What Can Rotterdam Teach Us About The Power Of Green Roofs

From May 26 to June 26, the Dutch city of Rotterdam is showcased the potential of rooftops. The Rotterdam Rooftop Walk connects rooftops through air bridges, offering a vision of the future of cities.

“Often, cities offer little space to develop more green spaces, and rooftops provide that space for plants and for biodiversity,” says Léon van Geest, director of the Rotterdam Dakendagen Festival.

Green rooftops can be an effective solution to urban heat islands, cities or specific neighborhoods within an urban area that have become much warmer than their surroundings due to absorbing and re-emitting the sun’s heat. Disenfranchised urban areas are particularly struggling with heat: A 2021 study in the Nature Communications journal found that heat exposure tends to be higher in lower-income neighborhoods, both in the U.S. and globally. Residents of these neighborhoods are also less likely to have air conditioning, or may be unable to afford to use it even if they do have it. This heat exposure aggravates inequality through risks such as higher heat-related mortality, heat strokes, dehydration, loss of labor productivity and decreased learning capacities, while also increasing costs for air-conditioning.

Enter green rooftops. They can take many shapes — lush gardens, potted plants, solar panels, urban farms, wildflower meadows and community spaces painted in light colors that absorb less sunlight — and can help combat heat islands in several ways. Plants and even trees on the roofs of buildings can drastically lower the urban heat island effect by reducing temperatures. According to the EPA, citywide ambient temperatures can be lowered by 5 degrees Fahrenheit with green roofs, reducing building energy use by up to 0.7 percent and thus lowering energy demand. Green roofs can also retain water and thereby prevent or reduce urban flooding. They filter greenhouse gas emissions and other pollutants from the air and attract more biodiversity.

Ashish Sharma, atmospheric scientist and graduate faculty at the University of Illinois Urbana-Champaign, researches how low-income neighborhoods could benefit from more green roofs in Chicago. In a 2018 paper on the role of green roofs in reducing heat stress in vulnerable urban communities, Sharma writes that “efforts to mitigate heat impacts, such as implementation of green roofs, offer solutions to issues of social justice as well.” Sharma and his team have identified several neighborhoods on the South Side and West Side of Chicago that are highly vulnerable to heat, have high electricity consumption, and also could benefit from green roofs to reduce temperatures. But the infrastructure is only one aspect of green roofs.

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