A groundbreaking study led by the University of Cambridge has discovered that while trees can significantly cool cities during the day, they can also trap heat at night. This unexpected finding urges urban planners to choose the right tree species and planting locations to effectively combat urban heat stress.
While urban trees have long been celebrated for their ability to cool city environments, a recent study reveals a surprising downside: planting the wrong species or in suboptimal locations can actually make cities hotter at night.
Researchers led by the University of Cambridge found that despite lowering pedestrian-level air temperatures by up to 12°C during the day, certain tree canopies can trap heat and raise nighttime temperatures.
“Our study busts the myth that trees are the ultimate panacea for overheating cities across the globe,” Ronita Bardhan, associate professor of sustainable built environment at the University of Cambridge, said in a news release. “Trees have a crucial role to play in cooling cities down, but we need to plant them much more strategically to maximize the benefits which they can provide.”
The study, published in Communications Earth & Environment, aggregated data from 182 studies across 110 global cities or regions, encompassing 17 different climates. This comprehensive assessment aimed to bridge gaps left by previous research, which typically focused on specific climates or regions.
Cooling by Day, Warming by Night
During the day, urban trees cool cities by blocking solar radiation, evaporating water through pores in their leaves and modifying airflow. However, at night, tree canopies can trap longwave radiation from the ground, primarily due to aerodynamic resistance and ‘stomatal closure’ — the closing of microscopic leaf pores in response to heat and drought stress.
Climate-Specific Results
The study found that urban trees tend to cool cities more in hot, dry climates, and less in hot, humid climates. For example, in tropical savanna regions like Nigeria, trees cooled cities by up to 12°C during the day but also increased nighttime temperatures by up to 0.8°C. Similarly, in temperate climates, trees can cool cities by 6°C but can cause a 1.5°C increase at night.
Trees performed well in arid climates, cooling urban areas by over 9°C and warming them at night by only 0.4°C. In contrast, in tropical rainforest climates, the daytime cooling effect was around 2°C, but the nighttime warming effect still reached 0.8°C.
Strategic Urban Greening
The researchers highlighted that the benefits of urban trees could be maximized by considering local climate conditions and urban layouts when selecting tree species. Cities with more open urban layouts, featuring a mix of evergreen and deciduous trees of varying sizes, experienced greater cooling. In arid regions, evergreen species proved more effective, especially in compact cities like Cairo and Dubai.
“Our study provides context-specific greening guidelines for urban planners to more effectively harness tree cooling in the face of global warming,” Bardhan added. “Our results emphasize that urban planners not only need to give cities more green spaces, they need to plant the right mix of trees in optimal positions to maximize cooling benefits.”
The study also emphasized that trees should not be the sole solution for urban cooling. Other measures such as solar shading and reflective materials remain crucial.
Planning for Future Cities
The research team developed an interactive database and map to help urban planners estimate the cooling efficacy of different strategies based on data from cities with similar climates and urban structures. This tool aims to guide cities in implementing more effective urban greening approaches.
“Urban planners should plan for future warmer climates by choosing resilient species, which will continue to thrive and maintain cooling benefits,” concluded Bardhan.
As cities worldwide grapple with rising temperatures and urban heat stress, these findings underscore the importance of strategic tree planting in urban planning to create cooler, more livable environments.