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It seems as though we're having record breaking snowfalls and extreme cold temperatures across Canada - if global warming is real, why are we getting such cold weather this winter?

Snow shovelling. Photo: Photolux Commercial Studio, © Environment Canada, 2002. Click to enlarge.

The average yearly temperature for the surface of the Earth has increased by 0.74°C over the past 100 years (1906-2005). However, this strong global warming does not mean that in all regions, and in all seasons, temperatures will be above normal each year.

On the contrary, weather patterns are affected by many different factors, some of which operate on short time scales. As a result, the weather experienced in anyone place will continue to vary from week to week, month to month, season to season and year to year. A cold spell in one region of Canada, or even in many regions of Canada at the same time, can still occur even though the Earth's climate as a whole is warming.

Also, it is important to look at the actual data on record to see whether or not perceptions of unusually cold, snowy conditions are supported by the data. An analysis of December temperatures shows that for the country as a whole, December 2007 was not particularly cold. In fact, the average temperature in Canada during December 2007 was only 0.1°C below the "normal" temperature for this month (where the "normal" is the average value over the 30 year period between 1951-1980). While this past December was slightly colder than normal, 2007 as a whole was warmer than normal in Canada. Globally, 2007 is tied for the second warmest year on record according to NASA's most recent global temperature trend analysis. Snowfall in December was well above normal in the eastern half of Canada and near normal elsewhere.

One factor that may be playing a part in producing cold, snowy conditions in some parts of Canada is that the year 2007 was the beginning of a moderate La Niña event - the counterpart to the more familiar phenomenon called El Niño. Although El Niño and La Niña events refer to particular conditions prevailing in the equatorial Pacific region, weather patterns around the world are affected by such events. La Niña winters tend to be colder and snowier than normal in Canada, particularly in the west.

Although weather is by its very nature variable - and we will undoubtedly experience very cold weather at times in the coming years - continued global warming will result in fewer record breaking cold events. To make an analogy, global warming will load the dice in favour of more warm events, so that the probability of getting record breaking cold weather will diminish with time. Precipitation across Canada is, however, expected to increase as the climate warms, particularly in wintertime, which could lead to greater snowfalls in some areas.

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