Carbon Brief reports on how “smart” surfaces can be used to cool cities in the face of worsening extreme heat.
Last month, new research showed that increasing the reflectivity of surfaces at San Francisco International airport could cut the risk of heat stress in its outdoor workers and see fewer working hours lost at the height of summer.
At its heart, the idea is a simple one – making the ground surface and roofs of buildings more reflective means they absorb less of the sun’s energy. This, in turn, means they warm up less and radiate less heat back into the surroundings.
This helps offset the “urban heat island” effect, where temperatures in cities are consistently higher than in the surrounding countryside due to heat-trapping urban infrastructure.
Heat inequality
The airport study was conducted by the Smart Surfaces Coalition, a not-for-profit organisation that works with cities to use “smart” surfaces – including reflective, porous and green surfaces, solar panels and urban trees – to help manage the impacts of extreme heat and rainfall.
The coalition is working with 11 “partner cities” in the US – plus Bhopal in India – to drive adoption of smart surfaces through citywide policies and local implementation projects.
According to coalition founder Greg Kats, cities have typically followed “business as usual” for public construction projects – opting for “dark, impervious surfaces of tarmac” at the lowest cost, which “absorbs 90% of the incoming sunlight”.
As well as leaving cities at higher risk of getting “hotter and hotter and more prone to flooding”, these choices have contributed to urban inequality, Kats explained to Carbon Brief.
In the case of Baltimore on the US east coast, “at the same time of day in the summer afternoon” there will be a “14F [7.8C] temperature difference between wealthy, treed areas and low-income, dark, impervious areas”, he said.
This “structural inequality is a legacy of redlining” – long-abolished discriminatory lending practices in the US – and is a “hallmark of American cities”, Kats added.
Urban cooling
In partner cities, the coalition is conducting citywide surface mapping and heat modelling, developing cost-benefit tools and helping cities craft policies and building codes.
In Baltimore, for example, installing smart surfaces could bring more than 5F (2.8C) of citywide cooling, coalition analysis has shown, with benefits that outweigh the costs at a ratio of 10 to 1.
These measures also reduce the need for air conditioning, Kats explained:
“For 1F of ambient temperature reduction, it reduces an air conditioning electricity bill by 4-5%, so a 5F reduction is a 20-25% reduction in electricity bills.”
As a result, smart surfaces are a “really big deal” for climate mitigation, Kats said. And it taps into another aspect of the urban island effect – a key contribution comes from human activity, such as the use of cars and air conditioning.
Air conditioners cool the air inside buildings by pumping waste heat outside, so “you have this perverse effect of private cooling heating the city, which means more air conditioning”. Kats explained:
“So you've got two different pathways for a city: do you do citywide cooling, or do you keep relying on the private sector to buy more air conditioning?”
While cool surfaces do have their barriers and challenges, Kats argued that they offer the “only viable, scalable, cost-effective, urban-cooling strategy”, adding: “There really isn’t anything else.”
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