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Permanently frozen ground which occupies some 26% of the Earth's land surface under thermal conditions where temperatures below 0 °C have persisted for at least two consecutive winters and the intervening summer. Considerable thicknesses may develop, e.g. 600 m on the North Slope of Alaska, and 1400 m in Siberia, but these are partly relict from the last glaciation. Permafrost may contain an unfrozen unit, called ‘talik’, and may be overlain by an active layer.

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