What Is Permafrost?
Permafrost is soil, rocks, or sediments that have been below the freezing point of water (32 °F) for two or more years. The ground must constantly remain below the freezing point for two years or more for it to be considered as permafrost. Most permafrost is found in the Arctic and Antarctic regions at a high latitude. However, the alpine permafrost exists at a higher altitude and a much lower latitude. Permafrost accounts for about 0.2% of the Earth’s water and occupies about 24% of the exposed land in the Northern Hemisphere. Permafrost also occurs in subsea on the continental shelf surrounding the Arctic Ocean.
Extent of Permafrost
Permafrost exists in layers on the ground. Permafrost exists beneath the top layer of soil or sediments which freezes and thaws annually forming the active layer. The thickness of the active layer varies with the season but it averages 0.3 to 4 meters thick. The deepest layer of the permafrost occurs where the underground heat maintains the temperature above freezing. Below the permafrost is the Talik layer which is made up of unfrozen soil or sediments. The extent of the formation of permafrost depends on the climate of an area. The depth of permafrost can exceed 4,600 feet in areas that have continuous permafrost and harsh winters. The thickness varies with location but averages 2 to 13 feet. A considerable size of land in the Arctic region is covered by permafrost.
Where is Permafrost Found?
Although permafrost is most common in high latitude locations that are close to the north and south poles, permafrost can also occur in various regions across the world. Slightly more than a third of Northern Hemisphere permafrost is found in parts of North America, mainly in northern Canada, Alaska, and Greenland. Most of the permafrost in the region occurs in Siberia, Far East of Russia, Mongolia, China, and Tibetan. In the Southern Hemisphere, permafrost occurs in Antarctica, its islands, and in the Andes Mountains. Continuous permafrost is formed in areas where the ground is cold enough throughout the year including Northern Scandinavia and the Far East of European Russia. Discontinuous or sporadic permafrost is formed in locations where temperatures only fall below the freezing point in certain areas such as in the shade or on the shielded part of a mountain. Seasonal permafrost forms during the cold season and disappear during the warm season.
Manifestation of Permafrost
Permafrost extends to the depth where the geothermal heat generated from the earth and the mean annual temperature achieve an equilibrium of 32 °F. Permafrost may reach a base depth of 4,880 feet in Siberia. The ice content of permafrost may exceed 2500%, leading to what is commonly referred to as massive ice. Massive ice can range in composition, from pure ice to sediment ice. Massive ice varies in thickness and can range from 2 meters to 10 meters. It is categorized into buried surface ice and intrasedimental ice. Buried surface ice may be derived from snow or frozen lakes while intrasedimental ice is formed by freezing of subterranean water. Permafrost also manifests in various large-scale landforms such as palsas and pingos. Palsas is a frozen heave containing permanently frozen ice lenses. Pingos are mounds of earth-covered ice that can reach 230 feet high and 2,000 feet wide.
Ecological Consequences of Permafrost
Permafrost puts constraint upon the plant rooting zones which leads to no or little vegetation cover in a permafrost region. Consequently, it affects the species dependent on plants and animals whose habitat is limited by the permafrost. The worldwide permafrost regions contain over 1500 billion tons of organic material that have built up over several decades. The amount of carbon contained in the permafrost is four times the amount that has been released into the atmosphere due to human activities. Should the carbon enter the atmosphere, it would speed up the rate of global warming with a significant amount emerging as methane. A layer of active permafrost may contain one billion bacteria cells. Some of the bacteria cannot be cultured in laboratories but their identity can be revealed by DNA techniques.
Effects of Climate Change
Climate change affects the interaction of the climate regime above and below the ground. However, any change in temperature at the surface of the ground takes time to affect the permafrost underneath. For thick permafrost, it may take hundreds of years for the temperature change to affect it, while for thin permafrost it may take years to decades. According to the Geological Survey of Canada, freezing has been significantly reduced during the cold season throughout North America’s permafrost region. The coastal and eastern areas of Canada have begun to experience increased warm season thawing of permafrost suggesting reduced depth and quantity of permafrost. Consequently, the incidences of seasonal permafrost have increased due to the decrease in the amount of permanent permafrost. Although several studies and research show a warming trend throughout the permafrost zone, areas such as Russia permafrost region has not experienced significant changes. The changes in permafrost regions are as a result of increased air temperature and reduced snow cover.
Impact of Melting Permafrost
Melting permafrost has major consequences on the surface and subsurface of the impacting infrastructure and ecosystem. Thawing permafrost on soils with potential instability may have a serious implication on the landscape. Erosion and landslides are common consequences of permafrost degradation. Erosion is especially worrisome in coastal areas. Permafrost and the ground surrounding it is very vulnerable to erosive agents such as wind and water. As permafrost thaws, the friction required between the thawing and frozen permafrost to maintain stability is likely to vanish which can lead to the sliding of permafrost. The sliding of the permafrost results in a landslide. Melting of permafrost may also lead to ground subsidence. It occurs when the soil and permafrost previously held together by ice collapse resulting in irregular hollows. Ground subsidence may negatively impact the vegetation cover of the affected area. Subsidence may also compromise the infrastructure built on the permafrost. Foundations of structure that were not designed to accommodate changes in permafrost are likely to shift and drop.
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