As the climate warms and average temperatures rise, cities will feel the brunt of the extra heat. Rooftop gardens and greenery can help with that, according to a new study from NASA’s Goddard Institute for Space Studies (GISS) in New York.

Roof-mounted bright surfaces or plants can help absorb and reflect part of the heat being poured into cities by sunlight. A team at GISS quantified the impact of ‘green roofs’ — roofs full of plant cover — in reducing ambient temperatures using three buildings outfitted this way in Chicago. Although the effect of such buildings on overall temperatures in their area is smaller than that of a plant-filled lot, the team reports that there are noticeable advantages to be had from changing a building’s roof from black-tar or simple concrete to a garden.

Greening it up

“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,” said Christian Braneon, a climate scientist and civil engineer at Columbia University and GISS. “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.”

Cities are significantly hotter than the areas around them, a phenomenon known as the urban heat island effect. This is due to the huge masses of asphalt, concrete, and other heat-trapping materials that cover every inch of a city. The combined effect of these surfaces — which do not reflect or absorb heat the same way plants do, instead storing it and releasing it later — can make cities around 10 degrees Fahrenheit (6 degrees Celsius) hotter. City areas with fewer green spaces or trees can become even hotter than this. Those who have trouble bearing the heat and those who cannot afford to insulate themselves from it — older adults, low-income communities, and some communities of color — are affected by these higher temperatures more heavily.

Green roofs can help mitigate this heat by providing extra plant cover to cityscapes. They can use extensive greenery (shallow soil, low-maintenance plants) or intensive greenery (requiring deeper layers of soil but able to support more diverse plants and trees).

The team partnered with Chicago’s Public Health and Planning and Development departments for the research. Together, they studied three sites that had green roofs installed in the early 2000s: Millennium Park, City Hall, and a Walmart shopping center. Imagery from the Landsat 5 satellite, captured between 1990 and 2011, was used to track changes in land surface temperatures and vegetation cover at these sites alongside control sites that were nearby but did not have green roofs. Ultimately, they found that two of the buildings had a cooling effect over time, but the third – replacing a previous green space – was less effective.

Millennium Park, whose roof is an intensive mix of plants and is located near Lake Michigan (which acts as a heat sink), showed significantly lower average temperatures than its surroundings after the roof was installed (in 2004). Out of all three sites, this was the only one where the roof completely mitigated the warming caused by climate change over the study period.

City Hall also has an intensive roof, which had been installed in 2002. Temperatures did drop (compared to the control site), but not enough to fully counter climate change; average temperatures here were on the rise by the end of the study period.

For the Walmart site, the findings paint a different picture. While the Millennium Park and City Hall sites had green roofs added onto existing buildings, the Walmart site had been newly-built during the study period. Despite having an extensive green roof, it replaced a vacant, green lot, meaning the overall plant cover of the area actually dropped.

“In a lot of places, you might be developing an area that didn’t have something there before; it just had overgrown vegetation,” said Braneon, who co-leads urban research for the Climate Impacts Group at GISS. “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. 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.”

The findings showcase why larger studies, with greater sample sizes, are needed to determine how to best employ green roofs. The results so far show that there is definitely room for them and that they do work. As cities grow and the climate warms, using green roofs will become an increasingly important element in our strategies to cope with excess heat.

The simple analysis, publicly available data, and model for working directly with cities could help urban planners assess the viability of green roofs in their own areas.

“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,” McConnell said.

“Traditionally, civil engineers and urban planners assumed a stationary climate,” Braneon added. “The whole practice is built on the premise that we can look at the past to assess risk in the future. Everything is being flipped on its head due to climate change, so I’m hoping to do more work that changes how civil engineers and urban planners practice.”

The paper “A quasi-experimental approach for evaluating the heat mitigation effects of green roofs in Chicago, Illinois” has been published in the journal Sustainable Cities and Society.