Re-Framing Future Risks of Extreme Heat in the United States

The goal of this study is to reframe the analysis and discussion of extreme heat projections to improve communication of future extreme heat risks in the United States. We combine existing data from 31 of the Coupled Model Intercomparison Project, Phase 5 (CMIP5) models to examine future exposure to extreme heat for global average temperatures of 1.5°C, 2°C, 3°C, and 4°C above a pre-industrial 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°C global temperature rise, parts of the country may never see summertime temperatures in excess of 100°F, but virtually all of the country is projected to experience more than four 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°C world by almost a generation relative to the higher scenario, and prevents the most extreme impacts projected for a 4°C world.