Cooling down cities could make a big difference to global warming

Image: russavia via Flickr.

Cities may only occupy about 2 per cent of the world’s habitable land, but they are big drivers of global climate change. That's because they're usually hotter than rural areas; a phenomenon which is known as the “urban heat island" effect. 

Cities are hotter for a number of reasons. Traffic pollution creates a greenhouse effect that keeps heat in at night. The lack of trees, meanwhile, means cities lose their ability to absorb heat and convert it into nutrients. Paving and tarmac quickly release the heat they retain back into the air, and rainwater has to be drained away in sewer systems, which deprives the area of the cooling effect of rain-soaked soil.

Then there are people. They both generate body heat and heat buildings to keep themselves warm – or use air-conditioning to cool them down. Aircon means they are transferring warmer air into the streets outside, so it adds to the city’s warmth just as much as heating systems would. 

As cities expand in size and more people live in them, these warming factors have gradually been exacerbated. In the south of England, the difference between rural areas and London is as much as 6°C. In Glasgow, where the population has been on the decline until recently, the difference can still be as much as 8°C.

In hotter parts of the world, the situation is reaching breaking point. Colombo in Sri Lanka has seen people migrating away in substantial numbers to live in cooler areas. The searing heat in Phoenix, Arizona, may prevent the city from continued expansion. Even in more temperate cities like London or Paris, unexpected heatwaves can kill hundreds and even thousands of people.

Phoenix, Arizona. Image: DPPed  at WIkimedia Commons.

The global warming debate

Discussions about global warming tend to overlook the contribution of urban growth to the problem, and instead concentrate on what is happening to the temperature across the world.

Policy developers aiming to fight global warming overlook the fact that by focusing on ways to make cities cooler, they might contribute in a big way to the solution – perhaps much more so than by focusing on global carbon-reduction agreements that either fail or end up badly watered down. Given the forecasts for climate change out to 2050, this it's beginning to look like policymakers have missed a trick when it comes to cities.

The good news is that cities have been living with the effects of local warming for decades. By observing different cities around the world, we can see what needs to happen – the problem lies in getting those cities that do less to focus on doing more.

One size doesn’t fit all

The solutions in hotter and cooler climates are different. Research in warm, humid Colombo shows excessive amounts of solar radiation. But because of the availability of abundant water from year-round rainfall and a large amount of urban vegetation, there is much less temperature difference between the city centre and rural surroundings. This suggests that were this not in place, the migration problem could be even worse.

The research found that you a big difference can be made to the climate in tropical cities, whether humid or arid, through shade.

Colombo, Sri Lanka would be far warmer were it not for all the tree cover. Image: Diana Montero Melis.

This is not about shading buildings per se (nor is this desirable) but to encourage an urban geometry that makes the spaces between buildings naturally shaded without compromising buildings' ability to draw in the sunlight as and when required.

Achieving this when the tropical sun is so high in the sky means that you have to use an intelligent combination of building heights and geometry together with elements such as canopies, awnings and urban vegetation.

With care and attention to detail, built-up areas can combine good shading with generous urban vegetation to cool neighbourhoods to temperatures that are even below those of rural areas. This is good news given the continued acceleration of urban growth in many tropical cities and rising concentrations of people. And even a couple of degrees' difference can make a city unbearable in an area that is already hot.

London and New York are a good examples of what cities in cooler areas can do to make a difference. Their heat island policies include things like planning requirements to plant trees, reduce paved areas in parking lots and reduce traffic. But these sorts of policies are still quite rare across the board, and you rarely see similar policies in hotter climes. Singapore is one of very few tropical cities that prioritise traffic control, for example.

Finally a word on colder cities such as Glasgow, where I am based and have been involved in work aiming to cool down the city. This may not seem very necessary when the temperature is not particularly high, but we need to bear in mind that it is likely to get hotter in the coming decades. For example our simulations suggest that if you increase tree cover by 20 per cent, you could eliminate a third to half of the expected urban heat increase by 2050. This sort of intervention is well worth considering.

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

Rohinton Emmanuel is a Professor of Sustainable Design & Construction at Glasgow Caledonian University.

 
 
 
 

What would an extended Glasgow Subway look like?

West Street station. Image: Finlay McWalter/Wikimedia Commons.

There are many notable things about Glasgow’s historic Subway.

It is the third oldest in the world. It is the only one in the UK that runs entirely underground. It runs on a rare 4ft gauge. For reasons passing human understanding, it shuts at teatime on a Sunday.

But more significantly, it’s the only metro system never to have been expanded since its original development. A couple of stations have come and gone in the 122 years since the Subway opened (and promptly shut again following a serious accident before the first day was out). But Glasgow’s Subway has remained a frustratingly closed loop. Indeed, while a Scottish newspaper recently estimated there have been more than 50 proposed new stations for Glasgow's iconic Subway since it first opened, all we’ve had are a couple of replacements for closed stops. 

The original route map. Image: SPT.

It’s not for a lack of trying, or at the least discussion. Glasgow’s SNP-led council pledged a major expansion of the Subway as part of their election pledge last year, for example, vowing to find the funding to take the network beyond the existing route.

All this sounds very familiar, of course. A decade ago, with the 2014 Commonwealth Games in mind, operators SPT began looking into a near-£3bn expansion of the Subway into the East End of the city, primarily to serve the new Velodrome complex and Celtic Park.

In the end, the plans — like so many discussed for expanding the Subway – failed to materialised, despite then SPT chairman Alistair Watson claiming at the time: “We will deliver the East End extension for 2014. I am being unequivocal about that.”

As detailed previously on CityMetric, that extension would have seen seven new stations being opened along a second, eastern-centric loop, crossing over with the original Subway at two city centre sites. Had that gone ahead, we would by now have had a new route looking something like this:

The 2007 proposals for an eastern circle. Image: Iain Hepburn.

St Mungo’s would have been close to Glasgow Cathedral. Onslow, presumably located on or near Onslow Drive, would have principally served Dennistoun, as would have a link-up with the existing Duke St overground station.

Gorbals, benefiting from the ongoing redevelopment and residential expansion that’s all but erased it’s No Mean City reputation, would have gained a station, while Newhall would have been next to Glasgow Green. Dalmarnock station would, like Duke Street, become an interchange with Scotrail’s services, while crucially Celtic Park would have gained the final stop, serving both the football stadium, the nearby Emirates Arena and velodrome, and the Forge shopping centre.


Those plans, though, were drawn up more than a decade ago. And if the SNP administration is serious about looking again at the expansion of the Subway, then there’s more than a few changes needing made to those plans.

For starters, one stop at the far end of the loop serving Celtic, the new sports arenas and the Forge feels a bit like underselling the area, particularly with so much new residential development nearby.

Two feels more realistic: one serving the Forge and the rest of Dennistoun, and the other sited on London Road to serve the mass volumes of football and sports traffic. And if Ibrox can have a stop, then it seems churlish not to give the other of the Old Firm clubs their own named halt.

That’s another thing. The naming of the proposed stations is… arbitrary, to say the least. You’d struggle to find many Glaswegians who’d immediately identify where Newhall or Onslow were, off the top of their head. 

The former, especially, seems like there’s a more natural alternative name, Glasgow Green; while the latter, with a second Forge stop also serving Dennistoun, would perhaps benefit from named for the nearby Alexandra Place and park.

(Actually, if we’re renaming stations from their unlikely original choices, let’s say goodbye Hillhead and a big hiya to Byres Road on the original Subway while we’re at it…)

So, what would a realistic, 2017-developed version of that original 2007 proposal give us? Probably something like this:

Better. Image: Iain Hepburn.

One glaring issue with the original 2007 study was the crossover with the… let’s call it the Western Subway. The original proposal had St Enoch and Buchanan St as the crossover points, meaning that, if you wanted to go out east from, say, the Shields Road park and ride, you had to go into town and double back. 

Using Bridge Street as a third interchange feels a more realistic, and sensible, approach to alleviating city centre crowding and making the journey convenient for folk travelling directly from west to east.

There’s a good case to be made for another south east of the river station, depending on where the Gorbals stop is sited. But these are austere times and with the cost of the expansion now likely more than £5bn at current rates, an expanded Bridge Street would do much of that legwork.

Putting all that together, you’d end up with something looking like this:

 

Ooooh. Image: Iain Hepburn.

Ahead of last year’s election, SNP councillor Kenny McLean vowed the party “[would] look at possible extension of the Subway and consider innovative funding methods, such as City Bonds, to fund this work. The subway is over 120 years old. It is high time that we look to connect communities in the north and east of Glasgow.”

Whether Glasgow could raise the £5bn it would probably need to make the 2007 proposal, or an updated variation of it remains, to be seen. And this still doesn’t solve how many places are left off the system. While a line all the way out to Glasgow Airport is unrealistic – after all, an overground rail service to the airport from Paisley has failed to materialise after 30 years of discussion and planning – there’s plenty of places in the city not well served by the Subway, from Maryhill in the north to Hampden in the south, or the riverside developments that have seen flats replace factories and new media hubs, museums and hotels line the Clyde.


Image: Iain Hepburn.

Key city landmarks like the Barrowlands, the Riverside Museum – with its own, fake, vintage subway stop, or the Merchant City are woefully underserved by the subway. But their incorporation – or connection with a Glasgow Crossrail – seems a very expensive pipe dream.

Instead, two adjoining loops, one to Ibrox and one to Celtic Park, seems the most plausible future for an extended Subway. At least colour coding the lines would be easy…

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