Climate change’s impact on health, specifically increasing temperatures, has become a prominent field of study worldwide. Although its importance is growing, decision makers still have little knowledge on the subject. Developing indicators to monitor spatial and temporal trends of health impacts due to climate change is a vital advancement needed to encourage policy adaptations. This research proposes an approach to producing annual estimates for heat-related mortality as an indicator to support these policies. The first step was to develop temperature-mortality relationships for each of the 96 metropolitan French departments, for the summer months (June–September) between 2014 and 2022. Several approaches were tested to control for a possible influence of the COVID-19 pandemic since 2020. The temperature-mortality relationships were used to compute the annual mortality attributable to heat for the same years, and for 2023.

Heat-related risks were slighly higher after the pandemic; an increase from 19.8 °C to 28.5 °C was associated with a relative risk of 1.25 [CI 95% 1.21:1.30] in 2004–2019, and 1.31 [1.24:1.38] in 2020–2022. Between 2014 and 2023, 37,825 deaths [IC 95% 34,273: 40,483] were attributable to heat. The largest impacts were observed in 2022 (6,969 [6277: 7445]), 2023 (5167 [4587; 5551]), and 2019 (4441 [4086: 4717]). The annual indicator of heat-related mortality documents the mortality impact of heat during the summer and during extreme heat waves. It shows that the impact is increasing, despite major prevention efforts. This call for a more ambitious, transformative adaptation to climate change.