Geoscience Reference
In-Depth Information
13.5 Inferring past climates from glacial evidence
Many desert uplands now devoid of ice bear traces of previous glacial activity, and
where extensive ice caps are present in arid regions such as Patagonia, they show
evidence of having been far more extensive in recent times (Murray et al.,
2012
).
For ice to form and persist in desert mountains, the temperature must be sufficiently
low in winter for precipitation to occur in the form of snow and sufficiently low in
summer for the snow cover to persist and accumulate. There are thus two prerequisites
for permanent ice to develop: relatively low winter and summer temperatures and
relatively high rates of precipitation in the form of snow. The lower limit of permanent
snow is defined as the equilibrium snow-line, or the zone in which summer ablation
and winter snow accumulation are in balance, which coincides tolerably well with
the lower limit of cirque glaciation (Flint,
1971
). The equilibrium line altitude (ELA)
is a useful index with which to compare changes in glacial activity over time in any
one area and between different regions. In arid mountains, the ELA can vary widely,
depending on changes in precipitation. A decrease in snowfall will lead to a sharp
increase in the ELA in arid areas. In more humid mountains, changes in temperature
become more important influences on the ELA. Osmaston (
2005
) has provided a
useful review of different methods of estimating the ELA and has stressed the need
to examine as many individual glaciers as possible on any one mountain, given that
aspect, local topography and local climate can cause substantial variation in the ELA
in any one region or massif.
Because the snow-line is controlled by two independent variables - precipitation
and temperature - we cannot use the lower limits of cirques and glacial moraines
to reconstruct either precipitation or temperature. In high mountains in arid regions,
where temperatures at high elevations are often low enough for any precipitation to
fall as snow, precipitation is often the limiting factor controlling the volume of ice
that is likely to accumulate. However, high mountains tend to create their own climate
in the form of orographic precipitation. Air masses coming into contact with a high
mountain will rise, becoming adiabatically cooler as they do, so that water vapour will
reach dew point and condense as rain or ice crystals. The adiabatic lapse rate varies
with water vapour content, but cooling rates of 0.65
C/100 m are not uncommon. If we
are to obtain the least ambiguous evidence of past climatic change in desert uplands
from erosional and depositional glacial landforms, some independent measure of past
temperature is necessary.
°
13.6 Use of periglacial features to reconstruct temperature changes
One reasonably straightforward approach to resolving this dilemma is to use the lower
limit of periglacial solifluction deposits to estimate past temperature. For example,
if the present lower limit of seasonal freeze-thaw activity on the mountain slopes
