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.

 
 
 
 

Never mind Brexit: TfL just released new tube map showing an interchange at Camden Town!!!

Mmmmm tube-y goodness. Image: TfL.

Crossrail has just been given a £1bn bail out. This, according to the Financial TImes’s Jim Pickard, is on top of the £600m bailout in July and £300m loan in October.

That, even with the pound crashing as it is right now, is quite a lot of money. It’s bad, especially at a time when there is still seemingly not a penny available to make sure trains can actually run in the north.

But the world is quite depressing enough today, so let’s focus on something happier. On Saturday night – obviously peak time for cartographic news – Transport for London emailed me to let me know it would be updating the tube map, to show more street-level interchanges:

Connections between several pairs of stations that are near to each other, but have traditionally not been shown as interchanges, now appear on the map for the first time. These include:

  • Camden Road and Camden Town
  • Euston and Euston Square
  • Finchley Road and Finchley Road & Frognal
  • Kenton and Northwick Park
  • New Cross and New Cross Gate
  • Seven Sisters and South Tottenham
  • Swiss Cottage and South Hampstead

The stations shown meet a set of criteria that has been used to help determine which should be included. This criteria includes stations less than a 700m or a 10 minute walk apart, where there is an easy, well-lit, signposted walking route and where making the change opens up additional travel options.

The results are, well, this:

In addition, interchanges between stations have traditionally appeared on the Tube map as two solid lines, irrespective of whether they are internal or external (which means customers need to leave the station and then re-enter for the station or stop they need). This approach has now been updated and shows a clear distinction between the two types, with external interchanges now being depicted by a dashed line, linking the two stations or stops.

And lo, it came to pass:

I have slightly mixed feelings about this, in all honesty. On the positive side: I think generally showing useful street-level interchanges as A Good Thing. I’ve thought for years that Camden Road/Camden Town in particular was one worth highlighting, as it opens up a huge number of north-east travel options (Finchley to Hackney, say), and apps like CityMapper tell you to use it already.


And yet, now they’ve actually done it, I’m suddenly not sure. That interchange is pretty useful if you’re an able bodied person who doesn’t mind navigating crowds or crossing roads – but the map gives you no indication that it’s a harder interchange than, say, Wanstead Park to Forest Gate.

The new map also doesn’t tell you how far you’re going to be walking at street level. I can see the argument that a 400m walk shouldn’t disqualify something as an interchange – you can end up walking that far inside certain stations (Green Park, Bank/Monument), and the map shows them as interchanges. But the new version makes no effort to distinguish between 100m walks (West Hampstead) and 700m ones (Northwick Park-Kenton), which it probably should.

I’m also slightly baffled by some of the specific choices. Is Finchley Road-Finchley Road & Frognal really a useful interchange, when there’s an easier and more direct version, one stop up the line? No hang on West Hampstead isn’t on the Metropolitan line isn’t it? So that’s what it’s about.

Okay, a better one: if you’re switching from District to Central lines in the City, you’re generally better off alighting at Cannon Street, rather than Monument, for Bank – honestly, it’s a 90 second walk to the new entrance on Walbrook. Yet that one isn’t there. What gives?

The complete new tube map. The full version is on TfL’s website, here.

On balance, showing more possible interchanges on the map is a positive change. But it doesn’t negate the need for a fundamental rethink of how the tube map looks and what it is for. And it’s not, I fear, enough to distract from the Crossrail problem.

Jonn Elledge is the editor of CityMetric. He is on Twitter as @jonnelledge and on Facebook as JonnElledgeWrites.