This article is about frozen ground. For other meanings, see Permafrost (disambiguation).
In geology, permafrost or permafrost soil is soil that stays in a frozen state for more than two years in a row.
The extent of permafrost can vary as the climate changes, approximately 20% of the Earth's land mass is underlain by permafrost. Seasonal frost commonly overlays permafrost and is called the active layer as it will thaw during the summer. The active layer can support plant life, permafrost will not. Thickness of the active layer varies by year and location but is typically 0.6 - 4 m (2 to 12 feet) thick. Thickness of the permafrost can go to hundreds of metres (440 m (1330 feet) measured at Barrow, Alaska).
Continuous and discontinuous permafrost
Permafrost will form in any climate where the mean annual air temperature is less than the freezing point of water - unless there is sufficient snowfall to form glaciers or ice sheets. However, typically the temperature of the ground will on average be less variable from season to season that the air temperature, and they also tend to increase with depth. Thus, if the mean annual air temperature is only slightly below 0°C (32°F), permafrost will form only in spots that are sheltered - usually with a northerly aspect. This creates what is known as discontinuous permafrost. Usually, permafrost will remain discontinuous in a climate where the mean annual temperature is between -5 and 0°C (23 to 32°F). There are exceptions in unglaciated Siberia and Alaska where the present depth of permafrost is a relict of climatic conditions during glacial ages where winters were up to 11°C (20°F) colder than those of today. At mean annual temperatures below -5°C (23°F) the influence of aspect can never be sufficient to thaw permafrost and a zone of continuous permafrost forms.
A line of continuous permafrost in the Northern Hemisphere is formed from the furthest-northward points at which permafrost sometimes melts or is interrupted by regions without permafrost; north of this line all land is covered by permafrost or glacial ice. The "line" of continuous permafrost lies further north at some longitudes than others and can gradually move northward or southward due to regional climactic changes. In the southern hemisphere, most of the equivalent line would fall within the Southern Ocean if there were land there; most if not all of the Antarctic continent is covered not with frozen soil but with glacial ice.
Construction on permafrost
Building on permafrost is difficult due to the heat of the building (or pipeline) melting the permafrost and sinking downwards. This sinking problem has three common solutions: using foundations on wood piles, building on a thick gravel pad (usually 1 to 2 m (4 to 6 feet) thick), or using anhydrous ammonia heat pipes. The Trans-Alaska Pipeline System uses insulated heat pipes to keep the pipeline from sinking into the permafrost.
At the Permafrost Research Institute in Yakutsk, it has been found that sinking of large buildings into the frozen earth (known to the Yakuts before Yakutsk was even founded) can be prevented effectively by means of stilts extended down to a depth of about fifteen metres or more. At this depth the temperature does not change with the seasons but remains at about -5°C.
Permafrost extent
Measurement of the depth and extent of permafrost may be an indicator of global warming as recent years (1998 and 2001) have seen record thawing of permafrost in Alaska and Siberia. In the Yukon, the zone of continuous permafrost has moved 100 kilometres poleward since 1899. However accurate records only go back 30 years. It is thought that the thawing of permafrost could exacerbate global warming through the release of methane and other hydrocarbons which are powerful greenhouse gases. It also could encourage erosion because permafrost lends considerable stability to the barren slopes in the Arctic.
At the Last Glacial Maximum, continuous permafrost covered a much greater area than it does today, covering all of ice-free Europe south to about Szeged and the Sea of Azov (then dry land) and China south to Beijing. In North America, only an extremely narrow belt of permafrost existed south of the ice sheet at about the latitude of New Jersey through southern Iowa and northern Missouri. In the southern hemisphere, there is some evidence for former permafrost from this period in central Otago and Argentine Patagonia, but it was probably discontinuous except at the very highest latitudes.
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