Prioritizing urban heat adaptation infrastructure based on multiple outcomes: Comfort, health and energy

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, co-benefits, costs, and externalities. Studies to date examine insufficiently diverse outcomes and use  inconsistent methodologies, limiting quantitative comparison between adaptation strategies and hindering our ability to assess optimal combinations of heat adaptation infrastructure in cities.

To assess the impact of urban heat infrastructures in a rigorous, comprehensive framework, we apply an urbanized meteorological model (WRF) with the newly integrated multi-layer 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 radiative environment impacting pedestrians. We evaluate the performance of five heat adaptation strategies (street trees, cool roofs, green roofs, rooftop photovoltaics, 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 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 through shade, fully offsetting heat stress increases in all cities studied, even under a high-emissions end-of-century climate scenario. Cool and green roofs moderately reduce heat stress and energy use. Alternatively, rooftop photovoltaics 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 photovoltaics as a highly complementary and effective means of adaptive mitigation across different climates and neighbourhood densities.