Although in the public imagination the term “permafrost” is often associated with massive ice buried under the ground, it does not imply the presence of the frozen water. Frozen ground may be "dry"; the term "permafrost" refers to any sub-surface materials that remain below 0 °С for at least two consecutive years. Regions where permafrost underlies all or part of the ground surface occupy about 25 per cent of land area in the Northern Hemisphere, of which about 16.7 million km2 is located in north-eastern Eurasia and 10.2 million km2 in North America.
Permafrost is divided into continuous (more than 90 per cent of land is underlain by the frozen ground), discontinuous (50 to 90 per cent), and sporadic zones (less than 50 per cent) depending on its areal continuity. Permafrost zones differ in ground temperature, depth of seasonal (summer) thawing, and mechanical strength of the frozen ground. Mean annual ground temperature is typically between –8 to –13°С in the northernmost zone of continuous permafrost, –3 to –7 °С in the discontinuous zone, and 0 to –2 °С in the southern sporadic zone. Permafrost thickness varies from as much as 1500 meters in unglaciated parts of Siberia and 740 meters in northern Alaska to less than a meter in the southern zone, and is typically between 100-800 m, 25-100 m, and 10-50 m in the continuous, discontinuous and sporadic zones, respectively.
In Russia, the total area of permafrost distribution is about 10.7x106 km2 that makes approximately more than 60% of territory. Several large cities (Yakutsk, Noril'sk, Vorkuta) with populations of more than a hundred thousand and large river ports are built upon permafrost.
The thawing of permafrost is one of the symbols of modern global warming. Owing to its potential for settlement, the thawing of permafrost constitutes a significant environmental hazard in high-latitude regions. The consequences of this problem can be learned by the observational data.
Permafrost mainly characterized by ground temperature and active layer thickness. The active layer of permafrost is an upper layer that thaws every summer and refreezes every winter.
Direct observational data about the long-term response of permafrost to climatic variations are not available except for a few locations. The longest record of permafrost temperature is available in Yakutsk from the 116.4- m-deep well.
In the early 1990s the Circumpolar Active Layer Monitoring (CALM) program was launched to monitor active-layer processes and thickness, using a variety of methodologies and sampling designs. Currently CALM involves more than 140 sites in both polar regions and in high mountain environments elsewhere, on the territory of Russia there are more than 20 CALM sites.
It is reasonable to refer to empirical data obtained over the past 25 years since the beginning of warming. During this period, the mean global air temperature increased by about 0.4 °С, and, in the permafrost region, an increase of the temperature was more significant (1.0–1.5 °С), which was to result in changes of the permafrost. According to the observational data, permafrost temperature in the Central Yakutia has increased by 1-1.5 °С and in the Western Siberia by 1.0 °С since the beginning of 1970-s. Such changes undoubtedly are due to global processes as degradation of ice-rich permafrost has also been documented in northern and central Alaska, where temperature of the upper layers of frozen ground has increased by 2-4°С since the beginning of 20th century till the 1980s and by 3°С during the next 20 years. In the North-west Canada the upper permafrost layer has become warmer by 2°С over the past two decades. The regions of the special interest are “anomalous” regions with prolonged cooling trends against the global warming. Such anomalous region is the north-east of Canada. It is important to note that even this region the temperature of the upper layers of frozen ground has increased almost by 2°С since the beginning of 1990s, which is the evidence of dominant role of the global warming in these changes.