Wetlands, parks and botanical gardens among the best ways to cool cities during heatwaves

Review led by the University of Surrey reveals that botanical gardens can cool the city air by 5°C during heatwaves.

Oxford Botanic Garden

Comprehensive research review conducted by the Global Centre for Clean Air Research (GCCAR) into the heat-mitigating effects of green spaces during heatwaves, found that botanical gardens are the most effective.

The combination of urbanisation and global warming leads to urban overheating and compounds the frequency and intensity of extreme heat events due to climate change. Yet, the risk of urban overheating can be mitigated by urban green-blue-grey infrastructures (GBGI), such as parks, wetlands, and engineered greening, which have the potential to effectively reduce summer air temperatures. Despite many reviews, the evidence bases on quantified GBGI cooling benefits remains partial and the practical recommendations for implementation are unclear.

Professor Prashant Kumar, Co-Director, Institute for Sustainability, Professor and Chair in Air Quality and Health; Founding Director, Global Centre for Clean Air Research (GCARE) said: “We have known for some time that green spaces and water can cool cities down. However, this study provides us the most comprehensive picture yet. What’s more – we can explain why. From trees providing shade, to evaporating water cooling the air”.

The GCCAR systematic literature review synthesised the evidence base for heat mitigation and related co-benefits, identifies knowledge gaps, and proposes recommendations for their implementation to maximise their benefits. The research involved the screening of 27,486 papers, 202 were reviewed, based on 51 GBGI types categorised under 10 main divisions. According to the researchers, green walls, parks and street trees have been well-researched for their urban cooling capabilities but other categories such as zoological gardens, golf courses, private gardens and allotments continue to receive negligible attention.

The researchers found that while success depends heavily on local factors, they identified some general patterns. Among the key findings, the following green spaces and waterways cooled the air considerably:

  • Botanical gardens (5.0±3.5°C)
  • Wetlands (4.9±3.2°C)
  • Green walls (4.1±4.2°C)
  • Street trees (3.8±3.1°C)
  • Vegetated balconies (3.8±2.7°C)

The findings also indicated that, up to a point, the bigger the park, the bigger the cooling effect. Cities can unlock greater benefits by connecting green spaces into ‘green corridors’ which can also remove carbon emissions and help prevent flooding.

Prof Maria de Fatima Andrade of the atmospheric sciences department at the University of São Paulo, Brazil, who co-authored the report said: “Our paper confirms just how many ways there are to keep cool. But it also reveals how much work is left to do. Institutions around the world need to invest in the right research – because what’s very clear from our study is that there is no one-size-fits-all solution. It depends on what works for your community.”

According to Professor Kumar, the research will help town planners around the world to confront the challenges of global heating. “By implementing just some of the measures we describe, cities can become more resilient, and their citizens can be healthier and happier too”, he added.