Geology Reference
In-Depth Information
3
Periglacial Climates
A number of cold-climate environments exist in which frost action is important. Their
combined spatial extent constitutes the periglacial domain. Their extent is diffi cult to
establish because all gradations exist, from environments in which frost-action proc-
esses dominate and where all, or a major part, of the landscape depends upon such
processes, to those in which frost-action processes are subservient to others.
Ground temperature is as important as air temperature in infl uencing the periglacial
landscape. Snow, wind, vegetation, soil type, and ground-thermal properties all infl u-
ence ground temperature.
The periglacial domain is a component of the cryosphere. Global climate models
(GCMs) predict warming will be greatest in high latitudes and that cold-climate envi-
ronments will be signifi cantly affected.
3.1. BOUNDARY CONDITIONS
The boundary between periglacial and non-periglacial conditions is arbitrary and, to a
large extent, varies according to the criteria used.
The presence or absence of perennially-frozen ground would appear to be a reasonably
easy boundary defi nition. Unfortunately, the outer limits of the discontinuous permafrost
zone are often hard to delineate. The location of the treeline is a second relatively unam-
biguous boundary defi nition, since most investigators equate severe periglacial conditions
with tundra or polar desert ecozones. However, the treeline is not static but either advanc-
ing or retreating depending upon environmental and regional climatic conditions. More-
over, the treeline is not a line but rather a zone, sometimes over 50-100 km wide in places,
lying between the biological limit of continuous forest and the absolute limit of tree species
(see Figure 1.2).
For our purposes, and in keeping with a simple frost-action defi nition, we adopt an
empirical defi nition and defi ne the periglacial domain as including all areas where the
mean annual air temperature is less than
3 °C. This closely follows the limits proposed
by Williams (1961) for solifl uction and patterned ground. Because it includes not only
areas where frost-action conditions dominate but also areas that are marginally periglacial
in character, it gives some idea of the maximum extent of the periglacial domain. We may
further subdivide the periglacial domain by the
+
2 °C mean annual air temperature into
environments in which frost action dominates (mean annual air temperature less than
−
−
2 °C) and those in which frost-action occurs but does not necessarily dominate (mean
annual air temperature between
−
2 °C and
+
3 °C).
