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Perspectives on the June 2021 heatwave and wildfires

Aseem Sharma (NRCan, University of Alberta), Piyush Jain (NRCan), Mike Flannigan (University of Alberta), John Abatzoglou (University of California, Merced)


A dangerous heatwave continues to bring extreme temperatures to western North America, greatly increasing fire risk. In the last two days alone (as of Friday, July 2nd) over 100 new wildfires have started in British Columbia (BC Wildfire). A research collaboration between the University of Alberta, the Canadian Forest Service, and the University of California Merced has been looking at how these large-scale heatwave events are related to wildfire activity in western Canada.

As part of this research, we have developed an algorithm for identifying and tracking heatwaves by considering upper atmosphere patterns (at heights of 5-6km above the earth’s surface). Our definition of a heatwave is based on identifying so-called persistent positive anomalies in these patterns, which cause higher temperatures at the surface.

The animation below shows the evolution of the 2021 heatwave based on a global weather forecast (June 19th to July 4th).

Evolution of the 2021 heatwave.

The colors indicate 500mb geopotential height anomalies (differences from normal). The blue outline identifies and tracks the heatwave by comparing the current pattern to historical data. The red and orange triangles indicate fire start locations. The magenta dot shows the location of Lytton, BC, a village that was destroyed by wildfire shortly after recording Canada’s highest ever temperature of 49.6 C. Forecast data is from the Global Forecast System (GFS, U.S. National Weather Service).

From our research, we have found that western North America experiences, on average, four such heatwave events (corresponding to blocking ridges) every June. Compared to all June heatwaves identified in the last 42 years (1979-2020), the June heatwave of 2021 is 35% larger and 36% more intense. Compared to the average climatology of all fire season heatwaves (May to September) of the same period, the June heatwave of 2021 is 31% larger and 32% more intense.

The intensity of the current heatwave is extremely rare for western North America, being the largest observed event in June in the last 42 years. Such an event is estimated to occur only once in over 1000 years during June; however, even during the entire fire season (May to September), such an event would still occur only once every ~20 years.

The magnitude of the heatwaves during the month of June for 1979-2020.

Historically, the presence of a heatwave event, on average, increases the likelihood of wildfire occurrence by a factor of 7. We have further estimated the probability of wildfire occurrence during or immediately after the June 2021 heatwave increased further by 34% when compared with previous heatwaves.

Fire data provided by John Little (Canadian Forest Service).

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