Background
The authors introduce their study by noting that "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," and that "transient climate change simulations performed with both Global Climate Models and Regional Climate Models suggest increased frequencies of extreme high temperature events."

What was done
In a study designed to look for the predicted increases in heat waves in Southern Quebec (Canada) over the last six decades of the 20th century, when climate alarmists claim the planet warmed at a rate and to a level unprecedented over the last couple of millennia - with some claiming an even longer such time period (Hansen et al., 2006) - Khaliq et al. assessed temporal changes in the frequency of occurrence and durations of heat waves based on data acquired at seven weather stations located in southern Quebec (La Tuque, Quebec, Maniwaki, Drummondville, Montreal, Les Cedres, Sherbrooke) for the 60-year period 1941-2000.

What was learned
For heat spells defined in terms of daily maximum air temperature, the majority of extreme events showed "a negative time-trend with statistically significant decreases (at 10% level)," while almost all of the heat spells defined in terms of daily minimum air temperature showed "a positive time-trend with many strong increases (i.e., statistically significant at 5% level) at all of the stations."

What it means
As for what their findings imply, Khaliq et al. say "a possible interpretation of the observed trends is that the maximum temperature values are getting less hot and minimum temperature values are getting less cold with time." And how could anyone argue with them? What they say would seem to be the only "possible" explanation for what they observed. In addition, they report that over Canada as a whole, the study of Bonsal et al. (2001) "on the characteristics of extreme temperatures has shown no significant trend over the course of the [entire, well almost] 20th century (1900-1998)."

References
Bonsal, B.R., Zhang, X., Vincent, L.A. and Hogg, W.D. 2001. Characteristics of daily extreme temperatures over Canada. Journal of Climate 14: 1959-1976.

Hansen, J., Sato, M., Ruedy, R., Lo, K., Lea, D.W. and Medina-Elizade, M. 2006. Global temperature change. Proceedings of the National Academy of Sciences USA 103: 14,288-14,293.