13.2 Trends in active-layer thickness in the NWT

Last Updated: 
2014

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 NWT8. 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 Valley8.

The Circumpolar Active-Layer Monitoring Program (CALM) has formalized a large grid sampling design.

Active layer: The surface layer of earth materials in permafrost terrain than thaws and refreezes each year. Several important biological, hydrologic, pedologic, geomorphic and biogeochemical processes operate within this surface layer that overlies the permafrost.

Frost probes: Measurement involves using a small-diameter metal rod to probe for the top of the 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 on 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 days and inter-annual variations in soil moisture11.

In areas underlain by ice-rich permafrost, the deepening of the active-layer can cause surface subsidence, which in turn can affect surface micro-relief5, infrastructure7, and ecology3. 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 cm8.  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 tundra9.

Active-layer thickness has responded significantly to past1 and recent climate change13. 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. Mackay5 in the Mackenzie Delta region. Mackay5 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 succession6. These studies show the benefits of collecting complementary data sets when implementing long-term monitoring programs.

Several researchers9,12 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 days4,10 and mean summer air temperature14 , in addition to thermal conductivity of soils10 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 record13. (See the NATURAL CLIMATE FLUCTUATIONS focal point for more information on El Niño and CLIMATE AND WEATHER 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 projected2. 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

  • Information on the locations of active-layer monitoring in the NWT visit the CALM website
  • The NRCan website may be visited for further information on NWT sites 
  • Nixon8 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 to the Mackenzie Delta region5,6. Overviews of the active-layer monitoring network in the NWT and some recent results are available in peer-reviewed publications9,12.
  • 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. 

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 alone9.  Frost probes designed by Mackay12 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.

 

Found an error or have a question? Contact the team at NWTSOER@gov.nt.ca.


References:

Ref. 1.  Burn C.R. 1997. Cryostratigraphy, paleogeography, and climate change during the early Holocene warm interval, western Arctic coast, Canada. Canadian Journal of Earth Sciences 34: 912-925.

Ref. 2. Duchesne., C., J.F. Wright, and M. Ednie. 2008. High-resolution numerical modeling of climate change impacts to permafrost in the vicinities of Inuvik, Norman Wells, and Fort Simpson, NT, Canada. In: Proceedings of the Ninth International Conference on Permafrost (Vol 1). D.L. Kane and K.M. Hinkel, Eds. Institute of Northern Engineering, University of Alaska at Fairbanks: Fairbanks, Alaska, pp. 385-390.

Ref. 3. Kokelj, S.V., Burn, C.R., Tarnocai, C. 2007. The structure and dynamics of earth hummocks in the subarctic forest near Inuvik, Northwest Territories, Canada. Arctic, Antarctic and Alpine Research 39: 99-109.

Ref. 4. Mackay, J.R. 1973. A frost tube for the determination of freezing in the active layer above permafrost. Canadian Geotechnical Journal 10: 392-396.

Ref. 5. Mackay, J.R. 1995. Active layer changes (1968 to 1993) following the forest-tundra fire near Inuvik, N.W.T., Canada. Arctic and Alpine Research 27: 323-336.

Ref. 6. Mackay, J.R. and C.R. Burn. 2002. The first 20 years (1978-1979 to 1998-1999) of active-layer development, Illisarvik experimental drained lake site, western Arctic coast, Canada. Canadian Journal of Earth Sciences 39: 1657-1674.

Ref. 7. Mackay, J.R. 1970. Disturbances to the tundra and forest tundra environment of the western Arctic. Canadian Geotechnical Journal 7: 420-432.

Ref. 8. Nixon, F.M. 2000. Thaw-depth monitoring. In: The physical environment of the Mackenzie Valley, Northwest Territories: a baseline for the assessment of environmental change. L.D. Dyke and G.R. Brooks, Eds. Geological Survey of Canada, Natural Resources Canada, Bulletin 547, pp. 119-126.

Ref. 9. Nixon, F.M., C. Tarnocai, and L. Kutny. 2003. Long-term active layer monitoring: Mackenzie Valley, northwest Canada. In: Proceedings of the Eight international Conference on Permafrost (Vol 2)., M. Philips, S.M. Springman, and L.U. Arenson, Eds., A.A. Balkema, Swets & Zeitlinger, Lisse, The Netherlands, pp. 821-826.

Ref. 10. Riseborough, D.W., S.A. Wolfe, and C. Duchesne. 2013. Permafrost modelling in northern Great Slave region, Northwest Territories, Phase 1: Climate data evaluation and 1-d sensitivity analysis; Geological Survey of Canada, Open File 7333, 50 p.

Ref. 11. Smith, S.L., S.A. Wolfe, D. Riseborough, and F.M. Nixon. 2009. Active-layer characteristics and correlations to recent summer air temperature climatic indices, Mackenzie Valley, Canada. Permafrost and Periglacial Processes 20: 201-220.

Ref. 12. Tarnocai, C., F.M. Nixon, and L. Kutny. 2004. Circumpolar-Active-Layer-Monitoring (CALM) sites in the Mackenzie Valley, northwestern Canada. Permafrost and Periglacial Processes 15: 141-153.

Ref. 13. Wolfe S.A., E. Kotler, and F.M. Nixon. 2000. Recent warming impacts in the Mackenzie Delta, Northwest Territories, and northern Yukon Territory coastal areas. Current Research, Geological Survey of Canada Paper, 2000-B1, 11 p.

Ref. 14. Yoshikawa, K. and L.D. Hinzman. 2003. Shrinking thermokarst ponds and groundwater dynamics in discontinuous permafrost. Permafrost Periglacial Processes 14: 151-160.