Hot In The City: Why The Wrong Trees In The Wrong Places Matters

Forbes

5 days ago

Cooling down: New York has recorded temperatures of nearly 100 degrees Fahrenheit
In the last days of last month, the death toll tripled in several major European cities in the face of sweltering heat. Milan, Paris, Barcelona and London were the worst hit. With more than two thirds of the world's population expected to live in cities by 2050, the race to cool them down is on. Tree planting has become an important part of urban heat mitigation efforts.
But a Cambridge University-led study shows that planting the wrong species or combinations of trees may not just reduce the benefits, but actually make things worse, elevating night time temperature.
The scientists found that planting trees in urban areas can lower pedestrian-height air temperature by up to 12 degrees centigrade (53.5 F). Introducing trees reduced the highest temperature in 83% of the cities studied. However, the extent will vary, depending on urban layout and species' traits.
'Our study challenges the common perception that trees are the ultimate panacea for overheating cities worldwide,' says Professor Ronita Bardhan from the University of Cambridge. 'Trees have a crucial role to play in cooling cities down, but we need to plant them much more strategically to get the best results.'
For instance, adds Bardhan, 'The cooling behaviour of a species in a rural environment may be very different in an urban setting. It also matters where these trees are placed. In more sparsely populated areas they will do well, but in heavily built-up areas, they won't.'
Until now, we've known relatively little about unique tree cooling mechanisms and how these interact with different urban features. Previous research has tended to focus on specific climates or regions, relying on fragmented case studies.
The first comprehensive global assessment of urban tree cooling has changed all that. The authors looked at a wide range of studies over 17 climates in 100 cities and regions for the period 2010 – 2023. Understanding why planting matters is the first step in getting green right.
How Trees Cool Cities
In daytime, trees help reduce temperature by blocking solar radiation while water evaporation and foliage alters the airflow. In some circumstances, however, the 'wrong' trees can also increase air temperature at night.
'When the aerodynamic resistance is high and the leaf stomata close, heat dissipation through transpiration is reduced, limiting the escape of heat energy,' Bardhan says. 'This typically happens at night. If there is a high humidity load, this humid-warm air can be trapped and re-circulate beneath the tree canopy.'
The scientists found that urban trees in compact settings are most effective in cooling cities in hot and dry climates but are less effective in hot, humid ones.
In tropical wet and dry climates with distinct wet and dry seasons, such as central Africa, parts of South America and northern Australia, trees can be very effective in cooling cities by day. In Nigeria, trees can cool temperatures by as much as 12 degrees centigrade. But at night, trees warmed cities the most, by up to 0.8 degrees.
Evergreens beside Dubai's International Financial Centre
A lush green canopy hangs over parts of Dubai City. Among the acacias, neem, olive, palm and other desert trees the evening air carries the scent of Indian jasmine. Last year the municipality planted 216,500 new trees as part of its ambitious 2023 Quality of Life Strategy.
Bardhan and colleagues found that trees performed well in arid climates such as Dubai's, cooling cities by just over 9 degrees and warming them at night by 0.4 degrees.
Elsewhere, however, in a tropical rain forest climate with higher humidity, daytime cooling was only 2 degrees. Closer to home, trees in more temperate climates, such as London, can cool cities by 6 degrees but warm them up by 1.5 degrees.
In many hot countries, trees tend to be evergreen. By including deciduous trees in the mix, the cooling effect can often be greater. Recent planting schemes in Saudi Arabia have incorporated deciduous as well as evergreens.
The nature of the local urban environment will have an impact on the potential cooling while the idea balance of temperate and deciduous trees will vary. Urban layouts such as London's are more likely to benefit from the inclusion of deciduous trees alongside evergreens, but to a lesser extent than in Saudi Arabia.
'If you plant deciduous trees in a densely populated part of London, they won't perform in the same way as in the wild,' Bardhan observes. 'It's also important where these trees are placed. Trees may perform well in sparsely populated areas, but in dense urban settings the limited cooling impact reflects failed design.
"To be effective for the climate, cities need to treat nature as a fundamental design variable, rather than an afterthought.'
Selecting the optimum type and location for trees in an urban setting is critical.
Few cities can afford projects on the scale of Dubai's and those who may need the right trees most because of global warming – are often the poorest.
Bardhan and her colleagues at Cambridge have come up with a solution. They are developing a prototype shading device, inspired by nature's way of cooling. It's already been tested out successfully in dense urban areas.
'It mimics everything that natural trees do,' explains Bardhan 'We can control humidity along with temperature unlike standard shading devices that only regulate temperature. The design also allows the release of trapped radiation at night, unlike a tree.'
The shading device can resemble a tree or take other forms, such as a vending cart. Rather than just being a luxury in affluent neighbourhoods, shade and cooling can come to schoolchildren and the most vulnerable communities at relatively low cost. As Bardhan says, it's climate action put into reality.
With the data provided by the Cambridge study, urban planners have an invaluable tool that could transform efforts to reduce heat.
You can see the cooling effect of trees in the cities researched on the interactive map here.