Urban Heat: Trees Alone Won't Cool Cities Water And Space Are Crucial

Trees are often considered a natural solution to reducing the heat in cities during hot summer days. They offer shade and help lower the temperature of the air, making urban areas more pleasant. However, the effectiveness of trees in cooling their surroundings largely depends on one key factor: water.

A recent study examined different neighbourhoods in Zurich to understand how well trees cool urban areas and what conditions are necessary for them to perform best. And it shows that trees need moist soil to cool the air effectively. The research, led by environmental engineers from ETH Zurich and Eawag, was published in npj Urban Sustainability.

Moist Soil

Water is crucial because when the soil around trees is dry, they cannot cool the air much. However, if the soil is sufficiently wet, trees can cool the air through evaporation, similar to how damp sponge cools the air as it dries. The study also found that evaporation from the ground itself contributes to cooling the air, not just the trees.

Properly watered trees, in areas with more space and greenery, can create cooler zones where people can escape the heat. In contrast, in densely built-up areas with limited space for plants, even a large number of trees may not be enough to fully counteract the heat on very hot days. Therefore, the ability of trees to cool an area depends on both the amount of space they have to grow and the availability of water.

Cooling Effect of Trees

Trees provide more shade than shrubs, and they cool the air more effectively. Watering the soil around trees helps sustain this cooling effect by supporting evaporation. Researchers recommend using smart watering systems that adjust based on weather conditions to keep the soil moist during heatwaves without wasting water.

The research aims to help cities find better ways to use trees and green spaces to combat heat. Simply planting trees is not enough; they need proper care, especially in hot, dry weather. Cities should focus on planting trees in locations where they have enough space and water to perform optimally.

Why It Matters

Experts say that more research is needed on different tree types, soil conditions, and tree ages to find the best strategies for keeping the environment cool.

However, even with proper watering and smart tree planting, trees alone cannot fully solve the problem of extreme heat in cities. On the hottest days, trees may not provide enough cooling to fully protect people from heat stress.

Lucas Gobatti, the lead author and a doctoral student at ETH Zurich, states, "On heat wave days in Zurich, it will not be possible to create heat shelters that fully solve heat by only using trees and water."

He suggests that cities should also develop other ways for people to stay cool, such as opening public buildings where people can rest, drink water, and escape the heat.

Trees are not the ultimate solution to fight climate crisis, but they are a valuable tool for cooling cities, and their success depends on water and space. Cities must plan carefully to water and maintain trees and green spaces, and also consider strategies beyond greenery to protect people during the hottest times.

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