A new study from UNSW Sydney has cast doubt on the effectiveness of urban tree planting as a remedy for urban heat during severe weather events.
The research, featured in Sustainable Cities and Society, reveals that the cooling benefits provided by trees are markedly diminished during extreme heat, questioning the traditional approach of using tree planting to combat urban overheating.
Key Findings
Reduced Cooling Efficiency
The study indicates that the cooling effects of trees, which are often emphasized in climate models, are less effective than previously believed during heatwaves.
Trees generally lower temperatures through a process known as transpiration, where they release water vapor, absorb heat, and cool the surrounding air.
However, under intense heat, trees minimize moisture loss by reducing sap flow, which can decrease their cooling effectiveness by up to 60 percent.
Impact on Climate Models
Utilizing data from a global tree sap flow database and conducting field experiments in Sydney, researchers focused on eucalyptus trees and found that two-thirds of the trees studied were inaccurately estimated in their cooling capabilities during heatwaves.
This discrepancy suggests that current climate models might overstate the cooling potential of urban greenery in extreme conditions.
Health and Environmental Risks
Extreme temperatures can cause trees to release high levels of Biogenic Volatile Organic Compounds (BVOCs), which, when combined with other pollutants, may negatively impact air quality.
This finding underscores the necessity for careful selection of tree species according to their temperature tolerance to avoid potential health hazards.
Implications for Urban Planning
Importance of Irrigation and Water Management
The study highlights the critical role of proper irrigation in maintaining the cooling potential of urban trees. Professor Mattheos Santamouris, a co-author of the study, advocates for incorporating comprehensive water management plans into tree planting strategies.
This may include utilizing alternative water sources, such as wastewater, to support the irrigation needs of urban trees.
Genetic Modification and Future Solutions
Although research is ongoing into developing genetically modified trees that can withstand higher temperatures, such innovations are still at least a decade away from practical application.
In the interim, urban planners should base their heat mitigation strategies on the latest scientific data to ensure their effectiveness.
Moving Forward
The study reveals the limitations of relying solely on tree planting to address urban heat. As global temperatures rise, with projections suggesting urban heatwaves could be up to 6 degrees Celsius hotter by the end of the century, policymakers need to refine their mitigation approaches.
This includes considering the selection of tree species, their irrigation requirements, and the potential impacts on air quality.
Professor Santamouris cautions that without integrating up-to-date scientific insights into urban planning, substantial investments in tree planting might not achieve the anticipated cooling benefits and could potentially result in negative health effects.
Conclusion
The UNSW Sydney study emphasizes the complexity of mitigating urban heat through tree planting alone. To effectively combat rising temperatures, it is essential to adopt a multifaceted approach that includes updated scientific research and comprehensive urban planning strategies.