What does heat wave adaptation look like for cities in a warming world?

What is it about?

One of the most pressing issues that cities face due to climate change are longer and stronger heat waves. Urban areas must now respond to these events which have a wide range of impacts to comfort, health, and energy use. Our study looked at how three major North American cities can best adapt to heat waves and mitigate these impacts. We studied the effects of having tree-lined streets (i.e. “street trees”), cool (reflective) roofs, green roofs, rooftop solar panels, and reflective pavements on outdoor heat stress, air conditioning (AC) energy use, and ventilation of air pollution. We found that cool roofs and green roofs moderately reduce both heat stress and AC use. However, street trees reduce heat stress four times as effectively as any other strategy in this study, although their canopies do also appear to exacerbate air pollution ventilation. We also found that rooftop solar panels can generate, on average, more than enough electricity for AC, but they only marginally reduce heat stress. The combination of street trees and rooftop solar panels provides a highly effective and complementary strategy both in the cities of today and of tomorrow. As an important additional co-benefit, by implementing these heat wave adaptation strategies, cities would not only be responding to climate change -- but they would also play a part in mitigating it by sequestering carbon and reducing emissions.

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Photo by Michael Kahn on Unsplash

Photo by Michael Kahn on Unsplash

Why is it important?

There has been a considerable amount of prior research regarding how effectively a single adaptation strategy, such as cool roofs, reduces air temperature. Due to the complexity of its calculation, heat stress is often less studied, even though it is more strongly associated with human health and comfort. Our study is one of just a small few that examines a range of outcomes and trade-offs from heat adaptation infrastructure, especially in a quantitative manner, using models that capture physical processes at all scales. One of the more uniquely advanced modeling approaches in our study was how we handled the street trees. We took into account not only their effects on air temperature, but also on wind, turbulence, humidity, and—critically—shade. Not only does this study examine a range of adaptation strategies and outcomes, it also does so for three major cities with distinct climates: Toronto, Phoenix, and Miami. Looking at multiple outcomes, multiple cities, multiple strategies, and multiple climate scenarios, this work applies advanced modeling of urban processes in an innovative way to paint a more complete picture of how to better prioritize urban heat adaptation. However, as each city is unique and faces different challenges, the optimal implementation of heat adaptation strategies needs to be place-based. We urge cities to keep in mind all benefits, co-benefits, costs, and externalities as they adapt to and develop in a rapidly changing climate.