How to reduce the risk of urban overheating by green-blue-grey infrastructure (GBGI)

The rise in urbanisation and global warming is causing cities to overheat and making extreme heat events more frequent and intense. However, the risk of urban overheating can be reduced by using urban GBGI, such as parks, wetlands, and engineered greening, which has the potential to effectively lower summer air temperatures. Professor Guo Hai of the Department of Civil and Environmental Engineering has contributed to a global study that reveals the effectiveness of green interventions for sustainable cooling.

GBGI can cool cities

According to the Intergovernmental Panel on Climate Change, 70% of global carbon dioxide emissions come from urban activities. The implementation of GBGI can not only cool cities, but also decrease energy consumption and improve urban residents’ quality of life. In addition, it can enhance biodiversity, reduce climate change impacts, and promote overall sustainability.

The study conducted by Professor Guo and other researchers around the world revealed regional and cityspecific variations in the effectiveness of GBGI in mitigating urban heat.

Overall, GBGI can lower urban temperatures by as much as 18.9°C in Europe, 17.7°C in Asia, 12°C in North America, and 9.63°C in Australia.

While green and blue infrastructures have proven to be highly effective in lowering air temperatures globally, grey infrastructures in Asian cities, especially roof gardens and pergolas, are found to be the most effective for urban cooling. Roof gardens reduce temperature significantly by 17.7°C in Singapore, 16.2°C in Japan, and 10.8°C in South Korea.

The study also showed notable effects of various GBGI features in mitigating urban heat in cities across Mainland China. The most effective means include botanical gardens, wetlands, green walls, and attenuation ponds. In Beijing, botanical gardens could result in a temperature decrease of up to 10°C, while those in the Shaanxi province contributed to a decrease of only 2.7°C.

In Hong Kong, parks, green roofs and golf courses were found to play substantial roles in cooling urban heat, resulting in temperature reductions of 4.9°C, 4.9°C and 4.2°C respectively.

A nine-stage framework for policymakers

The research team has introduced a nine-stage framework to facilitate the implementation of GBGI:

A nine-stage framework of GBGI

Professor Guo explains that this framework serves as a strategic roadmap, optimising GBGI implementation to maximise benefits. Policymakers should conduct thorough investigations and planning tailored to the specific context and needs of their cities.

In Asia, the widespread development of GBGI is a response to challenges posed by rapid urbanisation and cultural preferences for green spaces for community activities, along with environmental goals focused on biodiversity conservation, improved air quality, and mitigating the urban heat island effect. Future GBGI implementation should take a holistic approach to optimise their multifunctional benefits and effectively address sustainability goals.

The study findings have been published in the international interdisciplinary journal The Innovation.

Professor Guo’s team found that constructed grey infrastructures, especially roof gardens and pergolas, are the most beneficial for urban cooling in Asian cities.