Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 191

neighbors (Gillespie and Molnar 1995; Benn and Owen 1998; Thompson et al. 2005).
It is, of course, too soon to know where this will end. Is it just another small deviation
from the norm, or are we in fact near the end of the interglacial period and on the verge
of another ice age? If the Milankovitch cycles (long-term variations in the Earth's orbit)
are responsible for ice age/inter-glacial variations over the last few million years, we
should expect to return to glacial conditions over tens of thousands of years, in spite of
the current concern over global warming (Imbrie et al. 1992, 1993; Raymo and Nisan-
cioglu 2003; Jouzel et al. 2007; Bintanja and van de Wai 2008).
Glacier Thermodynamics and Hydrology
Glaciers can be classified based on thermal conditions at the surface and at the base of
the ice. Polar glaciers (or cold-based glaciers ) remain well frozen throughout, are frozen
to the rock of their beds, and move very slowly. Temperate glaciers (or warm-based
glaciers ) remain at the pressure melting point, are warm based, except for the winter
freezing of a thin upper layer in many instances, and hence are not frozen to their bed.
Temperate glaciers are, therefore, able to slide over their bed and move more rapidly.
Polythermal glaciers are the intermediate condition; these are composed of both warm-
based and cold-based sections. Polythermal glaciers can be further subdivided into let-
ter designations—a through f—depending on the relative distribution of cold and warm
ice within (Benn and Evans 2010). While this classification carries important implica-
tions for the behavior of glaciers in different climates, it is simplistic. Often, a single
glacier that flows through a large altitudinal range may display multiple thermal condi-
tions, and it is important to avoid lumping entire glaciers into a single thermal classific-
ation (Boulton 1972; Sugden 1977; Denton and Hughes 1981; Robin 1976; Blatter and
Hutter 1991; Paterson 1994; Barry and Gan 2011). The portions of glaciers at the pres-
sure melting point can cause the development of complex internal meltwater routing
systems that take surface meltwater and feed it through a network of englacial and bas-
al tunnels (Ilken and Bindschadler 1986). Ultimately, a large volume of water emerges
as an outlet stream at the snout of the glacier. Such glacier rivers are augmented in
summer by rainfall that percolates throughout the glacier. Since the flow of surface wa-
ter into these glacial plumbing systems is strongly dependent upon temperature and
radiation, the discharge from the outlet stream exhibits considerable diurnal variation.
Maximum melt occurs well after the daily maximum temperature, and minimum melt
occurs after sunrise, following the typical diurnal temperature swing (Gerrard 1990).
Given the lag time in response through the internal channels of the ice, similar dis-
charge fluctuations propagate downstream with a significant time lag. Thus, incautious
hikers who wade glacier rivers in the early morning may be faced with a serious prob-
lem when attempting to retrace their steps late in the day.