Trends in Tropical Nights and their Effects on Mortality in Switzerland across 50 years

Abstract

Despite its temperate climate, increasing temperature and more frequent and severe heat waves in Switzerland are leading to more heat-related health burden. Especially, high nocturnal temperature, usually associated to urban heat island effect, reduces the well-being of the society. We aimed to assess the spatiotemporal patterns in the frequency, the exposed population to Tropical Nights and its effect on daily mortality in Switzerland. We derived the number of TN (night temperature ≥20°C) per district using district-specific population-weighted temperature series based on high resolution hourly mean temperature (ERA5-Land reanalysis data set) between 1969-2019. Through a spatiotemporal analysis, we assessed the change of TN frequency as well as the exposed population per district and decade. We then estimated the TN-mortality association by canton using conditional quasi-Poisson regression analysis using data on all-cause mortality at district level from 1980-2019. The model accounted for long-term and seasonal patterns and daily mean temperature. We found an overall increase in the frequency of TN and exposed population in Switzerland between 1969-2019, mainly in the surrounding areas of the main cities of Lausanne, Geneva, Basel, Lugano and region of Zurich, and during the last 2 decades. Across Cantons, no clear spatial patterns were found in the level of vulnerability. In particular, TNs were associated with an increase of 37-22% in the risk of mortality in the cantons of Vaud (RR:1.37 [CI:1.19-1.59]), Zurich (1.33 [0.99-1.79]), Lucerne (1.33 [0.95-1.87]) and Solothurn (1.22 [0.88-1.69]), while a negative association was observed in Ticino (0.51 [0.37-0.7]), Basel-Land (0.4 [0.24-0.65]) and Thurgau (0.65 [0.5-0.85]). A null association was found in the remaining Cantons. Our findings indicate that TN is a relevant health hazard for a large part of the Swiss population with potentially larger impacts in the future due to climate change and increasing urbanization. Climate Change, Tropical Night, Mortality, Human Health, Switzerland