Cool roofs are more effective than green roofs, rooftop solar panels, and tree planting at reducing urban temperatures on hot summer days, according to a new modeling study focused on the Greater London area.
As the planet heats up, researchers have been trying to find ways to mitigate the urban heat island effect—the tendency for cities to be several degrees warmer than surrounding areas—in order to reduce discomfort and health risks for urban dwellers during heat spells.
But few studies have directly compared how different strategies would fare if rolled out city-wide. The new study suggests “that cool roofs seem to outperform all other commonly proposed adaptation and mitigation strategies to urban heat,” says study team member Oscar Brousse, who studies urban climatology and health at University College London in the UK.
Brousse and his collaborators used a computer model to gauge the effectiveness of different cooling methods at reducing urban outdoor temperatures. The researchers investigated both the theoretical effect of deploying the interventions on all residential, commercial, and industrial buildings across Greater London, and the effect of deploying them as widely as reasonably feasible given the constraints of the current built environment.
At city scale, cool roof technology—painting roofs white or covering them with a reflective coating_could reduce outdoor temperatures by 1.2 °C on average, and up to 2.0 °C in some areas, the researchers report in the journal Geophysical Research Letters.
By contrast, replacing grassy areas with trees would cool the city by just 0.3 °C on average. This strategy would also increase humidity in the city, which can make the heat more difficult for humans to bear.
Green roofs—planting vegetation such as sedum on rooftops—would be a wash temperature-wise, the researchers found. “Green roofs do reduce the temperature on average during the day according to our model but never to the same level as cool roofs,” Brousse says. “At night, they seem to heat up the city, which is why on average their effect is null in terms of temperature reduction.”
Installing solar panels across the whole city could reduce temperatures by 0.5 °C, the researchers calculated. This result was surprising, Brousse reports: the temperature effect of solar panels “is still something that is debated within our fields of study since solar panels do heat up a lot during the day,” he says. “We assume that they nonetheless remove a significant amount of the solar energy to produce electricity, an energy that would otherwise be stored in the buildings and heat up their surroundings.”
The feasible rollout of these adaptation approaches offers less rosy picture, though. “Sadly, the current prospect in practicable extensions of green roofs and solar photovoltaic panels over London are not expected to decrease temperatures at all,” says Brousse.
Still, the team found that a feasible rollout of solar panels would easily provide enough electricity to run air conditioning in all of London’s indoor spaces. (Air conditioning would heat up the outdoor air by an average of 0.15 °C city wide, and up to 1 °C in dense areas of central London, according to the model.)
The new analysis only modeled the effect of these interventions over the course of two hot summer days, so more research is necessary to determine how they would perform over the long term and in different seasons, Brousse says. Investigations of cities in different climate zones and with different urban forms are also needed. The study is part of a larger project to investigate ways to reduce the urban heat island effect across cities in different parts of the world, he reports.
Source: Brousse O. et al. “Cool Roofs Could Be Most Effective at Reducing Outdoor Urban Temperatures in London (United Kingdom) Compared With Other Roof Top and Vegetation Interventions: A Mesoscale Urban Climate Modeling Study.” Geophysical Research Letters 2024
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