July 16, 2019

Southwest Region Areas to Endure Seven Weeks or More a Year When “Feels Like” Temperature Exceeds 105 Degrees

Dangerous Heat to Soar Across Entire US “Breaking” National Weather Service Heat Index Scale, Posing Unprecedented Health Risks

WASHINGTON (July 16, 2019)—Increases in potentially lethal heat driven by climate change will affect every state in the contiguous U.S. in the decades ahead, according to a new report and accompanying peer-reviewed study in Environmental Research Communications, both by the Union of Concerned Scientists, released today. Few places would be unaffected by extreme heat conditions by midcentury and only a few mountainous regions would remain extreme heat refuges by the century’s end.

Without global action to reduce heat-trapping emissions, the number of days per year when the heat index—or “feels like” temperature—exceeds 100 degrees Fahrenheit would more than double from historical levels to an average of 36 across the country by midcentury and increase four-fold to an average of 54 by late century. The average number of days per year nationwide with a heat index above 105 degrees Fahrenheit would more than quadruple to 24 by midcentury and increase eight-fold to 40 by late century.

The analysis, titled “Killer Heat in the United States: Climate Choices and the Future of Dangerously Hot Days” shows a hotter future that’s hard to imagine today. Nearly everywhere, people will experience more days of dangerous heat even in the next few decades. By the end of the century, with no action to reduce global emissions, parts of Florida and Texas would experience the equivalent of at least five months per year on average when the “feels like” temperature exceeds 100 degrees Fahrenheit, with most of these days even surpassing 105 degrees. On some days, conditions would be so extreme that they exceed the upper limit of the National Weather Service heat-index scale and a heat index would be incalculable. Such “off-the-charts” conditions could pose unprecedented health risks.

The analysis calculated the frequency of days with heat index thresholds above 90 degrees Fahrenheit—the point at which outdoor workers generally become susceptible to heat-related illness—as well as above 100 and 105 degrees Fahrenheit, when the National Weather Service (NWS) generally recommends issuing heat advisories and excessive heat warnings, respectively. The number of high heat-index days was calculated by averaging projections from 18 high-resolution climate models between April and October. The report looked at these conditions for three possible futures. The “no action scenario” assumes carbon emissions continue to rise and the global average temperature increases nearly 4.3 degrees Celsius (about 8 degrees Fahrenheit) above pre-industrial levels by century’s end. The “slow action scenario” assumes carbon emissions start declining at midcentury and the global average temperature rises 2.4 degrees Celsius (4.3 degrees Fahrenheit) by century’s end. In the “rapid action scenario,” global average warming is limited to 2 degrees Celsius (3.6 degrees Fahrenheit)—in line with the Paris Agreement. The analysis also examined the number of people in each state that would be exposed to a heat index above 90 degrees Fahrenheit, 100 degrees Fahrenheit, 105 degrees Fahrenheit, or off-the-charts conditions for an average equivalent of at least one week, one month, or two months per year. All population data presented here, including for future projections, is based on the most recent U.S. Census conducted in 2010 and does not account for population growth or changes in distribution.

The U.S. Southwest region, as defined by the latest U.S. National Climate Assessment, would see an average of 35 days per year with a heat index above 100 degrees Fahrenheit, 22 days per year with a heat index above 105 degrees Fahrenheit and 10 “off-the-charts” days per year by the end of the century if no action is taken to reduce global warming emissions, with Arizona, California and New Mexico seeing the largest rise in extreme heat. The sobering results for all Southwest states, assuming no global action to reduce emissions, are below.

Arizona

  • Historically, there have been 52 days per year on average with a heat index above 90 degrees Fahrenheit, the worker safety threshold. This would increase to 90 days per year on average by midcentury and 119 by the century’s end.
  • Historically, there have been 15 days per year on average with a heat index above 100 degrees Fahrenheit. This would increase to 37 days per year on average by midcentury and 56 by the century’s end. Of the cities with a population of 50,000 or more in the state, Avondale-Goodyear, Casa Grande and Yuma would experience the highest frequency of these days. Limiting warming to 2 degrees Celsius above pre-industrial levels would cap the frequency of such days at an average of 31 per year on average.
  • By the end of the century, nearly 5.8 million people would be exposed to a heat index above 100 degrees Fahrenheit for the equivalent of two months or more per year. By limiting warming to 2 degrees Celsius, roughly 510,000 of those residents would avoid such days of extreme conditions.
  • Historically, there has been an average of five days per year with a heat index above 105 degrees Fahrenheit. This would increase to 21 days per year on average by midcentury and 35 by the century’s end. Limiting warming to 2 degrees Celsius above pre-industrial levels would cap the frequency of such days at 17 per year on average.
  • By the end of the century, an estimated 5.7 million people would be exposed to a heat index above 105 degrees Fahrenheit for the equivalent of a month or more per year. By limiting warming to 2 degrees Celsius, roughly 1 million of those residents would avoid such days of extreme conditions.
  • Historically, the state as a whole has experienced zero “off-the-charts” heat days in an average year. This would increase to four days per year on average by midcentury and nine by the end of the century. Limiting warming to 2 degrees Celsius would cap the frequency of such days at three per year on average. 
  • By the end of the century, an estimated 5.5 million people would endure “off-the-charts” heat days for the equivalent of a week or more per year. Historically, fewer than 2,000 people nationwide have experienced such conditions in an average year. By limiting warming to 2 degrees Celsius, nearly 1.5 million of those residents would avoid such days of extreme conditions.

California

  • Historically, there have been 40 days per year on average with a heat index above 90 degrees Fahrenheit, the worker safety threshold. This would increase to 68 days per year on average by midcentury and 92 by the century’s end.
  • Historically, there have been nine days per year on average with a heat index above 100 degrees Fahrenheit. This would increase to 26 days per year on average by midcentury and 43 by the century’s end. Of the cities with a population of 50,000 or more in the state, El Centro-Calexico, Indio-Cathedral City and Yuma would experience the highest frequency of these days. Limiting warming to 2 degrees Celsius above pre-industrial levels would cap the frequency of such days at an average of 21 per year.
  • By the end of the century, an estimated 10.7 million people would be exposed to a heat index above 100 degrees Fahrenheit for the equivalent of two months or more per year. By limiting warming to 2 degrees Celsius, roughly 10 million of those residents would avoid such days of extreme conditions. 
  • Historically, there has been an average of three days per year with a heat index above 105 degrees Fahrenheit. This would increase to 13 days per year on average by midcentury and 25 by the century’s end. Limiting warming to 2 degrees Celsius above pre-industrial levels would cap the frequency of such days at an average of nine per year.
  • By the end of the century, an estimated 11 million people would be exposed to a heat index above 105 degrees Fahrenheit for the equivalent of a month or more per year. By limiting warming to 2 degrees Celsius, roughly 10.3 million of those residents would avoid such days of extreme conditions.
  • Historically, the state as a whole has experienced zero “off-the-charts” heat days in an average year. This would increase to two days per year on average by midcentury and five by the end of the century. Limiting warming to 2 degrees Celsius would cap the frequency of such days at one per year on average. 
  • By the end of the century, an estimated 5.1 million people would endure “off-the-charts” heat days for the equivalent of a week or more per year. Historically, fewer than 2,000 people nationwide have experienced such conditions in an average year. By limiting warming to 2 degrees Celsius, roughly 4.5 million California residents would avoid such days of extreme conditions.

Colorado

  • Historically, there have been seven days per year on average with a heat index above 90 degrees Fahrenheit. This would increase to 26 days per year on average by midcentury and 48 by the century’s end. Limiting warming to 2 degrees Celsius above pre-industrial levels would cap the frequency of such days at 22 per year on average.
  • By the end of the century, an estimated 3.9 million people would be exposed to a heat index above 90 degrees Fahrenheit for the equivalent of two months or more per year. By limiting warming to 2 degrees Celsius, roughly 3.7 million of those residents would avoid such days of extreme conditions.
  • Historically, there have been zero days per year on average with a heat index above 100 degrees Fahrenheit. This would increase to three days per year on average by midcentury and 12 by the century’s end. Of the cities with a population of 50,000 or more in the state, Grand Junction, Greeley and Pueblo would experience the highest frequency of these days. Limiting warming to 2 degrees Celsius above pre-industrial levels would cap the frequency of such days at one per year on average.
  • By the end of the century, an estimated 270,000 people would be exposed to a heat index above 100 degrees Fahrenheit for one month or more per year. By limiting warming to 2 degrees Celsius, all residents would avoid such days of extreme conditions. 

Nevada

  • Historically, there have been 12 days per year on average with a heat index above 90 degrees Fahrenheit. This would increase to 39 days per year on average by midcentury and 68 by the century’s end.
  • Historically, there has been one day per year on average with a heat index above 100 degrees Fahrenheit. This would increase to six days per year on average by midcentury and 16 by the century’s end. Of the cities with a population of 50,000 or more in the state, Las Vegas-Henderson would experience the highest frequency of these days. Limiting warming to 2 degrees Celsius above pre-industrial levels would cap the frequency of such days at four per year on average.
  • By the end of the century, an estimated 2 million people would be exposed to a heat index above 100 degrees Fahrenheit for the equivalent of two months or more per year. By limiting warming to 2 degrees Celsius, roughly 1.1 million of those residents would avoid such days of extreme conditions. 
  • Historically, there has been an average of zero days per year with a heat index above 105 degrees Fahrenheit. This would increase to two days per year on average by midcentury and five by the century’s end. Limiting warming to 2 degrees Celsius above pre-industrial levels would cap the frequency of such days at one per year on average.
  • By the end of the century, an estimated 2 million people would be exposed to a heat index above 105 degrees Fahrenheit for the equivalent of a month or more per year. By limiting warming to 2 degrees Celsius, roughly 1.6 million of those residents would avoid such days of extreme conditions.
  • By the end of the century, an estimated 1.3 million people, mostly in the southernmost part of the state and including the Las Vegas-Henderson metro area, would endure “off-the-charts” heat days for the equivalent of a week or more per year. Historically, fewer than 2,000 people nationwide have experienced such conditions in an average year. By limiting warming to 2 degrees Celsius, nearly all Nevada residents would avoid such days of extreme conditions.

New Mexico

  • Historically, there have been 17 days per year on average with a heat index above 90 degrees Fahrenheit. This would increase to 51 days per year on average by midcentury and 86 by the century’s end.
  • Historically, there have been zero days per year on average with a heat index above 100 degrees Fahrenheit. This would increase to eight days per year on average by midcentury and 25 by the century’s end. Of the cities with a population of 50,000 or more in the state, Albuquerque, Las Cruces and Los Lunas would experience the highest frequency of these days. Limiting warming to 2 degrees Celsius above pre-industrial levels would cap the frequency of such days at four per year on average.
  • By the end of the century, an estimated 1 million people would be exposed to a heat index above 100 degrees Fahrenheit for the equivalent of one month or more per year. By limiting warming to 2 degrees Celsius, roughly 930,000 of these residents would avoid such days of extreme conditions.
  • Historically, there has been an average of zero days per year with a heat index above 105 degrees Fahrenheit. This would increase to one day per year on average by midcentury and nine by the century’s end. Limiting warming to 2 degrees Celsius above pre-industrial levels would cap the frequency of such days at one per year on average.
  • By the end of the century, an estimated 230,000 people would be exposed to a heat index above 105 degrees Fahrenheit for the equivalent of a month or more. By limiting warming to 2 degrees Celsius, all residents would avoid such days of extreme conditions.

Utah

  • Historically, there have been 10 days per year on average with a heat index above 90 degrees Fahrenheit. This would increase to 38 days per year on average by midcentury and 65 by the century’s end.
  • Historically, there have been zero days per year on average with a heat index above 100 degrees Fahrenheit. This would increase to four days per year on average by midcentury and 15 by the century’s end. Of the cities with a population of 50,000 or more in the state, Provo-Orem, Salt Lake City-West Valley City and St. George would experience the highest frequency of these days. Limiting warming to 2 degrees Celsius above pre-industrial levels would cap the frequency of such days at two per year on average.
  • By the end of the century, an estimated 760,000 people would be exposed to a heat index above 100 degrees Fahrenheit for the equivalent of a month or more per year. By limiting warming to 2 degrees Celsius, roughly 670,000 of those residents would avoid such days of extreme conditions. 
  • Historically, there has been an average of zero days per year with a heat index above 105 degrees Fahrenheit. This would increase to one day per year on average by midcentury and four by the century’s end. Limiting warming to 2 degrees Celsius above pre-industrial levels would cap the frequency of such days at zero per year on average.
  • By the end of the century, an estimated 110,000 people would be exposed to a heat index above 105 degrees Fahrenheit for the equivalent of a month or more. By limiting warming to 2 degrees Celsius, all residents would avoid such days of extreme conditions.

The report clearly shows how actions taken, or not taken, within the next few years to reduce emissions will help determine how hot and humid our future becomes. If the goal of the Paris Agreement is met and future global average warming is limited to 2 degrees Celsius, by late century the United States would see half the number of days per year, on average, with a heat index above 105 degrees Fahrenheit, and almost 115 million fewer people would experience the equivalent of a week or more of “off-the-charts” heat days. The longer the U.S. and other countries wait to drastically reduce emissions, the less feasible it will be to realize the “rapid action scenario” analyzed.

Governors and state legislators have begun moving toward 100 percent clean energy and Congress is considering a range of energy and climate policies—including renewable energy standards, climate resilient infrastructure and innovation incentives, which may see bipartisan support—that could help keep the worst at bay. Economists have also advised putting a price on carbon emissions to properly account for damages from the fossil-fuel-based economy and signal intentions to protect the environment.

In addition, the report includes a range of preparedness recommendations for governments, including: investing in heat-resilient infrastructure; creating heat adaptation and emergency response plans; expanding funding for programs to provide cooling assistance to low- and fixed-income households; directing the Occupational Safety and Health Administration (OSHA) to set up protective occupational standards for workers during extreme heat; requiring utilities to keep power on for residents during extreme heat events; and investing in research, data tools and public communication to better predict extreme heat and keep people safe.

To view the report PDF, click here.

Spreadsheets with our data on extreme heat are available and can be sorted by city, by county, by state, by region and by population

To get the results for your city or county by using our interactive widget, click here.

To use the interactive mapping tool, click here. The map allows you to learn more about extreme heat in specific counties. When you zoom in, the maps become more detailed.

For all other materials, including regional press releases, our methodology document and Spanish-language materials, click here

To listen to the latest UCS “Got Science” podcast on this new report, click here.

The Union of Concerned Scientists puts rigorous, independent science to work to solve our planet's most pressing problems. Joining with people across the country, we combine technical analysis and effective advocacy to create innovative, practical solutions for a healthy, safe, and sustainable future.