What is permafrost?
Permafrost is perennially frozen earth composed of substrate, including soil, gravel, sand, and decayed plant and animal material known as organic matter. Permafrost can contain large amounts of ice (up to 80%), and has been frozen continuously for at least two years; sometimes up to hundreds of thousands of years.[1]
Permafrost is found anywhere temperatures are low enough underground to stay below freezing. It can be found even in warm latitudes and in high mountain ranges, but is primarily in the Arctic and sub-Arctic, underlying the tundra and large swaths of boreal forest.
Researcher standing in front of an icy permafrost cliff. Photo by: Becky Tachihara
In the far north, much of the land is covered by a thin layer of soil that freezes and thaws based on the seasons—the active layer. Permafrost extends below the active layer, and can range in thickness from less than 1 meter to greater than 1,500 meters.[2] The deepest continuous permafrost ever found exists in Siberia—a layer that extends down 1650 meters.[2]
Thaw vs melt
An important distinction to be aware of when discussing permafrost degradation is that permafrost thaws, not melts. Permafrost can contain ice, which melts into liquid water when warmed, but as the frozen soil, gravel, sand, and organic matter in permafrost get warmer, they thaw, rather than melt or liquefy.[3] You can think of the difference as similar to a warming freezer. Ice cubes in the freezer would melt, but your frozen meats and vegetables would thaw.
Watch this optional video to see ice-rich permafrost soil cores thawing in a laboratory:
This is an additional video, hosted on YouTube.
Video courtesy of Brendan O’Neill
Different types of permafrost
Not all permafrost is exactly the same—scientists have assigned different terminology to permafrost with different characteristics.
Epigenetic and syngenetic permafrost
Epigenetic permafrost is permafrost that forms when earth material already accumulated by wind, water, gravity, or plant and microbial activity over thousands of years is frozen.[4] This also means that before the soil is frozen, it is exposed to microbes that have a chance to start decomposing organic matter before the soil becomes permafrost.
In contrast, syngenetic permafrost is permafrost that forms in a cold climate at the same time as earth material is being deposited by wind, water, or gravity.[4] Because of this, syngenetic permafrost can have larger ice wedges, which can cause major disturbances when this permafrost thaws. There can also be more carbon stored in syngenetic permafrost than in epigenetic permafrost, and this carbon can decompose quickly when thawed, since the soils were less exposed to microbes before becoming frozen.
Continuous and discontinuous permafrost
Continuous permafrost is permafrost that exists in a consistent swath, extending both vertically and horizontally, and is found in the high north, except under large bodies of water.[2]
The city of Yakutsk on the Lena River in east Siberia is an example of where continuous permafrost can be found.
Farther south, the continuous permafrost landscape transitions into discontinuous permafrost. Discontinuous permafrost is broken up into separate patches and is dependent upon moisture, soil type, sun exposure, and vegetation.[2]
The city of Fairbanks, Alaska is an example of where discontinuous permafrost can be found.
Yedoma permafrost and glaciers
Yedoma permafrost is a type of permafrost from the Pleistocene age (formed 1.8 million to 10,000 years ago). It contains a large amount of organic matter, and is made up of 50-90% ice.[5] Due to its high ice content, Yedoma permafrost is often confused with glaciers, which are massive bodies of perennial ice that slowly advance and recede over land that are defined by their size, but Yedoma isn’t a glacier.[6] All of the yedoma on Earth contains more than 25% of the carbon in the entire permafrost region, even though the yedoma domain only covers 7% of that region.[7]
References
1National Snow and Ice Data Center. 2022. Why Frozen Ground Matters. [online] Available at: https://nsidc.org/learn/parts-cryosphere/frozen-ground-permafrost/why-frozen-ground-matters [Accessed 15 August 2022].
2Energyeducation.ca. 2022. Permafrost – Energy Education. [online] Available at: https://energyeducation.ca/encyclopedia/Permafrost#cite_note-RE3-3 [Accessed 15 August 2022].
3Grosse, G., Romanovsky, V., Nelson, F. E., Brown, J., and Lewkowicz, A. G. (2010), Why Permafrost Is Thawing, Not Melting, Eos Trans. AGU, 91( 9), 87– 87, doi:10.1029/2010EO090003.
4Permafrost: General Facts [Internet]. erdc.usace.army.mil. [cited 15 August 2022]. Available from: https://www.erdc.usace.army.mil/CRREL/Permafrost-Tunnel-Research-Facility/General-Facts/
5National Snow and Ice Data Center. Cryosphere glossary. [online] Available at: https://nsidc.org/learn/cryosphere-glossary/yedoma-permafrost [Accessed 15 August 2022].
6What is a glacier? U.S. Geological Survey [Internet]. Usgs.gov. [cited 15 August 2022]. Available from: https://www.usgs.gov/faqs/what-glacier
7Strauss, J., Schirrmeister, L., Grosse, G., Fortier, D., Hugelius, G., Knoblauch, C., et. al. 2017. Deep Yedoma permafrost: A synthesis of depositional characteristics and carbon vulnerability. Earth-Science Reviews 172. doi:10.1016/j.earscirev.2017.07.007
Thawing Permafrost: Science, Policy, and Environmental Justice in the Arctic
Thawing Permafrost: Science, Policy, and Environmental Justice in the Arctic
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