Permafrost and Periglacial Landscapes

Permafrost, as defined by the International Permafrost Association, is any ground — soil, sediment, or rock — that remains at or below 0°C for at least two consecutive years. It underlies an estimated 15 percent of the exposed land area of the Northern Hemisphere, roughly 23 million square kilometers according to the widely used Circum-Arctic Map of Permafrost compiled by Brown et al. and later refined by Zhang et al. (2008) in Permafrost and Periglacial Processes. Permafrost extent is classified into continuous zones, where more than 90 percent of the ground is frozen, through discontinuous, sporadic, and isolated zones with progressively less coverage. Above the permafrost table lies the active layer, a surface zone that thaws each summer and refreezes each winter, typically ranging from under half a meter in the coldest continuous-permafrost areas to several meters at the southern margins of discontinuous permafrost.