What is permafrost?

Question: Melting permafrost seems to be in the news a lot lately. What is permafrost and what happens when it melts?

Answer: Permafrost, which is mostly located at high latitudes close to the North and South Poles and at high altitudes in mountainous regions, is defined as a perennially frozen, subsurface layer that remains below zero degrees Celsius for at least two consecutive years.

Permafrost is thawing across the Arctic, causing land surface to subside or change shape. In this photo taken on July 7, 2014 in Gates of the Arctic National Park, a bank of this lake thawed, allowing the Okokmilaga River to cut through and drain it. Photo: National Park Service Climate Change Response.

While the term “frozen” implies the presence of ice (water), extremely arid soils—like some Antarctic soils—can be permafrost, as can solid rock.

That’s because the definition of permafrost is based on temperature alone, and the subsurface layer can be composed of organic matter, soil, sediment, rock, or any combination of these.

In addition, a seasonally frozen “active layer,” which thickens during the winter cooling and thins during the summer melt, often overlies the permafrost.

Thawing…

In the past decade, research has revealed that a global temperature rise will result in the thawing of permafrost, and the scientific community continues to study the potential consequences of this thaw for the sustainability of the permafrost ecosystem.

Why this thawing is important is that more than one hundred billion tons of carbon is estimated to be stored in the permafrost layer. Several unique processes of organic matter accrual—including peat accumulation, intermittent burial by windblown and waterborne sediments, and “cryoturbation” (churning of permafrost due to freezing and thawing)—coupled with the low temperatures of high-latitudes, have sequestered organic carbon in circumpolar soils for millennia.

In fact, more carbon is stored in the permafrost layer than in the earth’s atmosphere.

…and its consequences

If the air temperature rises by even few degrees, increased thawing of the permafrost layer can emit potentially enormous amounts of greenhouse gases such as carbon dioxide and methane.

In Alaska and other northern places, buildings are often constructed on stilts to keep heat from the building from melting the permafrost below. Photo by Travis (www.flickr.com)

Of the two greenhouse gases, methane is a much more powerful greenhouse gas, with a greater warming potential per unit mass compared to carbon dioxide. All of this will have a positive feedback to the current warming of high latitudes.

Secondly, the frozen permafrost layer is a structural unit, the integrity of which is extremely important for human settlements. Thawing of the permafrost layer can cause irregular subsidence of the land surface, thereby undermining the structural integrity of infrastructures such as buildings, roadways, railroads, and utilities such as water and sewer pipes. In addition, permafrost thaw can contribute to erosion and landslides.

Evidence of melting

Observations of temperatures at different depths in the subsurface layer are used to characterize the thickness and dynamics of the permafrost layer. These observations have been carried out in various Arctic locations (Alaska, Siberia, Russia, and Canada) since the mid 1970s. These measurements show an increase in permafrost temperatures over the past three plus decades, with the actual rates of temperature rise being location specific.

Recently, there has been particular interest in ascertaining the amount of the methane that could be released due to thawing of the permafrost layer, and how this might affect global temperature patterns.

It would be best if the permafrost remained frozen, as a thaw would not be pleasant to humankind. But only through scientific studies can we understand how widespread thawing of perennially frozen regions will affect us.

–Answered by Venkat Lakshmi, University of South Carolina

This post is part of a series highlighting the importance of the “vadose zone”: the region of aerated soil or rock above the water table. The vadose zone varies in thickness, from being absent in some areas to many hundreds of feet thick in others. Water within this interval, which is moving downward under the influence of gravity, is called vadose water, or gravitational water. The Soil Science Society of America publishes a journal dedicated to studies of this region, called the Vadose Zone Journal.