Climate change is making the planet warmer, but a new report says there's something worse on the horizon: extreme heat.
A study by the Union of Concerned Scientists published Tuesday found the number of days in Florida with a heat index over 100 degrees Fahrenheit would more than quadruple to 105 days by mid century. The number would rise to 144 days by the end of the century, the report found.
Nationwide, the number of days would double by mid century from averages recorded between 1971 and 2000.
That misery would be felt by more and more people as cooler places grow hotter, said Juan Declet-Barreto, an environmental social scientist with the Union of Concerned Scientists and the study's lead author.
"If we don't take rapid action on climate now, many more people in the U.S. are going to be experiencing many more days of killer heat," he said.
The number of people experiencing 30 days or more with a heat index over 105 degrees will jump from 900,000 to 90 million - a third of the nation's population, the study said. That includes 17 million people in Florida. Historically, that number fell well below 50,000, the report said.
The study was published Tuesday in the peer-reviewed journal Environmental Research Communications. It used data from 18 different models that forecast temperature and humidity in cities over 50,000. While other studies have examined increased temperatures driven by a warming planet, Declet-Barreto said this study differs because it incorporates humidity, which can make it feel hotter. In its heat warnings, the National Weather Service uses the same heat index because it more accurately reflects risks, the study said.
"Think of...experiencing a temperature of 80 degrees in a dry, hot place like Arizona. And then think about what that temperature would feel like in a place like Florida, when you factor in the humidity of 68%," he said. "That feels like temperature becomes a lot higher. "
Extreme heat is considered the leading cause of weather-related fatalities, the study said. Between 1999 and 2010, more than 7,400 deaths in the U.S. were blamed on heat.
It's particularly dangerous during Florida's hot summers when hurricanes can trigger widespread power outages. After Irma, the report noted 17 deaths were blamed on extreme heat, including 14 residents at a Broward County nursing home.
This summer has also been a bruising one for South Florida. University of Miami reseacher Brian McNoldy, who tracks temperature records, tweeted that Miami reached 98 degrees twice so far this year for the first time on record. Temperatures topped 95 degrees on 10 days by July 2.
A certain amount of increased temperatures is already locked into the atmosphere because of increased emissions. But the study found that if temperature increases can be kept to the 2-degree Celsius limit laid out in the Paris climate accord, the U.S. would see half as many more days of extreme heat.
"We can’t just think that we can delay action until the late century," Declet-Barreto said, "because that is just going to get us into a catastrophic consequences."