4.3.11. Glaciers and Small Ice Caps
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
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.