Mitigating Urban Heat Islands

Strategies to reduce the magnified effects of global warming in cities

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In 2018, the United Nations published a report that stated: ‘Today, 55 per cent of the world’s population lives in urban areas, a proportion that is expected to increase to 68 per cent by 2050.’ These figures don’t bode well for city-dwellers already afflicted by the environmental condition, urban heat island (UHI) effect.

First identified in the 19th century, UHI effect is the warming of urban environments approximately three to four degrees more than surrounding rural areas. The main cause of UHI effect is the presence of dark, dense surfaces such as concrete and bitumen that store heat by day and release it at night. Thermal images reveal that hotspots in cities are roads and car parks rather than gardens and trees. Another contributor to UHI effect is air-conditioning, increasingly found in both buildings and vehicles. This cooling technology which has provided urbanites with thermal comfort, paradoxically, generates heat.

While there is no conclusive link between UHI effect and climate change, science has disputed this. A recent piece in the International Journal of Environmental Research and Public Health in 2015 indicated that UHI effect contributes to global warming in urban areas in the Yangtze River Delta in China. In any case, there is little doubt that it will intensify the effects of global warming in cities, causing discomfort for city-dwellers and negatively affecting health if we fail to reduce temperatures in periods of extreme heat. In 2009, for example, 374 people died in Melbourne, Australia during a heatwave — more than Victoria’s annual road toll.

Some coating systems with very high total solar reflectance absorbs and radiates less heat, reducing the Urban Heat Island effect.
UNStudio’s The Coolest White is a coating system with very high total solar reflectance that absorbs and radiates less heat, reducing the temperature inside and outside of buildings. Image courtesy UNStudio.

Major steps are being taken to counter UHI effect. ‘The world is getting hotter, more urban,’ says Ben van Berkel, principal architect at Dutch practice UNStudio. ‘So we need to come up with design solutions.’ UNStudio, with Swiss paint manufacturer Monopol Colors, recently created an extremely reflective white paint called The Coolest White, intended to reduce the heat that buildings absorb. This is particularly pertinent as UHI effect is aggravated by tall buildings forming so-called urban canyons whose multiple surfaces retain heat and make cities hotter.

The paint, which is 2.5 times more resilient than a standard polyester coating, also reduces high temperatures indoors caused by UHI effect. As Tim Kröger, Laboratory Manager at Monopol Colors, points out: ‘In Europe we can solve this by opening windows but there are other countries that cannot do so and use air-conditioning’.

Urban Heat Island effect is particularly prevalent in tropical Singapore, the world’s third most densely populated country
Land surface temperature map of Singapore (24 May 2018) illustrating the Urban Heat Island effect. Image © Conrad Philipp, Chao Ren and Sailin Zhong, Cooling Singapore, CIVAL at Singapore-ETH Centre, Source: NASA 2018

UHI effect is particularly prevalent in tropical Singapore, the world’s third most densely populated country. The country is also a signatory of the Paris Agreement of 2016 that calls for capping the rise in the global average temperature to ‘well below 2°C above pre-industrial levels’. Studies have shown that its urban areas are up to 7°C higher than its less built up neighbourhoods. In response, in 2017 the National Research Foundation launched Cooling Singapore, a joint initiative involving government stakeholders, the private sector and academia to mitigate UHI effect by covering roads and buildings with pale, reflective surfaces, varying buildings’ heights to encourage winds to cool them, creating green areas and installing canopies and shutters.

Work on mitigating UHI effect has also been carried out by Ecosistema Urbano, a design and consulting company based in Madrid and Miami. ‘We’re creating a large public space in Málaga, Spain,’ explains co-founder Belinda Tato. ‘It is connected to the University of Málaga and its aim is to bring indoor academic activities outdoors’. The structure incorporates a digital layer that will allow people to alter the area’s ‘climatic configuration’ via an app that activates fans and water atomisers powered by solar energy. ‘This reduces temperatures locally and contributes to reducing UHI effect in the city,’ says Tato.

The need to mitigate Urban Heat Island effect is acutely felt in the US, where demand for air-conditioning has risen steeply
Ecosistema Urbano’s new boulevard proposal for the University of Málaga features a solar-powered climatic and digital hub with open air classrooms and common spaces that can be climatically adjusted to offer cooler temperatures during hotter months.

The need to mitigate UHI effect is acutely felt in the US, where demand for air-conditioning has risen steeply. Tree-planting schemes have been initiated in many states and cities, including Sacramento and Utah. Yet tree-planting is problematic: some US cities have planted so many that women have felt unsafe walking on the streets due to reduced visibility.

Meanwhile, Green Roofs for Healthy Cities (GRHC) in Toronto increases awareness of the environmental benefits of green roofs, although as founder Steve Peck notes: ‘You need a lot of green roofs to cool a city, so these should be combined with measures such as urban forest development and reflective surfaces, which save energy costs and water and reduce air pollution.’ Yet reflective surfaces aren’t always the answer: there’s the danger that on very sunny days, bright sunlight bouncing off pavements can dazzle drivers.

Although at an experimental stage, the intelligent ideas being trialled in this arena can prove helpful in mitigating the warming effects of climate change in dense urban centres.

Main image: Image of Ecosistema Urbano’s digitally controlled climatic hub for UMA Campus Ciudad in Málaga, Spain.

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