Summer heat waves are becoming harsher across urban areas. Concrete, asphalt, and dense buildings trap heat, creating what scientists call the urban-heat-island effect.

For city dwellers, this translates into hotter days and nights, increased discomfort, and higher health risks. As global temperatures climb, researchers and planners are looking closely at natural ways to cool cities.

Urban trees are often viewed as allies against sweltering heat. Their shade can offer instant relief, while their leaves help cool air through evaporation. But new research shows that trees alone are not guaranteed to ease urban heat.

A study led by environmental engineers at ETH Zurich and Eawag highlights that the key factor lies beneath the surface: soil moisture.

Water drives cooling power

The research investigated several neighborhoods in Zurich to measure cooling potential. Findings revealed that trees noticeably lower surrounding air temperatures only when the soil around them is moist.

Dry soil limits their cooling power. The researchers also found that evaporation from the ground itself plays a vital role, working much like a damp sponge slowly drying out.

The study also compared city layouts. In Zurich’s less dense areas, where green space is more available, watered trees created zones with little heat stress.

In compact neighborhoods, though, even extensive greening had limits, especially during extreme heat. Cooling potential depends not only on vegetation but also on water supply and available space.

Shade, evaporation, and water

Cooling in cities comes mainly from two mechanisms: shade and evaporation. Shade reduces radiant heat from buildings and streets, while evaporation removes heat by transforming water into vapor.

The ETH study found that shade was often the stronger cooling force, but irrigated soils added significant benefits through evaporation. Soil moisture kept leaves functioning, yet once temperatures climbed beyond certain thresholds, leaves closed their pores, reducing evaporative cooling.

To maintain this cooling effect, steady irrigation was essential. For Zurich’s soil, bringing one cubic meter (about 35 cubic feet) from dry to optimal moisture required nearly 100 liters of water (about 26 gallons).

Daily upkeep ranged from 2 to 4 liters (0.5 to 1 gallon) per square meter during summer. Larger tree patches helped conserve water by shading the soil, while automated irrigation systems guided by weather forecasts offered a more efficient way to keep moisture levels stable.

Balancing trees, heat, and water

The study also noted that water management must strike a balance. Heavy irrigation improves cooling but can increase flood risk if soils remain oversaturated.

Frequent, light watering combined with good drainage works better than heavy irrigation.

Another trade-off involves mulching. While mulch conserves water and benefits plants, it suppresses soil evaporation, which plays a key role in cooling at pedestrian level. The researchers suggest using tree grates as a compromise, allowing evaporation while still protecting roots.

Cities share heat lessons

Zurich’s case is not unique. Cities like Singapore and Melbourne have pioneered integrated blue-green infrastructure, combining water features with greenery to create cooler urban pockets. These efforts show that irrigation and vegetation design must work hand in hand.

In desert cities such as Phoenix, irrigation of urban trees has already been shown to reduce local air temperatures by several degrees. Yet the challenge remains to balance cooling benefits with sustainable water use, especially in water-scarce regions.

“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,” said Lucas Gobatti, first author and doctoral student at ETH Zurich and Eawag.

During extreme events, even fully irrigated large tree patches could not eliminate heat stress.

Preparing for extreme heat

The study stresses that planting trees without considering water supply and soil conditions will fall short. Intelligent water management, such as irrigation systems that adapt to rainfall and heat forecasts, is vital.

But other measures must complement greenery, such as cooling centers, shaded public spaces, and indoor shelters.

“The city should also consider improving heat adaptation plans, following examples such as Barcelona, where citizens are given access to public buildings to cool down, rehydrate, and reduce heat exhaustion during heat waves,” said Gobatti.

Green meets blue

The findings reinforce that urban cooling requires combined strategies. Trees provide shade and comfort, but water management, species choice, and urban design play equally important roles.

Future research is exploring how drought-resistant trees or varying planting patterns might influence cooling.

What remains clear is that climate resilience in cities will depend on integrated approaches – green infrastructure paired with thoughtful water use and inclusive public-health planning.

The study is published in the journal npj Urban Sustainability.

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