As cities worldwide grapple with extreme heat and the escalating “urban heat island” effect, new research is refining the strategy for using trees to cool urban environments. While urban forestry is booming driven by reports from networks like C40 Cities where extreme heat is reported as the main risk scientists are warning that simply planting “any tree in any location” is insufficient and may even be counterproductive at night.
Trees cool their surroundings through three primary mechanisms: providing shade, emitting water via evapotranspiration (similar to human sweating), and altering airflow. A new comprehensive global assessment from the University of Cambridge, which analyzed 182 studies since 2010, found that urban trees can lower pedestrian-level air temperature by up to 12°C . The study noted that in 83% of the {110} global cities examined, temperatures in the hottest month were reduced to below 26°C after trees were planted.
However the study, led by Associate Professor Ronita Bardhan, challenges the myth that trees are an “ultimate panacea.” It revealed that the wrong combination of tree species, urban layout, and climate can fail to reduce temperatures or even trap heat radiating from the ground at night.
- Hot and Dry Climates: Trees are most effective here, providing over 9°C of daytime cooling, but modestly warming the city by 0.4°C at night.
- Hot and Humid Climates: Trees were less successful; in Nigeria, they cooled cities by up to 12°C during the day but caused warming of up to 0.8°C at night.
- Temperate Areas: Cooling was a modest 6°C during the day, but nighttime temperatures could be raised by 1.5°C.
Dr. Bardhan stresses that this trade-off is critical for health, as “night-time recovery is critical for health,” and the decision to plant must depend on local context and vulnerability. The research suggests that open low-rise cities benefit most while compact layouts may favor evergreens for consistent shade.
Maximizing Benefits and Addressing Inequality
Further research from Cardiff University and the Chinese University of Hong Kong, Shenzhen, supports the idea that the greatest cooling effect is achieved with significant coverage. Their study found that increasing tree canopy cover from 1% to 38% in Cardiff would reduce average ground-level temperatures from 24°C to 19.7°C.
Cities are also focusing on equity, as leafy neighborhoods are often wealthier, leaving low-income areas with the least green space. Cities like Guadalajara, Mexico, are now using “vulnerability mapping” combining heat maps with demographic data to target tree planting in areas most at risk from extreme heat.
To overcome financial barriers, cities are exploring innovative solutions. Freetown, Sierra Leone’s “Freetown the Treetown” initiative pays residents to plant and maintain geotagged trees, seeking funding through multilateral institutions and voluntary carbon markets. Meanwhile, Medellin, Colombia, created 30 “green corridors” by training citizens from disadvantaged backgrounds to plant 8,800 trees, an effort credited with reducing city temperatures by 2°C.
The Water Challenge
A major constraint on urban tree expansion, particularly in the hottest and driest places, is water demand. Research by The Nature Conservancy found that maximizing canopy cover in 61 large cities could increase aggregate water demand by 3,200 million cubic meters per year. This could be reduced to 1,500 million cubic meters by planting drought-tolerant species.
As exemplified by the 2011 Texas drought, which killed an estimated 5.6 million trees, planning must include provisions for emergency watering using alternative sources like wastewater or stormwater. Experts warn that because trees take decades to mature and provide full cooling benefits, the focus must be on both planting new trees and actively caring for existing urban canopy to ensure communities remain cool and healthy for decades to come.