When cities overheat: Urban heat exposure under the Swiss Climate CH2025 Scenarios

Abstract

Urban populations are exposed to more intense heat than people living in surrounding rural areas. The combination of high daytime temperatures and inadequate cooling at night in cities increases the risk of heat-related morbidity and mortality. In this study, we assess the exposure to heat in urban and rural locations in five Swiss cities: Basel, Bern, Geneva, Lausanne, and Zurich, under current and future climate conditions. Future projections are based on the Swiss Climate CH2025 scenarios for three Global Warming Levels (GWLs). To quantify heat exposure, we calculate several temperature-based indices and apply the MeteoSwiss heat warning framework to evaluate changes in the frequency and intensity of heatwaves and hot spells. Additionally, based on climate scenarios, we select the worst-case scenario for the warmest week in summer under three different GWLs to explore the potential implications for heat-related risks. Our results suggest that projected global warming is expected to further increase daytime and nighttime temperatures in summer, with greater heat exposure in urban areas than in rural areas. Heatwaves are likely to become more frequent and intense in a warming climate, and in urban areas they will last two to four days longer and will reach higher warning levels than in surroundings. Prolonged heatwaves and reduced nighttime cooling are likely to exacerbate urban overheating and increase heat-related health risks. Our findings also emphasize the importance of efficient heat warning systems to facilitate timely public communication and the preparedness of public health systems in a warmer world.