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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 permafrost
The line of continuous permafrost in the Northern Hemisphere is the edge made of the furthest-northward points at which the permafrost sometimes melts or is interrupted by regions without permafrost; north of the 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.
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. However accurate records only go back 30 years.
See also global warming controversy, tundra.
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