PNNL

Abstract

Globally, cities face increasing extreme heat, impacting comfort, health, and energy consumption. Infrastructure-based heat adaptation strategies can improve these outcomes, but each strategy has a unique mix of benefits and drawbacks. Here, we apply an urbanized meteorological model (WRF) with the newly integrated multilayer BEP-Tree street tree model to dynamically downscale Earth System Model projections and a 3-D microclimate model (TUF-Pedestrian) to simulate the street-scale radiation environment impacting pedestrians. We evaluate the performance of five heat adaptation strategies (street trees, cool roofs, green roofs, rooftop photovoltaics (PV), and reflective pavements) during extreme heat events in three cities with contrasting background climates (Toronto, Phoenix, and Miami), under contemporary and end-of-century projected climates, based on three metrics: outdoor heat stress, air conditioning (AC) energy use, and ventilation of vehicular air pollution. No single adaptation strategy improves all three outcomes. While street trees inhibit ventilation, they reduce outdoor heat stress four times more effectively than the next best strategy via shade provision, fully offsetting heat stress increases under a high-emissions end-of-century climate scenario in all cities studied. Cool roofs and green roofs moderately reduce heat stress and energy use. Alternatively, rooftop PV with energy storage can generate sufficient power for space cooling but have marginal effects on heat stress. Reflective pavements are the least effective across metrics. Where the ventilation of street-level emissions is of less concern, our results clearly support the combination of street trees and rooftop PV as a highly complementary and effective means of adaptive mitigation across different climates and neighborhood densities. Significance statement We evaluate how three cities in different climates (Phoenix, Toronto, and Miami) can best adapt to increasingly prevalent and intense heatwaves. We assess the effects of street trees, cool roofs, green roofs, rooftop solar panels, and reflective pavements on outdoor heat stress, air conditioning (AC) energy use, and ventilation of air pollution during contemporary and future heatwaves. Street trees shade pedestrians, reducing heat stress four times more effectively than any other strategy. Rooftop solar panels with energy storage can generate enough power for AC systems. Cool and green roofs moderately improve outcomes, unlike cool pavements. Where air quality effects of street-level emissions are of less concern, the combination of street trees and rooftop solar panels is optimal across the cities studied.