Glacier mass balance II: Ablation losses

Abstract

The negative or loss factors in mass balance are examined, mainly processes, controls and rates of ablation on glacier surfaces and some other contributors to water yields from glacier basins. Limited but actual ablation measurements exist for three of the large glaciers, Baltoro, Biafo and Batura. They are described and interpreted in relation to differing nourishment and debris covers. Ablation is largely due to solar radiation but regulated by surface conditions in ablation zones. The length of the ablation season is important, as are the extent of exposed ice and of debris covers. These vary strongly with elevation, leading to verticality effects. Quantities and duration of seasonal snow cover also increase with elevation. A further control is the ‘carapace’ of refrozen meltwater, snow and icy layers superimposed on ablation zone ice in the fall and spring shoulder seasons. It also migrates vertically. The interactions of elevation and seasonal snow cover, debris mantles and ice surface topography are keys to specific and net annual ablation losses. When snow and the icy carapace are removed, when debris covers are thin or absent and when weather conditions are favourable, ablation rates are high and similar over a broad range of elevations. Hence, ablation season length and differences in surface conditions are critical for specific and total losses. On lower ablation zone areas, heavy debris mantles protect ice against the longer ablation seasons and higher air temperatures. However, areas of clean-to-dusty or dirty ice, thin or scattered debris, are more extensive and much more critical for net and total ablation losses and water yields. Thinner debris covers, dust, dirt, cold-tolerant algae and so-called cryoconite fragments can drive ablation rates much higher than for clean ice, in places more than double. These are found predominantly in the mid- and upper ablation zones. They are more sensitive to weather and climatic variability than where there are heavy debris mantles. However, geomorphological events in glacier basins, notably rock falls and massive rock slope failures, can suddenly alter and increase the protective role of debris covers for some decades as they are transported down the glacier. They are recurrent phenomena in the Karakoram which, along with motion instabilities and glacier surges, may alter mass balance or glacier advance and retreat.