Official heat warnings miss situations with a detectable societal heat response in European countries

The frequency and intensity of heat waves are increasing globally. In this context it is not clear how the related impacts differ between countries with different vulnerability and exposure characteristics, such as overall climatic conditions and implemented adaptation strategies, e.g. urban planning and planting trees to reduce heat island effect, air conditioning, communication with the public about danger of heat waves. The release of official heat warnings two to six days in advance plays an important role in notifying the population. However, it remains uncertain to which extent they capture days with societal heat responses. Here, we analyze and compare the response of several societal metrics (Google search attention, excess mortality, press attention) to hot temperatures in twelve European countries. We identify country-specific temperature thresholds above which societal responses increase. We find higher thresholds in warmer countries, indicating overall lower heat vulnerability in southern Europe. Meanwhile, we find similar numbers of societally relevant hot days across countries, computed as the sum of days on which more than 50 % of a country's population experiencing temperatures above the detected thresholds. This indicates that the reduced vulnerability and exposure found for warmer countries are counteracted by hotter heat waves. Finally, the determined number of societally relevant hot days generally exceeds the number of days with heat warnings in five investigated countries: Belgium, Germany, Netherlands, Romania, and Sweden. This suggests that lower temperature thresholds would be better aligned with detectable societal responses and should therefore be considered in the context of warning systems.