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extreme heat, climate change, global warming, heat index, temperature, humidity, heat event,

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This UCS report and 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. Chapter 1 1 Introduction 2 Examining Future Extreme Heat and Emissions Choices 3 A Snapshot of Results Chapter 2 4 The Heat Index: What Extreme Heat “Feels Like” 4 How and Why the National Weather Service Uses Heat Index Thresholds Chapter 3 8 How Heat Harms Our Bodies 8 Heat-Related Illnesses and Deaths 9 Child Bodies 9 Elderly Bodies 10 Bodies with Special Conditions and Needs Chapter 4 11 Findings: The Future of Dangerously Hot Days 13 Midcentury Results (2036–2065) 17 Late-Century Results (2070–2099) Chapter 5 22 Implications: How the Heat We Create Threatens Us All—but Some More Than Others 22 Outdoor Workers 24 City Dwellers 24 Rural Residents 25 People and Neighborhoods with Low Income or Experiencing Poverty 25 People Exposed to Other Extremes Chapter 6 26 Our Challenge and Our Choices: Limiting Extreme Heat and Its Accompanying Harm 26 Keeping People Safe from Extreme Heat 28 Investing in Heat-Smart Infrastructure 29 Investing in Climate-Smart Power Systems 29 Putting the Nation on a Rapid Path to Reduced Emissions 30 Holding the Line against an Unrecognizably Hot Future 32 Appendix: Methodology 32 What Models Did We Use in This Analysis? 32 What Emissions Scenarios Did We Use? 32 How Did We Project Days with Extreme Heat Index Values? 32 What Are the Key Caveats, Limitations, and Assumptions?


Union of Concerned Scientists



Kristina Dahl, Erika Spander-Siegfried, Rachel Licker, Astrid Caldas, John Abatzoglou, Nicholas Mailloux, Rachel Cleetus, Shana Udvardy, Juan Declet-Barreto, Pamela Worth

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