As a handful of wildfires in British Columbia have already prompted evacuation orders and the wildfire season kicks off in earnest, what can we say about the role of global warming in causing them in this part of the world?
Scientists are reluctant to say that any single wildfire is ‘caused’ by climate change. There are so many factors involved in extreme weather events and natural disasters. And whatever role human influences might play lies atop natural variability. Instead, they argue, we should be asking: How much of a role do anthropogenic (human-originating) forces play, noting that there are many factors involved in causing extreme weather events with human influences lying atop natural variability.
Researchers in the emerging field of attribution studies use a combination of observations and models to consider what we could expect to happen in the absence of global warming, what is termed a “counterfactual” scenario. The counterfactual is then compared to observations of what has actually happened. The difference between the world (or region) as it actually is and the world in the counterfactual scenario shows how the probability of an extreme weather event has been altered as the result of human influences.
We know from many attribution studies exploring wildfires that boreal forests around the world and in Canada will likely see the fire season lengthen as a result of climate change. The duration, severity and frequency of fires go up, and the area that is burnt will grow. Researchers have even been able to estimate the capacity of firefighters to contain fires given their number and size, and have concluded that the expected larger number of fires will also likely escape the best efforts of fire management agencies under a warmer climate.
But very few studies have attempted to assess the human influence on fire risk in western Canada more specifically. One of the challenges is that as one scales the analysis from the global down to the regional level, it is harder to detect an anthropogenic signal amidst the noise of natural variability.
Researchers with the Pacific Climate Impacts Consortium and Environment and Climate Change Canada however were able to leap over this obstacle in essence by “taking apart” a key element of Canadian wildfire management. The Canadian government back in 1968 developed a wildfire assessment tool called the Canadian Forest Fire Danger Rating System (CFFDRS). This national system for rating the risk of wildfires has been refined over the years and today is employed around the world by other wildfire-prone nations.
The CFFDRS involves a suite of quantitative and qualitative indices, 14 of which the researchers broke down into their component elements, such as numeric ratings of the moisture content of surface fuels and decaying litter, potential fire spread rate, and fire weather (such as windiness). But these indices in turn depend on a series of variables such as air temperature, humidity, precipitation and wind speed, all of which are likely to alter as the climate changes. Thus by inputting ranges of values for what such variables are likely to be under future global warming projections—whose relationship to increased greenhouse gases have been robustly described—but downscaled for western Canada, the researchers were able to carry out a more trustworthy attribution analysis, across the longest period for which data was available (1960-2014).
The researchers looked at all of western Canada, divided up into different “homogenous fire zones” (HFR – regions with relatively uniform climate, geological history and physical geography). But they focused on one HFR in particular: that surrounding Fort McMurray, site of the devastating 2016 fire that destroyed more than 3000 homes and caused roughly $10 billion in damages. In this region, they found that during the current decade (2011-2020), extreme fire events are 1.5-6 times more likely than they would have been in the absence of human emissions.
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