Killer Heat: Projections of Extreme Heat for the Twenty-First Century Provide Local-Scale Tools for Communities to Act on Climate

Community resilience planners relying on nominal temperature projections under different emissions scenarios could benefit from knowing the “feels like” temperature to better protect public health. We developed national-scale projections of the future frequency of days when the heat index—a combination of temperature and relative humidity also known as the “feels like” temperature—is above 90°F, 100°F, or 105°F for midcentury (2036-2065) and late century (2070-2099) under a higher emissions scenario (RCP 8.5), a lower emissions scenario (RCP 4.5), and a scenario in which future global average warming is limited to 3.6°F above preindustrial levels. We also projected the number of “off-the-charts” days, when the combination of heat and humidity is above what the National Weather Service formula was developed for, and heat is so extreme that it poses increased risks to human health and lives. This research projects that nearly all communities across the contiguous United States will see significantly more extreme, dangerous, and even life-threatening heat throughout the 21^st century if rapid action is not taken to reduce greenhouse gas emissions. We will examine the regions and communities that are at particular risk of exposure to this extreme heat, and present the suite of tools related to the report, including interactive maps that display the number of projected days when the heat index is above the aforementioned thresholds by county and by congressional district. Results can also be downloaded in spreadsheet format by region, state, county, urban area, and population exposure. All these are useful tools for communities, policymakers, and other stakeholders to make the case for emissions reductions and for local, state, and federal preparedness for the future extreme heat we cannot avoid.