Ancient permafrost and past permafrost in the Northern Hemisphere

The existence and dynamics of permafrost depend on the prevailing climate conditions. Therefore, the study of ancient permafrost (existing since the Pleistocene or earlier) and past permafrost (Late Pleistocene or older permafrost that no longer exists) and their dynamics may inform about climate and environmental changes in the past. In this chapter, we provide a brief overview of characteristics, detection and dating methods of ancient and past permafrost, before presenting a spatial and temporal history of permafrost in the middle and high northern latitudes. While the first permafrost may have formed about 3 million years ago, the late Pliocene and Early Pleistocene were characterized by frequent thawing and new formation of permafrost. It was not until the Middle and Late Pleistocene that permafrost became more persistent and widespread due to prolonged cooling. The most ancient dated permafrost formed between 800 and 600 ka in Yukon/Canada and East Siberia. Interglacial warming after the last ice age has led to massive thawing of permafrost and large areas in Europe, Asia and America are now characterized by traces of past permafrost.