While everyone feels the impacts of extreme heat, cities and towns are usually 1–3°C hotter than the surrounding countryside. In some cases, the difference can be even greater, with certain cities experiencing temperatures up to 7°C higher than others in hot conditions.
If it gets very hot, the residents may face greater risks from heat stress, dehydration, and other heat-related illnesses, which can be lethal. The vulnerable groups such as children, older adults, and outdoor workers are especially at high risk to these dangers.
What can be the antidote then if the world’s cities are to cope with rising temperatures? One of the most promising solutions comes from nature itself: trees and green spaces. Yet an important question remains: do trees cool every city in the same way?
Unsurprisingly, trees have to be seen as part of a broader strategy when searching for effective ways to stay cool, but it is not the whole answer. In some cases, the wrong type of vegetation or poor planting design can reduce the cooling benefits of urban greenery that cities hope to achieve, because they can vary significantly depending on local climate conditions, highlighting the need for climate-smart urban greening strategies tailored to each city’s unique environment.
Why Focus on Climate-Smart Urban Greening When Cities Are Becoming Hotter: How Trees and Green Spaces Can Cool Cities Naturally
According to the United Nations report, more than half of the world’s population (55%) now lives in urban areas. By 2050, that figure is projected to rise to 68%. As cities continue to expand, they are facing an increasingly hotter future.
Climate change is driving more frequent and intense heatwaves, while the urban heat island effect further amplifies temperatures, making cities significantly warmer than their surrounding rural areas.
Urban areas are covered with materials such as concrete buildings, bricks, asphalt roads, and densely populated neighborhoods. This creates environments that absorb and retain heat throughout the day.
The surfaces then slowly release the stored heat throughout the evening and night, keeping temperatures elevated long after sunset. This phenomenon, also known as the urban heat island effect, make cities significantly warmer than the surrounding rural areas.
Reducing urban heat is, therefore, becoming a major priority for city planners, environmental scientists, and governments worldwide. In order to combat this growing challenge, the cities around the world are planting more trees and expanding green spaces. But Recent research suggests that planting trees alone may not always be enough. The key lies in climate-smart urban greening.
Why Trees Cool Different Cities Differently
Trees offer a simple way to counteract urban heat by acting like natural air conditioners, almost halving the heat trapped by the urban heat island effect. Their cooling power comes primarily from two natural processes:
- Shade: Tree canopies block sunlight from reaching roads, sidewalks, and buildings. This reduces the amount of heat absorbed by these surfaces and lowers the temperature experienced by pedestrians in the first place.
- Transpiration: Trees release water vapor through their leaves in a process called transpiration. As the water evaporates, it cools the surrounding air, much like perspiration cools the human body.
Because of these benefits, tree planting has become one of the most widely adopted strategies for reducing urban heat. Their role in lowering the surrounding temperatures can make a noticeable difference, particularly on sizzling summer days. But it is far from even. Researchers recently examined how different types of urban vegetation influence outdoor heat in three cities with distinct climates:
- Melbourne, Australia
- Munich, Germany
- Hong Kong, China
Rather than relying solely on computer simulations, scientists conducted field measurements during summer. They compared open urban areas without vegetation, areas with trees only, and areas with layered vegetation consisting of trees, shrubs, and ground cover.
The researchers measured not only air temperature but also mean radiant temperature – the heat radiating from roads, walls, and surrounding surfaces that directly affects how hot people feel. Their findings offer valuable lessons for cities worldwide.
Melbourne: The Power of Shade
In Melbourne, street trees significantly reduced radiant heat exposure. Researchers found that shaded streets could lower radiant temperatures experienced by pedestrians by more than 18°C compared with open streets. Interestingly, air temperatures did not always decrease dramatically.
Yet people felt much cooler because they were protected from direct sunlight and heat radiating from surrounding surfaces. This highlights an important fact that human comfort depends on more than the temperature shown on a weather forecast.
Munich: Layered Vegetation Delivers Greater Benefits
Munich demonstrated the strongest benefits from layered vegetation systems. Areas containing trees, shrubs, and ground cover reduced afternoon heat stress by nearly 8°C compared with more open landscapes. The combination of vegetation types created multiple cooling mechanisms.
Trees provided shade, shrubs helped reduce heat reflection from the ground, and ground cover protected soil from direct solar heating. Together, these layers created a more effective cooling environment than trees alone.
Hong Kong: When More Green Isn’t Always Better
Hong Kong presented a more complex picture. Vegetation certainly helped cool urban spaces, particularly where overlapping tree canopies created extensive shade. However, dense vegetation sometimes increased humidity levels.
In humid climates, additional moisture released by plants can make outdoor spaces feel uncomfortable because sweat evaporates less efficiently from the human body. As a result, some heavily vegetated areas felt warmer than expected despite lower temperatures.
This finding demonstrates that successful urban greening depends heavily on local climate conditions.
Why One-Size-Fits-All Solutions Fail
Many cities establish ambitious tree-planting targets without considering local environmental factors. However, the research revealed that several variables influence whether urban greening succeeds:
Climate: Dry climates benefit greatly from vegetation-induced cooling through evaporation. Humid climates may experience reduced benefits due to increased moisture in the air.
Airflow: Trees and shrubs can sometimes block wind circulation. In narrow streets, dense vegetation may trap warm air and reduce natural ventilation.
Street Design: Building height, road width, and surrounding infrastructure affect how vegetation interacts with sunlight and airflow.
Vegetation Structure: Different combinations of trees, shrubs, and ground cover create different cooling effects. The arrangement often matters as much as the number of plants.
The Importance of Climate-Smart Urban Greening
The climate-smart urban greening is an approach that carefully considers local climate, vegetation types, and urban design to maximize cooling and environmental benefits. Instead of focusing solely on planting large numbers of trees, cities should design green spaces according to local environmental conditions. Climate-smart urban greening involves:
- Selecting suitable plant species for local climates
- Combining trees with shrubs and ground cover where appropriate
- Preserving airflow in dense urban environments
- Creating shade where people walk and gather
- Integrating biodiversity and human comfort into planning decisions
This approach not only reduces heat but also supports wildlife, improves air quality, and enhances residents’ quality of life.
Benefits Beyond Cooling
Urban vegetation provides numerous environmental and social benefits beyond temperature reduction. These include:
- Improved air quality
- Reduced stormwater runoff
- Increased biodiversity
- Carbon storage
- Noise reduction
- Enhanced mental well-being
- More attractive public spaces
Studies consistently show that access to green spaces can reduce stress, improve mood, and strengthen community connections.
Looking Ahead
As climate change continues to challenge cities worldwide, urban greening will remain an essential adaptation strategy. However, the success of these efforts will depend on thoughtful design rather than simple tree-count targets.
The experiences of Melbourne, Munich, and Hong Kong demonstrate that nature offers powerful solutions to urban heat, but those solutions must be adapted to local conditions. The future of cooler and healthier cities lies not merely in planting more trees, but in creating climate-smart green spaces that balance shade, airflow, biodiversity, and human comfort. By working with nature intelligently, cities can become more resilient, sustainable, and enjoyable places to live for generations to come.
