Reframing future risks of extreme heat in the United States

Phase 5 models to examine future exposure to extreme heat for global average temperatures of 1.5, 2, 3, and 4 degrees C above a preindustrial baseline. We find that throughout the United States, historically rare extreme heat events become increasingly common in the future as global temperatures rise and that the depiction of exposure depends in large part on whether extreme heat is defined by absolute or relative metrics.

For example, for a 4 degrees C global temperature rise, parts of the country may never see summertime temperatures in excess of 100 degrees F, but virtually all of the country is projected to experience more than 4 weeks per summer with temperatures exceeding their historical summertime maximum. All of the extreme temperature metrics we explored become more severe with increasing global average temperatures. However, a moderate climate scenario delays the impacts projected for a 3 degrees C world by almost a generation relative to the higher scenario and prevents the most extreme impacts projected for a 4 degrees C world.