3.2 Trends in length of growing season and snow cover

Last Updated: 
August 13, 2015


This indicator tracks two important climatic drivers of ecosystem change - the length of the growing season and the length of snow cover season. 

The start of the growing season is defined as the date when mean temperatures are greater than 5ºC over five consecutive days in spring. The end of the growing season is when the inverse condition occurs1. Snow cover duration is the number of days with 2 cm or more of snow on the ground. 

The information is presented by ecozones analysed by climate experts at the Climate Research Division, Environment Canada as a study1 produced for Canada’s Ecosystems Status and Trends1 Report.

Flowers on the tundra
The growing season in the Arctic is 25 to 128 days long as measured at 20 weather stations in 1950-2007. Photo credit: Korpack/DUC.

NWT focus

Summers in the NWT are short but intense with long hours of sunlight. The number of days with snow cover affects how wildlife and humans respond to our changing winters. Many NWT species have adapted to short intense summers and long periods of snow cover by migrating, hibernating or changing behaviour and food sources to optimize survival and reproduction. Changes in the growing and snow seasons may have an impact on wildlife behavior and, ultimately, on their distribution and survival. Species previously not capable of surviving in our ecosystems may find new habitats in the NWT if there are less severe snow seasons and longer growing seasons. Species adapted to cold may show population declines. The effects of changes in both the growing and snow seasons on wildlife can be tracked using this indicator as studies become available.

Current view: status and trend

Overall, spring is arriving earlier, which lengthene the growing season and shortens the snow season. It remains unclear if this is occurring in every ecozone as no data is available for some analyses. The length of the data record also may not be long enough to detect some changes.

Ecozone Growing Season* Snow Season**
Arctic No change Shorter by 5.1 days in Spring 
Taiga Shield West No change in length but an earlier growing season is noted (by 11 days)
Taiga Plains Longer by 9 days  Shorter by 12.3 days in Spring 
Taiga Cordillera  Longer by 28 days  ?

*          Only significant changes are reported over the period from 1950-2007. Based
           on  five weather stations.
**        Only significant changes are reported over the period from 1950-2006.
        Data records are not long enough to analyze trends for these ecozones.
West: Western portion only, excluding the Quebec section.

NWT animals have adapted to short intense summers and long periods of snow cover by migrating, hibernating or changing behaviour and food sources to optimize survival and reproduction. Plants have adapted to fire, short intense growing seasons and long cold, dry winters. Changes in temperature, snow cover and the length of the growing and snow seasons will have an impact on wildlife behaviour and growth patterns, and, ultimately, on their distribution and survival. Species previously not capable of surving in our ecosystems may find new habitats in the NWT if  snow seasons become less severe and growing seasons are longer. Species adapted to cold may show population declines.

The effects of change in our climate on biodiversity are being tracked. Studies show  some changes are measureable but subtle. Other studies show  a warming climate is enhancing, in a cumulative fashion, the effect of other changes in land use and disturbance on our biodiversity. A summary of current findings is presented here.

Warming, Permafrost, and Tundra Vegetation
Warming effects on permafrost in the Southern Arctic (near the Mackenzie Delta) is presented in the PERMAFROST focal point. Permafrost is degrading and altering vegetation by creating disturbed areas that are nutrient rich, which enhances growth of green alders. Thaw slumps resulting from permafrost degradation are increasing2 and can accelerate the effects of climate change on tundra vegetation3.       

Degrading permafrost increases slumping, which results in changes in tundra vegetation including more shrubs.

Warm Winters and Parasites
Warming is changing the ecology of parasites already in the North and  facilitating invasion of new parasites from elsewhere4. See the WILDLIFE focal point for more details on parasites in Dall’s sheep.

A nematode was recently discovered in Dall’s sheep in the NWT (2001). A climate-parasite model predicts with climate warming areas further north would be suitable for more types of parasites4. The effects of changes in parasites and disease loads in northern wildlife need to be monitored.
Winter Icing and Peary Caribou
The effects of changes in precipitation, including rain events on snow, in the high Arctic on Peary Caribou5 are presented in the SPECIES AT RISK focal point. 

Peary caribou populations can decline dramatically during icing events, which prevent access to their food in winter. Banks Island Peary caribou.
Sea Ice decline and Polar Bear
Reductions in sea ice in the southern Beaufort sea are linked to increasing proportion of the polar bear population coming on land during the fall open-water period and an increase in the amount of time individual bears spend on land4. The declining polar bear population in the southern Beaufort Sea can be linked to sea ice declines. It is expected the Northern Beaufort sea population may not decline in the short term as sea ice there is not changing6. More details are presented in the SPECIES AT RISK focal point.

Polar bear in the southern Beaufort Sea. Photo credit: CASES   
Longer fire season but no trends in the number and extant of fires
Trends in forest fires are presented in the VEGETATION focal point. By the 1990s onwards more fires are occurring in early spring and late summer compared to the 1960s. This change in the extent of the “fire season” is partly due to changes in climate. In the NWT, the number of fires and the area burned each year are variable, and increased in the 1990s, but decreased in the 2000s. Climate change models are predicting an increase in forest fires in the next decades. 

Forest fires in the Taiga Plains, NWT in July 2005.
Species moving north
Trends in movements of mammals are presented in the WILDLIFE focal point. As well, some species of birds  common further south are extending their range in the NWT. The best known of these range extension is for the Black-billed Magpie. This species is now found as far north as in the Sahtu, surviving our winters there7. A warming winter climate best explains these rapid range extensions. Winter warming is also linked to the rapid range extension of some forest pests, such as the Mountain Pine Beetle8,9, a species not present yet in the NWT but closely monitored by ENR. Trends in forest pests are presented in the VEGETATION focal point. 

Black-billed Magpie. Photo credit: G. Court 
A Climate Mismatch

A mismatch between breeding and availability of food for some species of migratory birds nesting in the Arctic and boreal forest is thought to be the main cause of declines in populations10.

 “…a dependence on photoperiod as a breeding cue could limit the ability of Lesser scaup … to adjust timing of nest initiation with changes in climate and invertebrate phenology” 
Quote from:  DeVink et al. 2008. Auk (125).                                            

Lesser Scaup male and female in the Taiga Shield. Photo credit: J. Nagy 

Looking around

These changes are predicted by climate change scenarios and are part of a trend toward spring warming and earlier snow melt observed in the northern hemisphere1, 11

Looking forward

The rate of change in temperature and precipitation is faster than climate change models predictions. Normal climatic variations do not fully explain these warming winters and changes in precipitation. There is evidence natural climate fluctuations, such El Nino, enhances the effects of continued warming in the Arctic causing, in some years, rapid and less predictable changes in northern ecosystems, such as a record low in Arctic sea ice in September 2007.   

Technical notes

The selection of 5 °C as the minimum temperature requirement for the growing season is arbitrary. The actual growing season is different for different plant species. The relative changes in the start, end and length of the growing season, however, are relevant to all species and ecosystems1. Trends are statistically significant at 5% level. See Climate Research Division study for details in statistical procedures1. The trend analyses are based on weather stations in the entire ecozone in Canada, including some in the NWT. The number of stations used for analysis are for each ecozone, for growing season (gs) and snow season (Snow): Arctic: 20 (gs); 26 (snow), Taiga Shield West: 5 (gs); 0 (snow), Taiga Plains: 6 (gs), 11 (snow); Taiga Cordillera: 1 (gs), 0 (snow).

For more information

Other focal points

Contact us

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

Reference List

Ref. 1- Zhange, X. et al. 2008. Canadian Climate Trends Ecosystem Status and Trends Report. Climate Research Division, Environment Canada.

Ref. 2 - Lantz, T.C. and S.V. Kokelj. 2008. Increasing rates of retrogressive thaw slump activity in the Mackenzie Delta region, NWT Canada. Geophysical Research Letters 35:L06502.

Ref. 3 - Lantz, T.C., S.V. Kokelj, S.E. Gergel, and G.H.R. Henry. 2009. Relative impacts of disturbance and temperature: persistent changes in microenvironment and vegetation in retrogressive thaw slumps. Gloval Changing Biology 15: 1664-1675.

Ref. 4 - Kutz, S. et al. 2009. The Arctic as a model for anticipating, preventing and mitigating climate change impacts on hose-parasite interactions. Veterinary Parasitology 163: 217-228.

Ref. 5 - COSEWIC. 2004. COSEWIC assessment and update status report on the Peary caribou Rangifer tarandus pearyi and the barren-ground caribou Rangifer tarandus groenlandicus (Dolphin and Union population) in Canada, Ottawa. 

Ref. 6 - COSEWIC. 2008. COSEWIC assessment of update status report on the Polar Bear Ursus maritimus in Canada, Ottawa.

Ref. 7 - Working Group on General Status of NWT Species. 2005. NWT Species Infobase 2005-2010. Version 2007.2. Yellowknife, GNWT.

Ref. 8 - Canadian Forest Service N.R.C. 2008. Forest forward - Moving beyond the pine beetle. Natural Resources Canada.

Ref. 9 - Robertson, C., T.A. Nelson, D.E. Jelinski, M.A. Wulder, and B. Boots. 2009. Spatial-temporal analysis of species range expansion: the case of the mountain pine beetle, Dendroctonus ponderosae. Journal of Biogeography 36: 1446-1458.

Ref. 10 - Tulp, I., and H. Schekkerman. 2008. Has prey availability for Arctic birds advanced with climate change? Hindcasting the abundance of tundra arthropods using weather and seasonal variation. Arctic 61: 48-60.

Ref. 11 - Arctic Council. 2004. Arctic Climate Impact Assessment. Arctic Council.