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Permafrost is soil, rock or sediment that is frozen for more than two consecutive years. In practice, this means that permafrost occurs at a mean annual temperature of −2 °C (28.4 °F) or below. In the coldest regions, the depth of continuous permafrost can exceed 1,400 m (4,600 ft). [22]
The red dotted-to-solid line depicts the average temperature profile with depth of soil in a permafrost region. The trumpet-shaped lines at the top show seasonal maximum and minimum temperatures in the "active layer", which commences at the depth where the maximum annual temperature intersects 0 °C. The active layer is seasonally frozen.
The maximum frost depth observed in the contiguous United States ranges from 0 to 8 feet (2.4 m). [1] Below that depth, the temperature varies, but is always above 0 °C (32 °F). Alternatively, in Arctic and Antarctic locations the freezing depth is so deep that it becomes year-round permafrost, and the term "thaw depth" is used
In ecology, roots of plants cannot penetrate beyond the active layer, which places restrictions on which plants can grow in permafrost. In construction, the thaw depth is a major factor in ensuring the structural integrity of the objects in question. The primary factor that determines the thaw depth is the maximal air temperature.
Reconstruction of a paleo-surface revealed that the slump had carved at the time into the rolling topography to a depth of up to 73 m. The current size of the slump was then >69 ha, and it had thawed >25 × 106 m³ of ice-rich permafrost through 2016.
With time and continued increases in air temperature or snow depth, this talik layer becomes thicker and thicker and the deep permafrost layer eventually disappears. Findings from a scientific study suggest more common occurrence of open taliks within areas of fault zones and areas influenced by large rivers. [1]
continuous permafrost (underlying 90 to 100% of the landscape) discontinuous permafrost (50 to 90%) sporadic permafrost (0 to 50%) In the discontinuous and sporadic zones permafrost distribution is complex and patchy, and permafrost-free terrain is common. The depth of permafrost varies from less than one meter to more than 1,500 m (4,900 ft). [23]
For this reason, permafrost environments are extremely vulnerable to climate change in the Arctic. Permafrost degradation caused by climate warming is indicated by increased mean annual ground temperature, increased active layer thickness, talik and thermokarst development and disappearance of permafrost islands. [19]