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