Trees can significantly cool urban areas during the day, but recent research indicates that their canopies may trap heat and raise nighttime temperatures. The study’s goal is to assist urban planners in selecting the optimal types of trees and their locations to alleviate urban heat stress.
Global temperatures in cities are on the rise, leading to severe issues such as health problems, increased energy consumption for cooling, and social inequalities tied to heat, alongside challenges for urban infrastructure.
Some cities are already taking steps to address this, with tree planting being a key strategy. However, a study led by the University of Cambridge warns that improperly chosen tree species or their unsuitable placements can diminish their positive effects.
This study, published in Communications Earth & Environment, shows that urban trees can reduce the air temperature at pedestrian level by as much as 12°C. The researchers discovered that the presence of trees lowered peak monthly temperatures below 26°C in 83% of the studied cities, achieving a ‘thermal comfort threshold’. Nonetheless, the effectiveness of this cooling varies widely across different parts of the world due to factors such as tree species, urban design, and climatic conditions.
“Our study dispels the misconception that trees are a cure-all for overheating cities worldwide,” stated Dr. Ronita Bardhan, Associate Professor of Sustainable Built Environment at Cambridge.
“While trees are vital for cooling urban environments, it is important to plant them strategically to maximize their benefits.”
Prior research on urban trees’ cooling effects often focused on specific climates or regions and reviewed case studies in a piecemeal fashion, creating significant gaps in understanding the unique cooling mechanisms of trees and their interaction with various urban characteristics.
To address this, the authors of the current study examined 182 studies related to 17 climates across 110 cities or regions worldwide, published from 2010 to 2023. This provided a comprehensive global analysis of urban tree cooling.
During the daytime, trees cool cities through three primary mechanisms: blocking sunlight, evaporating water through leaf pores, and altering airflow with their foliage. At nighttime, however, tree canopies can hold in longwave radiation emitted from the ground, due to aerodynamic resistance and ‘stomatal closure’—the closing of tiny pores on leaves in response to heat and drought stress.
Variation by climate type
The research indicates that urban trees tend to cool cities more effectively in hot and dry climates than in hot and humid ones.
In ‘tropical wet and dry or savanna’ climates, for instance, trees can cool urban areas by up to 12°C, as observed in Nigeria, but they can also raise nighttime temperatures by as much as 0.8°C in this same climate.
In arid climates, trees cool areas by just over 9°C while increasing temperatures at night by 0.4°C.
In climates with tropical rainforests, the daytime cooling effect was reduced to around 2°C, with a nighttime increase of 0.8°C. In temperate areas, trees were shown to cool cities by up to 6°C while contributing to a 1.5°C increase at night.
Using trees more strategically
The study suggests that cities with more open layouts are likely to host a mix of evergreen and deciduous trees of varying sizes, leading to greater cooling in temperate, continental, and tropical climates.
Utilizing a combination of tree types in these areas often results in an additional 0.5°C of cooling compared to cities populated by only one type, as mixed trees provide seasonal shading and sunlight control, offering cooling across different heights.
Conversely, the researchers found that evergreen species are more prevalent and effective in compact urban settings like Cairo or Dubai, where specific conditions apply.
Overall, trees were found to be more effective in cooling open, low-rise urban areas in dry climates. In these open spaces, improvements in cooling by about 0.4°C can be achieved, due to more substantial tree canopies and a diverse mix of tree species.
“Our study offers tailored greening strategies for urban planners to better utilize tree cooling amidst global warming challenges,” Dr. Bardhan remarked.
“The findings highlight that urban planners must not only increase green spaces but also ensure the right mix of tree species is planted in optimal locations to maximize cooling benefits.”
“To prepare for warmer future climates, planners should select resilient tree species that will thrive and sustain cooling effects,” added Dr. Bardhan, a Fellow at Selwyn College, Cambridge.
Matching trees to urban forms
The study further asserts that the choice of tree species and their placements must align with urban architecture. Factors such as the design of street canyons, local climate zones, aspect ratios, and the visible sky ratio must all be thoughtfully considered to maximize the effect of trees.
While having a higher tree canopy cover in street canyons typically enhances cooling effects, excessively dense coverage may retain heat at the pedestrian level, particularly in compact urban areas with high temperatures. In these situations, it is advisable to use narrower tree species and spaced planting.
The researchers stress that trees alone are insufficient to cool cities and suggest that additional solutions, including solar shading and reflective materials, will remain crucial.
Moreover, the team has developed an interactive database and map to assist users in estimating the cooling effectiveness of various strategies based on data from cities with similar climates and urban profiles.
