Background/Objectives

The events of summer 2023 underscore how climate change is escalating heat-related risks in both urban and rural areas, with underserved and low-income communities particularly vulnerable to extreme heat. Planners, emergency responders, and decision-makers need high quality climate information to prepare for more frequent and more severe heat events and mitigate their impact on individuals and communities. Heat index is an oft-used measure to help contextualize the combined impacts of temperature and humidity on the human body; however, recent research suggests the traditional derivation of heat index leads to an underestimation of heat index when temperatures and humidity are particularly high. Thus, new methods for calculating heat index have been proposed to better account for extremes (see Lu and Romps, 2022). The purpose of this project is to explore how new estimations of heat index affect climate projections under different emissions scenarios, with takeaways for practice.

Approach/Activities

We use high-resolution (12 km) dynamically downscaled climate projections, some of which is housed on the Climate Risk and Resilience Portal (ClimRR), to explore traditional and novel estimations of heat index. We calculate heat index across the continental United States using both the traditional method for estimating heat index (currently used by the National Weather Service) and the method proposed by Lu and Romps (2022). We do these calculations for both a historical period (1995-2004) as well as two climate scenarios (RCP4.5 and RCP8.5) at mid-century (2045-2054) and end-of-century (2085-2094). Using both sets of heat index measures, we conduct regional comparisons as well as in-depth evaluations of several representative communities to understand how an updated methodology will influence heat wave analysis and on-the-ground decision-making.

Results/Lessons Learned

Our study finds that the updated Lu and Romps heat index methodology displays markedly greater changes by mid- and end-of-century than the traditional method, particularly under the RCP8.5 scenario. Communities in the southern and eastern US, as well as the Midwest, project to undergo significant changes in heat index impacts. Seasonal extremes are particularly high in the Midwest and Mississippi Delta regions, with summer seasonal highs of over 135°F by mid-century (under RCP8.5).

We connect these findings back to planning and practice, highlighting the takeaways that can help emergency responders better prepare for and respond to extreme heat impacts. We also discuss the important building-, infrastructure-, and community-level actions that can be taken by local officials, infrastructure operators, policymakers, and other key decision-makers to mitigate heat-related risks and protect vulnerable populations.