Map of the earth’s cryosphere, image by NASA, November, 2011.

What, exactly, is permafrost?

According to wunderground.com, 24% of the globe is covered by permafrost. As can be seen by the above map of the earth’s cryosphere, most of that permafrost is in the northern hemisphere. It is found in the high latitudes near the north and south poles, and also in high altitudes, such as the Alps and the Himalayas.

Permafrost is permanently frozen soil, sediment, or rock. Its classification is solely based on temperature, not moisture or ground cover. The ground must remain at or below 0 degrees Celcius/Centegrade for at least two years in order to be considered permafrost. Although new permafrost is forming, it can be over thousands of years old. For example, some of the permafrost in western Canada’s boreal peatlands has been there since the Little Ice Age of the 1600’s (Turetsky et al., 2007).

Permafrost has layers, of which frozen ground is just one portion (Figure 1).

• The active layer is ground that is seasonally frozen, typically lying above the perennially frozen permafrost layer.
• Talik is unfrozen ground that lies below the permafrost and between the active layer and [perenially frozen] permafrost.

Bullet point formatting added by Science15.com. Source: Wunderground.com.

The diagram below shows that the talik (unfrozen ground which formerly was permafrost) may interrupt the layers.

When talik completely separates areas (not just layers) of permafrost, which separation occurs in warmer latitudes (generally, below latitude 64°N–the Arctic Circle is at 66.5°N), and where ground has geologically shifted, the permafrost is “discontinuous.”

Model of permafrost soil layers.

Image Attribution: By Uros Djuric (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons. English text in this model of permafrost soil layers has been added by Science15.com.

To see a video explaining how scientists are investigating permafrost in Alaska, visit the next page.