April 9, 2007

Canadian Heat Waves Declining?

Filed under: Heat Waves

No popular presentation of global warming is complete without images of people suffering from the effects of a heat wave. It seems so simple – the world is getting hotter, temperatures are rising everywhere, and therefore, heat waves will be longer, more frequent, and more severe. There are heat waves somewhere on the planet at any moment, so one would never run out of fresh material for such a story. Add in giant killer heat waves in Chicago and/or Europe, claim tens of thousands of deaths on those ever-increasing heat waves, and another scary global warming story emerges. Heat waves put a human face on suffering thanks to global warming, and if you include sweltering pets and animals at the zoo, the story is further embellished. Add in the familiar lines about the heat waves differentially impacting the elderly, the poor, and children, and the story is nearly complete. Obviously, blame the industrial nations (particularly the United States) for all the misery just for some icing on the cake.

We have covered heat waves many times in the past at World Climate Report, but another article has appeared in a recent issue of Theoretical and Applied Climatology that we must call to your attention. A team of scientists from various institutions in Quebec decided to examine trends in the number of summer-season heat spells and the number of summer-season hot days in southern Quebec over the past 60 years. Canada is in the mid-to-high latitudes where climate models predict enhanced warming compared to the rest of the planet, so one might logically expect to see an increase in the heat spells. The title of the article reveals that the focus is on observed changes in heat spells in southern Quebec, but the results may surprise the global warming advocates of that country.

The Khaliq et al. team starts out with familiar language noting that “Extreme climate phenomenon, such as occurrence of heat spells, is an active area of investigation because of its possible adverse effects on public health and economy. Also, these extreme climate events are receiving increased attention because of the possibility of increases in their frequency and severity in future climate as a result of enhanced concentrations of greenhouse gases in the atmosphere and associated atmospheric warming. Transient climate change simulations performed with both Global Climate Models (GCMs) and Regional Climate Models (RCMs) suggest increased frequencies of extreme high temperature events and decreases in extreme low temperature events for the 21st century.” There is no doubt that these concerns permeate the popular global warming literature, but after these introductory remarks, they reveal some interesting results from past research.

They review a recent study for Canada showing that extreme temperatures have “shown no significant trend over the course of the 20th century (1900–1998) for the higher percentiles of daily summer maxima.” The researchers of the previous study “concluded that the number of extreme hot days showed little change, in spite of the increase in mean annual temperature by 0.9°C between 1900 and 1998.” That is the first clue that the darn empirical observations may not be supportive of the expected rise in heat waves in Canada.

The Khaliq et al. team gathered daily maximum and minimum temperature records from seven stations in southern Quebec from 1941 to 2000. They note that “extreme climate events can be quantified from the viewpoint of their rareness, intensity, duration and impacts on socio-economic sectors.” Obviously, a summer heat spell in southern Quebec might be viewed as a summer cold snap in Phoenix or Las Vegas, so definitions of a “heat spell” at any location can be tricky. The research team developed a number of definitions of heat spells and the number of hot days based on thresholds of maximum and minimum daily summer temperatures.

In discussing the trends in maximum temperatures in the summer, they found a “negative trend at all stations” while minimum temperatures in summer “in general have become less cold with time.” In their own words, they state “that the summer-season minimum temperatures are getting less cold and extreme hot days are getting less hot in southern Quebec based on the analysis of observations recorded over the last six decades of the 20th century.”

With respect to ≥1-day or ≥3-day number of heat spells based on maximum temperature or seasonal hot days based on maximum temperature thresholds, they found “significantly decreasing trends” for four of the seven stations, and they note that “This number is far higher than that would be expected just by chance.” The trends with respect to minimum temperatures were a mixed bag with some increases and some decreases depending on the various definitions. Their work makes it perfectly obvious that heat waves in southern Quebec, particularly heat waves defined by maximum daily temperatures, have become substantially less frequent over the 1941 to 2000 study period.

The results of this study are unlikely to be championed by the global warming crusade, especially by the pro-global warming majority in Quebec. But the results should come as no surprise to anyone who looks into the details of climate model predictions given the buildup of greenhouse gases. The models universally show that warming should be most pronounced in mid-to-high latitude continental locations like Quebec, but the warming is predicted to be far greater in winter than summer. Furthermore, the predictions are for greater warming in the minimum temperature, not the maximum temperatures, and if one examines the predictions in more depth, the models generally say little about maximum temperatures in summer.

Should southern Quebec suffer from a heat wave this coming summer, headlines around the world will point to global warming as the cause, and zero attention will be placed on the results of Khaliq et al. who have shown that heat waves in their study area have declined in frequency and magnitude since 1941. Once again, the observational record of weather and climate stands in the way of highly-popularized predictions about the effects of the ongoing buildup of greenhouse gases.


Khaliq, M.N., P. Gachon, A. St-Hilaire, T.B.M.J. Ouarda, and B. Bobeé, 2007. Southern Quebec (Canada) summer-season heat spells over the 1941–2000 period: an assessment of observed changes. Theoretical and Applied Climatology, 88, 83–101.

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