Read the Sydney Morning Herald article The Maps that show the tree-free heat islands in your city by Bianca Hall, 19 November 2024.

Australia’s two largest capital cities have far too little tree canopy, which increases the cities’ heat island effects and leads to increased rates of obesity, depression, anxiety and heatstroke.

An international study of more than 2.5 million buildings across eight global capital cities has found just 3 per cent of buildings in inner Melbourne had adequate neighbourhood tree canopy cover, while 17 per cent of buildings had enough shade from tree canopies in Sydney.

In central Sydney, it’s possible to see at least three trees from 84 per cent of buildings, while Melburnians can see at least three trees in 44 per cent of buildings.

Lead researcher Dr Thami Croeser said governments should work to urgently improve canopy shade to cool cities, where heat islands are increasing the effects of climate change, trapping hot air and radiant heat among buildings.

“We know depression, anxiety, obesity and heatstroke are more prevalent in urban areas lacking access to shady tree canopy and green open spaces,” said Croeser, from the RMIT Centre for Urban Research.

This, researchers have established, is because hot and inhospitable cities increase people’s stress, and discourage physical activity.

“We have given 70-80 per cent of our streets to parking, road lanes and moving cars around, and any time there’s a little conflict with that, the trees tend to come out second.”

Croeser and his research partners in Melbourne, and Munich and Freising, Germany, applied a new framework called the 3-30-300 benchmark over eight global cities: Sydney, Melbourne, Amsterdam, New York City, Seattle, Singapore, Denver and Buenos Aires.

The benchmark – increasingly being adopted in urban planning in Europe – suggests every home, school and business should be within view of at least three trees, be in a neighbourhood with at least 30 per cent canopy cover and be within 300 metres of a park.

While buildings in inner Sydney and Melbourne were often within view of three trees, having adequate canopy cover was rare.

“It’s the first time we’ve got a real benchmark on how much nature we need in cities … and I was just so shocked to realise how far we fall short,” Croeser said.

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

“We put trees in a hole in the footpath,” Croeser said. “That footpath is made of crushed rock underneath the paving, and there’s almost no water getting to that tree, and the soil underneath is very limited.

“So you’ve got trees that, effectively, are starving or thirsty all the time, and their roots are struggling to find a place to grow.”

Karyn Brown has lived in Sydney’s Waterloo public housing estate, surrounded by mature eucalypts and melaleucas, for 25 years.

For Brown, the beauty of her home has been enhanced by the coolness and shade offered by the trees and the wildlife they bring.

“Oh, I love it. I particularly like the birds, they sit on my balcony sometimes on the fourth floor … eastern rosellas, magpies, white cockatoos and corellas.”

But the estate is earmarked for redevelopment, which will almost certainly mean the loss of many mature trees. “Most of the trees will be taken away, and most of the new buildings will go right up to the footpath,” she said.

Nicole Vickridge is an urban horticulture master’s student at the University of Melbourne and a researcher with NGO Sweltering Cities, which campaigns against built forms creating extreme urban heat.

Vickridge said tree-lined urban environments didn’t just lower urban temperatures, but were also beneficial for city dwellers’ emotional and physical health.

“You want to spend longer when you’re walking along a beautiful shady street,” she said.

“It creates spaces where you want to linger and interact with other people, with a shopfront; a place where you might want to pause and have a seat; and none of those activities are much fun on a sweltering sidewalk with exhaust fumes that’s hot and dry and often dusty.”

The 3-30-300 concept was conceived by Dutch urban forestry expert Professor Cecil Konijnendijk in 2022 and is gaining momentum internationally in urban planning circles.

Konijnendijk said the benchmark was the “bare minimum” of what is required to cool cities and encourage active transport such as walking and cycling.

For Croeser, a complete rethink of the way cities are designed – and retrofitted – is required.

“Studies say we actually need at least 40 per cent canopy cover to substantially lower daytime air temperatures, so the ‘30’ metric is the bare minimum – most buildings we studied don’t even reach that goal,” he said.

“If there’s an exciting call to action, it’s that our streets might need to look quite different in the future.”