Identify features unique to areas with permanently frozen soils and explain environmental changes taking place in those areas.

Periglacial means “around glaciers,” but the term refers to all unglaciated areas at high latitudes and high elevations subject to persistent and intense freezing. As a result of climate change, periglacial environments are one of the most rapidly changing physical systems on Earth. As they change, they are also capable of creating significant climate feedbacks that could further change the global climate system.

In many regions of Alaska, northern Canada, and Siberia, the ground just beneath the surface veneer of plants and seasonally thawed gelisol soils (see Section 9.1) is perpetually frozen, forming permafrost. Permafrost is ground that remains below freezing continuously for two years or more (Figure 17.2). Not all permafrost is composed of ice. If there is no water in the soil, there will be no ice, only unconsolidated rock fragments at a temperature below freezing. Loose frozen sand, for example, is also permafrost.

Although permafrost, by definition, remains frozen year-round, in most regions the topmost portion of the ground, called the active layer, thaws each summer and refreezes in fall. The depth of the active layer decreases farther north and at higher elevations in mountainous areas. When the active layer thaws, ice turns to liquid water, forming lakes and wetlands called bogs (Figure 17.3A). The boreal forest and northern tundra biomes (see Section 8.3) are found in areas with permafrost.

About 25% of the soils in the Northern Hemisphere have permafrost. At high latitudes, permafrost is typically unbroken, or continuous. Farther south, permafrost may become isolated, sporadic, or discontinuous (Figure 17.3B).

548

Most permafrost is found at 60 degrees north latitude and higher, but it can be found at all latitudes at high elevations. Near the equator permafrost is found at elevations higher than 5,000 m (16,400 ft). Mount Kilimanjaro in Kenya, for example, located at 3 degrees south latitude, has permafrost soil. The thickest permafrost, 1,650 m (5,445 ft) thick, is found in northeastern Russia. The permafrost there is over a half million years old.

The Southern Hemisphere has relatively little permafrost due to its lack of land at high latitudes. Only 0.3% of Antarctica’s land is not covered by the Antarctic ice sheet, and all of that exposed land is permafrost. The Patagonian ice fields in South America and the highlands of the Southern Alps in New Zealand have discontinuous permafrost.

As we saw in Section 6.4, the cryosphere is changing rapidly in response to warming of the atmosphere. Rapid changes are occurring in periglacial environments, particularly in the active layer, as ground temperatures increase (Picture This).

549

Several landforms and phenomena are found only in periglacial environments. Two periglacial landforms are pingos and patterned ground. A pingo is a hill with a core of ice (Figure 17.4A). Pingos form as liquid water from below is forced up through a layer of permafrost. As the water pushes through the permafrost, it forms a mound. Pingos grow at a rate of a few centimeters per year. Patterned ground is formed as freeze-thaw cycles slowly wedge soil apart. Over time, polygon shapes develop (Figure 17.4B).

Trees growing on the active layer above permafrost are shallow-rooted and unstable because their roots cannot penetrate the permafrost just below. When the active layer deepens during particularly warm summers, the trees may become tilted, resulting in a drunken forest (Figure 17.5).

Structures that radiate heat, such as railways and pipelines, thaw the permafrost beneath them. When permafrost thaws, it becomes unstable, and structures built on it sink into the ground. Such structures may therefore be elevated above the ground to avoid these problems.

550

The 1,300 km (800 mi) Trans-Alaska Pipeline moves heated oil from the North Slope of Alaska all the way to the Pacific coast. Where it passes over permafrost, it has been elevated about 2 m (6.5 ft) above the ground to prevent melting of the permafrost, which would cause the pipe to rupture. China’s Qinghai–Tibet railway is also engineered for permafrost conditions (Figure 17.6).