urban rooftop garden

Roof gardens shown to counter urban heat island effects

Image credit: Dreamstime

Rooftop gardens and greenery can help to alleviate severe heat in cities, climate scientists have said.

Heat is often intensified or amplified in cities, a phenomenon known as the urban heat island effect. Asphalt, concrete and similar materials absorb and retain significantly more heat than vegetation, so temperatures in urban areas are often 5°C hotter than surrounding suburbs or rural regions.

In neighbourhoods with fewer trees and green spaces, this heat often disproportionately affects older adults, low-income communities and some communities of colour.

A team at Nasa’s Goddard Institute for Space Studies (GISS) have used freely available satellite data to measure the effectiveness of architectural changes designed to reduce urban heat. These include replacing black tar and other dark-coloured roofing materials with bright, Sun-reflecting surfaces or “green roofs” full of plant cover.

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).

The team studied three sites in Chicago 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 of three green roofs in the study reduced temperatures, but results indicated that effectiveness may depend on location and plant diversity, among other factors.

“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 GISS climate scientist.

“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.”

Using imagery captured by the Landsat 5 satellite between 1990 and 2011, the researchers compared changes in land surface temperatures and vegetation abundance at the study sites, as well as nearby control sites without green roofs.

Results from the three sites were mixed. Millennium Park, which has an intensive mix of plants and is located near Lake Michigan, showed significantly lower average temperatures after its green roof was installed in 2004. It was the only site where the roof fully mitigated climate warming over the study period.

City Hall, also an intensive site, had a green roof installed in 2002. Its temperatures after green roof installation were lower than those at the control site, but they were rising toward the end of the study period.

The Walmart site told a different story. While the green roofs at Millennium Park and City Hall were added onto existing buildings, the supermarket was newly built during the study period. Even though the Walmart installed an extensive green roof, the conversion of the land from a vacant, grassy lot to a store meant the vegetation index for the landscape decreased.

The team also found that the benefits of green roofs depend on a variety of factors, from geographic region and plant diversity to rooftop structure and the cooling efficiency of the building itself.

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.

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