Snow cover trends in Canada
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Mark Henry, Environment Accounts and Statistics Division
This article examines the average area covered by snow (snow cover extent) nationally for the 39 year period from 1972 to 2010. Snow cover extent is examined annually and for the months of October and November, the period that marks the onset and expansion of snow cover for much of Canada, and April, May, and June, the spring snow melt period (see textbox "Background and methodology").
An ongoing data collaboration
This article is the fifth of an ongoing series of articles in EnviroStats showcasing data related to Canada's climate and the impacts of climate change. The series focuses on short statistical analyses of climate-related data. To date, the series has included trend analysis on glacier mass balance, temperature, precipitation and sea-ice. Previous articles can be found at www.statcan.gc.ca/bsolc/olc-cel/olc-cel?catno=16-002-X&chropg=1&lang=eng.
The articles in the series are the product of an ongoing collaboration between Statistics Canada, Environment Canada and Natural Resources Canada.
Data featured in the articles will be made available through the Statistics Canada website, both in free CANSIM data tables and through new articles re-examining trends in the data every few years.
Snow cover extent is considered an essential climate variable by the World Meteorological Organization-Global Climate Observing System. 1 Snow cover is also one of several variables used to support the work of the United Nations Framework Convention on Climate Change (UNFCCC) and the Intergovernmental Panel on Climate Change (IPCC). 2
Background and methodology
Time series data used for this article were derived from the National Oceanic and Atmospheric Administration (NOAA - USA) Climate Data Record (CDR) Northern Hemisphere gridded weekly snow chart dataset maintained by Rutgers University, and based on manual analysis of satellite imagery. 3 Rutgers University processes the weekly data into monthly series of snow cover fraction at each grid point (percent of days in month with snow cover) that were used to compute the monthly average snow cover extent over the Canadian land mass in each year over the 1972 to 2010 period. 4 The snow charts were compiled into a time series by the Climate Processes Section of the Climate Research Division at Environment Canada. 5 , 6
Snow cover extent is expressed in square kilometers.
The time series data were tested for the presence of serial correlation and for anomalous observations (outliers). A Statistical Analysis Software (SAS) procedure, PROC ARIMA, was used to compute the overall trend. The PROC ARIMA process produces a linear trend and the associated significance level adjusted for any existing serial correlation and anomalous observations. 7 All linear trends shown are statistically significant unless otherwise noted. 8
Snow cover extent
The year with the lowest average snow cover occurred in 1998, followed closely by 2010. The two highest averages occurred in 1972 and 1978 and were the only times the yearly averages exceeded 6 million square kilometers or about 62% of the area of Canada.
For October and November, months of snow cover onset and expansion, results did not indicate statistically significant trends (Chart 2).
The month of June has seen the largest decline in average snow cover extent, decreasing by 34% over the 39 year study period. Average May snow cover has declined by 13% 10 and April snow cover declined by 7% (Chart 3).