Permafrost Melts in Siberia
Permafrost, or permanently frozen ground, is soil, sediment, or rock that remains at or below 0°C for at least two years. It occurs both on land and beneath offshore Arctic continental shelves, and its thickness ranges from less than 1 meter to greater than 1,000 meters. Seasonally frozen ground is near-surface soil that freezes for more than 15 days per year. Intermittently frozen ground is near-surface soil that freezes from one to 15 days per year.Frozen ground data are critical to understanding environmental change, validating models, and building and maintaining structures in seasonal frost and permafrost regions. Climate models and observations have both pointed to likely permafrost thawing in the 21st century.Permafrost:Insights from a Northern Hemisphere permafrost map and recent observationsPermafrost regions occupy approximately 22.79 million square kilometers (about 24 percent of the exposed land surface) of the Northern Hemisphere (Zhang et al. 2003b). Permafrost occurs as far north as 84°N in northern Greenland, and as far south as 26°N in the Himalayas. Because reliable data on hemispheric-scale permafrost extent have only recently become available, this site provides just a snapshot of current permafrost conditions rather than time-series data.Exactly what is permafrost? Permafrost is not defined by soil moisture content, overlying snow cover, or location; it is defined solely by temperature. Any rock or soil remaining at or below 0°C for two or more years is permafrost. It can contain over 30 percent ice, or practically no ice at all. It can be overlain by several meters of snow, or little or no snow. Understanding permafrost is not only important to civil engineering and architecture, it is also a crucial part of studying global change and protecting the environment in cold regions. In terms of area, permafrost can be characterized as continuous, discontinuous, sporadic, or isolated, but because these are descriptive terms, the boundaries separating different permafrost zones can be vague (Zhang 2005).Determining the location and extent of permafrost is often difficult. The historical approach has been to assume that ground temperature equals the overlying air temperature, but ground and air temperatures usually differ. Even in areas where the mean annual air temperature is below freezing, permafrost may not exist. Land under glaciers, rivers, and streams is often free of permafrost, despite freezing air temperatures at the surface (Williams and Smith 1989).Geologists and geocryologists have mapped permafrost since the mid-20th century. In 1990, the International Permafrost Association (IPA) recognized the need for a single, unified map to summarize the distribution and properties of permafrost and ground ice in the Northern Hemisphere. The IPA map shows the distribution of permafrost and ground ice for the continental land masses, areas of mountain and plateau permafrost, sub-sea and relict permafrost, relative abundance of ice wedges, massive ice bodies and pingos, and for ranges of permafrost temperature and thickness (Brown et al. 1998).Most permafrost in the Northern Hemisphere occurs between latitudes of 60°N and 68°N. (North of 67°N, permafrost declines sharply, as the exposed land surface gives way to the Arctic Ocean.) There is also a significant amount of permafrost around 35°N, in the Qinghai-Xizang (Tibet) Plateau, and in the mountains of southwest Asia and the U.S. Rocky Mountains. About 37 percent of Northern Hemisphere permafrost occurs in western North America, mainly in Alaska and northern Canada between 165°W and 60°W. Most permafrost occurs in the Eastern Hemisphere, mainly in Siberia and the Far East of Russia, northern Mongolia, northeastern China, the Qinghai-Xizang (Tibet) Plateau, and surrounding mountains between 60°E and 180°E (Zhang et al. 1999).Proximity to large water bodies tends to reduce temperature extremes, which affects the distribution of permafrost. Scandinavia and Iceland, for instance, have relatively little permafrost (Williams and Smith 1989). Snow cover can play a varying role in the formation or survival of permafrost. In areas of continuous permafrost, seasonal snow cover can lead to warmer ground temperatures, while in areas of discontinuous or sporadic permafrost, the absence of snow cover can contribute to permafrost formation (Zhang 2005).
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