Risk assessment of counties in the contiguous United States impacted by increasing frequency of hazardous temperatures

Abstract Frequent exposure to extreme temperature events (ETEs) has implications for human health and ecosystems. In this study, we investigate counties in the contiguous United States (US) at risk of increasing frequency of hazardous temperatures, as well as the demographic exposure over time. Absolute and seasonally relative ETEs were defined based on a duration-intensity metric calculated from excess apparent temperature factors. Hazard was defined as a statistically significant increasing trend in ETEs at the county level; exposure was defined as the total county population; and vulnerability was characterized using the social vulnerability index, which captures socioeconomic, and household factors linked to limited capacity to prepare for, respond to, and recover from ETEs. Focusing on ETE trends from ERA5 data, our results show that out of the 3109 counties and county equivalents in the US, 20% were exposed to increasing frequency of absolute extreme heat events (EHE) and 37% to seasonally relative extreme heat events (REHE) that occur outside typical warm months. The most populous exposed counties in California, Texas, Arizona, and Florida have urban-dominant populations. In 2020, approximately 92 million and 139 million people were exposed to increasing frequency of EHE and REHE, respectively. Additionally, between 1980 and 2020, the total exposed population to EHE and REHE increased by 100% and 93%, respectively. Risk to demographic exposure was most prominent in urban-heavy counties, such as Los Angeles, California and Maricopa, Arizona. These findings highlight the necessity for targeted adaptation strategies in densely populated urban areas to mitigate the health and environmental risks associated with increasing ETEs, while also acknowledging the exposed rural and socially vulnerable counties that may face distinct challenges in terms of resource availability and infrastructure resilience.