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.

Air-conditioners

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.

 
 
 
 

“A story of incompetence, arrogance, privilege and power”: A brief history of the Garden Bridge

Ewwww. Image: Heatherwick.

Labour assembly member Tom Copley on a an ignominious history.

The publication last week of the final bill for Boris Johnson’s failed Garden Bridge has once again pushed this fiasco into the headlines.

As well as an eye-watering £43m bill for taxpayers for this Johnsonian indulgence, what has been revealed this week is astonishing profligacy by the arms-length vehicle established to deliver it: the Garden Bridge Trust. The line by line account of their spending reveals £161,000 spent on their website and £400,000 on a gala fundraising event, amongst many other eyebrow raising numbers. 

Bear in mind that back in 2012, Johnson promised that the bridge would be entirely privately funded. The bridge’s most ardent advocate, Joanna Lumley, called it a “tiara for the Thames” and “a gift for London”. Today, the project would seem the very opposite of a “gift”.

The London Assembly has been scrutinising this project since its inception, and I now chair a working group tasked with continuing our investigation. We are indebted to the work of local campaigners around Waterloo as well as Will Hurst of the Architects Journal, who has brought many of the scandals surrounding the project into the open, and who was the subject of an extraordinary public attack by Johnson for doing so.

Yet every revelation about this cursed project has thrown up more questions than it has answers, and it’s worth reminding ourselves just how shady and rotten the story of this project has been.

There was Johnson’s £10,000 taxpayer funded trip to San Francisco to drum up sponsorship for the Thomas Heatherwick garden bridge design, despite the fact that TfL had not at that point even tendered for a designer for the project.

The design contest itself was a sham, with one of the two other architects TfL begged to enter in an attempt to create the illusion of due process later saying they felt “used”. Heatherwick Studios was awarded the contract and made a total of £2.7m from taxpayers from the failed project.


Soon after the bridge’s engineering contract had been awarded to Arup, it was announced that TfL’s then managing director of planning, Richard de Cani, was departing TfL for a new job – at Arup. He continued to make key decisions relating to the project while working his notice period, a flagrant conflict of interest that wouldn’t have been allowed in the civil service. Arup received more than £13m of taxpayer cash from the failed project.

The tendering process attracted such concern that the then Transport Commissioner, Peter Hendy, ordered an internal audit of it. The resulting report was a whitewash, and a far more critical earlier draft was leaked to the London Assembly.

As concerns about the project grew, so did the interventions by the bridge’s powerful advocates to keep it on track. Boris Johnson signed a mayoral direction which watered down the conditions the Garden Bridge Trust had to meet in order to gain access to further public money, exposing taxpayers to further risk. When he was hauled in front of the London Assembly to explain this decision, after blustering for while he finally told me that he couldn’t remember.

David Cameron overruled the advice of senior civil servants in order to extend the project’s government credit line. And George Osborne was at one point even more keen on the Garden Bridge than Johnson himself. The then chancellor was criticised by the National Audit Office for bypassing usual channels in order to commit funding to it. Strangely, none of the project’s travails have made it onto the pages of the London Evening Standard, a paper he now edits. Nor did they under his predecessor Sarah Sands, now editor of the Today Programme, another firm advocate for the Garden Bridge.

By 2016 the project appeared to be in real trouble. Yet the Garden Bridge Trust ploughed ahead in the face of mounting risks. In February 2016, despite having not secured the land on the south bank to actually build the bridge on, nor satisfied all their planning consents, the Trust signed an engineering contract. That decision alone has cost the taxpayer £21m.

Minutes of the Trust’s board meetings that I secured from TfL (after much wailing and gnashing of teeth from the Trust itself) reveal that weeks beforehand Thomas Heatherwick had urged the trustees to sign the contract in order to demonstrate “momentum”.

Meanwhile TfL, which was represented at board meetings by Richard de Cani and so should’ve been well aware of the mounting risks to the project, astonishingly failed to act in interests of taxpayers by shutting the project down.

Indeed, TfL allowed further public money to be released for the project despite the Trust not having satisfied at least two of the six conditions that had been set by TfL in order to protect the public purse. The decision to approve funding was personally approved by Transport Commissioner Mike Brown, who has never provided an adequate explanation for his decision.

The story of the Garden Bridge project is one of incompetence, arrogance and recklessness, but also of privilege and power. This was “the great and the good” trying to rig the system to force upon London a plaything for themselves wrapped up as a gift.

The London Assembly is determined to hold those responsible to account, and we will particularly focus on TfL’s role in this mess. However, this is not just a London issue, but a national scandal. There is a growing case for a Parliamentary inquiry into the project, and I would urge the Public Accounts Committee to launch an investigation. 

The Garden Bridge may seem like small beer compared to Brexit. But there is a common thread: Boris Johnson. It should appal and outrage us that this man is still being talked about as a potential future Prime Minister. His most expensive vanity project, now dead in the water, perhaps serves as an unwelcome prophecy for what may be to come should he ever enter Number 10.

Tom Copley is a Labour member of the London Assembly.