The cooling capacity of cities from the presence of trees varies significantly across the world, depending on the characteristics of the planted species, the urban layout, and climatic conditions, according to a study.
Global warming is choking cities around the world, where urban heat stress already causes illnesses, deaths, high energy costs, social inequality problems and urban infrastructure issues.
For this reason, many cities have begun implementing heat mitigation strategies, such as planting trees.
However, planting the wrong species or using them in less ideal locations limits the benefits, according to a study from the University of Cambridge (United Kingdom) published on Tuesday in the journal Communications Earth & Environment.
The study found that urban trees can reduce air temperature at pedestrian level by up to 12 degrees Celsius.
In addition, they found that the presence of trees reduced monthly maximum temperatures below 26°C in 83% of the cities studied, thus reaching the “thermal comfort threshold”.
But the same study found that this cooling capacity varies significantly around the world.
“Our work undermines the myth that trees are the definitive panacea against overheating in cities around the world,” warned Ronita Bardhan, Professor of Environment at Cambridge.
The study aimed to help urban planners choose the best combinations of trees and planting sites to combat urban heat stress: “Trees play a crucial role in cooling cities, but we need to plant them far more strategically to maximize the benefits they can bring.”
Although trees can significantly cool some cities during the day, the investigation shows that the canopies of trees can also retain heat and increase temperatures at night.
To conduct the study, the team reviewed results from 182 previous investigations – relating to 17 climates in 110 cities or regions around the world – published between 2010 and 2023 and observed that urban trees tend to cool cities more in hot and dry climates, and less in hot and humid climates.
According to the study, trees performed well in arid climates, cooling cities by just over 9°C and warming them at night by 0.4°C, while in tropical rainforest climates, where there is more humidity, daytime cooling was reduced to about 2°C and the nighttime warming effect was 0.8°C.
Furthermore, in temperate climates, trees can cool cities by up to 6°C and warm them by up to 1.5°C, according to the study.
The investigation pointed out that cities with more open urban layouts are more likely to feature a mix of evergreen and deciduous trees of different sizes, which, according to the researchers, helps to trigger greater cooling in temperate, continental and tropical climates.
The combined use of trees in these climates generally cools by about 0.5°C more than in cities where only deciduous or evergreen trees exist.
This is because mixed-tree canopies can balance seasonal shade and sunlight, providing three-dimensional cooling at different heights.
In arid climates, however, the researchers found that evergreen species dominate and cool more effectively in the specific context of compact urban layouts, such as Cairo (Egypt) or Dubai (United Arab Emirates).
The researchers concluded that urban planners should not limit themselves to trees as a tool to cool cities, but should find other solutions, such as solar protection and reflective materials, which will continue to play an important role.
