Permafrost Distribution In The Northern Hemisphere

Permafrost is widespread in the northern part of the Northern Hemisphere, where it occurs in 85 percent of Alaska and 55 percent of Russia and Canada. In contrast, in the Southern Hemisphere, permafrost is probably found across all of Antarctica. Permafrost is more widespread and extends to greater depths in the north than in the south. It is 1,500 metres (5,000 feet) thick in northern Siberia, 740 metres (about 2,400 feet) thick in northern Alaska, and thins progressively toward the south.

Most permafrost can be differentiated into two broad zones, the continuous and the discontinuous, referring to the lateral continuity of permafrost. In the continuous zone of the far north, permafrost is nearly everywhere present except under the lakes and rivers that do not freeze to the bottom. The discontinuous zone includes numerous permafrost-free areas that increase progressively in size and number from north to south. Near the southern boundary, only rare patches of permafrost have been found to exist.

In addition to its widespread occurrence in the Arctic and subarctic areas of the Earth, permafrost also exists at lower latitudes in areas of high elevation. This type of perennially frozen ground is called Alpine permafrost. Although data from high plateaus and mountains are scarce, measurements taken below the active surface layer indicate zones where temperatures of o°C or colder persist for two or more years. The largest area of Alpine permafrost is in western China, where 1.5 million square km (580,000 square miles) of permafrost are known to exist. In the contiguous United States, Alpine permafrost is limited to about 100,000 square km (38,600 square miles) in the high mountains of the west. Permafrost occurs at elevations as low as 2,500 metres (8,200 feet) in the northern states and at about 3,500 metres (about 11,500 feet) in Arizona.

A unique occurrence of permafrost—one that has no analogue on land—lies under the Arctic Ocean, on the northern continental shelves of North America and Eurasia. This is known as subsea or offshore permafrost.

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