The recent Third Assessment Report (TAR) of the Intergovernmental Panel onClimate Change (IPCC) indicated that observed 20th century changes in severalclimatic extremes are qualitatively consistent with those expected due to increasedgreenhouse gases. However, a lack of adequate data and analyses make conclusiveevidence of changing extremes somewhat difficult, particularly, in a global sense.In Canada, extreme temperature events, especially those during winter, can havemany adverse environmental and economic impacts. In light of the aforementionedIPCC report, the main focus of this analysis is to examine observed trends andvariability in the frequency, duration, and intensity of winter (Jan–Feb–Mar) cold and warm spells over Canada during the second half of the 20th century.
Cold spell trends display substantial spatial variability across the country. From1950–1998, western Canada has experienced decreases in the frequency, duration, and intensity of cold spells, while in the east, distinct increases in the frequency and duration have occurred. These increases are likely associated with morefrequent occurrences of the positive phase of the North Atlantic Oscillation (NAO)during the last several decades. With regard to winter warm spells, significantincreases in both the frequency and duration of these episodes were observedacross most of Canada. One exception was found in the extreme northeasternregions, where warm spells are becoming shorter and less frequent. The resultsof this study are discussed within the context of climate warming expectations.