In less than 20 years, millions of people in the United States could be exposed to dangerous “off-the-charts” heat conditions of 127 degrees Fahrenheit or more, a startling new report has found. In 60 years over one-third of the population could be exposed to such conditions, “posing unprecedented health risks,” the report says.

This first United States county-by-county look at what climate change will do to temperature and humidity conditions in the coming decades finds few places that won’t be affected by extreme heat.

“We were very surprised at how steeply and quickly the number of days of dangerous heat increased in such a short time,” says Kristina Dahl, senior climate scientist at Union of Concerned Scientists (UCS), and co-author of