The research, led by Jia Wang, Weiqi Zhou, and Yuguo Qian from the Chinese Academy of Sciences and co-authored by Steward Pickett, urban ecologist at Cary Institute of Ecosystem Studies, aims to measure the cooling impact of trees at a citywide level.
"Trees offer many benefits to cities, and cooling is one of them," explained Pickett. "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. The shade provided by trees also helps with cooling."
Traditionally, studies on trees' cooling effects focused on small-scale impacts, such as a specific neighborhood. For instance, a 1% increase in tree canopy can lower temperatures in a local area by 0.04 to 0.57 degrees Celsius. However, for city planners managing entire urban environments, determining the cooling impact of tree cover on a city scale remained a challenge.
"That's valuable, but planners and decision makers are thinking about the whole city," Pickett stated. "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."
To bridge this knowledge gap, the research team used satellite images and temperature data from four diverse cities: Beijing and Shenzhen in China, along with Baltimore and Sacramento in the U.S. The cities were selected for their distinct climates: temperate for Baltimore and Beijing, subtropical for Shenzhen, and Mediterranean for Sacramento. Each city was mapped in small pixel units, roughly the size of city blocks, where land surface temperature and tree cover were analyzed. They then scaled up the analysis to neighborhood and city levels to understand how cooling efficiency changes across larger urban areas.
Results showed that as tree cover increases across larger areas, the cooling effect strengthens, although at a slower rate as scale increases. In Beijing, for example, a 1% increase in tree canopy at the block level reduced temperature by about 0.06 degrees, whereas at the city level, the same increase in canopy could reduce temperatures by roughly 0.18 degrees.
Greater cooling at larger scales is due to large clusters of trees providing a cumulative cooling effect. This scaling insight allows planners to calculate citywide tree cover goals more accurately, giving municipalities a critical tool to combat extreme urban heat.
Co-author Weiqi Zhou highlighted that "cooling efficiency follows a power law across scales - from as small as 120 by 120 meters to as large as regions covering the entire city. The relationship holds across all four of the studied cities, which are in very different climates. 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 researchers estimate that a 1% increase in Baltimore's tree canopy could reduce land surface temperatures by 0.23 C. Achieving a cooling effect of 1.5 C would require a 6.39% increase in tree cover.
While these findings offer city-level guidance, Pickett emphasized the importance of targeting specific areas for equitable tree distribution and community support. "This paper doesn't tell you where to put the trees," he noted. "That's another sort of analysis, which would have to involve a lot more social information and engagement with communities or with individual property owners."
Next steps for this research include expanding the analysis to additional cities and examining how tree cooling benefits might change as climate change intensifies heat and drought conditions in urban areas.
Research Report:A scaling law for predicting urban trees canopy cooling efficienc
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