posure to particulate matter results in numerous negative health outcomes (described above), as well as reduced life expectancy or death (Schwartz et al. 1996; Dominici et al. 2006; Pope et al. 2009; Puett et al. 2009).
New fire projections for Montana
Warmer spring and summer temperatures in Montana and reduced summer precipitation (see Section 2 regarding drought) create conditions conducive to wildfires. In grassland regions, wet spring conditions favor wildfire activity by promoting growth of understory vegetation, which is subsequently flammable during warm, dry summers and even into the fall and following winter (McKenzie and Littell 2017; Holden et al. 2018). When fires ignite under unusually warm, dry conditions—whether caused by humans or lightning—they spread faster, are harder to suppress, and end up burning larger areas than in summers with average climate conditions. Fire danger ratings—a five-class system widely used in wildfire management (low, moderate, high, very high, and extreme)—summarize the ways weather and climate conditions influence the likelihood and spread of wildfires (Dennison et al. 2014).
We compare the average number of summer days having extreme fire danger (the highest classification in the system) between a reference period (1971-2000) and mid century (2040-2069), the latter based on climate projections (Whitlock et al. 2017). The most extensive wildfire activity in Montana correlates well with extreme fire danger, and thus the latter provides a suitable representation of extensive wildfire activity and resultant smoke in the future.
Under RCP8.5 climate projections, the number of summer days having extreme fire danger increases across all counties, relative to the 1971-2000 reference period. In many counties, particularly in northwestern and south central Montana, the number of days with extreme fire danger increases by 10 days by mid century, doubling (100% increase) from 1971-2000 benchmark (Figure 3-7). More days with extreme fire danger implies a longer, more active fire season, which ultimately results in more area burned per year, and increased atmospheric particulate matter and smoke (Norby et al. 2010; Jenkins et al. 2014; Jolly et al. 2015). Wildfire smoke is and will be most common in western Montana, primarily because wildfires in forested areas produce large amounts of smoke (Westerling et al. 2006; IDDEQ 2013) (Figure 3-7).
In Montana, Department of Environmental Quality reports show that the number of communities or counties whose air quality rated as moderate, unhealthy, very unhealthy, or hazardous generally increased from 2010-2017 (Figure 3-8), as a result of increased acres burned by wildfires. However, these trends were followed by declining smoke in the below-average fire seasons of 2018 and 2019, but on track in 2020 to be similar to or exceed 2017. The number of smoke events in each health category shows similar trends, with western Montana valleys receiving the most unhealthy smoke days followed by the southeastern region of Montana. In 2017, the most recent year with regionally extensive wildfire activity, Seeley Lake, Lolo, Superior, and Frenchtown all had ten or more days where air quality was rated unhealthy, very unhealthy, or hazardous, with Seeley Lake experiencing 38 days with unhealthy and worse smoke conditions (see Section 2, plus sidebar in Section 4 titled The 2017 Seeley Lake Fires and Lung Function). Assuming that projected increases in extreme fire danger result in increased wildfire activity, Montana could expect an increase in summers exceeding moderate air quality standards, with more communities affected, especially in western Montana.
Increased fire danger for Montana in coming decades will likely result in decreased air quality in years with widespread wildfire activity, like 2017. Such years will be interspersed with years having average or below-average fire activity. Communities can adopt a suite of actions to mitigate adverse effects on citizens from poor air quality (McWethy et al. 2019; also, such actions are described in Section 5).
