The impact of combined changes in temperature and precipitation on observed and projected changes in fire weather

 

Changes in temperature and in precipitation each have impacts across many sectors. However, combined changes in temperature and precipitation can have additional impacts, and some sectors rely on information regarding concurrent changes in these two variables. An example is fire weather. Changing precipitation and temperature (along with changing wind) alter the risk of extreme wildfires that can result from hot, dry, and windy conditions. Understanding changes in both temperature and precipitation lends insight into changes in wildfire risk and how it might evolve in the future.

The Canadian Forest Fire Weather Index (FWI) System is a collection of indices that use weather variables, including temperature and precipitation, to characterize fire risk. It includes an index, labelled FWI, that synthesizes information from the collection of indices to quantify day-to-day changes in the risk of a spreading fire. A threshold of this index is often used to define days conducive to fire spread (Wang et al., 2015; Jain et al., 2017). In addition, three of the most commonly used indices are moisture codes, describing the dryness of different categories of fuels (Wotton, 2009). All of the FWI indices represent factors affecting fire potential, with larger values indicating greater fire potential, although the occurrence of a large wildfire also depends on ignition sources, fuel characteristics, and fire management actions.

A few studies have looked at trends in these indices across Canada. Large year-to-year variability in the FWI indices hinders detection of trends (Amiro et al., 2004; Girardin et al. 2004). Trends may sometimes be discerned from a very long record of data, as is the case with increases in the Drought Code21 in northern Canada and decreases in the Drought Code in western Canada and parts of eastern Canada during the 20th century (Girardin and Wotton, 2009). Another study found that the mean number of fire spread days across Canada increased over 1979 to 2002, although the trends varied regionally, and only some were significant (Jain et al., 2017). Despite inconsistent trends in the FWI indices, there has been a significant increase in annual area burned across Canada (Podur et al., 2002; Gillett et al., 2004).

Higher temperatures in the future will contribute to increased values of the FWI indices and, therefore, increased fire risk. The increase in precipitation that would be required to offset warming for most of the FWI indices exceeds both projected and reasonable precipitation changes (Flannigan et al. 2016). Increases in extreme values of the Duff Moisture Code22 are projected across most of the forested ecozones of Canada by 2090 (Wotton et al., 2010). Increases in fire spread days and extreme values of the FWI are projected, with the largest changes in the western Prairies (Wang et al., 2015). Several other studies also project increases in the FWI indices and the length of the fire season in Canada in the future (Flannigan et al., 2009; de Groot et al., 2013; Flannigan et al., 2013; Kochtubajda et al., 2006). Although the magnitude of projected changes varied among these studies, most project increases in the FWI indices that correspond to higher fire risk.