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Global study reveals most effective green features for urban cooling

Words:
Stephen Cousins

Green walls, vegetated balconies and street trees among interventions needed to tackle overheating intensified by climate breakdown, finds GCARE review

Green walls, vegetated balconies and street trees are among the most effective nature-based solutions for cooling urban areas in heatwaves, the latest research has revealed.

The comprehensive review of research into the heat-mitigating effects of green spaces was carried out by the Global Centre for Clean Air Research (GCARE) at the University of Surrey.

Intended to inform policymakers planning cities for a warming world, it encompasses more than 200 papers, based on 51 different types of urban green-blue-grey infrastructure (GBGI) used to reduce summer air temperatures, such as parks, wetlands, and engineered greening.

The most effective interventions were found to include green walls, which cut street temperatures by an average 4.1°C, and street trees and vegetated balconies, which each reduced temperatures by an average 3.8°C.

The most efficient air cooling was observed in larger green spaces, with botanical gardens found to cut temperatures by an average 5.0°C and wetlands by 4.9°C. Other effective solutions included city farms, enabling an average 3.5°C reduction, parks (-3.2°C), reservoirs (-2.9°C) and playgrounds (-2.9C).

According to Prashant Kumar, lead author of the study and director of GCARE, the research has specific implications for urban designers and masterplanners working in the UK.

‘In London, planners could prioritise the implementation of green roofs and walls on commercial buildings, which have been shown to reduce ambient temperatures and lower energy consumption by cooling buildings,’ Kumar told RIBAJ, adding that areas identified as vulnerable to heat stress could benefit from interventions such as ‘urban parks or green corridors to provide cooling benefits and bolster community resilience’.

The review found the types of green spaces used to combat overheating varied considerably between continents, reflecting varying environmental needs and urban challenges.

In Asia, wetlands and parks were the most frequently reported, whereas in Europe street trees, green walls and green roofs were most common. In North America, parks, street trees and wetlands made up almost two thirds of all the interventions described.

Kumar said architects should explore the integration of multiple GBGI measures to both amplify cooling and enhance socio-ecological benefits. ‘This involves combining green roofs with rainwater harvesting systems and diverse green infrastructure elements, like street trees, green roofs and permeable paving, to address various aspects of environmental sustainability while mitigating urban heat,’ he said.

The study also revealed the significance of stakeholder engagement in tackling urban overheating. ‘By engaging residents in the design and maintenance of community gardens or street tree planting programs, a sense of ownership is fostered, promoting collective responsibility for mitigating urban overheating,’ said Kumar.

With future climate projections signalling increasing air temperatures and lower precipitation in some regions, researchers found that priorities for different types of green spaces need to adapt accordingly.

In temperate zones, such as Europe and parts of China, designers may need to integrate parks, pocket parks, green walls, green roofs, lakes, and woodlands to address changing climate patterns. ‘Prioritizing larger water bodies like lakes and implementing green roofs can help sustainable water management and climate adaptation,’ said Kumar.

Work in regions like the Mediterranean may need to prioritise features like wetlands, green walls and street trees to mitigate temperature variations and enhance urban resilience.

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