Vulnerability to heat waves: Impact of urban expansion scenarios on urban heat island and heat stress in Paris (France)

Heat-wave risk in cities is determined both by regional climate evolutions and by cities size and shape change, because they can impact heat island effect. Land planning and urban transport policies, due to their long-lasting impact on cities shape, can therefore play a role in heat-wave risk mitigation. However, the link between these policies and resulting change in urban heat island is far from direct, and not yet clearly understood. Using Paris urban area as a case study, we show that evolutions towards a denser city could indeed lead to increased heat wave exposure when compared with urban sprawl evolutions. However, this increase depends strongly on the way heat-wave risk is measured, and on the indicators used to characterize heat island effect. Using an interdisciplinary modeling chain, including a socio-economic model of urban expansion and a physically-based model of urban climate, air temperature in the city during heat waves is simulated for five prospective scenarios of Paris urban area expansion and development. These scenarios differ in terms of land planning policies simulated, but are otherwise similar (e.g. in terms of total population evolution). Their impacts on urban heat island and population heat stress are evaluated and compared through different indicators. The urban heat island is always higher at night and hits preferentially the small area of the city centre, whereas it more widely affects the residential areas during the day. However, the concentration of population in the historic centre and the first suburbs makes these districts quite vulnerable. This is especially the case for the compact city scenario, whereas the greening strategies allow to mitigate urban heat island and consequently to insure better thermal comfort. The efficiency of such strategies is however strongly dependent on water availability for evapotranspiration of plants. Finally, it should be kept in mind that the method and choice of indicators are crucial in the process of adaptation strategies analysis.