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.
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