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Is Snow Cover Changing in Canada?

Ask any Canadian about snow and they will invariably recall it being much snowier when they were young! As pointed out in a recent feature article in EnRoute Magazine (A. Brocklehurst, The Great Snow Debate , December, 1999), there are many non-climatic reasons for this perception such as snow banks looking higher when you were small, improved snow clearing practices, and a tendency to remember extreme events. However, there is a factual basis to this perception for people who were born in the middle of the 20th Century and did much of their skiing during the 1970s: historical reconstruction of snow cover over North America show that snow cover was at its highest 20th Century levels from the 1950s to the 1970s (Fig. 1), then experienced a rapid decrease during the warm decades of the 1980s and 1990s.
Historical variation in NA March snow cover extent reconstructed from snow depth observations
Fig. 1: Historical variation in NA March snow cover extent reconstructed from snow depth observations. Source: Brown (2000).
[ other months ]

Analysis of Canadian daily snow depth observations which have been systematically collected and archived since the mid-1950s revealed that recent decreases in Canadian snow cover have mainly occurred during the second half of the snow season, and that the decreases have been strongest over western Canada (Fig. 2). Trend analysis over the post-1955 period indicates significant reductions in spring snow cover of 0.3 days/year over many regions of western Canada. This observation is confirmed by satellite-derived estimates of change in snow cover over the post-1972 period. The satellite trends also reveal major regional and seasonal differences in snow cover trends. For example, snow cover actually increased over some parts of eastern Canada over the 1972-2000 period.

Observed variation in spring season snow cover duration derived from daily snow depth observations
Fig. 2: Observed variation in spring season snow cover duration derived from daily snow depth observations.
[more regional trend info ]

But is this relatively recent trend to lower spring snow cover attributable to global warming?

This is the first question most people ask, but it is the one that is the most difficult to answer. Snow cover represents an integrated response to precipitation and temperature, and exhibits considerable variability from one year to the next, and over longer time-scales of decades-to-centuries. Sensitivity studies have clearly shown that there is a strong inverse correlation between continental and hemispheric air temperatures and corresponding snow cover extent (Groisman et al., 1994) i.e. the warmer it gets the less snow you have. This response is related to the positive "albedo feedback" between snow cover and air temperature through surface reflectance (less snow -> lower albedo -> more energy absorbed at surface -> faster melt and warmer air temperature -> less snow). This feedback mechanism is strongest in the spring period when incoming solar radiation input to the snowpack is highest, and exerts a measurable feedback to the earth radiative balance (Groisman et al., 1994). Analysis of 20th Century NH air temperature trends show greatest warming in the spring period that is consistent with significant long-term decreases in spring snow cover extent over Eurasia (Brown, 2000). This response is consistent with global climate model simulations and lends some support to the argument that global warming is responsible.

However, North American snow cover does not reveal the same clear temperature response, and it has been shown (Brown, 2000) that NA snow cover extent actually increased over much of the 20th Century in response to increasing precipitation. In addition, it has been well-documented that NA snow cover extent is more closely linked to atmospheric circulation patterns than large-scale temperatures (Brown, 1995) and Brown and Braaten (1998) showed that the late-20th Century low snow cover conditions observed over many regions of Canada were linked to a major shift in atmospheric circulation over the north Pacific and North America that occurred in 1976.

Another point to note is that while continental-scale snow covered extent is expected to retreat in response to global warming, it is not so clear what other aspects of the snow cover may do. Increased precipitation may lead to increased snow accumulation in cold climate regions, while warming will be accompanied by increased frequencies of mixed precipitation and rain-on-snow events which have implications for snowmelt, snow depth and snow density. Documenting and understanding these characteristics requires monitoring with surface-based observations in addition to satellite data.

Conclusion?

While climate change simulations suggest a marked northward retreat in snow cover over North America over the next 100 years, the natural variability of the climate system and atmospheric circulation will ensure that snow is still an important component of the winter climate in many regions of Canada, so don't go throwing away the snow shovel just yet!

References
  • Brown, R.D. 1995. Spatial and temporal variability of North American snow cover, 1971-1992. Proc. Eastern Snow Conf., 52nd Annual Meeting, 6-8 June 1995, Toronto, Ontario, 69-78.
  • Brown, R.D. and R.O. Braaten, 1998. Spatial and temporal variability of Canadian monthly snow depths, 1946-1995. Atmosphere-Ocean, 36, 37-45.
  • Brown, R.D., 2000: Northern Hemisphere snow cover variability and change, 1915-1997. J. Climate, 13, 2339-2355.
  • Groisman, P. Ya, T.R. Karl, and R.W. Knight, 1994: Changes of snow cover, temperature and radiative heat balance over the Northern Hemisphere. J. Climate, 7, 1633-1656.