In geology, permafrost is soil that stays at or below 0 °C (32 °F) for two or more years. It is also called cryotic soil. Most permafrost is on land close to the North and South poles. Other permafrost is found in in high mountain areas. 
- Schuur, Ted. "Thawing Permafrost Would Accelerate Global Warming. Thawing Arctic tundra will likely speed up climate change for a century or more. The question is: How drastically?". Scientific American (December 2016): 56-61. https://www.scientificamerican.com/article/thawing-permafrost-would-accelerate-global-warming. Retrieved 25 November 2016.