Walking along the Rimrocks above Billings. Photo courtesy of Scott Bischke.


Robert Byron, Bruce Maxwell, Nick Silverman, Philip Higuera, Madison Boone, and Dave McWethy


The consequences of climate change in Montana—including more frequent and intense heat waves; and increases in spring flooding, late-summer drought, extreme weather events, and wildfires—have recognized potential to impact human health (Ebi et al. 2018). Though little studied for Montana to date, climate change is known to be adversely impacting global health, including some Americans, now. For example, an increase in climate-related extreme events has led to increased emergency room visits and hospital admissions in the US (Ebi et al. 2018). Those adverse impacts are expected to increase over coming decades (Wuebbles et al. 2017).

Figure 3-1 depicts how climate pressures (also called stressors), following multiple exposure pathways, can lead to specific health outcomes for people. Those outcomes, discussed in this section, include heat-related illness; vector-borne diseases; mental health impacts; physical trauma, injuries, and death; respiratory, cardiovascular, and gastrointestinal conditions; and adverse effects on pregnancies and birth outcomes.

Figure 3-1. Ways climate change is harming, and will increasingly harm, human health (Salas et al. 2018).


While climate change affects all parts of the world, the severity of specific health impacts will vary by the specific climate pressure, as well as by location. Coastal regions, for example, will endure the most damage from sea-level rise, whereas inland states like Montana will be impacted most by the adverse effects of heat waves and reduced air and water quality (from wildfire smoke, desertification, land degradation, and other factors [Hughes and Diaz 2008; Sharratt et al. 2017]).[1] In the subsections that follow, we describe some of the exposure pathways and health outcomes resulting from climate change as shown in Figure 3-1. Some of the discussion may apply globally, but we emphasize those pathways and outcomes more likely to impact Montanans.


Extreme Heat

Climate change is increasing the intensity and frequency of heat waves (Meehi and Tebaldi 2004; Reid et al. 2009). Globally, 157 million more people were exposed to extreme heat in 2017 than in 2000 (Watts et al. 2018). Assuming no changes in the current trend of emissions, the Union of Concerned Scientists projects that by mid century “the number of people [in the United States] experiencing 30 or more days with a heat index above 105°F in an average year will increase from just under 900,000 to more than 90 million—nearly one-third of the US population” (UCS 2019). Similarly, by the end of the century parts of Montana could see as many as 54 additional days over 90oF (32oC) (Whitlock et al. 2017; Section 2).

Multiple studies have demonstrated increased mortality associated with heat waves (Knowlton et al. 2009; Ostro et al. 2009; Isaksen et al. 2016), which is the major cause of weather-related deaths in the United States (NWS undated) (see sidebar). Gubernot et al. (2015) find that people who work in agriculture or construction jobs have the highest risk for heat-related death; that risk may be even higher for employees of Montana businesses such as farms and small construction firms that have no backup personnel. This finding should concern Montanans given that agriculture and construction are integral to our state’s economy.

In addition, American Indian ceremonial practices (e.g., sundances) become more difficult during extreme heat and expose participants to risk for heat-related conditions (Doyle et al. 2013).



Which is More Deadly—Extreme Heat or Extreme Cold?

This seemingly simple question actually has no clear answer. Temperature extremes in both directions kill hundreds of people in the US each year, but determining the actual death toll is subject to large errors. Two US agencies, using different methods and datasets, came to opposite conclusions about which is more deadly:

  • NOAA’s monthly publication Storm Data (NOAA-NCEIa) regularly reports many more deaths per year as heat-related than as cold-related (NOAA-NCEIb). For example, their annual summary for 2011 indicates 206 deaths from extreme heat versus 29 from extreme cold.
  • The Centers for Disease Control and Prevention’s National Center for Health Statistics analyzed death certificates in the US and came to the opposite conclusion, finding roughly twice as many people die of “excessive cold” conditions than of “excessive heat” (Berko et al. 2014). About 1300 deaths per year from 2006-2010 were coded as resulting from extreme cold exposure, whereas 670 deaths per year were attributed to extreme heat.

The different findings depend, at least in part, on whether the focus is on acute or short-term responses to extreme weather or, alternatively, seasonal differences in estimates of daily mortality. Abnormally cold conditions may have little effect on estimates of daily mortality; rather deaths in winter increase as a result of non-weather-related diseases, like influenza. In addition, a recent study noted that a large majority of cold-related deaths occur under moderately cold conditions, so a reduction in extreme cold due to global warming is not expected to cause a large reduction in cold-related deaths (Gasparrini et al. 2015). It is also widely accepted that direct attribution (in medical records) of cause of death underestimates the number of people who die from temperature extremes (Sarofim et al. 2016). For example, a person who dies from a heart attack that resulted from heat or cold exposure is listed only as having died from a heart attack, even though extreme temperature exposure may have hastened or triggered the attack.

Looking to the future, the Fourth National Climate Assessment found that under an upper-bound (RCP8.5) emissions scenario we should expect a large increase in US extreme temperature deaths. In 49 large US cities, representing one-third of the nation’s population, the report projected that changes in extreme hot and extreme cold temperatures would result in 9300 additional premature deaths per year by 2090, at a cost of $140 billion per year in 2015 dollars (Ebi et al. 2018). Since the study only covered populations in large cities, Montana was not included. These mortality estimates may be reduced through adaptation efforts such as acclimatization programs (e.g., ensuring adequate heating and cooling systems, and improved insulation). The report also considered the reduction in extreme cold deaths expected in a warmer climate; approximately 100 fewer cold deaths per year were expected in the US by 2050, with no further reduction in cold deaths between 2050 and 2090 (Ebi et al. 2018)

The bottom line is that high and low temperature extremes affect vulnerable populations, such as the unwell, the poor, the young, and the elderly, among others. Understanding the extreme-temperature risk for the future requires additional in-depth study of the interaction between temperature-related deaths and socioeconomic factors. With additional information, we can avoid adverse policy outcomes and achieve effective adaptation strategies.



Human impacts from excessive heat

Heat affects humans in a variety of ways, impinging on multiple body systems. As depicted in Figure 3-2, elevated temperatures directly cause heat-related conditions ranging from muscle cramps to heat exhaustion to heat stroke, the latter of which is deadly if not treated promptly (Becker and Stewart 2011; Epstein and Yanovich 2019). In addition, elevated temperatures have been associated with increases in respiratory disease (Anderson et al. 2013; Ho et al. 2015); heart disease and strokes (Knowlton et al. 2009; Wang et al. 2016; Achebak et al. 2018); and fluid and electrolyte disorders, kidney disease, and kidney failure (Bobb et al. 2014; Ross et al. 2018; Sorensen and Garcia-Trabanino 2019). A study by Li et al. (2012) found evidence of a potential high-temperature threshold for several age groups (15–64 yr, 75–84 yr, and >85 yr) and specific heat-related causes for hospital admissions that included endocrine, genitourinary, and renal morbidities, as well as accidental injuries and intentional self-harm.

Figure 3-2. How heat affects our bodies. When temperature and humidity climb during extreme heat events, the body’s cooling mechanisms become less effective. The symptoms shown here—ranging from minor annoyances to truly life-threatening issues—include both those that are indicative of heat-related illness and those that are signs of preexisting conditions exacerbated by extreme heat. Figure and caption used with permission from The Union of Concerned Scientists (UCS 2019).


Elevated temperatures are also associated with increased risk of preterm labor (Auger et al. 2014; Ha et al. 2016; Avalos et al. 2017), as well as sudden infant death/sudden unexpected infant death syndromes (SIDS/SUID) (Jhun et al. 2017). Elevated temperatures also cause worsening mental health, as described in the Mental Health subsection later in Section 3.

The severity and duration of high temperatures influence the health impacts resulting from extreme heat. In addition, air pollution and population vulnerability (Basu 2009) compound these impacts. Vulnerability is also a function of the typical temperatures to which a population is exposed, plus the extent to which a population adapts (Curriero et al. 2002; Baccini et al. 2008; Ho et al. 2015).

Workers, especially those who work outdoors or in hot indoor environments, are at increased risk of heat stress and other heat-related disorders, occupational injuries, and reduced productivity at work (Levy and Roelofs 2019). Workers in the agricultural sector face increased risk of heat strain and dehydration[2] due to repeated exposures to high air temperatures, arduous physical exertion, and limited fluid intake. These risk factors may result in acute kidney injury (Moyce et al. 2017), as well as increased heat-related traumatic injuries (Spector et al. 2016). Spector et al. (2016) suggest that efforts should be made to address heat-related illness and prevent occupational injury for high-risk populations exposed to high temperatures and high physical exertion.

The impacts of increased air temperature may increase workers’ exposure to hazardous chemicals and, thus, the adverse health effects of those chemicals (Spector et al. 2016). Global warming is also increasing ground-level ozone concentrations with adverse effects on outdoor workers (Levy and Roelofs 2019).

Assessing heat impacts specific to Montana

Multiple studies—covering wide ranges of temperature, time, and geographic area—have sought to assess the health impacts of extreme heat (e.g., Morabito et al. 2014; Zhang et al. 2014). Some studies estimate how heat-related mortality differs between rural and urban landscapes (reviewed by Ho et al. 2015). Other studies develop indices to identify heat-vulnerable populations (Vescovi et al. 2005; Reid et al. 2009; Reid et al. 2012; Chuang and Gober 2015).

Human health vulnerability to heat is most often expressed as a combination of three factors: adaptive capacity, exposure to heat, and sensitivity to heat (Smit and Wandel 2006; Füssel 2010; Inostroza et al. 2016) (see sidebar). In our analysis, heat vulnerability was calculated as a combination of a) the historical land-surface temperature and future projections of heat to describe exposure; and b) county-level[3] socioeconomic factors to describe sensitivity and adaptive capacity (see Appendix A for details). We will refer to the climate impacts as exposure, and the socioeconomic factors as sensitivity, with sensitivity inclusive of adaptive capacity, but no health factors were directly included due to incomplete county health factor data.



Factors Considered in Determining Vulnerability to Extreme Heat

Vulnerability, in the present context, is the extent to which a person is susceptible to the impacts of climate change. As described in the text and below, many factors go into calculating a person’s vulnerability to extreme heat. Three key terms bear definition:

  • adaptive capacity.—The ability of a person (or society) to cope with climate change.
  • exposure.—The type and magnitude of a climate change.
  • sensitivity.—How easily affected a person is by climate change.

Vulnerability (the “V” in the diagram below) is assumed to be the intersection of adaptive capacity, exposure, and sensitivity (Smit and Wandel 2006; Füssel 2010; Inostroza et al. 2016). We combine adaptive capacity and sensitivity into one group, which is based on socioeconomic factors (see Appendix A for details). We continue to use the intersection of the three areas to determine vulnerability; and assume adaptive capacity is simply implicit in the socioeconomic layers that comprise our sensitivity factor.




Heat exposure

Heat exposure is modeled as the combination of historical land-surface temperatures and projected heat. To determine the county average land-surface temperature on days of extreme heat, we use the 95th percentile of daily average land-surface temperatures from 2000-2019 (Figure 3-3). For future projected heat, we use the heat index of each county (UCS 2019) generated from an ensemble of general circulation models.[4] The heat index includes a combination of temperature and humidity to create a “feels-like” heat rating.

Figure 3-3. The county average land-surface temperature on days with extreme heat from 2000-2019. Note: Land-surface temperatures are higher than air temperatures measured at weather stations. The scale to the right shows color-coding for temperature in oF.



We base our calculation of sensitivity to heat on socioeconomic variables from the US Census Bureau, 2013-2017 American Community Survey 5-Year Estimates (US Census Bureau undated).[5]  Socioeconomic factors include age, the average income for people in the county, the percent of county population under the poverty line, the percent of poverty households with children, the percent of population employed in construction and production (implying predominantly outdoor workers with greatest exposure to heat), the percent of population unemployed, and type of housing (see further details in Appendix A).

Heat vulnerability

Human exposure to heat has been quantified and mapped, although only a few studies have combined socioeconomic sensitivity with heat exposure data to evaluate human vulnerability to the health risks associated with extreme heat (Buscail et al. 2012; Ho et al. 2015). Most studies that examine health vulnerability to heat use aggregate data that matches the spatial units from which sensitivity information is derived by the US Census Bureau (Ho et al. 2015). For example, most socioeconomic factors are measured at the county scale so exposure measurements (i.e., land-surface temperature and change in future heat) must be averaged across a county. Despite this complication, heat vulnerability is a useful measure to relate climate to human health impacts (see further details in Appendix A). Municipalities can use heat vulnerability information to guide heat mitigation interventions, such as establishment of green or reflective roofs, urban parks, and water features (see Section 5).

Little Bighorn River swimming hole at Crow Agency. Photo courtesy of John Doyle.


By combining exposure and sensitivity data, we identified counties with populations most vulnerable to extreme temperatures. Vulnerability was split into four main categories represented in Figure 3-4. The darker the color the higher the vulnerability to heat.

Figure 3-4. Montana heat vulnerability ratings for mid century, using multi-criteria decision analysis and the RCP8.5 emission scenario (explained in Section 2). Darker shading indicates higher vulnerability. See text and Appendix A for details.


Of Montana’s 56 counties, one county has a high vulnerability rating, while 17 have a medium-high rating, 12 have a medium-low rating, and 26 have a low rating. A strong west-to-east increasing trend in vulnerability exists. That trend largely represents the patterns of exposure (i.e., land surface temperatures and projected heat). Roosevelt County ranked 29th for historical extreme heat and 51st for future predicted heat (higher rankings indicate higher exposure), making it one of the top five counties for projected extreme heat in Montana. Along with its high heat ranking, other factors make the population of Roosevelt County vulnerable to the negative health impacts of heat. Those include relatively high unemployment, low median income, high poverty levels, and many production jobs (agriculture and energy sector).

Local temperature trends

To understand the local effects of extreme temperatures, we examined daily temperature data from 38 NOAA weather stations across Montana. We determined the maximum and minimum temperature trends in summer months (June, July and August) at each station. The temperature records at individual stations may not coincide or reveal the same patterns as the Montana climate region results indicated in the MCA, which were averaged (Whitlock et al. 2017). Our intent was to examine recent temperatures and their trends at a scale where human health may be monitored and impacted in the future. Our findings are:

  • Summer maximum temperature.—Fifteen out of the 38 stations across Montana indicate an increasing trend in summer (June, July, August) maximum temperatures over the last 30 yr. The easternmost stations show no trend in maximum temperatures. No stations have a negative trend in maximum summer temperatures (i.e., cooling) over the last 30 yr (1990-2019).
  • Summer minimum temperature.—From 1990-2019 minimum summer night temperature has an increasing trend at 15 of the 38 stations across Montana, and a decreasing trend at four stations. All of the decreasing trends in summer night temperature (i.e., cooling) are in western Montana.  Five of the 15 stations with increasing summer maximum temperature also show increasing trends in summer night temperature; three are in urban centers. In other words, stations that show a trend towards increasing temperature during the day also experience a trend towards warming night conditions. Warm nights represent a burden on the human thermoregulatory system, particularly for the elderly and those who rely on natural ventilation (e.g., open windows) to reduce body temperature (Mills et al. 2015).

The number of continuous days and nights at high temperature results in cumulative, negative heat impacts on people. In a study that looked at large cities at sea level, human mortality increased when extremely hot nights followed hot days (Xu and Tong 2017). While a different scenario than faced by Montanans, increased human mortality has the potential to become a concern here given projected increased extreme temperatures.

  • Winter minimum temperature.—Winter (December, January, February) minimum temperatures indicate an increasing trend (i.e., winter progressively getting warmer) at only two of 38 stations, both in southwestern Montana, and four stations indicate decreasing trends in winter minimum temperature from 1990-2019. Over the longer period of 1970-2019 (50 yr), 11 out of 35 stations show winters warming while no stations indicate decreasing winter temperature. Thus, the time span for analysis influences the trend, as do the months included in a season. For example, when winter is expanded to include November and March in a different study (not published), 13 out of 15 weather stations show winters getting warmer.

Increasing temperature trends over the last 30 yr at individual weather stations are not limited to the larger cities in Montana where heat islands[6] might exist. Small rural towns are also vulnerable to increasing heat and, based on limited healthcare facilities and other factors (see Section 4), may include citizens who are already vulnerable to extreme conditions. The number of summer days with maximum temperatures over 90oF (32oC) is projected to increase 11-33 days by mid century (Whitlock et al. 2017).

Air Quality Issues


Degraded air quality, or air pollution, refers to myriad substances, both human-made and naturally occurring, including tobacco smoke, carbon dioxide, nitrous oxides, sulfur dioxide, particulate matter, ground-level ozone, and volatile organic compounds. Outdoor air pollution, some forms of which are also global warming gases, is estimated to contribute to 64,200 premature deaths in the US annually (Watts et al. 2019). Outdoor air pollution is expected to worsen with climate change (Orru et al. 2014).

Across the US, human health concerns related to air quality are dominated by particulate matter and ground-level ozone (American Lung Association undated; Ward and Smith 2001; Tao et al. 2005).

Particulate matter (PM)—especially PM2.5 and PM10, describing particulates under 2.5 μm and 10 μm diameter, respectively—has been associated with multiple health effects. Those effects include cardiovascular, respiratory, and immunological problems, the latter manifesting as increased risk for pneumonia (Rappold et al. 2011; Reid et al. 2016b; Tinling et al. 2016), and neurological issues, in the form of greater risk for dementia (reviewed by Peters et al. 2019). Particulate matter has been found to penetrate the blood-brain barrier—i.e., move into the human brain—and cross the placenta in pregnant women (Bové et al. 2019). It has also been associated with possible increased risk of preterm birth (DeFranco et al. 2016; Trasande et al. 2016), low birth weight (Fleischer et al. 2014), and increased medication use in children with asthma (Gielen et al. 1997).

Ground-level ozone forms when sunlight and heat act on nitrous oxides and volatile organic compounds (Rani et al. 2011). It has been associated with asthma exacerbations, increased hospitalizations, and premature mortality (Bell et al. 2004; Zanobetti and Schwartz 2008; Di et al. 2017), as well as with preterm birth (Olsson et al. 2013). Ground-level ozone is expected to increase with warming temperatures (Melillo et al. 2014).

In Montana, ozone levels are generally rated low (<50) on the USEPA air quality index (AQI), but particulate matter levels are seasonally elevated (>100 AQI) in some areas (AirNow undated).[7] Ten counties in Montana, for example, received grades of “F” for particulate matter in the American Lung Association State of the Air 2020 report card (American Lung Association 2020).[8] Some local areas have elevated particulate-matter levels during winter months due to inversion layers and high woodstove usage. In eastern Montana, particulate matter is expected to increase as summer drought increases dust emissions in agricultural areas where land gets tilled for crops (Gage et al. 2016) and tillage is employed as a common summer fallow practice (Cook et al. 2014; Dawson et al. 2014). Wildfires also contribute significantly to high levels of particulate matter, as we discuss in the next subsection.

Separately, but related to air quality concerns, pollen counts in the United States are increasing with projections for doubling by 2040 (Zhang et al. 2015) due to warming temperatures with longer growing seasons (Ziska et al. 2019), and higher atmospheric carbon dioxide levels (Ziska and Caulfield 2000). In addition to worsening seasonal allergies, this increase in pollen poses risks of triggering exacerbations of asthma, which affects 9.1% and 6.8% of Montana adults and children, respectively (MTDPHHS 2013).

Smoky skies near Missoula, from Mount Sentinel. Photo courtesy of Philip Higuera.


Wildfires and wildfire smoke

Between 1997 and 2006, 339,000 human deaths per year globally were attributed to smoke from landscape fires, which includes forest fires, peat fires, grass fires, prescribed burns, agricultural burns, and tropical deforestation burning (Johnston et al. 2012). Deaths and injuries occur to people caught in fast-moving fires, as well as to firefighters, emergency response personnel, and others assisting with fire management. Wildfires can result in large-scale, temporary evacuations, or permanent displacement following the destruction of homes or even entire towns (Insurance Information Institute undated). The 2019/2020 fires in Australia, for example, killed 33 people across the country; in New South Wales alone over 2000 homes were destroyed (BBC News 2020). Closer to home, the November 2018 Paradise Fire in California killed 85 people, displaced hundreds, and destroyed over 18,000 buildings (Vox 2019). Large fires with adverse impacts on communities throughout the western US, including Montana, are increasing as measured in numbers of acres burned (Figure 3-6) because a) climate warming produces longer, drier fire seasons and extensive burning; and b) patterns of human development are increasing human exposure to wildfires (Abotzoglou and Williams 2016; Radeloff et al. 2018).

Figure 3-5. Annual area burned by, and number of, wildfires in Montana 2001-2019 (NIFC undated).


In Montana, increased fire activity in recent decades has impacted people through an increase in hazardous air quality from wildfire smoke, originating locally or from distant sources (see, for example, Figure 3-6). For the western US, including Montana, warmer and drier conditions during summer have contributed to longer fire seasons and more area burned by wildfires, producing more and longer smoke events (Dennison et al. 2014; Westerling 2016; Dalton et al. 2017). Smoke events in the western US from 2004–2009 were associated with a 7.2% increase in respiratory hospital admissions among adults over 65 yr of age (Liu et al. 2017).

Figure 3-6. NASA composite satellite image from 11 September 2017, showing fires in the Cascades and Rockies and smoke as far east as the Great Lakes. (Image source Wikipedia; used under Creative Commons attribution-share alike 4.0 international license [user “Bri”]. https://commons.wikimedia.org/wiki/File:2017_September_5_MODIS_Pacific_t....)


Human response data connecting heat and smoke to human health are limited for Montana. In nearby Boise, Idaho, however, seven of the last ten years have included smoke levels considered “unhealthy for sensitive groups” for at least a week during the fire season (IDDEQ 2013), causing cancellation of some school-related sports activities (Nolte et al. 2018). In 2020, large fast-moving fires in California, Colorado, Oregon, and Wyoming resulted in days with the air quality index rated “hazardous” in locations across the western US.

Air quality hazards from wildfire

Wildfire smoke worsens local air quality (Navarro et al. 2016), with substantial public health impacts in regions with large populations near heavily forested areas (Liu et al. 2015; Reid et al. 2016a; Fann et al. 2018). Smoke decreases visibility causing hazardous conditions (Yue et al. 2013), and can be transported hundreds of miles downwind (Dreessen et al. 2016; Kollanus et al. 2016).

Wildfire smoke contains many components that are hazardous to human health, including particulate matter (more below), polyaromatic hydrocarbons, carbon monoxide, nitrous oxide, aldehydes, benzene, among other components (Reisen et al. 2015; Adetona et al. 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.

Particulate matter

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 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).

Figure 3-7. Number of days with extreme fire danger over time in Montana. From top to bottom we show the historical pattern, the mid-century projection under RCP8.5 (upper-bound emission scenario), and the projected change in number of extreme fire days from 1971-2000 to mid century.


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.

Figure 3-8. The number of communities, counties, and regions in Montana where wildfire smoke led to varying air quality ratings (MTDEQ undated), and number of acres burned by wildfire each year.


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).

Water-related Illnesses

Climate change is affecting Montana’s water quantity and quality (Whitlock et al. 2017; MTDEQ 2019), each of which may, in turn, impact human health[9]. For example, spring floods, which are expected to increase because of earlier snowmelt or extreme precipitation events (see Section 2), affect people’s health in a variety of ways. Immediate impacts include injury, hypothermia, death from drowning, exposure to toxic substances and pathogens released by flood waters, and exacerbation of underlying conditions such as cardiovascular or pulmonary disease (due the flood itself or cleaning up afterwards). Later complications include respiratory and skin infections, vector-borne illnesses, and mental health conditions (Du et al. 2010; Ryan et al. 2015; Paterson et al. 2018).

Floods also increase the risk of mold formation in homes (Bell et al. 2017), an environmental stressor that can impact anyone but particularly American Indian/Alaska Native children (Barros et al. 2018). Even in the absence of flooding, elevated rates of gastrointestinal disease have been associated with precipitation events (Carlton et al. 2016; Levy et al. 2016), as well as with dry conditions (Alexander et al. 2013; Friedrich 2013; Fouladkhah et al. 2019), though the latter association is weaker.

Water quality issues resulting from climate change may also come about because of changes to precipitation runoff, land use, and the way agricultural lands are managed (MTDEQ undated). Changes to the timing and amount of runoff can impact stream turbidity and sediment deposition, plus non-point pollution runoff and subsequent increased stream concentrations of undesirable compounds. Late season droughts may result in higher concentrations of pollutants—including fertilizers and pesticides—in warmer waters, potentially leading to decreased levels of dissolved oxygen and/or increased algal blooms.

Harmful algal blooms, often caused by cyanobacteria[10], are special cause for concern. Cyanobacteria have increased across the US from under 100 incidents in 2013 to over 500 in 2019 (EWG undated), and are becoming increasingly common in Montana (MTDPHHSa undated). Many factors influence harmful algal blooms, including increased nutrients from agricultural runoff or sewage, changes in water circulation, and increased water temperatures (Paerl and Huisman 2009; Moore et al. 2011; Davidson et al. 2014). Based on climate model projections, Chapra et al. (2017) indicate that concentrations of cyanobacteria are likely to increase across the US, primarily due to water temperature increases and increased nutrient inputs. They predict that the mean number of days of harmful algal bloom occurrence, which is about 7 days/year per water body under current conditions, will increase to 16−23 days in 2050 and 18−39 days in 2090.

Harmful algal blooms can harm human health. Depending on the causative organism and type of exposure, maladies include gastrointestinal symptoms, muscle cramps, skin rashes, liver damage, and even death. In addition, harmful algal blooms can impair municipal water supplies, hamper recreational activity, and cause significant economic hardships (e.g., if tourism declines in the affected areas).



Food Security and Nutrition Concerns

Climate change threatens human health through its effects on agriculture. For Montana, the MCA describes that global warming will influence the distribution of weeds, pathogens, and insect pests and introduce new pests (Whitlock et al. 2017). These changes may alter the types and amount of pesticides used, potentially affecting the health of agricultural workers and others.

In addition, food quantity and quality, including nutrition, are directly and indirectly impacted by climate change (Gowda et al. 2018b) (see sidebar). Grains, for example, provide almost half the calories humans eat and wheat is a major crop in Montana. With increasing atmospheric CO2 levels, plants grow larger and store more carbohydrates (sugars), yet they contain less protein, zinc, iron, and other nutrient concentrations (Myers 2014).[11] Reduced nutrient quality leads to a number of health problems, including:

  • Inadequate nutrition can permanently affect the physical and mental development of children. In the US, low iron levels are experienced by one of out of every five children for part of their childhood (Irwin and Jeffrey 2001).
  • Low iron levels have been associated with decreased cognitive function (Jáuregui-Lobera 2014), reduced work capacity (Haas and Brownlie 2001), decreased quality of life, and reduced life expectancy (Shander et al. 2014).
  • Reduced protein decreases muscle mass.
  • Undernourished people have greater difficulty fighting infections.
  • Severe zinc deficiency can cause  diarrhea, weight loss, skin lesions, and decreased immunity (NCHS undated).



Climate Impacts on Food Availability and Nutrition

Food and nutrition concerns resulting from a warming world could outweigh all other human health impacts brought on by climate change (Springmann et al. 2016). Examples include:

  • Elevated CO2 levels can reduce the protein and micronutrient concentration in grains, putting larger swaths of the global population at risk for malnutrition and anemia (Medek et al. 2017; Smith and Myers 2018; Uddling et al. 2018).
  • Increased temperatures, both modest and extreme, can lower crop yields (Zhao et al. 2017; Vogel et al. 2019), as can drought (Lesk et al. 2016) and extreme weather events (Nelson et al. 2014).
  • Increased food prices are an economic result of lower crop yields, making some foods less available to those who may already be undernourished. Increased food prices disproportionately affect lower income people (Lake et al. 2012).
  • Montana has seen longer growing seasons, offset by summers becoming hotter and drier.  Hence, it is hard to project whether local food production will increase.



Climate change can also lower the overall quantity of food produced, resulting in multiple impacts including to traditional sources for American Indians (Zhao et al. 2017; Gowda et al. 2018b; see Section 4 sidebar: Changes Rippling Through Our Lives and Waters). The salt content of underground water sources (aquifers) and coastal lands globally is increasing due to greater withdrawal of groundwater and sea-level rise, making these regions less productive or non-productive. Some pests and weeds that lower the productivity of food crops can thrive better in a CO2-enriched atmosphere (Deutsch et al. 2018; Ziska et al. 2019). Increasing extreme weather (e.g., flood and precipitation events), growing season drought, and increasing fire severity can also be a detriment to food production.


Food and nutrition concerns resulting from a warming world could outweigh all other human health impacts brought on by climate change (Springmann et al. 2016).


Livestock, not just plants, are impacted by climate change. Milk production decreases as dairy cows are exposed to increased heat (reviewed in Whitlock et al. 2017; Summer et al. 2018). Deaths in all animals, including livestock, increase with excessive heat, and animals with dark coats are more sensitive to extreme heat. Heat waves make livestock less fertile (Takahashi 2012). Extreme weather events, such as flash floods and drought, also lead to increased animal mortality.

Cattle in Valley County. Photo courtesy of Erik Adams.


Vector-borne Disease

Vectors—primarily arthropods such as ticks, mosquitoes and fleas—are organisms that transmit diseases from one host to another. Globally, vector-borne diseases include Zika, plague, dengue, malaria, yellow fever, Chagas disease, and Chikungunya, among others. In the US, Rocky Mountain spotted fever, Lyme disease, and anaplasmosis, all carried by ticks, and West Nile virus, transmitted by mosquitoes, are most prominent. Zika, dengue, Chikungunya, and Eastern Equine Encephalitis are also of concern.

Overall, climate change is expected to increase the range of vectors, primarily ticks and mosquitoes in the US, thereby increasing the number of people exposed to the diseases that these arthropods transmit (Beard et al. 2016; Sonenshine 2018). Recent work by Rosenberg et al. (2018) shows that the number of vector-borne diseases tripled in the US between 2004 and 2016, with over 100,000 cases reported in 2016. Whereas the marked increase is unlikely to be fully attributable to climate change given the complexities of vector life cycles, disease prevalence, and human interactions (well discussed by Ogden 2017), neither can the contribution of climate change be discounted. Thus, concern is reasonable.

The interactions between climate, vectors, and pathogens are complex. For example, for certain mosquitoes increased temperatures boost the rate of reproduction and feeding, lengthen the breeding season, and shorten the maturation time for the pathogens they carry (Patz et al. 1996; Epstein et al. 1998; Epstein 2005). However, such complex relationships vary by locale and species. Temperatures that benefit mosquitoes may be detrimental to ticks; flooding may wash away larval stages of mosquitoes; drought may contribute to a decline in tick numbers. Rodent vectors, such as deer mice that carry Hantavirus, demonstrate equally variable relationships (Mordecai et al. 2012, 2017, 2019).

Lyme disease

According to the Centers for Disease Control and Prevention (CDC), Lyme disease is the most common vector-borne illness in the US, with approximately 300,000 total cases diagnosed annually (CDCa undated). Lyme disease is most prevalent in the Northeast, but has been spreading westward and northward into the Midwestern states and farther into Canada. The main vector, the deer or blacklegged tick, is not found in arid parts of the West, requiring more moisture to thrive. Although Lyme disease is the most common tick-borne disease in Montana—averaging 10 cases per year—all cases reported through 2018 came from people infected outside of Montana (MTDPHHSb undated).

West Nile virus

West Nile virus, first reported in the United States in 1999, is found in nearly every country and is the most common mosquito-borne illness in the United States, with 2647 cases and 167 deaths reported in 2018 (CDCb undated). West Nile virus is considered the most important cause of viral encephalitis globally (Chancey et al. 2015; Paz 2015), with increased spread projected with climate change (Paz 2015). Drier weather, especially drought, leads to  higher rates of West Nile virus infection in people (Wang et al. 2010; Paull et al. 2016).

Photo credit: James Gathany / Wikimedia


In the last several years, the northwest region of the US has seen an increase in some infectious diseases, including West Nile virus. The Washington Department of Health’s vector surveillance program observed earlier arrival of West Nile virus-carrying mosquitoes than some other states (likely associated with higher temperatures), and an increasing number of human infections, including some resulting in fatalities (WSDOH 2018). 


Figure 3-9. Map showing 2019 (last update 10/3/19) detections of West Nile virus in humans and horses, by county (MTDPHHSc undated).


Montana had no cases of West Nile virus until 2002. By 2018, the Montana Department of Public Health and Human Services (MTDPHHS) reported 51 cases across 18 counties. Nine counties reported mosquito pools[12] that tested positive for West Nile virus out of 30 counties tested (Figure 3-9). Three people have died from West Nile virus in Montana since it was first detected (MTDPHHSc undated). MCA projects longer droughts, warmer temperatures, and minimal change in average annual precipitation, likely concentrated in fewer precipitation events (Whitlock et al. 2017; also see Section 2), factors that favor both the virus and the primary vector, the Culex spp. mosquito (Epstein 2001; Harrigan et al. 2014; Paull et al. 2017). Thus, concern exists for climate change resulting in increased numbers of West Nile virus infections in Montana.



Climate Change and Mental Well-being—Perspectives from Montana Farmers and Ranchers
Meredith Howard, Selena Ahmed, Paul Lachapelle, and Mark B. Schure

While the physical and economic implications of climate change have been studied widely, including effects on agricultural systems, the mental health implications have been largely overlooked. Farmers and ranchers—who contribute to local, national, and global food security—represent an especially vulnerable and critical population for consideration. Montana State University researchers surveyed farmers and ranchers (n=125) in Montana using a mixed-method approach to investigate the relationship between climate change perceptions and mental well-being (Howard et al. 2020). Survey results revealed the following:

  • The majority of respondents (72%) agreed that climate change is having an adverse effect on their farm or ranch.
  • Nearly three quarters of respondents reported feeling moderate to high levels of anxiety when thinking about climate effects on their agricultural business.
  • The greater the reported perceived risk regarding climate change, the greater the level of reported anxiety.
  • The impact of climate change on farm/ranch profitability was perceived as a main cause of distress.

One Montana producer commented, “[I am] worried about crop losses of current and future years, and about where funds will come for dealing with climate change on the farm. As with most, my worries generally stem from financial stress.”

Another respondent shared, “Climate change contributes to my distress because it makes planning for long-term crops more difficult and predicting weather patterns less predictable.”

Public-health preparedness efforts are warranted to provide mental health support for the agricultural sector. Research and outreach efforts are further called for to promote the adoption of practices that mitigate climate risk and enhance personal and food-system resilience. Linkages between climate-induced anxiety and an increasingly aging and female farm and ranch population should also be studied. Policy discussions at various scales should be considered to address financial risk to agricultural enterprises in the face of increased climate variability.




Mental Health Concerns

The mental health impacts of climate change are profound and varied. Elevated temperatures have been related to worsening a) mental health status (Obradovich et al. 2018); b) diminished cognitive function (Laurent et al. 2018); c) increased violence (Clayton et al. 2017); d) increased interpersonal aggression in the form of domestic violence, abuse, and rapes (Hsiang et al. 2013); and e) suicide (Page et al. 2007). Even small increases in temperature, in one case comparing average monthly temperatures between 25-30°C (77-86°F) with those over 30°C (86°F),  can lead to significant increases in mental illness (Obradovich et al. 2018).

People with preexisting mental illness are at increased risk following weather-related events (Trombley et al. 2017), as are youth (Paulson et al. 2015; Orengo-Aguayo et al. 2019). Likewise, climate “surprises” (see Section 2) such as flooding (Lamond et al. 2015), wildfires, or storms have been shown to impact mental health. Those impacts include increases in post-traumatic stress disorder (PTSD), anxiety, depression, substance abuse, and suicidal thoughts. A sense of community loss and, in many cases, displacement can also result following sudden extreme events, or they may occur over time with slower, sustained climate change impacts (e.g., persistent drought or sea-level rise) (Hayes et al. 2018; Palinkas and Wong 2019).


Montana has one of the highest per capita suicide rates in the country (CDC 2018; AAS 2020a,b). This unfortunate reality, coupled with limited access to mental health professionals in most rural areas of the state, greatly heightens the importance of preparedness planning in Montana to address anxiety, stress, and other mental health issues exacerbated by climate change.


Montana has one of the highest per capita suicide rates in the country (CDC 2018; AAS 2020a,b), discussed in detail in Section 4. This unfortunate reality, coupled with limited access to mental health professionals in most rural areas of the state, greatly heightens the importance of preparedness planning in Montana to address anxiety, stress, and other mental health issues exacerbated by climate change (see sidebar). The Montana health workforce does not have widely available access to training on how to address climate anxiety and post-disaster trauma; nor does it have information on the tracking of climate stressors (Doppelt 2016).



[1] Montanans could feel indirect influences of sea-level rise, for example if coastal populations are forced to relocate inland or coastal grain export terminals important to the Montana economy must be moved.

[2] Dehydration, or the excessive loss of body water, can have many causes, including heat exposure, kidney disease, and diseases of the gastrointestinal tract that cause vomiting or diarrhea.

[3] Figure 2-1 provides a map showing and naming Montana’s 56 counties.

[4] Both heat index and general circulation models are described in Section 2.

[5] See Appendix A for a complete explanation of socioeconomic variables and their weighting to derive a heat vulnerability index.

[6] According to the US Environmental Protection Agency (USEPA undated): “Heat islands occur on the surface and in the atmosphere. On a hot, sunny summer day, the sun can heat dry, exposed urban surfaces, such as roofs and pavement, to temperatures 50–90°F (27–50°C) hotter than the air, while shaded or moist surfaces—often in more rural surroundings—remain close to air temperatures. Surface urban heat islands are typically present day and night, but tend to be strongest during the day when the sun is shining.”

[7] The USEPA developed the AQI scale as a simple way to convey how clean or polluted air is. The AQI scale has six levels: 0-50 good; 51-100 Moderate; 101-150 Unhealthy for sensitive groups; 151-200 Unhealthy; 201-300 Very unhealthy; 301-500 Hazardous.

[8] The report is based on data collected from 2016-2018. The ten Montana counties are Fergus, Flathead, Gallatin, Lewis and Clark, Lincoln, Missoula, Powder River, Ravalli, Rosebud, and Silver Bow.

[9] Ecological health may also be impacted, but here we focus on human health.

[10] Although commonly referred to as blue-green algae, cyanobacteria are a group of photosynthetic bacteria.

[11] Conversely, in some cases drought can increase the concentration of nutrients (Balla et al. 2011; Gooding et al. 2008).

[12] A mosquito pool is described by the West Umatilla Mosquito Control District as a collection of mosquitoes (usually about 50) of any given species or group (e.g., Culex spp. mosquitoes) that are likely to carry and potentially transmit a virus (WUMCD undated).



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Helena at -15oF (-26oC). Photo courtesy of Scott Bischke