The permafrost history of the high northern latitudes over the last two mil
lion years indicates that perennially frozen ground formed and thawed repea
tedly, probably in close synchronicity with the climate changes that led to
the expansion and subsequent shrinkage of continental ice sheets. The earl
y stages of the Pleistocene are the least known and the changes that occurr
ed in the Late Pleistocene and early Holocene are the best known.
Evidence that permafrost is degrading in response to the current global war
ming trend is difficult to ascertain. The dearest signals are probably prov
ided by changes in permafrost distribution in the sub-Arctic regions, at th
e extreme southern fringes of the discontinuous permafrost zone.