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