In this sense previous glaciation has very likely played an important role in the present existence of per-
mafrost in marginal areas. It is safe to conclude that little, if any, permafrost exists beneath nonpolar gla-
ciers, but once they withdraw, permafrost may rapidly form and grow. So, previously glaciated regions will
show a lesser volume of frozen ground than unglaciated regions with similar climatic histories. In this regard
it is significant that Canada was heavily glaciated while Russia (Siberia) had little permanent glaciation.
Thus, the permafrost thickness in Siberia is much greater than in Canada, although the climates are similar.
Since the energy balance involves meteorological conditions, surface vegetation, topography and soil
conditions, we can anticipate that we will find no simple correlation for the existence of permafrost in the
marginal or discontinuous regions. Permafrost does exist far south of the usually accepted limits in scattered
patches almost always associated with high altitudes and, thus, microclimates similar to the usual permafrost
regions.
Origin and existence of permafrost
In discussing the present distribution of permafrost, questions often arise concerning its origin and age.
These two concepts should be clearly differentiated, since they deal with two separate phenomena. The age
of a particular deposit of permafrost is the time that has elapsed since the freezing of the soil system. Actual-
ly, it may be very difficult or impossible to determine this age because thawing and freezing may cycle at
long intervals and different frequencies in different regions of the Earth. Thus, the ages of two "similar" de-
posits of permafrost may be quite different. In this regard, the presence of preserved animal remains may be
a reliable clue to the age of a deposit of frozen ground. The age of permafrost is a question of significance
and may be useful to paleontologists, paleobotanists, etc. The present thermal state of the permafrost--tem-
perature, degradation, aggradation, etc.--is of interest to engineers.
The origin of permafrost involves the question of the conditions under which it can form and grow. These
same conditions will explain the present existence of permafrost at a given location. As the conditions for
the origin of permafrost are dynamic, it is certainly possible that areas now lacking permafrost once had
these underlying frozen strata, and that the present regions of permafrost could once have been thawed. In
other words, the present existence of permafrost depends upon two things: the proper energy exchange con-
ditions and the thermodynamic state of the permafrost mass itself. The first of these conditions has little to
do with past climatic conditions, but the second is a function of the complete thermal history of the perma-
frost and is thus related to past climate. In this sense, it is incorrect to describe permafrost simply as a legacy
of the last great ice age. It is possible that some, perhaps most, permafrost had its origin at the beginning of
the Pleistocene era (Brown 1964), but this should not imply that the intervening thermal conditions were
without significance.
From the above discussion, it should be clear that the formation and existence of permafrost are related to
the present and historical conditions of energy exchange between the soil and the atmosphere. Nevertheless,
it is not possible to state these conditions in a simple, precise manner that will allow us to define unique per-
mafrost indicators.
Over a sufficiently long time span, the energy exchange will be periodic, and, averaged over a number of
periods, the net energy flow for a given soil volume will determine its thermodynamic state. Dynamic equi-
librium of the energy flows may exist such that the soil is perennially frozen or the thermodynamic state of
the soil may be varying because of an imbalance of the cyclic energy flows. The original formation of per-
mafrost depends upon a net periodic (yearly) loss of energy from the soil volume that must persist for many,
many years for the permafrost to attain great thicknesses. The maintenance of the present thermodynamic
state of permafrost requires only that the net energy flow averaged over a number of years be zero.
Paleotemperatures
The thermal history of permafrost is greatly influenced by the long-term temperature variations experi-
enced at its upper surface. The relative mean global temperature deduced from oxygen isotope data is shown
for the past 180 million years in Figure 1 (Eddy and Bradley 1991). There was probably little or no perma-
frost prior to the late Tertiary Period and certainly none for 100 million years prior to the long-term cooling
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