Barrat, A., de Arruda, G. F., Iacopini, I., & Moreno, Y. (2021). Social contagion on higher-order structures. ArXiv:2103.03709 [Physics]. http://arxiv.org/abs/2103.03709

Permafrost is ground that is permanently frozen. Permafrost contains soil, sand, and gravel, which are held together by ice (National Geographic). Permafrost can extend deep into the Earth, ranging in depths from 1 to 1000 meters. Frozen ground can be considered permafrost if it is frozen continuously at a temperature less than 0 degrees Celsius for two or more years (International Permafrost Association). Permafrost is common to places where temperatures stay below freezing, such as Siberia, Canada, Alaska, Greenland, among others (National Geographic). Permafrost can be continuous or discontinuous. Continuous permafrost is a solid sheet of permafrost, like in Siberia. Discontinuous permafrost exists when some permafrost areas remain frozen all year, but other areas of permafrost melt for a brief period of time during the summer. Such discontinuous permafrost exists in Canada. The melting of permafrost can be dangerous due to increased water levels and levels of erosion when the soil, gravel, and sand are no longer held together by ice. Measuring the temperature of deep permafrost can provide information about temperature changes in a region due to climate change (International Permafrost Association). In the 21st century, permafrost research had focused on monitoring the boundaries of permafrost and identifying melt regions. When permafrost melts, it melts from the top and bottom simultaneously. Areas of discontinuous permafrost create the most concern when considering climate change effects. Areas of continuous permafrost are not expected to melt for a very long period. Current permafrost research focuses on areas where permafrost is thin, as these areas are most likely to create issues for infrastructure.

References: "What is Permafrost?" International Permafrost Association. Accessed May 04, 2017. http://ipa.arcticportal.org/publications/occasional-publications/what-is-permafrost.

"Permafrost." National Geographic Society. October 09, 2012. Accessed May 04, 2017. https://www.nationalgeographic.org/encyclopedia/permafrost/.