Tundra and permafrost-dominated taiga

Permafrost is defined as any subsurface material that remains frozen continuously for at least two consecutive years. The seasonally thawed layer above is known as the 'active layer'. The permafrost regions occupy about 25% of the Earth's land area (Zhang et al., 2000), mostly situated in the tundra and taiga zones of the northern hemisphere. The distribution of permafrost is usually represented as a series of concentric zones in which permafrost is continuous (underlies more than 90% of surface), discontinuous (50–90%) and sporadic (less than 50%) (Brown et al., 1997) (Fig. 13.1). Permafrost occupies 85% of Alaska, more than 60% of Russia and 50% of Canada. Four million people are living in circumarctic/subantarctic regions, including 10% belonging to indigenous communities such as the Inuit, Sami (Lapps) and Chukchi (Bogoyavlenskiy and Siggner, 2004). Human concentrations are particularly important in northern Russia, with cities of over 100 000 inhabitants and large river ports (Fig. 13.2).