Some permafrost has stayed frozen for around 700,000 years
Permafrost is ground that stays below freezing for at least two years running, and the oldest has been frozen for roughly 700,000 years. It underlies about 11 percent of the world's land, reaching more than 1,500 metres deep in places. As it thaws, it releases ancient carbon, buckles buildings and threatens to leak buried pollution.
Around 18 million square kilometres sit on permafrost, including much of Alaska, Canada, Greenland and Siberia, plus high ground such as the Tibetan Plateau. The Southern Hemisphere has little, confined to mountains like the Patagonian Andes and New Zealand's Southern Alps. Above the frozen layer lies an active layer that thaws each summer and lets roots grow; in 2018 it averaged about 145 centimetres across the Northern Hemisphere, but under 50 centimetres in northeastern Siberia, Alaska and Greenland, and up to 10 metres in southern Norway and the Mongolian Plateau.
Its presence depends on subtle temperature thresholds. Where the mean annual air temperature falls below freezing, permafrost generally forms, though deep snow can insulate the ground enough to prevent it, as in humid forests of northern Scandinavia. Slightly below zero, it survives only in sheltered pockets, giving patchy, discontinuous zones. Just over half of the northern permafrost region is continuous, meaning 90 to 100 percent of the ground is frozen.
Frozen soil has locked away dead plant matter for millennia. As warming thaws it, microbes resume decomposition and release carbon dioxide or methane, feeding further warming. Exact amounts are uncertain, but scientists broadly agree the emissions will stay smaller than human ones and will not trigger runaway warming; yearly releases may rival global deforestation or the output of a large country such as Russia, the United States or China.
Thaw carries other hazards. Ice-rich ground can abruptly shift or collapse, and nearly 70 percent of infrastructure built on permafrost may be at risk by 2050, with costs possibly reaching tens of billions of dollars later this century. Between 13,000 and 20,000 toxic waste sites, along with natural mercury, lie in frozen ground and could leak. Fears of revived ancient pathogens are considered unlikely; one review rates the risk generally low.
Source: Permafrost
