Imma Perfetto
Cosmos science journalist
Two new studies published in the past week have presented new tools to help cities across the globe adapt to the increasing threat of dangerous heat.
The first, published in the journal Proceedings of the of the National Academy of Sciences (PNAS), offers a tool to determine how the cooling efficiency of tree cover varies from the neighbourhood to city level.
The second, published in PNAS Nexus, helps urban planners determine which interventions provide the best relief from the heat in a city’s most vulnerable areas.
The frequency and intensity of warm days and nights, and heatwaves, have increased globally since 1950. And, according to the Intergovernmental Panel on Climate Change’s (IPCC) Sixth Assessment Report, they will continue to increase in nearly all inhabited regions on Earth due to global warming.
Heat exposure greatly impacts human health and wellbeing. Cities compound this through the urban heat island effect, in which structures like roads and buildings absorb the sun’s heat more than natural landscapes and amplify already rising temperatures.
Steward Pickett, an urban ecologist at Cary Institute of Ecosystem Studies and co-author of the firststudy explains trees are good at cooling because they pump a lot of water from the ground into the air, and when that water evaporates at the leaf surface, it absorbs a vast amount of heat.
“That’s just the physics of evaporation,” Pickett says. “The shade provided by trees also helps with cooling.”
It’s been shown that when the urban tree canopy expands by 1% within neighbourhoods, nearby temperatures may decrease by 0.04 to 0.57°C. This is known as the cooling efficiency – the mathematical relationship between greenery and temperature.
“That’s valuable, but planners and decision makers are thinking about the whole city,” says Pickett.
“They’re asking, ‘How much tree canopy do we need for the whole city? What happens when we scale it up?’ And that information hasn’t been available.”
The team analysed satellite imagery and temperature data from 4 cities with different climates – Beijing and Shenzhen in China; Baltimore and Sacramento in the US.
The team discovered that the cooling efficiency of urban trees follows a “power law” relationship across all scales, providing urban planners and decision makers a tool to set science-based city-wide greening goals.
“The relationship holds across all 4 of the cities, which are in very different climates,” adds co-author Weiqi Zhou of the Chinese Academy of Sciences.
“This suggests that it could be used to predict the amount of additional tree cover needed to achieve specific heat mitigation and climate adaptation goals in cities worldwide.”
The second study suggests the varying impact of heat in different communities must be considered if urban “overheat” is to be mitigated equitably across all areas of a city during heatwaves.
The researchers from the University of Texas at Austin assessed the impact of urban heat mitigation strategies, such as reflective roofs, vegetated roofs, and urban tree cover, across communities in Houston.
They found that while painting roofs white was the best cooling option in places with low vulnerabilities, such as a generic city block, planting trees provided the best relief from the heat in Houston’s most socially vulnerable areas.
“This research lays the groundwork for city-specific goals aimed at creating community-oriented and diverse mitigation approaches,” the researchers write.
“By addressing the vulnerable communities, such as the elderly and underprivileged households, this study contributes toward efforts for safeguarding public health and advancing environmental justice for climate extremes.”
