Greening rooftops to reduce heat islands: How large is large enough?

Green roofs, with adequate water supply, have been proven as effective measures to reduce urban environmental temperature. The benefits of large-scale deployment of green roofs have been studied mainly through numerical simulations with unrealistic high penetration scenarios, where all rooftops across the entire metropolis is assumed to be retrofitted. In this study, the scale dependence of the cooling effect of green roofs is investigated with a coverage of 25% over buildings at local, city, or regional scales. We compared results at 6 major U.S. cities to assess the response of the scale dependence to geoclimatic conditions. High-resolution weather simulations reveal that the cooling of near-surface air temperature by green roofs increases non-linearly with the scale of deployment. The shape and geoclimatic setting (geographic and climatic characteristics) of metropolitan areas control the scaling that some city centers are not able to achieve a significant cooling by greening their own rooftops. Uniform deployment of green roofs at the regional scale, on the other hand, provides a substantial temperature reduction with a very low cooling efficiency per intervention area. Cities should carefully revisit the scale dependences of cooling benefit and efficiency of green roofs to develop resilient plans meeting their expectations