Permafrost
Permafrost
By Christy Ullrich
National Geographic Magazine
Photograph by Bernhard Edmaier, National Geographic December 2007
Permafrost is perennially frozen ground that remains at or below zero degrees Celsius (32 degrees Fahrenheit) for two or more years and forms in regions where the mean annual temperature is colder than zero degrees Celsius. Permafrost underlies about 20 percent of the land in the Northern Hemisphere and is widespread within the Arctic Ocean’s vast continental shelves and in parts of Antarctica. Most of the world’s permafrost has been frozen for millennia and can be up to 5,000 feet thick.
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