Botanical gardens ‘most effective’ green space at cooling streets in heatwaves

Researchers hope the findings will inform policymakers planning cities for a warming world

Few things are as soothing on a hot summer’s day as a walk through a beautiful botanical garden, but they are not just oases of calm. As climate breakdown fuels soaring temperatures, they could prove crucial in moderating the heat in the streets around them.

A comprehensive review of research into the heat-mitigating effects of green spaces during heatwaves has found that botanical gardens are the most effective. It is a finding the team at the Global Centre for Clean Air Research (GCCAR) hope will inform policymakers planning cities for a warming world.

They are setting up the Reclaim Network Plus, a global web of planners, academics, city officials, charities and businesses who will look at the evidence around the benefits of green and blue space in urban planning.

This particular research found that sites such as the Chelsea Physic Garden and Royal Botanic Gardens at Kew in London, or the Gardens by the Bay in Singapore, reduced air temperatures during heatwaves in the city streets around them by an average 5C.

Urban parks and wetlands have a similar effect, and even green walls, street trees and playgrounds were found to significantly mitigate temperatures. “We have known for some time that green spaces and water can cool cities down,” said the GCCAR director, Prof Prashant Kumar, the study’s lead author.

“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 whole idea of the network is to promote the implementation of blue and green infrastructure in urban environments,” he said. But the problem they have encountered is a lack of systematic research into the kinds of different urban green spaces, and their impacts on the environments around them.

“So the idea of this paper was to state the science actually in terms of what does exist in terms of green and blue spaces in cities, what information is available, where are the areas where there is a lack of research … and then if there is city planning then what kind of evidence base they can use to plan new green spaces better.”

Local factors played an important role, but generally the review found that the bigger the park, the bigger the cooling effect – at least up to a point. It also found that cities can unlock more benefits by linking green spaces together into “green corridors”.

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

A co-author of the report, Prof Maria de Fatima Andrade of the atmospheric sciences department at the University of São Paulo, Brazil, 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.”

Average cooling effect / variation

Botanical gardens: -5.0C / -2.2C to -10C

Wetlands: -4.7C / -1.2C to -12C

Rain gardens: -4.5C / -1.3C to -7C

Green walls: -4.1C / -0.1C to -18C

Street trees: -3.8C / -0.5C to -12C

City farms: -3.5C / -3.0C to -3.9C

Parks: -3.2C / -0.8C to -10C

Reservoirs: -2.9C / -1.8C to -5C

Playgrounds: -2.9C / -2.8C to -3C

Cover photo: Sites like the Chelsea Physic Garden reduced air temperatures in nearby streets by an average 5C. Photograph: Sandra Lousada