Urban Forests as Natural Air Conditioners: How Trees Cool European Cities

Redacción HC
10/09/2023

As European cities grapple with rising temperatures and increasingly frequent heatwaves, a simple yet powerful solution is emerging from the scientific community: plant more trees. While green spaces are often hailed as urban climate solutions, not all greenery is created equal. According to a landmark study published in Nature Communications, urban trees are significantly more effective at reducing surface temperatures than treeless green spaces, especially during extreme heat events.

This research, conducted by scientists from ETH Zurich and the University of Bern, provides the most comprehensive evidence to date on how tree cover shapes the urban microclimate across the continent.

The Urban Heat Island Effect: A Growing Urban Risk

Cities are inherently hotter than their surrounding rural areas due to the urban heat island (UHI) effect—a phenomenon driven by dense construction materials, limited airflow, and sparse vegetation. This temperature imbalance is particularly dangerous during heatwaves, leading to increased health risks, energy consumption, and air pollution.

While previous studies hinted that vegetation could counteract urban heating, this research addresses a crucial gap: How effective are trees specifically, and how do their cooling benefits vary by region?

A Pan-European Satellite Analysis

To answer this, researchers used satellite-based land surface temperature (LST) data to evaluate 293 European cities. They categorized urban surfaces into three types:

• Built-up areas (e.g., roads, buildings)
• Green spaces with trees
• Green spaces without trees (e.g., lawns or fields)

The team compared surface temperatures during normal summers and heat extremes, adjusting for local climate variables like latitude, altitude, and humidity.

Key findings include:

• Tree-covered areas were up to 12°C cooler than built-up zones in central Europe.
• In southern European cities, tree cover reduced LST by 0–4°C.
• Treeless green spaces had a cooling effect 2–4 times smaller than that of trees.

Trees vs. Turf: Not All Green is Equal

The study underscores a critical insight: only tree cover delivers substantial cooling. While lawns and open fields offer aesthetic and recreational benefits, they do little to mitigate heat at the surface level.

“The cooling impact of urban forests exceeds that of grass-covered areas by a factor of 2 to 4,” the authors note.

This is because trees not only provide shade but also evaporate water through their leaves, a process called evapotranspiration, which actively removes heat from the air and surrounding surfaces.

Regional Variability: Why Central Europe Gains More

Interestingly, the cooling benefits of urban trees were more pronounced in central Europe compared to southern regions. This might seem counterintuitive, given the hotter Mediterranean climate. However, the researchers suggest that higher solar radiation, lower cloud cover, and less vegetation stress in central areas enhance the cooling potential of trees.

This insight has major planning implications. Cities in different climatic zones will need tailored greening strategies to maximize impact.

Implications for Urban Policy and Planning

The study provides concrete evidence that strategic urban tree planting is one of the most effective tools for climate adaptation in cities. Its recommendations are clear:

1. Prioritize Urban Tree Cover

Municipalities should focus on planting and maintaining tree canopies, especially in densely built and vulnerable neighborhoods.

2. Integrate Cooling Data in Urban Design

Planners can use satellite LST data to identify urban heat hotspots and design targeted green interventions.

3. Favor Connectivity and Diversity

Green corridors and mixed vegetation types can improve airflow and extend cooling benefits beyond individual parks.

4. Adapt Tree Species to Local Climates

To ensure resilience, urban forestry programs must select drought-tolerant, low-maintenance species suited to regional conditions.

Public Health and Climate Resilience Benefits

Beyond reducing temperatures, urban trees offer a range of co-benefits:

• Lower heat-related illnesses and hospitalizations
• Improved air quality
• Reduced energy use for air conditioning
• Enhanced mental well-being and community cohesion

“Urban forests are not just climate tools—they’re public health infrastructure,” the researchers emphasize.

As Europe faces longer and more intense summers, these benefits will become increasingly critical.

Looking Ahead: Monitoring and Scaling Up

The study calls for ongoing thermal monitoring using satellite data to evaluate the effectiveness of urban greening programs over time. It also encourages collaboration across cities and countries to share best practices.

Cities like Vienna, Paris, and Amsterdam have already integrated tree-planting targets into their climate action plans. The next step is scaling up these efforts and ensuring that trees are viewed not as amenities—but as essential urban infrastructure.

Conclusion: Rethinking Urban Green Space

This research reshapes how we think about urban greening. It’s not just about adding green space—it’s about adding the right kind of green space. Grass and shrubs are no substitute for the towering power of trees when it comes to cooling our cities and protecting public health.

As European cities plan for an increasingly hot future, this study provides a science-backed mandate to plant smarter, not just greener.

Topics of interest

Climate