Hundreds of U.S. cities could see an entire month each year with heat-index temperatures soaring above 100 degrees by 2050 if global warming isn’t curtailed, a terrifying new study has predicted. “Our analysis shows a hotter future that’s hard to imagine today,” said study co-author Kristina Dahl, climate scientist at the Union of Concerned Scientists. “Nearly everywhere, people will experience more days of dangerous heat in the next few decades.” The heat index is what the temperature feels like to the human body when humidity is combined with the air temperature. The study predicts the number of days each year when it feels like it’s more than 100 degrees will more than double nationally, with conditions becoming so extreme on some days that they’d break the current upper limit of the heat index. “We have little to no experience with ‘off-the-charts’ heat in the U.S.,” said co-author Erika Spanger-Siegfried, adding they “could be deadly.”