Killer Heat in the United States

Climate Choices and the Future of Dangerously Hot Days

Published Jul 2, 2019


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 we act now to reduce heat-trapping emissions.

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

Dangerous heat

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 our interactive tool.


Dahl, Kristina, Erika Spanger-Siegfried, Rachel Licker, Astrid Caldas, John Abatzoglou, Nicholas Mailloux, Rachel Cleetus, Shana Udvardy, Juan Declet-Barreto, and Pamela Worth. 2019. Killer Heat in the United States: Climate Choices and the Future of Dangerously Hot Days. Cambridge, MA: Union of Concerned Scientists.