Strategies to heat mitigation during future extreme heat stress conditions in Lisbon, Portugal

Abstract This study assesses projected future thermal comfort conditions and the impact of Human Based Solutions (HBS) on heat stress mitigation, in a known hotspot in Lisbon. Three future scenarios were modelled in ENVI-met: Scenario A, in which no changes were made to the urban morphology; Scenario B, where existing trees were replaced by sun sails; and Scenario C, which involved the addition of sun sails to the existing tree layout. The results indicate a shift toward higher heat stress categories in the future, with minimum and maximum Universal Thermal Climate Index (UTCI) values projected to increase by approximately 9 °C and 5 °C, respectively. Present-day maximum UTCI levels are projected to become the average under shaded conditions in the future, highlighting the urgent need for effective mitigation strategies. Of the three scenarios, Scenario C was the most effective, lowering the minimum UTCI to approximately 34 °C, comparable to current maximum values. It achieved an average UTCI reduction of 0.5 °C, with a maximum decrease of 1.7 °C, and reduced dynamic Physiological Equivalent Temperature (dPET) by about 1 °C. While Scenarios A and B yielded similar thermal outcomes, their impacts differed: trees offered a spatially broader but less intense cooling effect, whereas sun sails provided more localised yet continuous and more impactful relief from heat stress. Despite the inherent uncertainties associated with climate modelling, these findings are consistent with the existing literature and reinforce the necessity for immediate intervention. Given that tree growth is a long-term process, HBS represent viable short-term mitigation options.