“The best time to plant a tree was 20 years ago. The second best time is today,” so the saying goes.
Trees have been touted as solutions to many environmental problems, from reducing air pollution to restoring biodiversity. But much planting in cities today is motivated by their potential to mitigate extreme heat.
“Most of our signatory cities are reporting extreme heat as their main risk currently, and adding green infrastructure such as trees is one of their key activities to tackle it,” says Samie Raichs Tovany, urban heat project officer at C40 Cities, a global network of cities committed to climate action.
Nearly half a million people are estimated to die annually from heat-related causes, primarily due to cardiovascular problems. Extra strain is put on the heart as the body tries to cool itself down by circulating large quantities of blood to the skin. This can prove fatal to those with chronic health problems, particularly the elderly.
Repeated exposure to heatwaves has even been shown to increase a person’s biological age to an extent comparable to regular smoking or alcohol consumption.
Trees cool their surroundings in three main ways. They create shade. They emit water from their leaves which absorbs heat as it evaporates much like human sweat. And they change airflow.
But there are significant gaps in the understanding of the cooling potential of urban trees and there is great variation in how particular tree species cool a given urban area. Many existing assessments are only really relevant to their particular locations and climates.
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A trend for a lot of cities in the coming decades is to plan for emergency watering of trees. You have to think about how to maintain the canopy you already have.
Rob McDonald, lead scientist for nature-based solutions, The Nature Conservancy
Tree panacea?
Researchers at the University of Cambridge recently attempted to provide the first comprehensive global assessment by analysing 182 studies published in journals since 2010. They concluded that urban trees can lower pedestrian-level air temperature by up to 12°C. After trees were planted, in 83 per cent of the 110 global cities studied “the air temperature of the hottest month was reduced to below 26°C”.
But it is not as simple as just planting any tree in any location and things get cooler. To maximise the potential of trees to reduce heat in cities, many factors need to be taken into account.
The Cambridge study reveals the importance of tree species, urban layout and climate conditions. It also shows that the wrong tree in the wrong place could not only fail to reduce temperatures but also trap heat radiating from the ground at night.
“Our study busts the myth that trees are the ultimate panacea for overheating cities across the globe,” says Ronita Bardhan, associate professor of Sustainable Built Environment at the University of Cambridge’s Department of Architecture.
The study found that urban trees are best at cooling cities in hot and dry climates, providing cooling of just over 9°C and warming them at night by 0.4°C. Trees in hot and humid climates were less successful. In Nigeria, trees cooled cities by up to 12°C during the day, but warmed cities at night by up to 0.8°C. In temperate areas, cooling in the day was a more modest 6°C but nighttime temperatures could be raised 1.5°C.
“Whether the substantial daytime cooling is worth the modest increase in night-time heat is not straightforward, since night-time recovery is critical for health,” Bardhan tells Dialogue Earth. “The trade-off depends on context, vulnerability and how households adapt.”
She explains that more open and low-rise cities tend to benefit more from trees, as they have more space to spread out and for air to flow. In compact city layouts, evergreens may be the best choice as they provide year-round shade.
Where trees species are selected appropriately, the evidence suggests the greater the area they stand over, known as “canopy cover”, the greater their cooling effect. A soon-to-be-published study by Cardiff University and the Chinese University of Hong Kong, Shenzhen found that average temperatures of 24°C at ground level in Cardiff would fall to 22°C with minor tree canopy cover of 1 per cent, 20°C with 15 per cent cover, and 19.7°C at 38 per cent cover.
“Any trees are useful to reduce temperatures, but you need to have quite large areas covered to maximise the benefit,” explains study co-author Phillip Jones, professor of architectural science at Cardiff.
This finding is echoed by a 2019 study in the mid-sized city of Madison in the US state of Wisconsin. It found the greatest cooling was achieved when canopy cover exceeded 40 per cent.
Targeting inequality
In order to protect the most vulnerable people from extreme heat, cities need to consider where new trees are planted.
Leafy neighbourhoods with access to parks are often the most desirable and expensive, leaving the poorest people living in areas with the least green space, exacerbating inequality. Low-income neighbourhoods in the US had less tree cover than high-income blocks in 92 per cent of nearly 6,000 communities assessed in a 2021 study by environmental non-profit The Nature Conservancy (TNC).
Members of C40 such as Guadalajara in Mexico have specifically targeted tree planting in areas with populations at most risk from extreme heat, says Tovany. To do this, they are using a technique known as “vulnerability mapping”, where maps of urban heat are combined with demographic data.
Even where it is determined that trees are a good option for cooling, money can be a major barrier for cash-strapped city administrators. Cities are now using a variety of innovative solutions to fund trees.
In Freetown, Sierra Leone, authorities hope to plant five million trees by 2030, to deal with environmental issues including heat stress. The city’s Freetown the Treetown initiative pays residents to plant and maintain trees, with the plants geotagged to help track their progress. The project has been partly financed through multilateral funding institution the Global Environment Facility, but the city is also considering obtaining additional investment through voluntary carbon markets.
In Colombia, the city of Medellin has trained citizens from disadvantaged backgrounds to plant 8,800 trees in 30 “green corridors” that have been credited with reducing city temperatures by 2°C.
Alternative options?
In some cases, trees might not be the most appropriate solution to urban heat, even if they theoretically provide the largest temperature reduction. A July 2025 study from Boston University compared trees to “cool roofs” designed to reflect sunlight, for example by being painted white.
It suggests temperature reductions from expanding tree canopy in Boston were 35 per cent higher than those from installing cool roofs. However, the roofs on average had more potential to reduce heat exposure because there were more opportunities to install them in parts of the city where populations were most vulnerable, and space for tree planting most limited.
The research team is now working to identify how the cooling efficiency of trees changes across different climates, and what other factors need to be taken into account, explains Ian Smith, a researcher at Boston University’s Department of Earth and Environment.
It is not just about heat of course. Trees provide other benefits to mental health, flood prevention, biodiversity and air quality, he points out. “There’s a balance between all of these different considerations in determining whether tree canopy expansion is the best solution to reduce heat in a city,” he adds.
One crucial factor is water.
“A lot of the hottest places might also be some of the driest, where in order to sustain the health of the urban canopy, and therefore the cooling benefits … we might have to strain local water supplies,” says Smith. “That makes decision-making a bit more complicated.”
Increasing tree canopy cover in 61 large cities to the maximum potential would increase aggregate water demand by 3,200 million cubic metres per year, according to research by TNC. If location-appropriate, drought-tolerant species were planted, water demand could be reduced to 1,500 million cubic metres per year.
The consequences of failing to get these calculations right were illustrated in cities across Texas in 2011, when an estimated 5.6 million trees, representing around 10 per cent of the urban tree canopy, died in a drought. Authorities had to spend US$560 million removing them.
So those aiming to increase tree cover in cities must also think about how they deal with water limitations, says Rob McDonald, TNC’s lead scientist for nature-based solutions. Where irrigation is essential, alternative sources of water will be needed, such as reusing wastewater or stormwater, he adds.
“A trend for a lot of cities in the coming decades is to plan for emergency watering of trees,” says McDonald. “You have to think about how to maintain the canopy you already have.”
Trees can take decades to mature and become large enough to provide shade and cooling benefits. As the planet heats up, keeping people cool and healthy will require caring for existing urban trees as well as planting new ones where appropriate, so that they are still cooling people for decades to come.
This article was originally published on Dialogue Earth under a Creative Commons licence.
