. 2016; Liu et al. 2016).
Exposure to wildfire smoke can result in emergency room visits for a variety of conditions, newly caused or aggravated, including asthma and chronic obstructive pulmonary disease (Rappold et al. 2011; Tinling et al. 2016; Cascio 2018); and cardiovascular conditions, including stroke, heart attack, and heart failure (Dennekamp et al. 2015; Haikerwal et al. 2015; Hutchinson et al. 2018; Wettstein et al. 2018). Additionally, wildfires can lead to rapid ozone formation and increased frequency of ozone pollution levels that exceed air quality standards (Jaffe et al. 2008; Jaffe and Widger 2012). Increased ozone pollution harms human health, as described previously.
Wildfires are estimated to have contributed about 18% of the total atmospheric particulate matter emissions in the US from 2004-2009 (Liu et al. 2015). Wildfires are projected to become the principal driver of summer particulate-matter concentrations in the western US (Ebi et al. 2018).
A study by Liu et al. (2016) shows that on days in the US exceeding regulatory particulate matter (PM2.5) standards, wildfires contributed an average of 71% of total particulate matter (PM2.5). Under future climate change, they estimate that more than 82 million individuals will experience a 57% increase in frequency and a 31% increase in intensity of smoke waves, defined as two or more consecutive days with high, wildfire-associated increases in particulate matter. They additionally project that wildfire-associated particulate matter will increase 160% by mid century under the RCP4.5 scenario (see Section 2).
Airborne particulate matter is expected to increase in Montana croplands with more soil-exposed fallow periods (Pi and Sharratt 2017). Wind erosion of soil in wheat production is likely to increase due to increased summer drought and changing precipitation patterns favoring winter wheat over spring wheat. Climate change is also likely to create more cropland abandonment leading to desertification and land degradation (Hughes and Diaz 2008).
Exposure 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 se