Valley glaciers and small ice caps represent storages of water over long time scales. Many rivers are supported by glacier melt, which maintains flows through the summer season. The state of a glacier is characterized by the relationship between the rate of accumulation of ice (from winter snowfall) and the rate of ablation or melt. Most, but not all, valley glaciers and small ice caps have been in general retreat since the end of the Little Ice Age, between 100 and 300 years agofor example, in Switzerland (Greene et al., 1999), Alaska (Rabus and Echelmeyer, 1998), the Canadian Rockies (Schindler, 2001), east Africa (Kaser and Noggler, 1991), South America (Ames and Hastenrath, 1996; see also Chapter 14), the arid region of northwest China (Liu et al., 1999), and tropical areas as a whole (Kaser, 1999). Temperature appears to be the primary control (Greene et al., 1999), and rates of retreat generally are accelerating (Haeberli et al., 1999). The World Glacier Monitoring Service (see http://www.geo.unizh.ch/wgms) monitors glacier mass balances and publishes annual reports on glacier fluctuations.
The effect of future climate change on valley glaciers and small ice caps depends on the extent to which higher temperatures are offset by increased winter accumulation. At the global scale, Gregory and Oerlemans (1998) simulate a general decline in valley glacier mass (and consequent rise in sea level), indicating that the effects of higher temperatures generally are more significant than those of additional winter accumulation. Model studies of individual glaciers have shown general retreat with global warming. Wallinga and van de Wal (1998) and Haerberli and Beniston (1998), for example, both simulated retreat in Alpine glaciers with higher temperatures and changes in winter accumulation. Davidovich and Ananichevas (1996) simulation results show retreat of Alaskan glaciers but also a substantial increase in mass exchange (and therefore rate of movement) as a result of increased winter accumulation.
Oerlemans et al. (1998) simulated the mass balance of 12 valley glaciers and small ice sheets distributed across the world. They found that most scenarios result in retreat (again showing that temperature changes are more important than precipitation changes) but showed that it was very difficult to generalize results because the rate of change depends very much on glacier hypsometry (i.e., variation in altitude across the glacier). Their simulations also show that, in the absence of a change in precipitation, a rise in temperature of 0.4°C per decade would virtually eliminate all of their study glaciers by 2100, but a rise of 0.1°C per decade would only lead to a reduction in glacier volume of 1020%.
Tropical glaciers are particularly exposed to global warming. Kaser et al. (1996) show that the equilibrium line altitude (ELA)the line separating the accumulation zone from the ablation zoneof a tropical glacier is relatively more sensitive to changes in air temperature than that of a mid-latitude glacier. This is because of the lack of seasonality in tropical temperatures and the fact that ablation is significant year-round. To illustrate, a 1°C rise in temperature during half of the year only will have a direct impact on total ablation, annual mass balance, and ELA of a tropical glacier. In the case of a mid-latitude glacier, this increase may occur during winter when temperatures may be well below freezing over much (if not all) of the glacier. As a result, there may be no significant change in ablation or position of the ELA, even though the annual temperature will have increased.
Glacier retreat has implications for downstream river flows. In rivers fed by glaciers, summer flows are supported by glacier melt (with the glacier contribution depending on the size of the glacier relative to basin area, as well as the rate of annual melt). If the glacier is in equilibrium, the amount of precipitation stored in winter is matched by melt during summer. However, as the glacier melts as a result of global warming, flows would be expected to increase during summeras water is released from long-term storagewhich may compensate for a reduction in precipitation. As the glacier gets smaller and the volume of melt reduces, summer flows will no longer be supported and will decline to below present levels. The duration of the period of increased flows will depend on glacier size and the rate at which the glacier melts; the smaller the glacier, the shorter lived the increase in flows and the sooner the onset of the reduction in summer flows.
Other reports in this collection