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Green approach cools overheating cities

Tree-lined streets aren’t just nice to look at: they could be a crucial tool in fighting urban heat, according to a new report from the Australian Conservation Foundation and the Monash Climate Change Communication Research Hub.

The report shared research on the effects of urban heat islands — a phenomenon where the built infrastructure in an area traps heat within cities, causing the temperature to rise even further — and on ways to mitigate it. The research found that by 2080, summer’s hottest days are likely to regularly be over 40 degrees Celsius in Brisbane and Melbourne, and up to 50 degrees in Sydney, with significant health and economic impacts.

Urban heat islands are caused by a combination of factors. High buildings and narrow streets trap heat in canyons, and dense or dark materials, such as concrete, asphalt, steel and glass, absorb and retain more heat.

The greater density of people in cities also increases the temperature, especially when they are using machinery such as air conditioners and fans to cool down. At the same time, cities have comparatively little vegetation and tree cover, which means there is less shade, and less moisture to cool the air.

The cumulative effect of these factors is as much as 13.5 degrees Celsius of additional heat. Because each local government area has unique geography, climate and existing green infrastructure, urban heat islands can be highly localised.

In Sydney, there was a significant disparity between suburbs in terms of both green cover and additional heat due to built infrastructure. The north shore suburb of Mosman, which has moderately high vegetation cover at 43%, experiences on average 2.2 degrees Celsius of additional heat from its built infrastructure. By contrast, the western suburb of Blacktown has only 22% vegetation cover and experiences an additional 5.8 degrees Celsius on average from its built infrastructure.

Heat spikes in urban heat islands can have impacts on health, infrastructure, the economy, wildlife and pollution and carbon emissions. As Australia’s climate has warmed, and heatwaves have increased in frequency, duration and severity, it is vital to identify ways to mitigate the effects of urban heat islands.

One of the simplest ways to address urban heat islands is to install green infrastructure. Green infrastructure is the “network of photosynthetic vegetation, including trees, shrubs and other plants in forests, parks, waterways, wetlands and gardens, and on green roofs and walls”.

Additional greenery helps address urban heat in at least three ways: it absorbs heat, the releases moisture, which helps to cool the air; and it shades surfaces, preventing them from storing and releasing heat.

The report is an important step in recognising the importance of urban green infrastructure and makes the case to local governments that it’s not just visually appealing, but also a good investment.