As you may know from personal experience, summers tend to be much hotter in the city than in the countryside. This phenomenon is called the “urban heat island” effect. This difference in temperature happens in large part because urbanized areas have a greater concentration of buildings and asphalted roads that absorb and trap the heat, whereas the countryside has lots of trees and grasses and other growing things.
More rooftop gardens for the win
There is something that could help cool down our cities, and that is more rooftop gardens. At least that’s according to a recent study from climate scientists at NASA’s Goddard Institute for Space Studies (GISS).
The study, published in Sustainable Cities and Society, echoes previous recommendations from scientists to replace tar and other dark-colored materials that have been used in roofing for decades. Additionally, the recent research brings new evidence that “green roofs” filled with different types of plants can significantly mitigate hot temperatures in cities.
“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 study co-author 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.”
As part of the study, the researchers looked at satellite images of three green rooftop sites that had been installed in Chicago in the early 2000s. They then compared those images with ones taken between 1990 and 2011. Particularly, they analyzed the changes in surface temperature and vegetation and compared them to control sites nearby that had no green roofs.
The researchers observed lower temperatures in two out of the three sites. Particularly, one of the sites had experienced significant temperature changes compared to the other two. According to the authors, factors like the structure of the rooftop itself, its location, as well as the diversity of plants on it, can influence the benefits of a particular green roof on its surrounding area.
The difference between a green roof and a rooftop garden
Whether it’s simply a green roof or a rooftop garden may also matter. Green roofs are generally lighter, as they typically have a thin layer of soil for plants that don’t need much maintenance and can be installed on rooftops that are slanted. Rooftop gardens, on the other hand, are heavier and require a flat surface and deeper soil to accommodate a variety of plant species, including trees and flowers.
With climate change expected to exacerbate the heat island effect, making our cities greener is a key coping strategy. In addition to adding more greenery on our rooftops, planting more trees in our urban areas can also help a great deal — especially when we know that city trees are very good at trapping CO2.