The Record-breaking 2023 Fire Season in Canada was driven by Extreme Weather

By most measures, the 2023 Canadian fire season has been record breaking; over 15 million hectares (Mha) have burned (as of August 24th), more than doubling the previous record of 7.6 Mha in 1989; emergency evacuations due to wildfires were ordered for approximately 200,000 people (as of August 17th) nationwide, double the previous record; multiple structures were lost in the outskirts of Halifax (Nova Scotia), and in Enterprise (Northern Territories) and West Kelowna (British Columbia); widespread smoke throughout North America with very poor air quality in many major urban centers, and there were several fire fighting-related deaths.

Significantly, much of this burning occurred throughout the boreal and temperate biomes of Canada, and in multiple provinces and territories covering a large geographic extent. In particular, British Columbia, Alberta, Nova Scotia, and Quebec all saw extensive fire activity in May and June, driven by a combination of antecedent drought and temperatures that were well above seasonal averages. These conditions were particularly atypical for the province of Quebec, where the largest fire in the province’s history (>1.1 Mha) burned through late spring and summer. Recent analysis by the World Weather Attribution group has found that fire seasons of this severity have been made 7 times more likely in the province of Quebec due to anthropogenic climate change.

An examination of the Canadian Fire Danger Rating System, calculated with available data from the ERA5 reanalysis, showed that record-breaking fire weather occurred for Quebec in June with record values (since 1940) of the mean monthly Drought code (DC) and Duff Moisture code (DMC), both indicative of drought and low fuel moisture; this resulted in record values of the Fire Weather Index (FWI), a proxy for fire intensity. Similarly, British Columbia had record breaking values of the DC and FWI in June. Alberta, in contrast, had record values of the Initial Spread Index (ISI) in May suggesting a more wind-driven fire regime. Interestingly, fuel moisture was affected by previous year drought conditions in British Columbia, Alberta and the Northern Territories but not in Quebec. The extreme fire weather conditions coupled with high levels of ignitions - both human and lightning - led to the observed widespread burning.

The drivers of the observed extreme fire weather are determined by an analysis of large-scale atmospheric patterns, which showed that the burning in both the West (British Columbia and Alberta) and East (Quebec and Nova Scotia) were each associated with the presence of multiple persistent atmospheric ridges in May and June. These patterns were evident in persistence of positive anomalies in the 500hPa geopotential heights. It is likely that the antecedent drought conditions combined with the presence of these atmospheric ridges, served to amplify the drying of soils through positive land-atmosphere feedbacks.