Cities suffer from the urban heat island effect and localized flooding. Urban planners often suggest adding parks and embracing construction practices like indoor and outdoor living walls and green roofs, practices that can have cooling effects and can lead to the absorption of stormwater runoff. But new research finds urban greening practices have different effects in different environments. Communities will be better served by identifying their greatest need and considering which greening practice will best address it.
There are a lot of potential benefits of urban greening, but there is also “a temptation to assume the benefits in one context can translate to another,” said Mark Cuthbert, lead author and a principal research fellow at the School of Earth and Environmental Sciences at Cardiff University in Wales. Cuthbert and his coauthors undertook the research to understand the effects of urban greening practices on different ecosystems.
One major finding is that in arid climates, urban greening works best when focused on reducing surface water runoff, whereas in more humid climates, cooling is more likely to be a stronger benefit. Because of this relationship, “common urban greening strategies cannot yield high performance simultaneously for addressing both urban heat island and urban flooding problems in most cities globally,” Cuthbert explained.
Other factors, such as substrate thickness, may determine how beneficial urban greening is. In general, thicker soils like those in urban parks have greater soil moisture storage capabilities and thus can provide some measure of both cooling and water retention benefits, in addition to being more robust in extreme weather events due to climate change. Thinner substrates like green roofs provide only “reasonable performance,” according to the study.
Harini Nagendra, who leads the Center for Climate Change and Sustainability at Azim Premji University in India, was not involved in the new study but highlighted the need for further research on the topic: “Tropical cities will need to follow different strategies from temperate ones. But most of these models and field studies have been done in temperate cities, so we don’t really know enough to devise best strategies for tropical cities.”
The Need for Local Planning and Community Engagement
Although an urban planner may want to install a park on the basis of its localized climate benefits, factoring in what community residents actually want in their neighborhoods “adds several layers of complexity” to urban greening policies and projects, Nagendra said. She is the coauthor of a January 2021 paper in Urban Forestry and Urban Greening that found inequities related to gender and income in accessing urban parks in Hyderabad, a city of more than 6 million in central India.
The study focused on four parks in the city and found that low-income groups were marginalized. For example, the parks all had entry fees, as well as regulations that prohibited flower harvesting. In addition, women residents raised safety concerns. Urban parks like those in Hyderabad must be “reimagined as commons, with access to all,” the paper concluded.
Creating green spaces can also contribute to social inequities through the process of gentrification, experts say. Isabelle Anguelovski, director of the Barcelona Laboratory for Urban Environmental Justice and Sustainability, said that green infrastructure can contribute to “higher land and home prices and therefore displacement of the most socially fragile residents due to processes of green gentrification.”
Cities should be both green and just, Anguelovski added, and a narrow perspective of greening that focuses on only the ecological benefits could risk the creation of social inequalities if it is not accompanied or even preceded by strong affordable housing and inclusive greening policies.
Earlier reports have highlighted how the involvement of local communities could help create environmentally just green spaces. Cuthbert added that implementation strategies need to “take account of a much wider set of social, cultural, and economic factors.”
“Is it an area where children are deprived of play spaces and there is high obesity? Do [residents] want safe open spaces for children to play?” Nagendra asked. “Or is it a food desert, where it is more important for them to have community gardening areas where people can grow their own food?”
Such greening projects “can’t be set in stone,” Nagendra said. The creation of green spaces “will set into motion social-ecological transformations in the landscape and in the community, all of which may not be anticipated in advance.” For example, in tropical areas, greening measures that inadvertently increase water absorption could lead to waterlogged areas where mosquitos thrive. The diseases they carry could then call for draining interventions, underscoring the importance of adaptation and constant reevaluation in greening practices.
Ultimately, the creation of green spaces requires “codesign between scientists, experts, and local residents, and that is an interactive process where there are no easy shortcuts,” Nagendra said.
—Rishika Pardikar (@rishpardikar), Science Writer