Permafrost

Permafrost is a thermal condition existing in any type of earth material that is perennially frozen. It occurs in ground in which the temperature remains below 0 degrees Celsius for at least two consecutive years. Affecting about 25 percent of the earth's surface, the condition hampers construction in the far north in regions such as Siberia and Arctic Canada.

Formation and Distribution

Permafrost, or permanently frozen ground, is a thickness of earth material such as soil, peat, or even bedrock, at a variable depth beneath the ground surface, in which a temperature below 0 degrees Celsius exists continuously for a number of years (from at least two years to several thousands of years). Permafrost is thus not a process or an effect, but rather a temperature condition of earth material on land or beneath continental shelves of the Arctic Ocean. Its occurrence is conditioned exclusively by temperature, irrespective of the presence of water or of the composition, lithology, and texture of the ground. However, unfrozen water horizons may exist within a body of permafrost as the result of various conditions such as pressure or impurities in the water.

With regard to the climatic history of the earth, cold conditions and glaciers are the exception, not the rule. Because they are so rare, they are referred to as “climatic accidents.” During the Pleistocene, advances of continental glaciers covered vast areas of Europe and North America. Glaciers continue to be significant landscape features, since they lock up as ice some 2.15 percent of the earth's waters. Permafrost is a relatively new condition introduced with the onset of the Pleistocene glacial events, and it will be maintained as long as the climate remains cold.

As the earth's climate has warmed during the past 20,000 years and the continental ice has receded, the location, distribution, and thickness of permafrost have changed. Today, the area underlain by permafrost is vast. It has been estimated that about 25 percent of the earth's surface is underlain by permafrost. One reason that vast areas of the earth possess permafrost is that the continents widen toward higher latitudes in North America and Eurasia. In the Northern Hemisphere, Mongolia, Russia, and China contain about 12.2 million square kilometers of land underlain with permafrost. North America and Greenland have 8.8 million square kilometers of permafrost. In the Southern Hemisphere, Antarctica accounts for 13.5 million square kilometers of permafrost. During the Pleistocene, perennially frozen ground had a different distribution. Permafrost produces soil and sediment structures that remain after the climate warms. Relict features such as ancient ice wedges can be identified, and former permafrost areas can be mapped. Evidence of preexisting permafrost conditions, and thus a record of climate change, has been extensively documented in Great Britain, southern Canada, and the central United States.

In terms of aerial distribution and continuity, permafrost may be classified as either continuous or discontinuous. At higher-latitude land circling the Arctic Ocean, the lateral extent of permafrost is uninterrupted nearly everywhere, except in areas of recent deposition and under large lakes that do not freeze to the bottom. Discontinuous permafrost includes areas of frozen ground separated by nonpermafrost areas. A transect from the Arctic Ocean southward, into Canada or Russia, reveals more and more permafrost-free areas. Farther south, in central Canada and central Russia, only patchy or sporadic zones of perennially frozen ground are to be found.

Permafrost is a unique condition because it is maintained very close to its melting point. Several thousand square kilometers are warmer than –3 degrees Celsius. If global warming continues, as many scientists predict, most discontinuous permafrost will eventually degrade. Russian scientists, in fact, have documented the northward retreat of permafrost near Archangel at a rate of 400 meters per year since 1837. In Canada, permafrost has retreated northward more than 100 kilometers since 1945, as ground temperatures have increased by 2 degrees Celsius in the northern prairie provinces. Changes in temperature may result from a local microclimate change, alteration of land cover, or changes in the atmosphere. Conversely, permafrost not only is degraded by global warming, but may also contribute to atmospheric change. Scientists suggest that thawing of Arctic terrain, which is rich in peat accumulation, may affect the chemistry of the atmosphere. It has been determined that one-fourth of the earth's terrestrial carbon is stored in organic matter in the permafrost and active layer. Long-term warming would release enormous quantities of greenhouse gases—methane and carbon dioxide in particular—accelerating global warming.

Vertical Characteristics

A cross section of excavated earth in the high Arctic reveals the vertical characteristics of permafrost. The thickness of permafrost is variable because of differences in air temperatures, occurrence of water, composition and texture of earth materials, and many other factors. The permafrost in Siberia may be at least 5,000 meters thick, whereas in Alaska thicknesses of 740 meters have been recorded. Farther south, thicknesses decrease as discontinuous permafrost becomes more common. The thickness of permafrost is in part determined by a balance between the increase of internal heat with depth and the heat loss from the earth's surface. If the thickness and depth of permafrost remain unchanged for many years, the heat loss at the surface and the heat at the base of the frozen ground are in equilibrium, and a steady state is maintained. At the surface, the ground seasonally thaws and freezes. This section of the permafrost is known as the “active layer.” Its thickness is controlled by numerous variables in addition to seasonal temperature variations, and it exhibits great variability in thickness. Vegetation, snow cover, albedo, water distribution, and human alteration of the surface can all act as insulators and affect the thickness of the active layer. However, in areas not disturbed by people, the ground will typically thaw to a depth of from 1 to 12 meters during the summer.

The deeper boundary separating the active layer from the permafrost is known as the “permafrost table” or “zero curtain.” Within the permafrost, unfrozen lenses known by the Russian term talik occur. These unfrozen bodies frequently contain water and function as protected water sources or aquifers in the Arctic environment. Talik lenses may be isolated bodies completely enclosed in impermeable permafrost and may result from a change in the thermal regime in permafrost. Conversely, talik may be laterally extensive even in continuous permafrost and is frequently a valuable source of groundwater. Talik lenses are larger in discontinuous permafrost, and the active layer thickens at the expense of the permafrost.

Below the permafrost table, the perennially frozen ground has low permeability. Maps and photographs of the Arctic reveal many lakes, swamps, and marshes, suggesting high precipitation. However, evaporation rates are low because of the cold air temperatures, and standing water occurs because the ground is frozen at depth. Throughout the region, precipitation is probably less than 35 centimeters annually, equivalent to that of marginal deserts. The abundant water and poor drainage result from the occurrence of permafrost at depth, not to high atmospheric precipitation. As seasonal warming and cooling occur, the land surface expands and contracts, particularly in continuous permafrost areas. In essence, the lowering of the ground temperature causes thermal contraction of the ground, and vertical fissures or frost cracks occur. Water filling and freezing in these cracks creates ice wedges in the permafrost that widen with seasonal expansion and contraction. Ice wedges are often 3 to 4 meters wide and extend 5 to 10 meters into the perennially frozen ground below the active layer. In areas of degradation, ice wedges in the thawing permafrost begin to melt, creating thermokarst features characterized by numerous depressions similar to karst plains. As temperatures rise above freezing, the active layer extends deeper and deeper and eventually may intercept an ice wedge. If large ice wedges begin to thaw, the ground subsides, creating steep-sided conical depressions. The perimeter of a depression, often composed of very moist sediment, will slump into the depression, thus making it wider. Water then fills the depression, creating a thermokarst, or thaw lake. Averaging about 3 meters across, the lakes occur in clusters, are elliptical in shape, and are all oriented at right angles to the prevailing wind direction.

Principal Terms

periglacial: originally restricted to regions modified by frost weathering such as tundra

Pleistocene: the geologic era between 2.58 million and 11,000 years ago, characterized by extensive continental glaciation and a colder climate

subsidence: a downward vertical movement of the ground, commonly caused by land-cover changes of the surface

talik: a layer of unfrozen ground in permafrost; water under pressure frequently occurs in talik layers

thermokarst: a group of landforms in flat areas of degrading permafrost and melting ice

Bibliography

Anderson, Bjørn G., and Harold W. Borns, Jr. The Ice Age World. Oslo: Scandinavian University Press, 1994. The well-illustrated text details the Quaternary history of North America and northern Europe over the last 2.5 million years. Contains an extended glossary, references list, and index. For the general reader as well as the serious student.

Ballantyne, C. F., and C. Harris. The Periglaciation of Great Britain. New York: Cambridge University Press, 1994. A good upper-level description of permafrost in Britain. The discussion ranges from basic freeze-and-thaw processes to permafrost conditions and distribution in the geologic past.

Bloom, Arthur. Geomorphology: A Systematic Analysis of Late Cenozoic Landforms. 3rd ed. Long Grove, Ill.: Waveland Press, 2004. This college-level text covers the basics of geomorphology. Includes three chapters on glaciation, cold climates, and permafrost. Index and bibliography.

Chernicoff, Stanley, and Dona Whitney. Geology: An Introduction to Physical Geology. Upper Saddle River, N.J.: Prentice Hall, 2006. This is a good overview of the scientific understanding of the geology of the earth and surface processes. Includes a section on the development and effects of permafrost. Includes a link to a Web site that provides regular updates on geologic events around the globe.

Clark, M. J., ed. Advances in Periglacial Geomorphology. New York: Wiley InterScience, 1988. A series of state-of-the-art reviews of numerous processes and landforms in cold climates. Emphasizes processes of formation; very technical. Suitable for advanced earth science students as a reference and literature source.

Grosse, G., et al. “Why Permafrost Is Thawing, Not Melting.” EOS 91 (2010): 87. Discusses terminology used in past and recent scientific communications and suggestions for future communications. The authors' recommendations provide the basis for an interesting debate over proper word usage to convey information beyond the scientific community. The article also provides a good description of the process of thawing, contrasting it to the physical process of melting.

Harris, S. A. The Permafrost Environment. Totowa, N.J.: Barnes & Noble, 1986. A primer on the consequences of construction in permafrost regions. Topics include the soil properties and methods of permafrost analysis. Written for contractors and environmental specialists.

Romanovsky, V., et al. “Permafrost Temperature Records: Indicators of Climate Change.” EOS, AGU Transactions 83 (2002): 589-594. Discusses the changes to the ecosystem brought on by permafrost thawing. The article provides information on permafrost zones and the methods used to study their degradation.

Schuur, Edward A. G., et al. “The Effect of Permafrost Thaw on Old Carbon Release and Net Carbon Exchange from Tundra.” Nature 459 (2009): 556-559. This article discusses the permafrost soils of the Arctic ecosystem. Provides a good understanding of how permafrost thaw alters the carbon cycle. Somewhat technical, so having a background in science is helpful for readers.

Williams, P. J., and M. W. Smith. The Frozen Earth: Fundamentals of Geology. New York: Cambridge University Press, 1991. A college-level textbook for science majors, emphasizing the physics of cold regions. Equations are used to illustrate significant physical ideas.