What's the most effective way of keeping a hot city cool?

Nantes during last summer's heat wave. Image: Getty.

The recent spate of heatwaves through eastern Australia has reminded us we’re in an Australian summer. On top of another record hot year globally, and as heatwaves become more frequent and intense, our cities are making us even hotter.

This is the urban heat island, where city temperatures can be significantly warmer than the surrounding rural regions. The question, then, is what we can do to keep our cities cooler.

Why are cities hotter?

The temperature difference is caused by a range of factors, including dense building materials absorbing more of the sun’s energy, fewer trees to provide shade, and less soil to cool by evaporation.

Buildings can also act like the hairs on a husky, reducing wind speeds and blocking thermal radiation up to the night sky. On top of that, waste heat from car engines, air-conditioners and other energy use adds to overall air temperatures.

Why does this matter? Even a small increase in air temperature pushes up overall energy demand, and about 25 per cent of our energy bills are for only 40 hours per year when the grid is most heavily used.

The most extreme heat events can buckle train lines, cause rolling blackouts and cost billions in lost productivity. And it’s not just bad for our wallets.

Heat stress can damage organs or exacerbate existing illnesses. Since 1900, extreme heat events have killed more Australians than bushfires, cyclones, earthquakes, floods and severe storms combined.

So, what can we do?

There are a number of things individuals can do to reduce the impact of heat in their homes, such as installing light coloured roofing material, insulation or an air-conditioner.

But it gets more complicated when considering the city as a whole, and how these small actions interact with each other and with the climate.


In heatwaves, air-conditioners save lives, allowing stressed bodies time to cool. But our homes can only be made cooler by blowing heat outside, along with the extra energy to run the system.

As well as increasing outside air temperatures in the short term, the fossil fuels burned add to global warming. A world cooled by air-conditioning probably isn’t the answer.

Trees and parks

Trees provide shade, but also cool the air, because evaporating water from leaves takes energy, reducing peak temperatures by 1-5° C.

Most city planners agree on the broad benefits of urban vegetation, with some metropolitan councils developing urban greening strategies.

However, urban trees can be a vexed issue for some councils; they use water, can be costly to maintain, can damage utilities and property, and can worsen air quality instead of improving it. Larger cities are often made up of dozens of councils; getting them to agree is a major challenge.

White roofs

We know that black surfaces get hotter in the sun, but demand for dark roof tiles still far outweighs demand for light colours. More reflective roofs can reduce a household’s energy bill, as well as the overall temperature of a city.

White roofs are most effective in warmer climates, because in cold climates, the cost savings in summer must be balanced with additional heating costs in winter.

Green roofs and walls

Green roofs and walls are building structures with integrated vegetation. They provide cooling benefits by shading buildings and through evaporation from leaves. They generally show less cooling benefit than white roofs, cost more to install and maintain, and use additional water and energy.

But they do look nice, improve biodiversity and make people happier.

Pavement watering

Prior to an extreme heatwave, it may be possible to reduce temperatures by wetting down building and road surfaces. It’s a traditional practice in Japan, and is now being considered in major cities like Paris.

But temperature and humidity are important factors in heat stress, so pavement watering should only be undertaken if the extra humidity does not increase heat stress.

Large scale rooftop solar

Solar panels convert energy from the sun into electricity, so less energy is required from the network overall. If enough roofs were covered with solar panels, could that lower air temperatures?

Probably a little. Other benefits include a reduction in the energy required for cooling (because the roofs are shaded by panels), and a stable, lower cost, decentralised renewable energy system.

Building density

A building with lots of thermal mass (think sturdy, double-brick home) can be an effective way to keep inside temperatures more stable. Heat is absorbed during the day and released at night. The same idea can work for an entire city.

An urban cool island can form in high-density cities like Hong Kong because tall buildings provide extra heat capacity and shade.

For similar reasons, the tight street layout of traditional Arabian and Mediterranean cities keep those streets cooler.

Shading structures

Installing light shading structures over streets, pavements and roofs can reduce the surface temperature of materials, and reduce the heat absorbed and radiated back into streets. Shading structures need to be designed so that they do not limit airflow, trapping heat and air pollution in streets.

Which is best?

To figure out what works best, we need to be able to model the physics of different strategies, in different types of cities and in different climates. We can then assess the economic and health impacts and decide on appropriate and plans that give us the biggest bang for our buck.

Here we have focused on heat in cities, but there are other important concerns like air quality or flooding.

In colder cities, an urban heat island could actually be a good thing. Each city is different; each requires a tailored and integrated plan developed over the entire metropolitan region, and then implemented locally by councils, businesses and households.The Conversation

Mathew Lipson is a PhD Candidate, and Melissa Hart the graduate director of the ARC Centre of Excellence for Climate System Science, at UNSW Australia.

This article was originally published on The Conversation. Read the original article.


We need a manifesto to put green spaces at the heart of our communities

Birkenhead Park, the oldest municipal park in the world. Image: Getty.

Parks and open spaces are arguably the most universal of all our public services. They are used by the entire community, from pre-school children through to retired adults.

Green space is a defining part of our local landscape and these publicly owned, civic spaces have something to offer to all as places to enjoy life – whether that’s reaching a sporting milestone, teaching grandchildren to cycle, engaging with nature, or simply walking a much loved dog. A Fields in Trust survey indicated that nearly a quarter of respondents (24 per cent) use their local park at least twice a week.

Yet unlike education or libraries, parks are a discretionary service which councils have no statutory duty to provide. There is no national audit of informal recreation space, making it difficult to track the losses of these vital assets. Whilst the number of visitors to parks is rising, investment has decreased and maintenance and upkeep has been reduced; local authority spending on open spaces fell by 14 per cent between 2010 and 2014.

The Heritage Lottery Fund State of UK Public Parks 2016 report reveals that 92 per cent of park managers reported cuts to their revenue budget over the past three years – and 95 per cent expect their revenue budget to be cut over the next three years.

In recent months, the nation's green spaces have had significant political attention through the Communities and Local Government Parliamentary Committee Inquiry and its subsequent report into Public Parks. Yet the dissolution of Parliament, before CLG Parks Minister Andrew Percy had formally responded to the report, risks the loss of impetus and the issue being overlooked in a crowded legislative programme of the next Parliament.

It’s in this context that Fields in Trust launched our Manifesto for Parks. This election presents an opportunity to ensure the UK's parks, playing fields and green spaces are seen as a vital national asset by the next government.

Since the 1920s, Fields in Trust has been protecting land for play, sport and recreation and campaigning about the importance of these spaces to the health and wellbeing of communities. We are concerned about the impact of building on green space sites in both urban and rural areas; local green spaces of all shapes and sizes are invaluable if we are to create a more active nation as the government's sports strategy aspires to. Whilst recognising the urgent need to build new homes, it is vital that all neighbourhoods, and particularly children, should be able to enjoy healthy active outdoor recreation within walking distance of home.

All local authorities have to make tough decisions over funding and there is a temptation for cash-strapped councils to irreversibly auction off assets; not only to generate immediate income, but also remove a longer term maintenance liability. Our parks are facing increasingly challenged futures and local authority funding cuts could have a damaging impact on the nation’s health.

We need to change the way public green space is conceived, not as a drain on spending that requires a considerable funding to maintain – but rather as an asset which can be deployed to achieve longer term savings. Numerous research studies have demonstrated the health impact of access to green space in encouraging physical exercise, promoting mental wellbeing, and providing a stress-free space to relax. They also contribute to the “liveability” of our towns and cities.

But they are undervalued and underfunded. Few public services have such a cross cutting impact as parks and green spaces. Creating more joined-up service provision is key to the work of Health and Wellbeing Boards. Changing the conversation to recognise the role that parks can play in funding the prevention rather than the cure is crucial to sustaining their future.

We need a practical response to delivering the key government objective of a more active nation which was articulated in the Department for Culture Media & Sport’s Sporting Future – A New Strategy for an Active Nation report. Our recent research in Rugby, Warwickshire shows that local access to green space leads to people feeling healthier and happier – and becoming more active as a result. A reduction in childhood obesity and inactive communities requires a combination of measures, but the goal of getting people more active will only be achieved if they have places to play. School playing fields, parks and open spaces are all crucial to ensuring communities can take part in physical activity yet they remain vulnerable to development.

Parks and open spaces contribute to the physical and mental health and well-being of our communities they provide civic spaces for the development of community cohesion through festivals and events. We need to ensure funding for parks is commensurate with their positive impact on communities. The General Election provides an opportunity to revalue our parks and playing fields.

At a time when green space is increasingly under threat of development for housing and employment, the need to secure places for play, sport, the enjoyment of nature and recreation has never been greater. Current concerns about health, child obesity, access to nature and mental wellbeing all require a green infrastructure for future generations to enjoy, forever.

Helen Griffiths is Chief Executive of Fields in Trust, a national charity working to improve the protection, provision and quality of outdoor recreational spaces for all communities in the UK. She tweets as @HEGriffiths.

You can read the Fields in Trust Manifesto here. And if you #LoveyourLocalPark, join the campaign to celebrate the UKs parks and green spaces – with Fields in Trust’s Have a Field Day on 8 July.