While developing green spaces can help to reduce ‘heat island’ effects in cities, a new study by the University of Cambridge suggests that planting the wrong sort of trees in the wrong locations can actually heat up cities at night.
The study, which was published in Communications Earth & Environment, found that the planting of trees in urban areas can lower pedestrian-level air temperature by up to 12°C, while in 83% of cities, introducing trees reduced peak monthly temperatures to below 26°C – achieving the ‘thermal comfort threshold’.
At the same time, however, it indicates that planting the wrong species or the wrong combination of trees in suboptimal locations or arrangements can limit their benefits, as tree canopies can also trap heat at night.
Variations by climate
The benefits of trees as a means to reduce the impact of global warming in cities varies depending on the climate. In hot and dry climates, trees offer significant cooling during the day but may increase nighttime temperatures slightly, according to the study.
In arid climates, for example, trees can cool cities by 9°C during the day but raise nighttime temperatures by 0.4°C, while in humid, tropical climates, the cooling effect is around 2°C, while the nighttime heating effect increases to 0.8°C.
‘Busts the myth’
“Our study busts the myth that trees are the ultimate panacea for overheating cities across the globe,” commented Dr Ronita Bardhan, associate professor of Sustainable Built Environment at Cambridge’s Dept. of Architecture. “Trees have a crucial role to play in cooling cities down but we need to plant them much more strategically to maximise the benefits which they can provide.”
As regards planning, the study suggests that in cities with open urban layouts, combining evergreen and deciduous trees generally results in better cooling, while cities with more compact layouts may benefit from evergreen species, which tend to cool more effectively in dry climates.
The authors took into account the findings of 182 studies, undertaken in 110 global cities and regions, which were published between 2010 and 2023.
“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 emphasise 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 maximise cooling benefits.” Read more here and here.
Read more: Lack of green spaces exacerbates extreme heat in Global South
