Temperatures in cities are rising across the globe and urban heat stress is already a major problem causing illness, death, a surge in energy use to cool buildings down, heat-related social inequality issues and problems with urban infrastructure. Some cities around the world, including in India, have already started implementing mitigation strategies, with tree planting prominent among them. But a University of Cambridge led study now warns that planting the wrong species or the wrong combination of trees in suboptimal locations or arrangements can limit their benefits.
Dr Ronita Bardhan, Associate Professor of Sustainable Built Environment at Cambridge University's Department of Architecture, explains: “Our study busts the myth that trees are the ultimate panacea for overheating cities across the globe.
“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.”
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The study, published in ‘Communications Earth & Environment’ found that urban trees can lower pedestrian-level air temperature by up to 12°C. Its authors found that the introduction of trees reduced peak monthly temperatures to below 26°C in 83 per cent of the cities studied, meeting the ‘thermal comfort threshold’. However, they also found that this cooling ability varies significantly around the world and is influenced by tree species traits, urban layout and climate conditions.
Previous research on the cooling effects of urban trees has focused on specific climates or regions, and considered case studies in a fragmented way, leaving major gaps in our knowledge about unique tree cooling mechanisms and how these interact with diverse urban features.
To overcome this, the authors of this study analysed the findings of 182 studies – concerning 17 climates in 110 global cities or regions – published between 2010 and 2023, offering the first comprehensive global assessment of urban tree cooling.
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During the day, trees cool cities in three ways: by blocking solar radiation; through evaporation of water via pores in their leaves; and by foliage aerodynamically changing airflow. At night, however, tree canopies can trap longwave radiation from the ground surface, due to aerodynamic resistance and ‘stomatal closure’ – the closing of microscopic pores on the surface of leaves partly in response to heat and drought stress.
The researchers emphasise that we cannot rely entirely on trees to cool cities, and that solutions such as solar shading and reflective materials will continue to play an important role.
*Info: University of Cambridge
