In the coming decades, more urban dwellers, larger and denser cities, and a warming climate will combine to result in many more people exposed to more deadly urban heat. In a new study, researchers set out to determine to what degree (literally) green roofs can make up for these trends.
“There are a wide variety of green roofs out there, which may differ by their plant species, irrigation methods, and rooftop structure, among other characteristics,” says study team member Kathryn McConnell, a graduate student at the Yale School of the Environment in New Haven, Connecticut. “We show that different types of green roofs have different capacities to mitigate urban heat.”
McConnell and her colleagues assessed the cooling effect of three green roofs in Chicago, Illinois. Since a deadly heat wave hit the city in 1995, Chicago has become a leader in installing green roofs, reflective roofs, and other heat-beating infrastructure.
The researchers analyzed the temperature and the amount of greenery at each site before and after the green roof installation, and in comparison to similar nearby sites lacking a green roof.
In the past, many studies of green roof effectiveness have lacked such a control, making it difficult to assess cause and effect, and to separate the effects of green roofs from other changes in the environment. Other studies that have included such controls tend to be costly and complicated.
In the new study, researchers developed a simple, low-cost approach using publicly available satellite data and open-source software to evaluate the green roofs. “I think our main contribution with this paper is illustrating a method that we hope will be scaled up to investigate a broader range of green roof types in different geographic regions,” McConnell says.
Two of the three green roofs in the study were effective at lowering temperatures, the researchers report in the journal Sustainable Cities and Society.
The best performer was the green roof at Millennium Park, one of the largest green roofs in the world at 99,000 square meters. It’s home to more than 300 plant species, including grasses, flowers, shrubs, and trees. Average temperatures at Millennium Park were lower than at the corresponding control site, and lower after installation of the green roof than before. This means that the green roof “was able to fully mitigate larger warming trends,” the researchers point out.
The green roof at City Hall is smaller than that at Millennium Park, about 1,900 square meters, but has an impressively diverse suite of plantings: more than 150 plant species, including a few trees. The City Hall green roof lowered temperatures relative to its nearby control site. But rising temperatures by the end of the study period signal that this roof garden couldn’t completely cancel out the effects of a warming climate.
The third green roof, 7,000 square meters in size, was installed on top of a Walmart Supercenter when the store was built in 2006. This site “allows us to test whether the green roof is able to mitigate the known warming effects of building densification,” the researchers write.
The Walmart site was previously a grassy vacant lot, and the amount of vegetation on the landscape decreased when the store was built—despite the green roof—the analysis shows. What’s more, temperatures at the Walmart were more or less the same as they were at the control site, suggesting that the store’s green roof had little cooling effect.
The Walmart green roof was made up of just three types of low-growing, drought-tolerant plants. The results echo other studies that have called into question the cooling abilities of green roofs with a similar structure, at least in certain climates.
But the researchers caution that more systematic studies encompassing a larger number of green roof sites will be necessary to fully work out how factors such as vegetation type, local climate, site characteristics, rooftop size and structure, and building energy consumption determine the effectiveness of green roofs. “There is a need for more research on climate mitigation strategies in small and mid-sized cities, particularly in Asia and Africa,” McConnell adds.
Source: McConnell K. et al. “A quasi-experimental approach for evaluating the heat mitigation effects of green roofs in Chicago, Illinois.” Sustainable Cities and Society 2022.
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