Arctic Science Explained: Permafrost
June 8, 2022
By Liz Weinberg
By Katherine Schexneider
Welcome to Arctic Science Explained! Each month, Collaborations community member Katherine Schexneider breaks down a topic related to Arctic science. If you have a topic you’d like to see featured, please email firstname.lastname@example.org. This month, Katherine takes on permafrost.
“Boggy and um…soggy?” That’s how my friend described what thawing permafrost would look like. Arctic permafrost is thawing, which is bad for the Arctic—but why? This blog defines permafrost and summarizes it with five terms, all new to Wilson, and maybe for you, too.
Permafrost is part of a healthy Arctic ecosystem. Found where the annual average air temperature is below 0℃, it is ice within the ground mixed with soil, decaying plant material, and bedrock. To be classified as permafrost, it must remain at or below 0℃ continuously for at least two years. It’s more variable than you might think, with different compositions of the materials, architecture, geographical distribution, and types of change occurring through the seasons. Permafrost changes through the year and due to climate warming, the teeter totter of thawing and freezing, and its carbon stores.
Coastal erosion reveals the extent of ice-rich permafrost underlying active layer on the Arctic Coastal Plain. Photo: Brandt Meixell/
Continuity zones. Scientists classify permafrost as continuous when it underlies a geographic region, found everywhere they probe the ground. Discontinuous means patchy, found in 50 to 90 percent of the region, usually away from south-facing slopes. Sporadic permafrost is even patchier, existing in a few spots, but not others. In Alaska, shrubby plants (tundra) overlie continuous zones, while boreal forests grow in discontinuous regions. In Siberia, continuous permafrost can underlie boreal forest.
Generally, the farther north we go, the more continuous zones we find. These zones can change as temperatures rise over the long-term, as you’ve probably already guessed: what was once continuous becomes a patchwork. This change in permafrost type can change the broader ecosystem, too, with forests beginning to replace the tundra. By tracking continuity, we assess and map where permafrost has thawed, and as we’ll see shortly, thawing affects the ecosystem, often in detrimental ways.
Researcher Ludda Ludwig prepares a permafrost ice core for transportation back to the lab. Photo: Stan Skotnicki (PolarTREC 2016), courtesy of
Active layer. Permafrost acts differently at different depths. The active layer lies above the permafrost and changes over the course of a year, thawing in summer and refreezing in winter. Plants decompose in the active layer when they die, releasing nutrients and carbon as part of an ecosystem cycle.
A cross-sectional diagram of a permafrost pit at Imnavait Creek, Alaska, clearly illustrates the active layer above the permafrost. Photo: David Walker (PolarTREC 2019), courtesy of
This active layer is usually about a foot and a half deep, but may be deeper in certain areas such as the Tibetan plateau, where it is eight feet deep in some areas. Summertime air temperatures, rain, and the thermal properties of the soils determine the active layer thickness. Wet, soggy soils conduct heat better than dry soils, causing deeper seasonal thaw. As temperatures warm over several years, the active layer gets deeper.
In addition to monitoring changing active layer thickness, scientists can monitor deep permafrost temperatures to detect local effects of climate change.
Talik formation. A talik (tah’-lick) is an area of ground that remains unfrozen year-round and is found separate from the active layer. It can be thought of as a thawed space suspended in permafrost. Some taliks are surrounded by frozen permafrost, while others form below melt ponds called thermokarst lakes.
We can locate taliks using probes, and they’re another valuable indicator of climate warming. They are growing in number. In her July 2021 presentation to ’s Permafrost Collaboration Team, Dr. Louise Farquharson (University of Alaska Fairbanks) noted that a study of 56 Alaska permafrost sites found 25 recently-formed taliks—a lot.
The drivers of talik formation are complex, but involve longer periods of thawing and increased snow cover, since snow is actually an insulator for the ground, keeping it relatively warm. When they form underneath highways, these roads—and there aren’t that many in Alaska–can sink, cutting off supply arteries to remote communities. Similarly, taliks threaten homes, buildings, and industrial structures. Rebuilding, whether it’s roads or buildings, is no easy task in the Arctic.
Carbon pools. All the decaying plants, animals, and microscopic organisms that make up much of permafrost contain carbon, a lot of it, about twice as much as is currently in our atmosphere and three times as much as in all the forests on our planet. As the active layer deepens, carbon and nutrients are released from the thawed permafrost layer.
Permafrost contains a significant amount of carbon. Image:
Plants can benefit from this process, taking up nutrients to support growth. But microbes in the soils decompose the thawed permafrost, releasing carbon dioxide and methane gas to the atmosphere in the process. When permafrost thaws, that carbon is released into the atmosphere, compounding the burden that human beings contribute by fossil fuel emissions. We are doing well to reduce our own carbon footprints and to move toward the elimination of fossil fuel usage, but much of the carbon burden cannot be so easily controlled, if at all.
Abrupt thaw. In addition to the more gradual active layer thickening that has been happening with climate warming over the last several decades, permafrost can also thaw rapidly, often several meters at once. The main cause is a high ice content in the permafrost, which when it thaws, causes the ground to collapse. When a massive ice wedge thaws, a pool of water can form on the surface, forming what is called a thermokarst, or thaw, lake. This melt water warms the ground underneath and beside it, further contributing to permafrost thaw. Ice-rich permafrost thaw can also form collapse-scar bogs and cause hillslope erosion.
Abrupt thaw events cause rapid carbon release orders of magnitude greater than that of gradual thaw. Abrupt thaws also destabilize the ground, lowering it and taking any natural or built structures with it. Abrupt thaw is accelerating and looks to double the amount of carbon release from permafrost this century.
Thawing permafrost is a huge threat to the Arctic ecosystem. Each time I step onto ground softened by rain, the boggy feeling reminds me of what is happening up North.
“Not so permanent permafrost…” Talk by Louise Farquharson, a geologist at , at the July 2021 Permafrost Collaboration Team Meeting.
, 2020 Arctic Report Card, Section on Coastal Permafrost Erosion.
National Academy of Sciences “Science Session: Thawing Arctic Permafrost–Regional and Global Impacts.” 87-minute panel discussion and Q+A.
Katherine Schexneider is a retired US Navy physician who now does volunteer work in Arctic research and climate change.
Note: Views expressed in this article are the author’s and do not necessarily reflect the views of the community.