Populations overheat as major cities fail canopy goals: new research

A new study led by RMIT University in Australia measuring access to nature for eight major global cities found most still have inadequate canopy cover, despite access to an abundance of trees.

Less than 30% of buildings in New York City, Amsterdam, Buenos Aires, Denver, central Sydney and central Melbourne were in neighbourhoods with adequate canopy cover.

New York and Amsterdam both scored almost 0% for canopy cover despite 92% and 50% of buildings having views of at least three trees, respectively.

The research, a collaboration with the Technical University of Munich, studied over 2.5 million buildings across eight cities using an emerging sustainable cities measure, known as the ‘3-30-300' rule. 

The rule states every house, school and workplace should have a view of at least three trees, be in a neighbourhood with at least 30% canopy cover, and be within 300 metres of a park. 

Only Seattle and Singapore passed the 30% canopy benchmark, with 45% and 75% of buildings in these cities enjoying adequate shade, respectively.

Access to parks was also patchy, with Singapore and Amsterdam scoring high while Buenos Aires and New York City scored poorly.

Lead researcher and RMIT University research fellow, Dr Thami Croeser, said it was concerning that most of the buildings in the study failed the 30% tree canopy test.

With 2023 being the hottest year on record and 25% of the global population experiencing dangerous levels of extreme heat, canopy cover was urgently needed to cool down our cities, he said.

“Previous research has shown depression, anxiety, obesity and heatstroke are more prevalent in urban areas that lack access to shady tree canopy and green open spaces,” said Croeser, from RMIT’s Centre for Urban Research.

“Canopy cover doesn’t just increase cooling, it can also reduce flood risk as well as benefit mental and physical health and support urban biodiversity.

“Studies say we actually need at least 40% canopy cover to substantially lower daytime air temperatures, so the ‘30’ metric is the absolute bare minimum - and most buildings we studied don't even reach that goal.”

Croeser said current ways of designing or retrofitting streets did not support healthy canopy growth as planning prioritised infrastructure such as cabling and pipelines over tree growth.

“We need to stop thinking that allocated spaces for buildings and roads are permanent when they could be reallocated to prioritise green infrastructure,” he said.

“30% canopy cover seems like a high bar if we keep doing things the same way, but it’s totally achievable if we change a bit of our practice.

“Currently, we put trees last, and if it gets in the way of cabling or pipes, we remove the tree or replace it with a sapling.

"Designing trees into streets early, and then figuring out win-win solutions to get in utilities and traffic access, is one of the big changes we need to make a difference.”

Trees currently tend to be planted in conditions that were not easy for them to grow in, Croeser said.

“The soil is compacted, there’s asphalt over them and when it rains, the water runs off into the gutters instead of into the soil.”

“Earlier research shows that if urban trees are planted in better quality soil with enough space for them to grow, where rainwater can run directly into the soil, it will help trees grow bigger faster to address our lack of canopy cover.

"Trees in urban environments are also removed and replaced with saplings, or pruned very heavily, so not many trees get the opportunity to grow into big old canopy trees except in a few lucky areas.”

A new nature access metric with potential

While the ‘3-30-300' rule, devised by Dutch urban forestry expert Professor Cecil Konijnendijk, is still relatively new in Australia, it is gaining momentum internationally, with at least six cities in Europe, the US and Canada implementing the measure in their urban forestry strategies.

Konijnendijk said he devised the benchmark to help set a ‘bare minimum’ for nature in cities.

“Getting more parks and trees into cities is complicated work, and I realised that a simple metric could take the mystery out of it and set a proper benchmark based on evidence,” he said. 

“I reviewed decades of science linking nature to human health – and found that views to nature, canopy cover and parks are all really essential if we want to be mentally healthy, physically active and safe from heatwave impacts.”

Professor Wolfgang Weisser, from the Technical University of Munich, said metrics that benchmark the adequacy of green infrastructure at a neighbourhood level in relation to human wellbeing were still rare.

“Some of the metrics we use now are not really sufficient whereas the ‘3-30-300' metric really demands that nature is brought to the areas that people actually live and work in,” Weisser said.

“A municipality with almost treeless streets and a few large, well-forested parks may score well on aggregated metrics of canopy and per-capita greenery but will be exposed as inadequate by the highly local ‘3’ and ‘30’ requirements.”

The researchers collaborated with Dutch firm Cobra Groeninzicht (Green Insights) to visualise the results.

“With the techniques our team has developed, we can calculate the ‘3-30-300’ benchmark for any city in the world,” said Dirk Voets, Senior Advisor Geospatial at Cobra Groeninzicht.

“Acute canopy deficits in global cities exposed by the 3-30-300 benchmark for urban nature”, with Thami Croeser, Roshan Sharma, Wolfgang Weisser and Sarah Bekessy, is published in Nature Communications. (DOI: 10.1038/s41467-024-53402-2))