NAGRA Opalinus Clay (…002 - Version 1
FEP 17.1.4 Permafrost

As the climate cools, air temperatures will fall substantially below freezing point for much of the year. If these conditions are sustained, then permafrost (permanently frozen ground) may develop, with depths up to a few hundred metres in currently temperate regions. If summer temperatures remain above freezing point, then a zone of seasonally unfrozen ground will remain above the permafrost layer. Significant groundwater discharge zones and the courses of major rivers and lakes are liable to remain open (taliks).