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- City features like asphalt and concrete absorb and retain a significant amount of heat, creating what is known as the urban heat island effect.
- This phenomenon leads cities to record temperatures up to 10 degrees Fahrenheit higher than neighboring suburbs or rural areas.
- Using satellite images, researchers measured the affect of green roofs at Chicago’s Millennium Park, City Hall and a Walmart shopping center and compared the changes in land surface temperatures and vegetation abundance at the sites to nearby buildings without green rooftops.
Recent research suggests that green roofs — rooftops covered with greenery or other sun-reflecting materials — can reduce heat in cities.
City features like asphalt and concrete absorb and retain a significant amount of heat, creating what is known as the urban heat island effect. This phenomenon leads cities to record temperatures up to 10 degrees Fahrenheit higher than neighboring suburbs or rural areas, an effect that research suggests might be mitigated with soil and a diversity of plants on city roofs.
NASA’s Goddard Institute for Space Studies (GISS) in New York partnered with Chicago’s Public Health and Planning and Development departments to study three sites that installed green roofs near the beginning of the century.
Researchers measured the affects of green roofs at Millennium Park, City Hall and a Walmart shopping center and compared the changes in land surface temperatures and vegetation abundance at the sites to nearby buildings without green rooftops.
“As cities grow and develop, they need to make good decisions about their infrastructure, because these decisions often last for 30 or 50 years or longer,” Christian Braneon, a climate scientist and civil engineer at Columbia University and GISS, said in a news release. “In the context of more frequent heat waves and more extreme heat, it’s important to understand how these urban design interventions can be effective.”
The installations at Millennium Park and City Hall showed a decrease in temperature over the decade, according to satellite images taken by Landsat 5 satellite between 1990 and 2011. Researchers said the site at Millennium Park, which is next to Lake Michigan and surrounding by an array of greenery, was the only site to completely mitigate warming over the study period.
City Hall also showed a reduction in warming during the study, but the team noted temperatures were on the rise toward the end. Researchers said that results from the Walmart site, which was built during the study period, might have been affected by a loss in overall vegetation during construction despite its new green rooftop.
“You might think that putting a green roof on your new building would make a significant impact. But what we see is that a lot of impervious material may also be added there – such as a parking lot around the building,” Braneon said. “As a result, you might reduce the impact of the parking lot, but you certainly haven’t created the cooling effect that the overgrown vegetation had.”
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Study authors said the overall benefits of green rooftops might be affected by a host of factors, including plant diversity, a building’s cooling efficiency, and the building’s regional location, adding they’re hopeful their research might be furthered and used by city planners in other regions.
“My hope would be that the methods we proposed show a low-cost way for folks working in less-resourced cities – who maybe don’t have access to a university or government researcher – to study their own communities,” the study’s lead author Kathryn McConnell said.
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