🌳 Trees can warm up cities, here's why

Faced with rising temperatures in cities, many municipalities are betting on tree planting. However, trees alone are not always sufficient, and poorly designed greening can even worsen discomfort during heat waves.

To understand this phenomenon, researchers compared field measurements in Melbourne, Munich, and Hong Kong. They tested how different forms of urban plantings modify the heat felt by pedestrians. The results show that the structure of vegetation is crucial: trees combined with shrubs and ground covers cool more effectively than isolated trees. Local climate and street design also play a key role.


The experiment measured not only air temperature but also the mean radiant temperature, which accounts for heat emitted by roads and walls. In Melbourne, street trees reduced this radiant heat by more than 18°C (32.4°F) compared to open streets. In Munich, layered plantings (trees, shrubs, and ground cover) lowered thermal stress by nearly 8°C (14.4°F) in the afternoon. Hong Kong showed more mixed results due to its humid climate.

The benefits of vegetation therefore heavily depend on the local environment. In dry cities, plants release water vapor that cools the air. But in already humid climates, this extra moisture can make outdoor life uncomfortable because sweat evaporates less efficiently. In Munich, dense vegetation sometimes reduced air circulation in narrow streets, trapping heat and pollutants.

These results show that more vegetation is not automatically better. Street width, air currents, and local climate condition the effectiveness of cooling. A tailored design is therefore essential to avoid unintended effects.

The solution is to design greening carefully. In parks, layered plantings provide strong cooling while supporting biodiversity. In dense streets, a balance between shade and ventilation must be found.