WASHINGTON - 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.

“Our analysis shows a hotter future that’s hard to imagine today,” said Kristina Dahl, senior climate scientist at UCS and co-author of the report “Killer Heat in the United States: Climate Choices and the Future of Dangerously Hot Days.” “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 conditions could pose unprecedented health risks.”

In the U.S., these “off-the-charts” days now occur only in the Sonoran Desert—located on the border of southern California and Arizona—where historically fewer than 2,000 residents have been exposed to the equivalent of a week or more of these conditions per year on average. By midcentury, these “off-the-charts” conditions would extend to other parts of the country, and areas currently home to more than 6 million people would be subjected to them for the equivalent of a week or more per year on average. By late century this would increase to areas where more than 118 million people—over one-third of the U.S. population—live.

“We have little to no experience with ‘off-the-charts’ heat in the U.S.,” said Erika Spanger-Siegfried, lead climate analyst at UCS and report co-author. “These conditions occur at or above a heat index of 127 degrees, depending on temperature and humidity. Exposure to conditions in that range makes it difficult for human bodies to cool themselves and could be deadly.”

Overall, the study showed that the Southeast and Southern Great Plains would bear the brunt of the extreme heat. With no action to reduce emissions, areas of states in these regions would experience the equivalent of three months per year on average by midcentury that feel hotter than 105 degrees Fahrenheit, possibly as hot as 115 degrees, 125 degrees, or worse. In this time frame, parts of those regions and the Midwest would experience “off-the-charts” heat days for the first time. By late century, communities in each state in the contiguous U.S. would experience days with a heat index exceeding 105 degrees Fahrenheit, with nearly one-third of the population enduring the equivalent of two months of such heat. Similarly, “off-the-charts” heat days would spread to communities in 47 states.

In addition, the analysis found that by midcentury with no reduction in global emissions:

• Four hundred and one sizeable U.S. cities—places with more than 50,000 residents—would experience the equivalent of a month or more on average per year when the heat index exceeds 90 degrees Fahrenheit compared to 239 cities historically.
• Two hundred fifty-one of those cities would experience the equivalent of a month or more per year on average with a heat index surpassing 100 degrees Fahrenheit compared to just 29 historically.
• One hundred and fifty-two cities, and more than 90 million people nationwide, would experience a heat index over 105 degrees Fahrenheit for the equivalent of a month or more per year on average. Only three sizeable cities—Yuma, Ariz. and El Centro and Indio, Calif.—and fewer than 1 million people nationwide routinely experience such conditions today.
• More than 6 million people would experience “off-the-charts” heat days for the equivalent of a week or more per year on average.

According to the analysis, by late century with no reduction in global emissions:

• Nearly all sizeable cities in the country—469 out of 481—would endure the equivalent of a month or more per year on average when the heat index exceeds 90 degrees Fahrenheit. Of those, 389 cities would experience the equivalent of a month or more per year with a heat index above 100 degrees Fahrenheit.
• About 300 cities—and more than 180 million people nationwide—would experience the equivalent of a month or more per year on average with a heat index exceeding 105 degrees Fahrenheit.
• Nearly two-thirds of the country by area would endure “off-the-charts” heat days at least once a year on average, with nearly 120 million people—more than one-third of the contiguous U.S. population—experiencing the equivalent of a week or more per year on average of these unprecedented conditions.
• Cities experiencing the most “off-the-charts” heat days would be: Yuma, Ariz. (46); El Centro-Calexico, Calif. (45); Casa Grande, Ariz. (40); Avondale-Goodyear, Ariz. (38); Indio-Cathedral City; Calif. (37); Phoenix-Mesa, Ariz. (32); Brownsville, Texas (31); Lake Jackson-Angleton, Texas (27); Lake Havasu City, Ariz. (26); Alexandria, La. (24); Conroe-The Woodlands, Texas (24); Harlingen, Texas (24); and Victoria, Texas (24).
• 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 with a heat index above 105 degrees Fahrenheit, on average, and almost 115 million fewer people would experience the equivalent of a week or more of “off-the-charts” heat days.

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. 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 rise in days with extreme heat will change life as we know it nationwide, but with significant regional differences,” said Rachel Licker, senior climate scientist at UCS and report co-author. “For example, in some regions currently unaccustomed to extreme heat—those such as the upper Midwest, Northeast and Northwest—the ability of people and infrastructure to cope with it is woefully inadequate. At the same time, people in states already experiencing extreme heat—including in the Southeast, Southern Great Plains and Southwest—have not seen heat like this. By late century, they may have to significantly alter ways of life to deal with the equivalent of up to five months a year with a heat index above—often way above—105 degrees. We don’t know what people would be able and willing to endure, but such heat could certainly drive large-scale relocation of residents toward cooler regions.”

The report notes that the rising heat could particularly affect outdoor workers and thus sectors depending on their labor.

“By the end of the century, on most days between April and October, construction workers in parts of Florida won’t be able to safely work outside during the day because the heat index would exceed 100 degrees,” said Dahl. “Likewise, agricultural centers such as Illinois and California’s Central Valley could struggle to keep farm workers safe, with the heat index exceeding 90 degrees and 100 degrees, respectively, for the equivalent of about three months a year. If farm workers are unable to work as a result of extreme heat, this could affect the productivity of farming enterprises.”

People exposed to the same heat event can have different levels of heat-related health risk, with children, elderly adults, people with special needs, and outdoor workers having higher risks of heat-related illness and death. City-dwellers contend with the urban heat island effect—a phenomenonwhere where heat-retaining materials and surfaces drive up temperatures, particularly at night—which can increase rates of heat-related illness. Meanwhile, residents of some rural areas may face higher risk of heat-related hospitalization and death given their distance from cooling centers and medical facilities.

“Low-income communities, communities of color and other vulnerable populations may be particularly at risk when exposed to extreme heat,” said Juan Declet-Barreto, climate scientist at UCS and report co-author. “Longstanding social and economic inequities have led to these communities often having more limited access to transportation, cooling centers, and health care, and they may lack air conditioning, or the financial resources to run it.”

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. 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.

“The best ways to avoid the worst impacts of an overheated future are to enact policies that rapidly reduce global warming emissions and to help communities prepare for the extreme heat that is already inevitable,” said Astrid Caldas, senior climate scientist at UCS and report co-author. “Extreme heat is one of the climate change impacts most responsive to emissions reductions, making it possible to limit how extreme our hotter future becomes for today’s children.”

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.

“To ensure a safe future, elected officials urgently need to transform our existing climate and energy policies,” said Rachel Cleetus, lead economist and policy director at UCS and report co-author. “Economists have 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.”

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.