Permafrost 101

What is permafrost?

• Permafrost is ground (soil or rock) that remains at a temperature of 0°C or lower for at least two consecutive years. Most permafrost in Canada also contains water in the form of ice.
• Permafrost is present where the climate is cold but the surface is not covered by glaciers.  It is found across the North and at high elevations in mountainous areas such as the Rocky Mountains in western Canada and the Chic-Choc Mountains in the East (Figure 1).
• The thickness of permafrost, its temperature, its ground ice content, and what proportion of the terrain it underlies, vary greatly. In the High Arctic (e.g. on Ellesmere Island), ground temperatures average as low as -15°C, permafrost is more than 700 m thick and is present everywhere except beneath deep lakes or rivers. In the subarctic (such as near Yellowknife), permafrost is present beneath about half of the landscape, its average temperature is usually between -1°C and 0°C and it may be only a few metres to a few tens of metres in thickness. Some of the southernmost occurrences of lowland permafrost in Canada occur on the Gulf of St. Lawrence near Blanc Sablon where the temperature is just below 0°C and the permafrost is less than 5 metres thick.

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Figure 1. Modified from Heginbottom et al. (1995). cartography by B. O’Neill. The National Atlas of Canada; Natural Resources Canada, Geomatics Canada, MCR Series no. 4177, 1995, 1 sheet, (Source: Natural Resources Canada)

What is ground ice?

• Ice present within the permafrost is called ground ice. It can be microscopic, hidden within the soil pores, or visible as discrete bodies of ice.
• Visible ground ice can appear in many forms.  The most common types are:
  • Segregated ice, present as lenses or layers, a few millimeters to a few centimeters in thickness;
  • Ice wedges, which can grow to several meters wide and 10 m deep beneath tundra polygons (Figure 2a);
  • Massive ice, present as layers or bodies of ice that can be 10 m or more in thickness (Figure 2b).
• The concentration of ice varies from thick bodies of pure ice to almost none at all. This variation has a big impact on how permafrost responds to climate change.

Figure 2. a) Example of ice wedge polygons (Copyright Government of Northwest Territories) b) Massive ice exposed in the headwall of a retrogressive thaw slump near Holmes Creek NWT (Photo by Trevor Lantz)

What is the active layer?

• The soil just beneath the ground surface thaws in the summer and refreezes in the winter. This is called the active layer (Figure 3).
• The active layer in Canada is generally 0.5 m to 2 m thick.
• The colder the climate, the thinner the active layer. Therefore, thin active layers can be found in the high arctic, whereas the thickest active layers occur near the southern limits of permafrost.

Figure 3. a) Trumpet curve of permafrost thermal profile. The maximum depth affected by the annual temperature variations is called the depth of zero annual amplitude; it varies with air temperature and the type of soil (Source: ADAPT). b) Slump exposing the ice-free active layer sitting above a large ice wedge. Photo credit: Benjamin Jones, USGS, Public Domain (Modified by NASA)

What is a talik?

• Permafrost generally is present immediately beneath the active layer but there may be an intervening layer, called a talik that is above 0°C all year-round. This means that it is not permafrost but it is also not part of the active layer (Figure 4).
• The existence of a talik is often a sign that the permafrost is degrading.
• Taliks may also occur at depth, for example generated by constant flow of water.
• Beneath large water bodies, a talik may form that connects all the way through the bottom of the permafrost. This is called a through talik.

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Figure 4. Changes in active layer thickness (ALT) in response to climate warming. ALT reaches a maximum prior to talik development. (Source: Connon et al.)

Why is permafrost changing?

• Permafrost is controlled by the energy balance at the ground surface.  If that changes, so will ground temperatures and the characteristics of the permafrost.
• Extra heat can come from a warming climate, deeper snow cover, or surface disturbance. All of these increase the thickness of the active layer, resulting in thaw of the top layers of the permafrost and if this continues for decades, potentially the loss of the entire permafrost body.
• Where permafrost contains a lot of ground ice, its thaw results in thermokarst processes, which include subsidence and landsliding.
• Changes in the permafrost and active layer may affect surface water drainage, which can then also affect the permafrost.

Why is permafrost important?

• Permafrost affects many aspects of life in the North. For example, it impacts:
  • The design and construction of buildings, roads, airports, dams, pipelines and electrical transmission towers, which must minimize the amount of additional heat passing into the ground so that it stays solidly frozen.
  • The costs of maintenance for transportation infrastructure constructed on top of permafrost, increasing them compared to those in non-permafrost areas (Figure 5).
  • Northern ecology, including wildlife, through its interaction with vegetation and drainage.
  • Northerners, who hunt and fish in permafrost areas.
  • The potential for hazardous landslides in areas of steep terrain like the mountains of the Yukon and NWT.
  • Future greenhouse gas additions to the atmosphere as organic material (mostly plant matter) currently stored in the frozen ground defrosts and is broken down by bacteria into carbon dioxide or methane. This is termed the permafrost carbon feedback.

Figure 5. Oblique overview of Dempster Highway km 27, NWT showing road embankment and permafrost thaw related disturbances (Source: van der Sluijs et al.).

Permafrost Engineering

Permafrost Engineering is the application of engineering principles in areas where permafrost may be present, and design conditions are affected by temperatures below 0°C. These conditions mostly exist in high northern and southern latitudes or at high elevations. In recent years, engineering projects and developments in cold environments has increased in importance due to access required to enable mining of natural resources, resource transportation (e.g., pipelines), or in improving infrastructure and accessibility (e.g., infrastructure corridors).

For engineering purposes, it is important to appreciate the physical differences in the materials that may be encountered and used as foundations or for construction such as snow, firn, and ice in special forms, including sea ice, glacier ice, pore ice, segregated ice, ground ice, ice shelves or icebergs. However, understanding the conditions that prevail in the cryosphere is also required for the design of conditions that are artificially induced, for example, artificial ground freezing used to increase the strength of the ground temporarily to build tunnels, caverns, or shafts (e.g. Thermosyphons)

Figure 6. Thermosyphons (Photo Courtesy of BGC Engineering Inc.)

In addition to challenges resulting from the complex material properties, other notable challenges are related to the logistics, such as the remoteness of a site, availability of site investigation equipment and construction material, or access in steep and mountain environments. Further the effects of the harsh climatic conditions, including cold temperatures, darkness at high latitude, or major diurnal air temperature variations and low oxygen levels in high mountain areas, are wearing on equipment and people. This often results in limited information for the design, such as site investigation or historical climate data. However, the latter is important in carrying out future projections and designing for potential effects from climate change. Since the cryosphere is often found in environments that are ecologically very sensitive, it is important to understand how a planned infrastructure affects the environment and it is essential to select an appropriate adaptation strategy.