Permafrost
Permafrost is an important physical component of the Northwest Territories. Permafrost has a profound influence over the hydrology, landscape and ecology of northern environments. The current and future conditions of permafrost are key considerations in the planning and management of virtually all infrastructure in the NWT. Permafrost underlies the majority of 
NWT communities. Subsea permafrost is also present in the Mackenzie Delta region.
The temperature and thickness of permafrost varies across the NWT with climate, vegetation cover and geology. Ground ice in permafrost gives rise to unique landforms such as pingos, tundra polygons and thaw slumps. Determining areas with ice-rich permafrost is important for planning sustainable infrastructure and predicting which landscapes are most sensitive to change if the permafrost thaws24. Some of the most abrupt changes to terrestrial and aquatic environments in the NWT have resulted from the degradation of ice-rich permafrost. Thawing of ice-rich permafrost results in a thermokarst landscape which can modify drainage9, cause lakes or wetlands to expand, or conversely to drain42, and vegetation communities to change irreparably. Landsliding or slumping can add large volumes of sediment into lakes and streams, raising suspended sediment concentrations and adversely impacting aquatic habitat30,40. Thaw slumping and permafrost degradation can also influence lake or stream water chemistry by exposing previously frozen materials to leaching or by increasing ground water flow17.
Terrestrial ecosystems are also modified by the growth or degradation of ground ice. For example, heaving of he ground can cause hummocky terrain and settlement may produce "drunken forests"15. Both thaw slumping and collapse of peatlands can change moisture and chemical conditions in soils16. Permafrost disturbances can also expose mineral soils for colonization by disturbance-adapted species, which may contribute to changes in the composition of surrounding vegetation communities in
natural20 and disturbed settings41. Thawing permafrost can also damage buildings, roads and other infrastructure facilities leading to increased maintenance and mitigation costs. There is also the possibility that thawing of permafrost could release significant amounts of carbon sequestered in permafrost.
Multidisciplinary approaches are required to understand northern environmental systems and distinguish normal variation from the effects of climate change, and natural or anthropogenic disturbance5, 38. The thermal state of permafrost is strongly influenced by climate, vegetation and snow5, 35. The development and degradation of ground ice has both geotechnical and ecological implications21, 23. The strong links between permafrost and other components of the environment places the discipline of permafrost science in a unique position to serve a unifying role in the investigation of northern physical and biological processes and their responses to global change11. The high priority of permafrost issues in development and management of northern infrastructure highlights the importance of the discipline in engineering and northern geoscience10.
Tracking ground temperatures and active-layer monitoring in the NWT can give early-warning information on the deterioration of permafrost. Delineating areas with ice-rich permafrost provides an indication of landscapes sensitive to change if the permafrost thaws. Trends in the areal extent of thermokarst provide information on landscape change. Together, monitoring of these indicators provides critical information to resource managers, planners, proponents of resource development projects and those charged with maintaining NWT infrastructure.
Permafrost: Rock or soil that remains below 0°C for at least two years. Surface conditions including vegetation, organic cover and snow thickness can influence permafrost temperatures. Permafrost thickness is related to the air temperature, soil characteristics and the geothermal gradient and to the geological history of the area.
Active layer: Surface layer of earth materials that thaws and refreezes on an annual basis.
Ice-rich permafrost: Ground-ice content greater than the saturated moisture content of thawed soil is called “excess ice”. Ground ice features include polygonal terrain underlain by a network of ice wedges and the distinct ice-cored conical pingos of the Mackenzie Delta region.
Thermokarst: Land surface that forms as ground subsides due to thaw of ice-rich permafrost. Thermokarst processes may cause lakes to enlarge, peatlands to collapse and landslides or thaw slumps to develop.
Retrogressive thaw slump: Develop due to thawing of ice-rich permafrost on slopes. Thawing turns exposed ice-rich permafrost into a mud slurry which falls to the base of the exposure and flows downslope.
Indicators related to Permafrost
13.1 Ground temperature in permafrost
This indicator measures the mean annual ground temperature characteristics of permafrost in the NWT. Monitoring permafrost temperatures provides planners, resource managers and engineers with valuable information on ground thermal regimes across the NWT.
Researchers in Natural Resources Canada, Aboriginal Affairs and Northern Development Canada and others26,4 are contributing significant data and information to increase understanding of ground thermal regimes in the NWT.
Natural Resources Canada compiles ground temperature data from across the Canadian North. These data can be accessed at http://www.gtnp.org36. Ground temperature information from the Mackenzie Delta region is also available at the Northwest Territories Geoscience Office.
Ground temperature: The purpose of the monitoring program should determine the depth and frequency of ground temperature collection. Studies investigating the influence of snow, vegetation or disturbances on soil conditions may monitor temperatures near the ground surface. Long-term tracking of changes in ground temperatures should focus on temperatures at a depth where there is no variation with seasons. A change in temperature at this depth indicates a shift in the ground thermal regime.
NWT Focus
Knowledge of ground temperature in permafrost zones is required for the design of northern infrastructure, assessing environmental impacts of these developments and planning mitigation. It is also important to understand how ongoing climate warming is impacting permafrost temperatures because the thermal state of frozen ground is closely linked to its physical stability and ecosystem integrity40.
Increasing air temperatures, changes in vegetation or increased snow cover can cause permafrost temperatures to increase and in some cases, permafrost to thaw. Since the relationships between air and ground temperature are influenced by other factors such as snow, vegetation and soil moisturee, it is important to take a holistic monitoring approach. This requires collection of complementary field data so factors driving changes in the thermal state of permafrost can be understood.
Current view: status and trend
In general, mean annual ground temperatures decrease and the thickness and aerial extent of permafrost increases poleward. Mean annual ground temperatures in tundra environments of northern NWT are below -6oC36 and permafrost may be several hundreds of metres thick33. The transition from continuous to discontinous permafrost roughly coincides the position of the subarctic boreal-tundra transitiion.
Permafrost temperatures across the NWT are increasing in response to current climate warming In the Mackenzie Delta region, mean permafrost temperatures have warmed by as much as 2oC since the early 1970s (see figure below)42.
Analysis of ground temperature trends from north (Canyon Creek) to south (Manners Creek) near Norman Wells. Figure 3 from Smith et al., (2005) (reproduced with permission from John Wiley & Sons, Ltd., Permafrost and Periglacial Processes).
Current research indicates the cumulative impacts of disturbance or ecological change will compound the impacts of climate warming on the thermal stability of permafrost. Disturbance of the forest cover in the southern discontinuous permafrost zones of the NWT can sufficiently alter the surface energy balance to stimulate the degradation of permafrost under contemporary climate conditions43,44. Thermal modelling has shown that the majority of permafrost warming at abandoned oil and gas infrastructure in the western Arctic can be attributed to the proliferation of tall shrubs and snow accumulation rather than to rising air temperatures45. This study also illustrates that shrubbier tundra and enhanced snow cover will likely accelerate the warming of permafrost anticipated with climate change.
Looking forward
It is anticipated that ground temperatures will continue to increase with future warming, but regional changes in vegetation or snow cover or proximity of sites to water may either enhance or slow the ground warming. The rate of ground warming typically slows as temperatures approach 0oC because of the large amounts of heat that must be removed from the ground as ice in the soils turns to water .
Planning and managing development of northern infrastructure and understanding environmental responses to climate warming requires information on permafrost termperatures. As snow cover, vegetation and soil organic cover all influence the ground thermal regime in addition to air temperatures, measurement of these parameters should complement a thermal monitoring program.
Looking around
Warming of permafrost is being reported in Alaska33. Some recent modelling projections of permafrost degradation probably overestimate the rates and magnitude of future thawing3, but both empirical and modelling evidence suggest that over the next several decades, continued climate warming will cause permafrost to warm and, in some areas, to thaw entirely.
Find more
Natural Resources Canada compiles ground temperature data from across the Canadian North. These data can be accessed at: http://www.gtnp.org.
Other focal points
13.2 Trends in active-layer thickness in the NWT
This indicator tracks changes in the thickness of the active layer in several locations in the NWT.
Active-layer thickness and thaw penetration (active-layer thaw + surface subsidence) is currently monitored at several locations throughout the NWT 31. When read periodically, frost tubes provide information about seasonal progression of thaw and maximum seasonal thaw. They have also been used successfully as part of NRCan’s long-term permafrost monitoring network in the Mackenzie Valley.
The Circumpolar Active-Layer Monitoring Program (CALM) has formalized a large grid sampling design, which is explained on the website http://www.udel.edu/Geography/calm/.
Active layer: The surface layer of earth materials that thaws and refreezes each year. Several biological, hydrologic, pedologic, geomorphic and biogeochemical processes operate within this shallow layer that overlies the permafrost.
Frost probes: Measurement involves using a small-diameter metal rod to probe for permafrost table.
Latent heat effects: The rate of ground warming or cooling typically slows as temperatures approach 0oC because of the large amounts of energy associated with changing water into ice or vice versa.
NWT Focus
Active-layer characteristics including temperature, thickness and moisture are influenced by climate, vegetation and soil conditions. These characteristics affect the geomorphology, hydrology and ecology of the landscape and are also relevant to planning, building and maintaining infrastructure. Measurement of active-layer thicknesses can provide information of spatial variability and, over the long-term, may detect change due to disturbance, ecological succession or climate. Year-to-year variation in active layer thickness is generally related to thawing degree days46.
In areas underlain by ice-rich permafrost, the deepening of the active-layer can cause surface subsidence, which in turn can affect surface micro-relief, infrastructure, and ecology15. Deepening of the active-layer can release nutrients sequestered in permafrost and may affect biogeochemical cycling. A deeper active-layer will increase the water storage capacity and provide greater amounts of soil for plant roots. With continued climate warming, or disturbance, the active-layer may not freeze back completely in winter. If these conditions persist, the permafrost will degrade. Slower freezing of the active-layer can impact winter land access and the length of the winter operating season. An unfrozen layer of soil between permafrost and the frozen ground surface can continue to convey water throughout the winter, impacting winter streamflow and chemistry and potentially contributing to the development of icings.
Current view: status and trend
In the NWT, there is a general southward increase in active-layer thickness with increasing thawing degree days. At a regional scale, variation in active-layer thickness is influenced by vegetation type, organic layer thickness, soil moisture and snow cover. In tundra soils, mean maximum thaw depths are generally less than 80 cm, while in southern NWT the thaw depths commonly exceed 100 cm31. However, in boreal forests of southern NWT, thicker organic ground cover inhibits frost penetration such that a greater number of thawing degree days are required to achieve similar active-layer thicknesses on the tundra32.
Active-layer thickness has responded significantly to past4 and recent climate change41. Due to the high degree of spatial variability in thaw depths and a limited observational record at most sites, conclusions regarding trends related to climate warming are difficult to make. The longest active-layer records in the NWT were collected by Dr. J.R. Mackay27 in the Mackenzie Delta region. Mackay27 monitored areas burned by wildfire in 1968 and nearby undisturbed forested sites. Data indicate that fire disturbance and subsequent ecological recovery had a dominant effect on active-layer thicknesses. Thaw depths increased by up to 100 cm following the fire to depths of 140 cm. The active layer at the burn has progressively thinned over the past 20 years due to the establishment of organic ground cover and the shading effect of shrubs. Active-layer thicknesses in some unburned areas also decreased due to a proliferation of moss and shrub cover at those sites. Active-layer thinning on the bed of a lake drained in 1978 is also largely attributed to ecological succession28. These studies show the benefits of collecting complementary data sets when implementing long-term monitoring programs.
Several researchers report a longer-term record of active-layer variation from the Mackenzie Valley and Delta areas. There is significant inter-annual variability and although records are too short to make inferences about temporal trends related to climate, it is found that thawing indexes based on thawing degree days25 and mean summer air temperature42 can explain variation in thaw depths.
The maximum thaw penetration measured at most monitoring sites in the Mackenzie Valley occurred in 1998, an El Niño year and the warmest on record. (See the NATURAL CLIMATE FLUCTUATIONS focal point for more information on El Niño, and WEATHER AND CLIMATE for more on summer 1998).
Looking forward
It is anticipated that active-layer thickness will increase with climate warming. Significant increases in thaw penetration from 2000 to 2055 are projected8. However, the rates of increase will be influenced by the ice content of underlying permafrost and feedbacks with changing vegetation cover and soil moisture.
In areas of ice-rich permafrost, maintaining long-term datasets will be critical to detecting changes in thaw depths. Furthermore, since thawing of ice-rich permafrost may cause surface subsidence rather than an increase in thaw depths, establishing stable reference points and assessing surface heave or subsidence is also important.
Looking around
Areas outside of the Mackenzie Valley are under-represented in the monitoring network. The effects of warming on active-layer deepening are confounded by the effects of vegetation cover and soil moisture and, therefore, monitoring should take place in various ecological terrain types within representative ecoregions throughout the NWT. Monitoring programs should take effort to collect complementary data on other environmental parameters which may influence active-layer development.
Find more
For information on the locations of active-layer monitoring in the NWT visit the CALM website at http://www.udel.edu/Geography/calm/.
Additionally, the NRCan website may be visited for further information on NWT sites www.gtnp.org.
Nixon also provides a table and maps of active layer monitoring sites along the Mackenzie Valley. Several papers written by Dr. J.R. Mackay provide examples of long-term active-layer monitoring, but these are limited primarily to the Mackenzie Delta region.
Overviews of the active layer monitoring network in the NWT and some recent results are available in peer-reviewed publications.
The CALM protocols are being adapted for community-based monitoring of the active-layer and will be published by the NWT Cumulative Impact Monitoring Program for future guidance. See http://www.nwtcimp.ca/.
Other focal points
For more indicators related to large scale changes in the NWT landscape and to vegetation, see the Focal Point LANDSCAPE CHANGES and VEGETATION.
Technical Notes
In areas where permafrost has a high ice content, the accumulative thaw depth (active-layer depth + subsidence) is a better indicator of changes in thawed soils than is measurement of active-layer thickness alone. Frost probes designed by Mackay can measure annual thaw penetration and maximum heave and subsidence of the ground surface (see Figure below).
Schematic of a frost tube for measuring active-layer thickness and surface heave or subsidence, all of which are required to determine thaw penetration. Coloured beads are added to the water/ice filled tube to estimate maximum thaw.
13.3 Trends in thermokarst in the NWT
This indicator tracks changes in thermokarst aerial extent in selected areas of the NWT.
Thermokarst: The processes of thawing ice-rish permafrost causing irregular settlement of the landscape. Thermokarst processes may cause lakes to enlarge, peatlands to collapse and landslides or retrogressive thaw slumps to develop.
Retrogressive thaw slump, Mackenzie Delta region (photo by T. Lantz and Steve Kokelj).
Increases in the aerial extent of thermokarst are detected using various remote sensing techniques including sequential analyses of aerial photographs19 or high resolution satellite imagery. Remote sensing technology using interferometric satellite radar to track ground surface subsidence has been developed by Canada Centre for Remote Sensing at NRCan47. This technique, which can detect seasonal and annual vertical displacement of even a few centimetres, can be used to monitor natural permafrost environments or built infrastructure.
Information for this indicators is obtained from an NRCan database with general information on the distribution of landsliding in the Mackenzie Valley (GSC open file D3917)1. Future changes in landsliding and thaw slumping can now be tracked using a recent baseline landslide inventory. There is limited data on thermokarst in peatlands, again with focus on the Mackenzie Valley.
NWT Focus
Thermokarst disturbances can impact infrastructure, so determining sensitive environments and rates of thermokarst activity are critical to planning development and infrastructure monitoring.
Thermokarst can impact aquatic systems by modifying drainage pathways, causing lakes to rapidly drain or, conversely, to increase their size and depth (See photographs for 1950 and 2004 in figure below). Degrading permafrost can also change water quality conditions and impact the structure and functioning of both terrestrial and aquatic ecosystems in the NWT.
Current view: status and trend
There are few longitudinal studies of thermokarst in the NWT. These studies show that the rates and extent of collapsed peatlands and retrogressive thaw slumping has increased over the last half of the 20th century. Disturbance such as fire can have a dominant impact on thermokarst processes at a local scale.
Looking forward
It is reasonable to predict continued and larger scale modification of terrestrial and aquatic ecosystems as the result of permafrost degradation due to climate warming.
Map of the retrogressive thaw slumps in the upland tundra study region east of the Mackenzie River Delta (Lantz and Kokelj, 2008). Areas bounded by a single line represent study plots where disturbances were mapped on aerial photographs taken in 1950, 1973 and 2004. The study plots affected by the 1968 fire are marked by a border of two solid lines.
Data from satellites may be used to evaluate landscape change in the NWT. There is potential to assess the distribution of thermokarst landscapes and select representative areas for longer-term change detection monitoring. Validation of results can be facilitated with targeted ground data collection. It is most ideal to establish keystone monitoring areas in sensitive terrain areas within various ecoregions, with focus on areas for which historical remotely sensed data exists.
Thermokarst lakes and slumps, Mackenzie Delta region. From Lantz TC and Kokelj SV (2008).
Looking around
A number of investigations in Alaska and northern Alberta and Manitoba have examined the evolution of thermokarst landscapes. Results from these studies all indicate an increase in the aerial extent of thermokarst features. Studies mapping permafrost degradation in Alaska have focused on describing the impacts on forested landscapes and tundra polygons and elsewhere in the Canadian North on peatland degradation, thaw slumping and stream drainage patterns. Researchers are investigating the effects of slumping on tall shrub ecology.
Find out more
Other focal points
- For more indicators related to landscape, see the Focal Point LANDSCAPE CHANGES.
- For more indicators related to plants, see the Focal Point VEGETATION.
Thawing of ice-rich permafrost in headwall of an active slump
(photo by Stephen Wolfe, NRCan).
References
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Updated: September 2011