There are an estimated 54,000 glaciers in the Hindu Kush Himalayas. These glaciers cover 60,000 square kilometers and serve as a major source of the water in the region’s rivers, including as much as 40 percent in the Indus River system. On the whole, the region’s glaciers appear to be shrinking substantially.
In simplest terms, a glacier is a large body of ice that is moving from a higher to a lower elevation. It has to be at least 30 meters thick for movement to take place, and the ice is always moving from the upper part of the glacier to the lower part, or snout. That’s true whether the glacier is advancing or retreating.
Glaciers form when precipitation falls as snow, remains in the same place year-round and accumulates enough new layers to transform into ice. Each year, new layers of snow bury the old layers, forcing the snow to re-crystallize. Slowly the grains grow larger and the air pockets between the grains get smaller, causing the snow to compact and increase in density. After about two winters, the snow turns into firn, an intermediate state between snow and glacier ice, about two-thirds as dense as water. Over time, with the continuation of the compression, air pockets become so tiny that firn becomes ice.
In the upper part of the glacier, or accumulation zone, more snow falls than melts during the course of a year, while in the ablation zone, in the lower part, more ice and snow melts than accumulates. Lost ice is replaced by ice from the accumulation zone through transport of ice from the upper part to the lower part with the help of gravity. But sometimes the math doesn’t work out in the glacier’s favor. The glacier mass balance, or difference between accumulation and ablation over a year, can either grow, shrink or stay the same based on environmental factors.
Why does that happen? Changes in the amount or the timing of precipitation – for instance, whether it falls as snow or summer rain – can make a difference. So can the amount of melting due to warming. A thin layer of soot pollutants on the glacier surface may increases heating, while a thick layer of debris can act as an insulator. In most cases, the rate at which a glacier retreats or advances depends on the rate at which the total amount of ice shrinks or grows, as well as the size and geometry of the glacier.
Most observations indicate that the Hindu Kush-Himalayan glaciers are shrinking and retreating. While there is local variation – for instance, some glaciers in the Karakorum range appear to be advancing – the trends indicate that we have only until to the middle of this century before continued deglaciation leads to a crisis in water availability.