A recent study conducted by researchers at UNSW Sydney has raised concerns about the effectiveness of urban tree planting initiatives in mitigating urban heat during extreme weather conditions.
The study, published in Sustainable Cities and Society, suggests that the cooling benefits of greenery are significantly reduced in sweltering conditions, challenging the conventional reliance on tree planting as a solution to urban overheating.
The study underscores the complexity of addressing urban heat through tree planting alone. As global temperatures continue to rise, with urban heatwaves projected to be up to 6 degrees Celsius hotter by the end of the century, it is crucial for policymakers to refine mitigation strategies.
This includes considering the types of trees planted, their irrigation needs, and potential impacts on air quality.
Professor Santamouris warns that without integrating scientific insights into urban planning, significant investments in tree planting may fail to deliver the desired cooling effects and could even lead to adverse health outcomes.
Key findings
- Reduced cooling potential: The study found that the ability of trees to cool the environment is overestimated by current climate models, particularly during heat waves. Trees typically reduce temperatures through transpiration, a process where they release water vapour, absorbing heat and cooling the surrounding air. However, in extreme heat, trees conserve moisture by reducing sap flow, thereby decreasing their cooling capacity by up to 60 per cent.
- Impact on climate models: The researchers used data from a global tree sap flow database and conducted field experiments in Sydney, focusing on eucalyptus trees. They discovered that two-thirds of the trees studied were overestimated in their cooling ability during heatwaves.
- Health and environmental concerns: Under extreme temperatures, trees can emit high concentrations of Biogenic Volatile Organic Compounds (BVOCs), which may harm air quality when interacting with other pollutants. This highlights the need for careful selection of tree species based on their temperature thresholds to avoid potential health risks.
Implications for urban planning
- Irrigation and water management: The study emphasises the importance of adequate irrigation to maintain the cooling potential of trees. Professor Mattheos Santamouris, co-author of the study, suggests that water management plans should be integral to tree planting strategies, potentially using alternative water sources like wastewater to support tree irrigation.
- Genetic modification and future prospects: While efforts are underway to develop genetically modified trees with higher temperature thresholds, these solutions are at least a decade away from being viable. In the meantime, urban planners must base heat mitigation strategies on the latest scientific evidence to ensure effectiveness.
