More than 70 per cent of Canada’s coastline is located in the Arctic and defined by ice. Most of our homeland is underlain by permafrost, and the southern limit of permafrost in Canada is moving northward.

Permafrost is often termed “permanently frozen ground,” but this description may not be the most accurate because permafrost is not necessarily “frozen” and, as recent studies have shown — and as Inuit have been experiencing first-hand — it may not always be permanent.

The formal definition of permafrost considers the temperature of the ground, with permafrost being defined as soil, sediment or rock that remains at or below 0 C for at least two consecutive years. The use of this thermal definition, as opposed to the term “frozen ground,” is mainly because “frozen” implies that ice is present in the soil or rock, whereas permafrost does not necessarily contain ice. This is often the case for bedrock permafrost, which does not necessarily contain frozen water but can exhibit different properties when below 0 C.

Permafrost is overlain by a layer of seasonally frozen ground that freezes and thaws each year called the active layer. This layer can be as thin as several centimetres in the High Arctic or many metres thick, but is at its deepest in late summer or early fall when ground temperatures are warmest. The thickness of the active layer at any given location is controlled by the amount of heat that penetrates the soil; as a result, regions with warmer summers generally have deeper active layers. However, snow cover, ground surface and vegetation properties also control how heat from the air is transferred into and through the ground. Local conditions often make active layers thinner or thicker than predicted for an area.

Just like a frozen piece of meat, permafrost thaws; it does not melt, though the ground ice within permafrost can melt. When permafrost contains ice, it takes much more energy to thaw the ground. Because of this, permafrost thaw can be a very slow process in areas containing ground ice. Even during a very warm year, it is possible that a deeper active layer thaw may not occur because of the amount of energy required to melt the ground ice. For the above reasons, determining whether changes in permafrost thicknesses and active layers are underway requires long-term ground temperature monitoring.

Permafrost was initially thought to be limited to areas where average annual air temperatures are less than 0 C, but in some very isolated environments permafrost has been found where average annual air temperatures exceed 2 C. The geographic distribution of permafrost is typically categorized at the broad (continental) scale into the following four zones:

Continuous zone

Permafrost underlies between 90 per cent and 100 per cent of the landscape in this zone and may be hundreds of metres thick. Permafrost exists in nearly all environments except beneath rivers, large lakes and other deep bodies of water. Mean annual air temperatures in this zone are typically below -8 C, and permafrost will actively form in newly exposed terrain.

Extensive discontinuous zone

Permafrost underlies between 50 per cent and 90 per cent of the landscape in this zone and can range from a few to hundreds of metres thick. Permafrost is widespread and exists in most environments but may not exist beneath unfavourable local conditions such as on south-facing slopes or beneath tall willows.

Sporadic discontinuous zone

Permafrost underlies between 10 per cent and 50 per cent of the landscape in this zone and can range from a few to more than a hundred metres thick. Permafrost is most often found in environments that favour its preservation, such as beneath north-facing slopes and in peatlands. Permafrost in this zone has typically formed under a previous climate regime when climate conditions were colder and potentially more favourable to its development. The distribution of permafrost in this zone is often related to snow depth, with deeper snow causing insolation of the ground and warmer ground temperatures.

Isolated patches zone

Permafrost in this zone underlies less than 10 per cent of the landscape. Although this class is found on most permafrost maps, it can be challenging to interpret because it combines regions having almost no permafrost with areas that have frequent small patches of permafrost. Permafrost in this zone typically has formed under prior climate conditions that were much more favourable to permafrost than current conditions, and has remained only in environments that are the most favourable for the maintenance of permafrost. The isolated patches zone often also includes permafrost in high mountain areas that can exist much farther south of where lowland permafrost exists.

Permafrost zones underlie about 24 per cent of the Northern Hemisphere area; therefore, permafrost impacts numerous processes in the environmental and human domains. Permafrost underlies many Inuit communities in the Arctic and Subarctic and should be considered during development of infrastructure and housing, particularly with projections of a warming climate. When permafrost with ground ice thaws, it can cause many changes to the local environment including cave-ins on the surface and changes in water flow and chemistry. The surface response of permafrost to thawing will depend on the amount of ground ice it contains. Areas with very little ground ice may show very little surface change if permafrost thaws, whereas areas with high quantities of ground ice can show extreme surface changes.

Recently, research has examined how thawing permafrost may release carbon stored in the ground into the atmosphere. Although there is still some debate as to how much carbon may be released and how fast, there is a consensus that over the next century carbon release from permafrost may begin to influence the climate.

Permafrost temperatures are increasing with few exceptions and these trends are projected to continue as the climate continues to warm. It is important to keep in mind that there can be considerable local variability in permafrost temperatures, especially in areas with an abundance of water bodies, seasonal flooding or variable snow conditions associated with variable or changing vegetation cover. Several northern communities have incorporated research on changing permafrost conditions into their coastal adaptation planning, and Inuit knowledge is widely recognized to hold valuable insights for understanding how the climate of the northern coastline is changing and documenting associated impacts.