If you grew up in New York City in the 1990s, you might have experienced a couple of days each year when the heat index—the combination of heat and humidity that explains how a temperature feels—climbed above 100 degrees. By the end of the century, it could feel that way as many as 42 days a year. In Atlanta, the number of days with a 100-degree-plus heat index could jump from 6 to 82; in Miami, it could go from 16 to 153.
A new report and a peer-reviewed study mapped out how extreme heat could increase across the country if global emissions continue on their current trajectory. “We know that extreme heat events are becoming more extreme and more frequent,” says Erika Spanger-Siegfried, lead climate analyst at the Union of Concerned Scientists, the nonprofit that worked on the study. “We wanted people to be able to see . . . what’s next, and to be able to take action to steer us in a different direction.”
By the middle of the century, under a business-as-usual scenario, the average number of days that feel hotter than 100 degrees would more than double. The average number of days that feel hotter than 105 would more than quadruple by midcentury, and by the end of the century, there would be eight times more than there have been historically. An interactive map shows the impacts in detail across the country, and a search tool shows the details for specific cities.
Extreme heat will hit the South hardest, where some areas could experience the equivalent of three months a year that feel hotter than 105 degrees. But the heat will impact almost everyone. By the middle of the century, if greenhouse gas emissions continue, around 400 cities will feel hotter than 90 degrees more than 30 days a year. Two hundred and fifty of those cities will feel hotter than 100 degrees for the equivalent of a month or more. Many areas will experience days so hot they’re “off the charts,” or hotter than the National Weather Service’s heat index, which currently tops out around 127 (depending on the combination of temperature and humidity).
“The kind of heat we’re talking about is not just an inconvenience, it’s dangerous,” says Spanger-Siegfried. Extreme heat kills more people in the United States than any other type of weather disaster. Even in areas where people are accustomed to heat, people may not be able to work outside. Children may not be able to play outside. In areas that have historically been cooler, air-conditioning may not be common.
The impacts of climate change are already apparent. Last month, the hottest June on record, cities that aren’t known for heat, like San Francisco, suffered through heat waves. This weekend, New York City could feel like it’s 110 degrees. And even with strong climate action now, it’s still going to get hotter. But the researchers point out that extreme heat—unlike other effects of climate change, like sea-level rise—does respond quickly to changes in emissions.
Right now, the U.S. isn’t on track to make the changes that are needed. “Many states and municipalities are struggling mightily to bring really important policies online to reduce emissions, but at the federal level, we are actively rolling back our most important policies to reduce emissions in the power sector, in the transportation sector,” says Spanger-Siegfried. “What we do at the state and local level is not enough if we’re rowing in the opposite direction at the federal level.”
The difference between a world in which emissions drop now and the current path is stark: Of 481 large cities in the U.S., without climate action, 204 cities would face a week or more of “off the charts” extreme heat, with a heat index over 127 degrees, by the middle of the century. Even with just slow climate action, the number of cities facing those temperatures would drop from 204 to just seven.