Permafrost

Permafrost is ground that remains at or below 0 °C for 2 years or more, and is therefore a geologic manifestation of climate. At present, permafrost underlies about 25% of the land surface of the Earth, extends beneath great portions of Alaska and Canada, Siberia, and the Tibetan Plateau, and is found at high elevation in mountains throughout the globe. The spatial extent of permafrost has varied with climate and global ice cover during the Quaternary. Permafrost terrain consists of a surface active layer, which freezes and thaws each year, underlain by perennially frozen ground. Variations in active-layer thickness over time commonly alter the nature and form of near-surface ground ice and the associated cryostratigraphy. Since ground temperatures are a product of both climate and surface conditions, changes in active-layer or permafrost conditions may be caused by alterations to vegetation or snow cover as well as climate. Data collected throughout the circumpolar North indicate that the temperature in near-surface permafrost has increased in response to climate warming. Active-layer deepening has accompanied the warming, especially in areas of continuous permafrost. At present, considerable quantities of carbon are stored in permafrost, both in peatlands and mineral soils. Release of such carbon following permafrost thawing may accentuate climate change.