What Alternatives Does Paris Have to Adapt to Future Heat Waves?

In 2003, the city of Paris faced a particularly severe heat wave that resulted in an exceptional excess mortality rate for the urban population. Subsequently, several research projects in partnership with public stakeholders investigated adaptation strategies that could be implemented to address the combined effects of climate change (leading to an increase in occurrence of heat waves) and the urban heat island phenomenon. Action levers have been tested at different scales to mitigate air temperature in the city under heat wave conditions: urban design strategies from the neighbourhood to the city scale based on implementation of reflective materials on buildings, greening, or pavement watering, but also large-scale territorial planning policies related to urban expansion and landscape planning around the city. All action levers tested for local-scale urban design provide a substantial cooling effect, but their efficiency varies spatially according to the urban typology. Greening is relevant in medium-density urban areas where vegetation coverage is enough to bring noticeable cooling through evapotranspiration. Reflective materials and pavement watering are most effective in central Paris, the first acting on the radiative balance of buildings and the second on surface evaporation at pavement level. All these levers can therefore be combined for an increased and additive efficiency. Larger-scale planning policies are another interesting way to adapt and regulate local climate, while bringing multiple other ecosystemic services. These two adaptation pathways finally lead to a comparable urban heat island attenuation of about 1.3–1.5 °C. Nonetheless, the studies highlighted the predominant effect of residents’ practices, particularly with regard to energy use, on the effectiveness of the strategies envisaged. In particular, massive use of air conditioning tends to increase street-level air temperature and degrade outdoor comfort conditions, and can consequently counterbalance all benefits of the other adaptation levers.