Past and present permafrost as an indicator of climate change

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.