Green Spaces to Mitigate Urban Heat

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Wednesday, July 22nd, 2015
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Several recent studies have shown that green spaces help to mitigate the urban heat island effect, which is expected to worsen in cities in the coming years.

A recent Asian Scientist study found that due to global warming, Sydney’s urban temperatures may rise by up to 3.7 degrees Celsius by 2050.

Climate change and global warming are expected to cause summer months to be much hotter and drier in many locations, increasing the urban heat island effect in cities, which will feel more heat than rural areas.

To combat the rising temperatures, more green spaces, trees and bodies of water are needed as they can help to lower temperatures and provide shade.

Researchers are encouraging city planning officials to adopt new guidelines to help reduce the effect of imminently rising temperatures.

Creating Urban Greenscapes

In a preparatory campaign to green the city, Sydney has seen a surge in green wall installations.

“There is an absolute fascination with green roofs and walls at the moment,” says City of Sydney green space project officer Lucy Sharman.

Sydney currently has a 15.5 per cent green canopy cover, which the council hopes to increase to 23.5 per cent by 2030.

Melbourne has a 23 per cent cover with a projected target of 40 per cent by 2040.

Sharman says Sydney is on track to reach its target, but says more green roofs and walls need to be installed.

“The more people that move to Sydney and to Melbourne and to Brisbane and other big cities, the more we have to think creatively about our open space,” she says.

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Sydney urban green space

The Urban Heat Island Effect

The heat island effect happens because the array of structures within a city stores more heat than open ground. Additional heat is added due to urban surfaces hindering the cooling effect of evaporation.

The term is used to describe warming in a localised urban area, usually due to large amounts of urban development and dark coloured surfaces such as roads and roofs. Cities are often several degrees hotter than surrounding rural areas.

Ambient temperatures rise because the sun’s heat is absorbed rather than deflected. The wide use of car engines and air conditioners also contribute to the urban heat island effect.

How to Prevent the Urban Heat Island Effect

There are several things that landscape architects and laymen can do to reduce the effects of increasing temperatures.

  1. Conservation of large plots of vegetation – grass, shrubs and trees are all natural cooling mechanisms that need to be protected
  2. Maintain open green space – reduce the development of hard surfaces in favour of urban green space
  3. Cooling rooftop gardens – a living roof as opposed to a non-living roof is better for cooling the structure below, reducing the need for air conditioning within the home
  4. Increase vegetation cover across landscape – re-vegetate open space and agricultural land
  5. Light coloured roofing – light roofs reflect energy from the sun rather than absorbing the heat like dark roofs
  6. Street trees – the shade provided by trees onto roads and buildings can greatly decrease temperatures
  7. Landscaping – increasing vegetation on the north, east and west sides of a building blocks solar radiation and reduces heat
  8. Water sensitive urban design – promotes water retention and slows the movement of water over the landscape allowing vegetation to cool

New research from Portugal has shown that even small green spaces can help mitigate urban heat and enhance cooling down over night.

A neighbourhood garden in Lisbon was studied to see if small green spaces demonstrate cooling benefits and the results were positive. The 0.24-hectare plot of land in a densely populated area was monitored over six hot summer days and results showed temperatures within the garden were significantly cooler than other locations nearby.

On the hottest day of study, the temperature within the garden was 6.9 degrees cooler than areas surrounding.

Overall the study showed that the impact of the heat on human physiology was significantly lower within the garden, and can even be beneficial for an extended distance beyond the green space.

opera house aerial view

Sydney to increase green canopy cover to mitigate the urban heat island effect.

Implementation of Green Space

The results of implementing more green space into our cities have yet to be seen.

“We have yet to establish if land managers and decision makers will be prepared to trade-off the increased benefits of additional green space against potential costs. These costs include a reduction in the amount of developable land, maintenance costs, storm damage, and an increase in insects,” says David Byrne, a lecturer at the Gold Coast’s Griffith University.

Sharman says Sydney council is currently conducting a hard cost-benefit analysis and admits that cost is a major boundary to installing green roofs and walls.

“One of the biggest barriers to people putting in green walls is really understanding how much it is actually going to cost,” she says, “and exactly what benefits they’re going to get out of it,” she says.

Sustainability expert Professor Stuart White of the University of Technology Sydney says small urban green spaces are not going to fix the air quality issues of Sydney on their own, but says they can be “an important contributor” in that battle.

“If we look 20 or 30 years into the future, we hope we’d see a lot more of these spaces,” he says. “Then it would start to make a significant difference to the urban heat island effect and to the thermal efficiency of buildings.”

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