Trends in tropical nights and their effects on mortality in Switzerland across 50 years

Abstract

Increasing temperatures and more frequent and severe heat waves in Switzerland are leading to a larger heat-related health burden. Additionally, high nighttime temperatures or tropical nights (TNs) also affect the well-being of the population. We aimed to assess the spatiotemporal patterns in the frequency and the exposed population to TNs, and its mortality effect in Switzerland. We identified the TNs (minimum nighttime temperature ≥ 20°C) in each district in Switzerland using population-weighted hourly temperature series (ERA5-Land reanalysis data set) between 1970–2019. We assessed the change in the frequency of TNs and the exposed population per district and decade through a spatiotemporal analysis. We then performed a case time series analysis to estimate the TN-mortality association (controlled for the daily mean temperature) by canton and for the main 8 cities using data on all-cause mortality at the district level between 1980–2018. We found an overall increase in the annual frequency of TN (from 90 to 2113 TNs per decade) and the population exposed (from 3.7 million to over 157 million population-TN per decade) in Switzerland between 1970–2019, mainly in the cities of Lausanne, Geneva, Basel, Lugano, and Zurich, and during the last two decades. The TN-mortality association was highly heterogeneous across cantons and cities. In particular, TNs were associated with an increase of 22–37% in the risk of mortality in the cantons of Vaud (Relative risk: 1.37 (95%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)), and a null association in the remaining cantons. Our findings indicate that TNs are a relevant health hazard for a large part of the Swiss population leading to potentially larger impacts in the future due to climate change and increasing urbanization.

Climatological analysis of urban heat island effects in Swiss cities

“…Figure 7 presents another standard metric used to investigate extreme events: the number of so‐called tropical nights, defined by MeteoSwiss (n.d.‐a) as nights when the temperature did not drop below 20°C. The increase in number of these nights has been connected with a change in the mortality rates in Switzerland (Rippstein et al, 2023), highlighting the importance of this metric. The first notable feature of Figure 7 is that Lugano has approximately twice the number of tropical nights as any other city.…”

“…More than half of the world lives in cities, and the rural population is stagnating and predicted to decline in the near future (United Nations, 2018). It therefore comes at no surprise, given the large amount of people exposed to the atmospheric environment of cities, that the relevance of a better understanding and modelling of the urban climate has steadily increased over the past few decades (see, e.g., Arnfield, 2003;Barlow et al, 2017;Masson et al, 2020;Myrup, 1969;Oke, 1974;Rizwan et al, 2008;Stewart, 2019 and references therein). The interest stems from the current discussion on sustainable urban design, and is in large part connected to the adverse health effects caused by higher urban temperatures (Gabriel & Endlicher, 2011;Kovats & Hajat, 2008;Rippstein et al, 2023) and pollution (Pope III et al, 2002).…”

“…The origin of the term dates back to 1929, but the reader is referred to Oke's comprehensive book (Oke et al, 2017) for an in-depth explanation of the phenomenon. A significant number of UHI studies focuses on large cities (Hart & Sailor, 2009;Imhoff et al, 2010;Peng et al, 2012;Rizwan et al, 2008;Roth, 2007;Sarkar & de Ridder, 2011;Varentsov et al, 2020;Zhou et al, 2014) where the differences between urban and rural areas are more marked, while fewer analyses can be found in the literature for small towns (Chieppa et al, 2018;Hutcheon et al, 1967;Magee et al, 1999;Zhou et al, 2017), where the effects are less dramatic. In fact, it was empirically shown that the UHI intensity is correlated with the logarithm of the urban area (Zhou et al, 2017).…”

“…It therefore comes at no surprise, given the large amount of people exposed to the atmospheric environment of cities, that the relevance of a better understanding and modelling of the urban climate has steadily increased over the past few decades (see, e.g., Arnfield, 2003; Barlow et al, 2017; Masson et al, 2020; Myrup, 1969; Oke, 1974; Rizwan et al, 2008; Stewart, 2019 and references therein). The interest stems from the current discussion on sustainable urban design, and is in large part connected to the adverse health effects caused by higher urban temperatures (Gabriel & Endlicher, 2011; Kovats & Hajat, 2008; Rippstein et al, 2023) and pollution (Pope III et al, 2002). The interaction between heat waves and urban areas is also becoming a prominent issue, especially since heatwaves are projected to increase with climate change (Kong et al, 2021) and have been shown to worsen the temperature in cities more than in rural areas (Li & Bou‐Zeid, 2013).…”

Tropical nights in the Mediterranean: A spatiotemporal analysis of trends from 1950 to 2022

“…Changes in the frequency of tropical nights, which result in discomfort and heat-related illnesses due to insufficient nocturnal rest, are an illustration of a pertinent human health concern. Especially, days with high temperatures frequently precede these warm nights, preventing the human body from recovering from diurnal thermal stress and leading to episodes of temperature-induced discomfort (Hajat et al, 2002;Ragettli et al, 2017;Rippstein et al, 2023;Royé, 2017;Royé et al, 2021). A study conducted in Barcelona, Spain, underscored an augmented mortality risk from natural, respiratory and cardiovascular causes during hot nights where temperatures surpassed 23 C (Royé, 2017).…”