The Union of Concerned Scientists released a report entitled “Killer Heat in the United States: Climate Choices and the Future of Dangerously Hot Days.” The report, funded in part by the MacArthur and Rockefeller Foundations, sought to estimate the frequency of days in which the heat index would rise above 90°F, 100°F or 105°F in the United States. The report also estimated the distribution and frequency of heat conditions so severe that the National Weather Service (NWS) would be unable to calculate a heat index.
In conducting analysis, researchers relied on eighteen global climate models to estimate fluctuations in the heat index. These models where then simplified to only consider the contiguous United States, and examined in the context of different climate scenarios in which varying action is taken to reduce greenhouse gas emissions. The three scenarios were based on representative concentration pathways (RCPs), used in the 5th Assessment Report of the Intergovernmental Panel on Climate Change from 2014. The scenarios helped estimate heat indexes for situations in which no action is taken to reduce global greenhouse gas emissions, slow action is taken, and rapid action is taken.
Researchers used the models to calculate the daily maximum heat indexes during the period from 2006 – 2099, in the months from April-October. This information was then used to estimate the number of days in which the heat index would be above 90°F, 100°F or 105°F in addition to the number of days in which the heat index would not be precisely measured.
Figure 1. How Temperature and Humidity Create the Heat Index
Heat is more harmful to human health when humidity is high because humid air hinders the evaporation of sweat, and thus reduces the body’s ability to cool itself. To determine the effect of both heat and humidity, the National Weather Service formulated the heat index based on the range of warm-season conditions we typically see on Earth. As our climate warms, we will increasingly find ourselves outside the range of reliably calculable heat index values, or, quite literally, off the charts. Colors reflect the categories of heat index conditions examined in this study.
A Snapshot of Results
Results show that, with no action to reduce heat-trapping emissions, by mid-century (2036–2065), the following changes would be likely in the United States, compared with average conditions in 1971–2000:
- The average number of days per year with a heat index above 100°F will more than double, while the number of days per year above 105°F will quadruple.
- More than one-third of the area of the United States will experience heat conditions once per year, on average, that are so extreme they exceed the current NWS heat index range—that is, they are literally off the charts.
- Nearly one-third of the nation’s 481 urban areas with a population of 50,000 people or more will experience an average of 30 or more days per year with a heat index above 105°F, a rise from just three cities historically (El Centro and Indio, California, and Yuma, Arizona).
- Assuming no changes in population, the number of people 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 U.S. population.
- Countrywide, more than 1,900 people per year have historically been exposed to the equivalent of a week or more of off-the-charts heat conditions; this number is projected to rise to more than 6 million people by mid- century—again, assuming no population changes.
Late in the century (2070–2099), with no action to reduce heat-trapping emissions, the following changes can be expected:
- The United States will experience, on average, four times as many days per year with a heat index above 100°F, and nearly eight times as many days per year above 105°F, as it has historically.
- At least once per year, on average, more than 60 percent of the United States by area will experience off-the- charts conditions that exceed the NWS heat index range and present mortal danger to people.
- More than 60 percent of urban areas in the United States—nearly 300 of 481—will experience an average of 30 or more days with a heat index above 105°F.
- The number of people who experience those same conditions—still assuming no population change—will increase to about 180 million people, roughly 60 percent of the population of the contiguous United States.
- The number of people exposed to the equivalent of a week or more of off-the-charts heat conditions will rise to roughly 120 million people, more than one-third of the population. 
Figure 2. Millions More People Will Face Extreme Heat by Midcentury
Taking no action or slow action to reduce global heat-trapping emissions would expose millions more US residents to an average of seven or more days per year of extreme heat index conditions by midcentury, even when assuming no changes in population
Findings from the report revealed that rapid action to reduce greenhouse gas emissions would allow many to avoid extreme heat conditions, while inaction would yield a grim future in which, “few refuges from extreme heat would remain.”
The report also highlighted how some will be more impacted by extreme heat conditions compared to others. Among the most at risk will be outdoor workers, low income urban communities, and individuals in rural areas. Researchers also noted that extreme heat events will become increasingly dangerous when occurring in conjunction with natural disasters, as severe heat will compound the damage associated with events like hurricanes and flooding.
In discussing ways to limit the impacts of extreme heat, the report outlined recommendations for the U.S. government, beginning with investment in heat-smart infrastructure and climate-smart power systems. The report notes, however, that the most impactful measure to limit extreme heat will emerge from concrete plans to reduce greenhouse gas emissions.
The full report can be accessed here:
 Kristina Dahl et al., Killer Heat in the United States: Climate Choices and the Future of Dangerously Hot Days (Union of Concerned Scientists, 2019), [pg.3], https://www.ucsusa.org/sites/default/files/attach/2019/07/killer-heat-analysis-full-report.pdf.