han 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 adaptat