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.

 
 
 
 

It’s time to rethink how the British railway network works

Nothing doing: commuters await a long-delayed train. Image: Getty.

The recent meltdowns on Northern and Thameslink not only left many passengers besides themselves with frustration about not being able to get to work on time, if at all. It also led to a firestorm of criticism and condemnation from politicians and media alike.

With the immediate shock of that first Monday morning of the meltdown passed, there’s a now a bigger debate about whether the way that rail services are provided for cities needs some far reaching reform. But before coming to that, the first thing to say – and as we set out in our Rail Cities UK report, launched today – is that the fundamentals for urban rail remain very strong.

Here’s why. All cities want to become denser, more dynamic places which attract the best people to the growth sectors of the economy (including the ‘flat white economy’ of media, communications and information). In order to achieve this, as well as to improve air quality, cities are also reducing space for motorised traffic in favour of space for people.

It’s very difficult to see how this can be achieved without expanding rail networks and their capacity. What’s more, if housing need is to be met without creating more sprawl and traffic congestion, then again its rail that will be key – because it opens up former rail-connected brownfield industrial sites, it extends commuting range, plus housing can be built above or around new or existing rail stations and interchanges.

In some ways there’s nothing new here. From Metroland to Docklands, successful cities have always grown with their rail networks. And to be fair, there is significant investment going into urban rail at present. Northern will get a lot better (the pacers are doomed) and both Merseyside and Tyne & Wear are getting a whole new fleet of trains for their urban rail networks.

However, much (but not all) of this investment is incremental, or replacing rolling stock on its last legs. It stops short of the wider vision for the rail cities that we need.


What would that look like in practice? There comes a point when the biggest cities need more cross-city routes, because running trains in and out of edge-of-centre termini can’t cope with the numbers. That explains the push for Crossrail 2 in London, but also the need for more cross-city capacity in cities like Birmingham (on the Snow Hill route) as well as in Manchester (on the Oxford Road to Manchester Piccadilly corridor, as well as a potential new underground route).

Tram-train technology can also help – allowing the lucky commuter that benefits to get on board at their local station and get off right outside their city centre office on main street in the city centre, rather than piling out at a Victorian railway terminal on the edge of that city centre.

Tram-trains aren’t the only tech fix available. Battery packs can extend the range of existing electric trains deeper into the “look ma, no wires” hinterlands, as well as allow trams to glide through city centres without the expensive clutter of overhead wires.

More mundane but equally useful work to increase capacity through signalling, station, track and junction work offers the opportunity to move to turn-up-and-go frequency networks with greater capacity and more reliability – networks that start to emulate the best of what comparable German rail cities already enjoy. Interlocking networks of long distance, regional express, regional, S-bahn, U-bahn, trams and buses, all under common ticketing.

But in talking about Germany and common ticketing I am now getting back to where I started around the debate on whether some fundamental change is needed on how urban rail networks are provided. Obviously there is a bigger national discussion going on about whether the current structure is just too layered, with too many costly interfaces and too fractured a chain of command. And in addition another, on whether the railway should be publicly or privately owned and operated.

But it’s been heartening to see the growing recognition that – regardless of how these debates are resolved – more devolution for urban and regional services should be part of any solution. That’s not only because fully devolved services have been out-performing comparators both operationally and in passenger satisfaction; it’s because local control rather than remote control from Whitehall will mean that the dots can be joined between rail and housing, between rail and the wider re-fashioning of city centres, and between rail and local communities (for example through repurposing stations as wider hubs for local community use, enterprises and housing). It will also allow for rail and the rest of local urban public transport networks to be part of one system, rather than be just on nodding terms as is all too often the case at present.

The crisis on Northern and Thameslink has been a miserable experience for rail users, affected cities and the rail industry. If any good has come out of it, it is that it shows how important rail is to cities, and opens up a space for some bigger thinking about what kind of rail cities we will need for the future – and how best we can make that happen.

Jonathan Bray is the Director of the Urban Transport Group which represents the transport authorities for the largest city regions. You can read the group’s full report here.