Future trends in compound concurrent heat extremes in Swiss cities - An assessment considering deep uncertainty and climate adaptation options

The interaction of multiple hazards across various spatial and temporal scales typically causes compound climate and extreme weather events. Compound concurrent hot day and night (CCHDNs) extremes that combine daytime and nighttime heat are of greater concern for health than individual hot days (HDs) or hot nights (HNs), even though their frequency is lower. We utilize a bottom-up exploratory approach to investigate how adaptation options and various unfolding future scenarios alleviate the impacts of the heatwaves and affect the frequency and intensity of CCHDNs. We use climate observations (1981–2020) and Switzerland's future climate model scenarios (CH2018) to analyze historical and future trends of the individual hot day followed by a hot night (HDNs, first metric), and the length and frequency of CCHDNs (second and third metrics) in the near-future (2020–2050) and far-future (2070–2100). Results show more frequent and lengthier HDNs in cities under all emission scenarios, notably significant under high emissions scenarios. The highest increase of HDNs occur in i) Lugano with 65.8 days (decade−1) in the historical period and 110 (371) days (decade−1) in near-future (far-future), ii) Geneva with historical 48 days (decade−1) to 108 (362) (decade−1), iii) Basel with 48–74 (217) days in the future, followed by iv) Bern with 15–44 (213) days and v) Zürich with 14–50 (217) days (decade−1) in the near-future and far-future, respectively. We consistently project that the CCHDNs in April–October become more likely and intense in all cities under all emission scenarios, with higher increases under the RCP8.5 scenario and after the 2050s. The frequency of compound extreme heatwaves (exceeding both historical thresholds of night and day temperatures) may increase by 3.5–7.8-fold and become 3.3–5.3-fold lengthier in all cities of Switzerland in the far-future. We find that the adaptation options targeting higher tolerance to increased minimum temperatures contribute more to reducing compound extreme events' frequency and intensity than adaptation options that address the maximum daily temperature.