Current Interventions Are Inadequate to Maintain Cities' Resilience During Concurrent Drought and Excessive Heat

Climate change is expected to intensify the global water cycle, affecting land-atmosphere feedbacks and surface water availability. This leads to prolonged droughts and excessive heat events, increasing vulnerability of cities to water scarcity and extreme heat. Here, we integrate data from regional climate simulations into an urban modeling approach that operates at an intraurban microscale. Using this approach, we investigate the concurrent effects of the 2019 European summer drought and an increase in extreme heat days under RCP2.6 (mitigation scenario) and RCP8.5 (business-as-usual scenario) on land-atmosphere interactions, evaporative cooling potential, and bioclimatic conditions in Innsbruck, Austria. Results indicate that water-limiting conditions such as those from summer 2019 impair evaporative capacities of ecological systems and augment diurnal and nocturnal heat transfer between the soil, surface and atmosphere in the city, if not irrigated extensively. Combined with the projected increase in daily maximum temperature of extreme heat days by 3.9 K under RCP8.5, we see the development of extreme human heat stress, with a mean Universal Thermal Climate Index (UTCI) exceeding 38°C across the study area. Additionally, we found that maintaining a prevailing evaporative cooling effect over an area requires a degree of surface sealing less than 11% and unrestricted water supply. We stress the urgency of integrated urban water management, including combined rain and greywater recycling, and innovative natural and technical climate change interventions for urban green space irrigation. These mitigation measures are necessary to avoid critical malfunctions in ecological systems related to human well-being under future climate trajectories.