Assessing the potential of urban wide greening for climate-resilience: The example of Vienna

• Increasing urban resilience towards climate change through large scale greening. • Large scale urban greening effectively reduces summer temperatures. • Strong potential of reducing the occurrence of tropical nights. • Increasing impact of greening with increasing summer temperatures due to the different heat capacity of sealed versus unsealed areas. Cities are facing multiple problems due to the combined effects of climate change and the growing urbanization. Especially the rising global temperatures worsen the urban heat island (UHI) effect and therefore demand sustainable counteracting measures. Nature-based solutions are effective adaptation measures since they balance the ambient air temperature and improve the overall well-being. However, evidence is scarce on the potential effects of greening a city on the number of tropical nights and heat wave days. In the presented study the effects of large-scale greening within Vienna are analysed to quantify their effect on prevailing summer temperatures and to display their potential in increasing cities’ resilience with respect to ongoing anthropogenic climate change. The analysis is done using the COSMO-CLM climate model, with a specific urban extension. The model runs, representing different percentages of greening within the city, were conducted at 1 km² resolution for four scenarios: First, the status quo, second a moderate greening, third a maximum greening and finally a simulation with all surfaces sealed (worst case). Using this setup, 17 years were simulated, thereby allowing to assess the long-term effect of urban greenery on climate indicators such as number of tropical nights or summer days, which is not feasible by urban or micro climate models. The results display a strong positive effect of the greening on the morning and afternoon temperatures and especially on the night temperatures due to the altered heat storage characteristics of sealed vs unsealed areas. Furthermore, it is shown that the positive effect of substantial greening increases with higher maximum temperatures, an effect that has not been investigated so far by other studies. On the contrast, sealing of currently open areas worsens the UHI effect significantly. The simulation results give an important indication for the value and importance of integrating green in urban planning, especially regarding an intensification of the Urban Heat Island effect through climate change.