Right now, there are only a few pockets in the United States where it is possible for the heat index to rise above 125 degrees Fahrenheit — a particularly dangerous threshold for human health. But by mid-century, a much larger area is at risk, sprawling from the Gulf Coast across a swath of the middle of the country, reaching as far north as southern Wisconsin.
A new report maps out where that might happen, along with an increased risk of more ordinary (but still risky) extreme heat, heatwaves and temperatures rising outside local norms. In a new tool, you can enter any US address and see both the risk from heat in your neighborhood now and in the middle of the century. It’s part of Risk Factor, a broader climate risk tool that also shows the risk from floods or wildfires for any US address.
The researchers call the area exposed to the worst case the extreme heat belt. “Currently, only about 50 counties have any estimate of reaching the 125-degree heat index in today’s environment,” says Jeremy Porter, director of research at the First Street Foundation, the nonprofit that created the tool. “But by 2053, that’s going to grow to about 1,000 counties . . . you go from having about 8 million people today with potential exposure to that level of heat to about 108 million in 30 years.”
When it gets this hot, it’s dangerous to be outside. “A heat index of 125 is the level at which our body is effectively unable to cool down over an extended period of time,” says Zachary Schlader, a professor at the Indiana University School of Public Health who studies the effects of heat on humans. body. “What would happen is that the temperature inside the body would just continue to rise, unless you were able to escape that environment.”
Of course, lower heat levels can also be fatal for vulnerable people, or people who have to work outside. And it’s already very hot. The summer of 2022 has broken heat records across the United States. In Salt Lake City, temperatures rose above 100 degrees on 18 days in July. Austin had 29 triple-digit days that month (and another 22 in May and June). Newark, New Jersey, has gone through four consecutive heat waves since late June. And cities like Boston, Denver and Portland, Maine, have broken daily heat records.
But as climate change continues to make heat waves longer and more intense, this summer is also likely to be cooler than most you’ll experience in your lifetime. The tool shows both the current risk from heat in American cities, and how the heat will increase in each location. “Heat ends up killing more people than any other [weather disaster]” says Porter. By one estimate, heat contributed to 1,577 deaths in the United States in 2021, but the number could be even higher; heat worsens other conditions, such as heart disease, and the link to heat is often not officially reported as a cause of death.
In the report, the research team looked at how many days each year each place has a heat index above 100 degrees. They also calculated how the number of these “dangerous days” will change by 2053, based on datasets from the federal government and others, combined with heat modeling. In Miami-Dade County, for example, there will be 41 additional three-digit days each year by mid-century. The number of heat waves, with several days of extreme heat, will also increase.
The researchers also mapped “local hot days,” or days above the local 98th percentile temperature. Someone who lives in a place that is typically cooler, such as Seattle, may start to see health effects at a lower temperature than someone who lives in Phoenix who is more used to heat (the person who lives in a historically cooler area is also less likely to have air conditioning). Someone who plans to move can use the tool to reconsider. For each address, the report also calculates how much more someone is likely to spend on air-conditioning bills in the future. “Your energy bills are going to be a lot higher,” says Porter.