Killer Heat in the United States: Climate Choices and the Future of Dangerously Hot Days

This UCS analysis provides a detailed view of how extreme heat events caused by dangerous combinations of temperature and humidity are likely to become more frequent and widespread in the United States over this century. It also describes the implications for everyday life in different regions of the country.We have analyzed where and how often in the contiguous United States the heat index—also known as the National Weather Service (NWS) "feels like" temperature—is expected to top 90°F, 100°F, or 105°F during future warm seasons (April through October). While there is no one standard definition of "extreme heat," in this report we refer to any individual days with conditions that exceed these thresholds as extreme heat days. We also analyzed the spread and frequency of heat conditions so extreme that the NWS formula cannot accurately calculate a corresponding heat index. The "feels like" temperatures in these cases are literally off the charts.We have conducted this analysis for three global climate scenarios associated with different levels of global heattrapping emissions and future warming. These scenarios reflect different levels of action to reduce global emissions, from effectively no action to rapid action. Even the scenario of rapid action to reduce emissions does not spare our communities a future of substantially increased extreme heat. For the greatest odds of securing a safe climate future for ourselves and the ecosystems we all depend on, we would need to take even more aggressive action, in the US and globally, than outlined in any of the scenarios used here. Our challenge is great, but the threat of not meeting it is far greater.