When it comes to coping with heat waves, our own cities are conspiring against us. Road surfaces, pavements and buildings all contribute to keeping urbanized environments 3°C to 4°C hotter than surrounding non-urbanized areas.
With heat waves like the ones that have just baked half of Australia to a crisp forecast to increase in frequency and intensity, city councils are taking the urban heat island effect very seriously.
“Some of the modeling studies have shown that we can often have an urban heat island magnitude — so that’s the difference between the temperature in the city versus the temperature in the non-urbanized surroundings — that can be greater than the types of temperature increases that we’re looking at with global warming,” said Melissa Hart, graduate director of the Australian Research Council’s Centre of Excellence for Climate System Science at the University of New South Wales.
Illustration: Kevin Sheu
The urban heat island effect occurs because the dense dark surfaces such as bitumen on roads and building materials used in cities accumulate and store heat during the day and then release it at night.
“That’s important, particularly during hot summer evenings; if the minimum temperatures are much warmer at night and not cooling down then that can have health implications,” Hart said.
And those health implications are staggering: In 2009, 374 people died across metropolitan Melbourne in one heatwave — more than Victoria State’s annual road toll.
One of the simplest solutions to reducing the urban heat island effect is to provide more shade, with trees.
In 2012, the Melbourne City Council launched an ambitious project to double the tree canopy cover from 22 percent to 40 percent by 2040, by planting about 3,000 new trees every year.
“On thermal images, you can see clearly the red hot spots of cities are streets, roads, car parks — wherever there is bitumen and concrete — and you can see the contrast with parks, garden and trees,” said Melbourne City Councilor Cathy Oke, chair of the council’s environment portfolio.
However, tree planting has its limitations: Trees cannot be planted in the middle of roads, they cannot necessarily be planted on private property and there are also potential issues with having too many trees.
Simon Toze, principal research scientist at the Commonwealth Scientific and Industrial Research Organisation Land and Water’s Urban Living Lab, gave the example of some US cities that went overboard on tree planting and as a result, women felt less safe walking around the streets.
“We want to make sure that what we do is not actually having a detrimental effect elsewhere,” Toze said, highlighting other issues, such as water use and bushfire risk that can have implications for tree planting efforts.
Another approach that can cut down on heat absorption is to consider different surface materials for roads and pavements.
As well as committing to a 50 percent increase in tree canopy cover by 2030, Sydney has begun a trial of lighter-colored pavement in one inner city street to see if this will reduce temperatures by reducing heat absorption.
However, lighter-colored pavement can be a problem in very sunny areas.
“On a bright day like today in Brisbane, the last thing you want is to be driving on the road with the sun coming down and bouncing off the pavement,” Toze said. “It’s a trade-off that we’ve got to work through.”
One alternative is green roads with a more porous surface that allows water to seep in and even grass to grow through, which in turn cuts down the amount of heat absorbed by the road surface.
Toze said it might be particularly useful for low-traffic areas that do not see heavy vehicles, although he admits they are notorious for trapping high heels.
A similar principle to green roads applies to green roofs and green walls, where the building is partly or fully covered by vegetation.
Sydney already has about 100 buildings with green roofs or green walls, including the award-winning One Central Park building on Broadway. This approach indirectly reduces urban heat by cooling the building itself and reducing its air-conditioning requirements, which in turn reduces the amount of waste heat released into the environment.
However, green roofs can also have unwanted side effects.
“Some recent work coming out of our center found that if you put green roofs on the rooftops all across Sydney you reduce the temperature, but you can actually increase the humidity a little bit,” Hart said. “That can mean you’ve got a slight increase in heat stress because of the combined influence of temperature and humidity.”
Contributors to the urban heat island effect and the potential solutions to it vary enormously from city to city, which is why modeling of individual cities is vital. What works in one city — like planting trees along the wide streets of Portland, Oregon — is not going to be as effective or even as practical in the narrow street canyons of Hong Kong, Hart said.
Their research on Sydney suggests the density and color of building materials is one of the more significant contributors to the heat island effect.
“This affects the amount of radiation from the sun that’s reflected straight back out rather than absorbed,” Hart said. “And so the simple matter of painting surfaces white or lighter colors, rather than the dark, can have a significant impact.”
There is another contributor that is less talked about, and that is people. Vehicles, the machinery used to make days more comfortable — such as air-conditioning and refrigeration — and even human bodies produce significant amounts of heat.
This anthropogenic heat is something Hart has argued that needs to be understood and dealt with.
“Obviously you can’t get rid of the people in a city, but there are ways we can mitigate that,” she said.
More public transport means fewer heat-producing cars on the roads.
Another issue is an overreliance on air-conditioning during hot periods.
“If we’re building buildings that can deal with these conditions a little bit better than they currently do and we don’t have to rely on air-conditioning so much, then you’ve got less energy consumption and less waste heat,” she said.
There are no simple solutions, but ignoring the problem is definitely not an option, Oke said.
“The reality is that the climate is changing, [and] that cities that are already hot will get hotter,” she said. “The cooler we can make our city now, it’s an insurance policy for the future.”
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