The United States is facing a potentially staggering expansion of dangerous heat over the coming decades. 

This analysis shows the rapid, widespread increases in extreme heat that are projected to occur across the country due to climate change, including conditions so extreme that a heat index cannot be measured. The analysis also finds that the intensity of the coming heat depends heavily on how quickly people act now to reduce heat-trapping emissions. 

The results highlight a stark choice: People can continue on the current path, where people fail to reduce emissions and extreme heat soars. Or people can take bold action now to dramatically reduce emissions and prevent the worst from becoming reality. 

For this national analysis, extreme heat is measured according to the heat index, the combination of temperature and humidity that creates the “feels like” temperature. 

The analysis includes four different heat index thresholds, each of which brings increasingly dangerous health risks: above 90°F, above 100°F, above 105°F, and "off the charts." (Off-the-charts days are so extreme they exceed the upper limits of the National Weather Service heat index scale, which starts topping out at or above a heat index of 127°F, depending on the combination of temperature and humidity.)

The report features three time frames—historical, midcentury, and late century—and three different scenarios of climate action. 

Location-specific results can be found using the interactive tool. Complete findings are available in the full report.