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Rooftop gardens: A way to turn down the heat in cities

by Madelein
Rooftop gardens: A way to turn down the heat in cities

Main image: Rooftop garden. Photo credit – Pixabay

For several decades, researchers have promoted replacing black tar and other dark-coloured roofing materials with bright, sun-reflecting surfaces or “green roofs” full of plant cover. Now they have used freely available satellite data to measure how effective these changes are.

The decade-old method

Heat is often intensified or amplified in cities. Asphalt, concrete and similar materials absorb and retain significantly more heat than vegetation, so temperatures in urban areas are often hotter than in surrounding suburbs or rural regions.

How green roofs differ

Green roofs are designed to harness the cooling power of plants to lower the temperature in city spaces. The greenery may be extensive (shallow soil, low-maintenance plants) or intensive (deeper soil, more diverse plants, and trees).

Heat exchange and water runoff of a green roof versus a traditional roof
Photo credit: US Environmental Protection Agency

In Chicago, the USA, the Geographic Information System Specialist (GISS) team studied three sites to see how green roofs affected surface temperatures around those buildings, and whether there was a difference between those sites and others nearby without green roofs. Two out of three green roofs in the study reduced temperatures, but results indicated that effectiveness may depend on location and plant diversity, among other factors.

Rooftop gardens and greenery can help ease some of the severe heat in cities, according to research from climate scientists at NASA’s Goddard Institute for Space Studies (GISS) in New York.

Studying green roofs in urban jungles

“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 heatwaves and more extreme heat, it’s important to understand how these urban design interventions can be effective.”

The benefits of green roofs depend on a variety of factors – from the geographic region and plant diversity to rooftop structure and the cooling efficiency of the building itself, the scientists said.

Studies with larger sample sizes are needed to tease apart these details, but this study represents a promising start. With urban heat island effects expected to intensify as Earth’s climate warms, it will become more important to understand these variables.

Proposed methods

The study method is designed to be used by other cities for further research, said lead author Kathryn McConnell, a doctoral candidate at Yale University’s School of the Environment. 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.”

Full acknowledgement and thanks go to www.sciencedaily.com for the information in this editorial.

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