Trees can retain heat in cities at night

Dec. 10 () –

Although trees can significantly cool some cities during the day, tree canopies can also trap heat and increase temperatures at night.

It’s the finding of a new study that aims to help urban planners choose the best tree combinations and planting locations to combat urban thermal stress.

Temperatures in cities are rising around the world and urban heat stress is already a major problem causing illness, death, an increase in energy use to cool buildings, heat-related social inequality issues and problems with urban infrastructure.

Some cities have already begun to implement mitigation strategies, among which tree planting stands out. But a study led by the University of Cambridge now warns that planting the wrong species or the wrong combination of trees in suboptimal locations or layouts may limit your benefits.

The study, published in Communications Earth & Environmentfound that urban trees can reduce pedestrian-level air temperatures by up to 12°C. Its authors found that the introduction of trees reduced maximum monthly temperatures below 26 °C in 83% of the cities studied, meeting the “thermal comfort threshold”. However, they also found that this cooling capacity varies significantly around the world and is influenced by the characteristics of tree species, urban design and climatic conditions.

“Our study debunks the myth that trees are the ultimate panacea for overheating cities around the world,” he said. in a statement Dr Ronita Bardhan, Associate Professor of Sustainable Built Environment at the University of Cambridge.

“Trees have a crucial role to play in cooling cities, but we need to plant them much more strategically to maximize the benefits they can provide.”

Previous research on the cooling effects of urban trees has focused on specific climates or regions and considered case studies in a fragmented manner, leaving important gaps in our knowledge about the unique cooling mechanisms of trees and how these They interact with various urban characteristics.

To overcome this, the authors of this study analyzed the findings of 182 studies, related to 17 climates in 110 global cities or regions, published between 2010 and 2023, offering the first comprehensive global assessment of urban tree cooling.

During the day, trees cool cities in three ways: by blocking solar radiation; Trees can absorb long-wave radiation from the soil surface by evaporating water through the pores in their leaves and by aerodynamically changing the airflow produced by the foliage. However, at night, tree canopies can trap long-wave radiation from the soil surface, due to aerodynamic drag and ‘stomatal closure’ (the closing of microscopic pores on the surface of leaves, partly in response to heat and drought stress).

The study found that urban trees generally cool cities more in hot, dry climates, and less in hot and humid climates.

In the ‘tropical wet and dry or savannah’ climate, trees can cool cities by up to 12°C, as recorded in Nigeria. However, it was in this same climate that trees also warmed cities more at night, up to 0.8°C.

The trees performed well in arid climates, cooling cities by just over 9°C and warming them by 0.4°C at night.

In rainforest climates, where humidity is higher, the daytime cooling effect was reduced to about 2°C, while the nighttime warming effect was 0.8°C.

In temperate climates, trees can cool cities by up to 6°C and warm them by up to 1.5°C.

The study notes that cities with more open urban layouts are more likely to have a mix of evergreen and deciduous trees of different sizes. The researchers found that this It tends to generate greater cooling in temperate, continental and tropical climates.

The combined use of trees in these climates generally results in 0.5°C more cooling than in cities where there are only deciduous or evergreen trees. This is because mixed trees can balance seasonal shading and sunlight, providing three-dimensional cooling at different heights.

However, in arid climates, researchers found that evergreen species dominate and cool more effectively in the specific context of compact urban layouts such as Cairo in Egypt or Dubai in the United Arab Emirates.

In general, trees cooled most effectively in open, low-lying cities in dry climates. In open urban designs, cooling can be improved by approximately 0.4°C because their larger green spaces allow for more and larger tree canopies and a greater mix of tree species.

“Our study provides context-specific greening guidelines for urban planners. make more effective use of tree cooling in the face of global warming“said Dr. Ronita Bardhan.

“Our results emphasize that urban planners not only need to give cities more green space, but also need to plant the right mix of trees in optimal positions to maximize the benefits of cooling.”

“Urban planners should plan for future warmer climates by choosing hardy species continue to thrive and maintain the benefits of cooling,” Dr Bardhan, a fellow at Selwyn College, Cambridge, said in a statement.

The study goes further and argues that the selection and location of species must be compatible with urban forms.