layer.

Reflect or absorb the sun's rays

A light-colored surface, like fresh snow, reflects much of the sun's energy and therefore has a high albedo value. Conversely, a dark surface reflects only a small portion of the sun's energy and therefore has a much lower albedo value, which leads to greater absorption by the ice, resulting in increased melting. To put it simply, the same phenomenon underlies our choice of summer clothing: we usually prefer white to black, precisely because the former, by reflecting sunlight, keeps us cooler, while the latter, by accumulating heat, would amplify the sensation of mugginess.

Furthermore, a significant decrease in albedo was highlighted in 2019 by a study published in Global and Planetary Change and conducted by researchers from the University of Milan, who analyzed 15 glaciers in the Ortles-Cevedale group, in the central Alps, between 1984 and 2011.

The causes of the phenomenon

The main cause of the darkening is climate change over the past few decades. Rising global temperatures are causing glaciers to rapidly retreat, exposing ever-larger rock surfaces, which are eroded by frost and temperature fluctuations, producing fragmented material. For example, on the glaciers of the Hunza Valley in the Karakoram Mountains, Pakistan, debris increased from 8 to 21% between 1990 and 2019. Similarly, rock cover on the glaciers of the Greater Caucasus increased from approximately 48 square kilometers in 1986 to approximately 79 in 2014. Another important factor contributing to the darkening is atmospheric dust, primarily of human origin, such as particulate matter from diesel engines, industrial activities, and forest fires.

According to some experts, however, there is at least one piece of positive news: when the ice cover exceeds 10-20 centimeters in thickness, it protects the underlying glacier, slowing the melting process, at least temporarily. But the outlook remains bleak.

The most significant examples

Black glaciers are found in many mountain ranges around the world, with a heterogeneous distribution: in polar regions or on low slopes, debris cover is almost absent, while on steep mountains it is more abundant. This is why the phenomenon is particularly present in Asia, where the Khumbu Glacier in Nepal, on the slope of Everest, is affected; the Ngozumpa Glacier in the eastern Himalayas; and the Baltoro Glacier in the Karakoram. It is also significant in the Andes, where it particularly affects the Ventisquero Negro Glacier on Mount Tronador in Argentina. Even Italy is not immune to this dynamic. In the Alps, the best-known cases are the Miage Glacier on Mont Blanc and the Belvedere Glacier on Monte Rosa.

A 2018 study based on satellite analysis estimated that approximately 4.4% of the surface of all glaciers on the planet, except Greenland and Antarctica, is covered by sediment. This global percentage, while small, actually equates to thousands of square kilometers of darkened ice sheets.

The impact on the environment

The presence of debris has various environmental impacts. First, it increases slope instability and the risk of landslides. Second, it favors the opening of cavities and depressions where water accumulates, forming lakes on the surface or along the edges of glaciers. This destabilizes the glacier's structure from within and accelerates its collapse, with the risk of flash flooding in the valleys below.