Understanding Glacier Changes: Elevation Matters

By Kenneth Hewitt, China Dialogue
Part II of a three-part series

As we saw in part one, climate change is obviously having different consequences in different mountain areas of Asia. The situation in the Karakoram must represent some distinctive conditions.

Three features of the regional environment seem critical. The first two relate to snowfall and the nourishment of these glaciers. They are intermediate in type between the summer accumulation (snowfall) glaciers of the greater Himalayas, and the winter accumulation glaciers of, say, the Caucasus and European Alps to the west. In each of the latter, more or less strong glacier retreat is reported.

Second, the zone of maximum precipitation in the Karakoram is much higher than in these and most other mountain ranges. It is also entirely within the accumulation zones of the glaciers. This relates to the third factor, the exceptional elevations and, especially, elevation range of these ice masses.

The glaciers of large and intermediate size originate at very high altitudes, and many of them descend much lower than elsewhere in the sub-tropics.

Five glaciers span more than 5,000 meters in elevation, 15 over 4,500 meters and more than 30 over 3,000 meters. In the Hunza valleys of the central Karakoram, glacier termini advance below 2,300 meters above sea level. Those on the north flank in the Yarkand drainage do not descend so low because the valleys are at greater elevations, but they include several descending more than 4, 000 meters, due to location in the very highest parts of the range around K2 (8,610 meters).

All of the glaciers recently observed to be growing are in these high-relief basins. Of special interest, but poorly understood, is how elevation and topography interact with the regional climatic influences to determine the behavior of the glaciers.

The regional climate of this south-western part of the Inner Asian mountains comes under the influence of three different, seasonally varying, weather systems.

First, the winter half of the year is dominated by a westerly or “sub-Mediterranean” circulation. Second, in summer, moisture comes from the Indian Ocean to the south and the climate becomes “sub-monsoonal”. Third, inner Asian high-pressure systems, especially involving the Tibetan Plateau, interact with the other two systems to affect storm paths and the incidence of clear weather. The last is critical, since direct solar radiation is responsible for 80% to 90% of melting on the glaciers.

Global climate change is expected to alter the absolute and relative roles of all three systems, a likely factor in recent developments that complicates forecasting of future glacier changes.

Meanwhile, investigations on the glaciers at higher elevations have revealed how different conditions are from the valley weather stations — mostly below 3,000 meters above sea level — whose records had dominated climatic interpretations.

Station records from the inhabited areas of the Karakoram show two-thirds or more of precipitation occurs in winter, mainly February through May. The average annual precipitation in these valleys is 150 millimetres to 300 millimeters water equivalent — an arid or semi-arid environment with severe summer drought. However, a very different story emerged from our measurements on the glaciers in the 1980s. At elevations above 4,800 meters, we found that snowfall amounts are roughly the same in summer and winter, with roughly equal amounts coming from the west and the Indian Ocean. Summer drought was not observed on the glaciers especially in their accumulation zones above 4,500 meters above sea level.

Moreover, the zone of maximum precipitation turned out to be between 5,000 meters and 6,000 meters above sea level — much higher than in, say, the eastern Himalaya or any other reports from tropical mountains. Moreover, accumulation zone snowfall is equivalent to between 1,000 millimeters and 2,000 millimeters of water; far more moisture than the valley stations suggest.