Urban Heat Island effect – how do we stop our cities from baking?
The Urban Heat Island effect was first described over a century ago as a phenomenon where air temperatures in densely built urban areas were higher than temperatures in surrounding rural areas. It was originally known as the ‘heat island’ and is considered the most well documented phenomenon of climatic modification. The temperature difference associated with an urban heat island has been found to usually be greater at night than during the day, and is most apparent when winds are weak. Several causes of the Urban Heat Island effect have been identified, however, the main cause is the modification of land surfaces using materials that effectively store short-wave radiation, such as concrete, ashphalt and bricks/roofing materials. Waste heat generated by energy usage is also considered a secondary contributor.
An obvious influence on long-term temperatures in urban areas is global warming, linked to hotter, longer and more regular heat waves. The latest report of the Climate Council finds hot weather in Adelaide, Melbourne and Canberra has already reached levels predicted for 2030, and eight of the hottest summers on record in Australia have occurred in the last 15 years. Assuming these trends continue it is obvious that rising temperatures in our urban centres is going to become an increasingly ‘hot’ issue.
The urban heat island effect can be detected throughout the year, but it is of particular concern to the public during summer. Higher surface air temperatures in summer are associated with air pollution, heat stress-related health issues, increases in electricity demand for air conditioning, and failure of infrastructure associated with public transport and roads. The federal government’s 2013 State of Australian Cities report also found people living in cities could be more susceptible to the effects of heatwaves. The report states that the heat island was ”caused by the prevalence in cities of heat-absorbing materials, such as dark-coloured pavements and roofs, concrete, urban canyons trapping hot air, and a lack of shade and green space”. It has been particularly apparent in Melbourne recently, as the Melbourne City Council found temperature variations of up to 4 degrees between the city centre and suburbs (check out the thermal image above!). The 2013 State of Australian Cities report also found that Melbourne lead the nation with the highest annual average number of heat-related deaths (approximately 200 a year compared to the state’s road toll of 242), and heat-related deaths in Australian cities are predicted to increase overall, especially in Perth and Brisbane.
Sustainable cities expert, Professor Steffen Lehmann from the University of South Australia, has stated that cities trap and store heat ”like a baking oven”. So what can be done to prevent us from getting roasted and mitigate the effects of the Urban Heat Island?
The main opportunities to reduce the urban heat island effect are considered to be:
- increased vegetation
- higher albedo surfaces
- higher albedo pavements
Typical examples of this are the use of green roofs and of lighter-coloured surfaces (roofs, roads and footpaths) in urban areas (also known as ‘cool roofs’), which reflect more sunlight and absorb less heat. The city of New York determined that the cooling potential per area was highest for street trees, followed by living roofs, light covered surface, and open space planting.
Tony Blackwell, a Winthrop Professor of Landscape Architecture at the University of Western Australia and Managing Director of Blackwell & Associates recently gave a presentation as part of New WAter Ways Water Sensitive Cities Speakers Series on his observations on the Urban Heat Island effect and proposals for tackling this issue. Tony’s research reflects on the comparative amounts of space dedicated to cars, as opposed to people, and considers that our streets should be considered as the primary focus for of the future. In particular, planting trees close to roads as well as in strategic locations around buildings, to benefit from the cooling and CO2 absorption effects. Tony also suggests that energy saving guidelines similar to those applied to all new buildings in Australia (NABERS, BASIX, NatHERS) be developed for streetscapes with the aim of reducing temperatures in urban areas by 5 degrees. In Perth approximately 25-40% of the area of a new subdivision is developed as road corridors compared to the 10% provided for public open space. Thus our streets may be considered a largely untapped opportunity to extend the health and environmental benefits offered by public open space.
In other research Professor Lehmann is supporting a campaign funded by Nursery and Garden Industry Australia to increase urban green spaces by 20 per cent by 2020. He also wants changes to Australia’s building rules to mandate more heat-resistant designs and materials. Interestingly for those of us living in Perth, have you noticed how many new developments have new homes built with black or dark roofing materials?
If we want to prevent our cities from being baked, a risk that is increasing as suggested by current climate patterns, then we have to seriously consider implementing some of these policies and strategies for our roads and homes.