Urban greening: between promise and reality

Faced with accelerating climate change, urban greening is emerging in many projects as an obvious response to urban overheating. But beyond the obvious, a crucial question remains: how effective are trees in cooling urban spaces and providing thermal comfort?

Contrary to simplistic narratives that equate tree planting with guaranteed environmental results, scientific studies provide a more nuanced view, based on measurement and analysis of local contexts.

Three determining factors: climate, urban form, and plant species characteristics

The thermal effects of trees in cities are not automatic. Their ability to mitigate urban heat islands depends on three interdependent parameters:

• Local climate: average temperature, air humidity, intensity of solar irradiance
• Urban morphology: building density, street orientation, sky view factor (openness of the sky)
• Tree characteristics: type of foliage (deciduous, evergreen), leaf density (LAI, LAD), height, crown shape

These factors together determine the effectiveness of the shading, evapotranspiration and ventilation mechanisms that allow plantings to modulate temperatures at pedestrian level.

What the scientific literature shows: quantified results

A recent meta-analysis, published in Communications Earth & Environment (Li et al., 2024), synthesizes the results of 182 studies covering 110 cities and 17 climate types. This review offers the best available insight to date into the real-world conditions for the thermal efficiency of urban vegetation.

Measured temperature reduction (ΔTair):

Note: in dense urban forms (LCZ 1-3), the effectiveness is lower, with risks of reverse effect at night (heat trapping).

Factors that amplify the cooling effect:

• mixed species (deciduous + evergreen) in temperate, continental and tropical climates
• dense and well-developed canopies (high LAI/LAD)
• installation in open areas, with good air circulation

Limiting factors:

• Stomatal closure (closing of leaf pores) in case of prolonged heatwave
• young plantations (very limited effectiveness before maturity)
• Over-density vegetation in a compact morphology, with low nighttime ventilation

Do not confuse greening with biodiversity, nor planting with climate efficiency

The choice of tree species, their density, their arrangement, and their suitability to the climatic context are essential technical parameters. Equating tree planting with the automatic production of biodiversity or a universal thermal solution is a dangerous oversimplification.

Biodiversity, like climate performance, cannot be decreed: it must be conceived, measured and evaluated with appropriate methods.

Evaluating strategies, certifying results: the role of the independent third party

In this context, independent evaluation of greening strategies is essential to move beyond mere intentions and assess their actual effectiveness. Performance indicators, measurement methods (ΔTair, UTCI, PET), and contextualization of results are all key tools for adjusting choices and avoiding false promises.

IRICE, as an independent certifier, bases its actions on this logic of methodological requirement, in service of the robustness of the environmental approaches of project owners, developers and communities.

Conclusion: Act methodically, not with slogans

Trees are a valuable tool for action. But like any tool, their effectiveness depends on the conditions under which they are used.

Planting is not proof. Measuring, adapting, certifying: this is how credible strategies to combat the urban heat island are built.

References

Li, H., Zhao, Y., Wang, C., Ürge-Vorsatz, D., Carmeliet, J., Bardhan, R. (2024). Cooling efficacy of trees across cities is determined by background climate, urban morphology, and tree trait. Communications Earth & Environment, 5:754. DOI: https://doi.org/10.1038/s43247-024-01908-4