Exploring Projected Changes in Heat Index Across the U.S. Using Dynamically Downscaled Projections

Heat index is a metric incorporating temperature and humidity to determine the conditions that a human body perceives, i.e., an “apparent temperature”. Combinations of high temperature and high humidity inhibit the human body’s ability to dissipate heat through sweat and evaporation, with prolonged exposure to high heat indices potentially posing serious health risks even for previously healthy individuals. This danger is exacerbated within vulnerable populations and for those with preexisting health conditions. The National Weather Service defines categories of risk associated with heat index regimes, where “extreme danger” is given for heat indices that exceed 125F. The Centers for Disease Control and Prevention also defines criteria that are based on heat index as guidance for curtailing outdoor activities (including employment, recreation, and other physical activity) to minimize health risks associated with exposure to high heat indices.

The summer of 2023 has been marked by several events featuring record-breaking heat indices across the United States, including many approaching and into the “extreme danger” category. Under climate change, there is likely to be an increased prevalence of extreme events resulting in dangerous heat indices throughout the United States. In this presentation, we will use dynamically downscaled regional climate projections to explore potential changes to the severity and recurrence of extreme heat indices. Using hourly projections of both temperature and humidity, we will provide a comparison of recent historical heat indices to those projected under multiple scenarios. Other metrics of extreme heat and health impacts, such as wet bulb global temperature, may be explored to provide additional context.

Disclaimer: The views expressed in this presentation are those of the authors and do not necessarily reflect the views or policies of the U.S. EPA.