Permafrost is permanently frozen soil—soil that remains at or below 0°C for at least two consecutive years—and occurs mostly in high latitudes. Permafrost can occur beneath the land surface (terrestrial permafrost) and beneath the seafloor (subsea permafrost).
The ice content and temperature of permafrost are the key parameters that determine its physical state. Scientists use permafrost temperature, measured at a depth where seasonal variations cease to occur, as an indicator of long-term change and to represent the mean annual ground temperature.
Permafrost temperatures have increased in most regions since the early 1980s in response to increased surface temperature and changing snow cover.
By 2100, scientists expect the area of permafrost near the surface to decrease by between 37 to 81 percent, depending on the level of 21st century greenhouse gas emissions.
Permafrost lays underneath about 80 percent of Alaska’s surface. It is thickest and widespread in northern Alaska, diminishing to a permafrost-free region in the far south. 70 percent of permafrost land in Alaska is vulnerable to land sinkage due to the steady rate of permafrost thaw.
In the arctic region of Alaska, permafrost warmed up to 5.4°F (3°C) from 1980 to 2000. Over the same period in the subarctic region—home to Alaska’s boreal forest—permafrost warmed 0.5-1.8°F (0.3°C-1°C) and is already beginning to thaw.
Some climate models project that near-surface permafrost will be lost entirely from large parts of Alaska by the end of this century. Some areas such as Fairbanks—Alaska’s second-largest city—are particularly vulnerable, because the ground temperature now hovers near the thaw point, making the permafrost on the verge of collapse and prone to thawing unevenly.