Can trees help cool our cities down?

Two men and a dog shelter from last year's Mumbai heatwave in the shade of a tree. Image: Getty.

In cities around the world, trees are often planted to help control temperatures and mitigate the effects of the “urban heat island”. But while trees have been called “nature’s air conditioners”, in practice, scientists often have difficulty demonstrating their cooling properties.

The most obvious way to measure the cooling effect of trees would be to compare the air temperature in parks with that in nearby streets. But this method often comes up with disappointing results: even in large, leafy parks, the daytime air temperature is usually less than 1°C cooler than in the stuffy streets – and at night the temperature in parks can actually be higher.

To explain this contradiction, we need to think more clearly about the physics of heat flows in our cities, and the scale of the measurements we are taking.

Shady days

Theoretically, trees can help provide cooling in two ways: by providing shade, and through a process known as evapotranspiration.

Locally, trees provide most of their cooling effect by shading. How warm we feel actually depends less on local air temperature, and more on how much electromagnetic radiation we emit to, and absorb from, our surroundings. A tree’s canopy acts like a parasol, blocking out up to 90 per cent of the sun’s radiation, and increasing the amount of heat that we lose to our surroundings by cooling the ground beneath us.

Shade cools the ground. Image: Author provided.

All up, the shade provided by trees can reduce our physiologically equivalent temperature (that is, how warm we feel our surroundings to be) by between seven and 15°C, depending on our latitude. So it’s no surprise that, in the height of summer, people throng to the delicious coolness of the shade provided by London parks, Parisian boulevards, and Mediterranean plazas.


Trees can also cool down buildings – especially when planted to the east or west – as their shade prevents solar radiation from penetrating windows, or heating up external walls. Experimental investigations and modelling studies in the USA have shown that shade from trees can reduce the air conditioning costs of detached houses by 20 to 30 per cent.

But air conditioning is more common in some places than in others: for example, while three out of four Australian households have an air conditioner, they’re much less common in Northern Europe, leaving the population there more vulnerable to the harms of urban heat. During the 2003 European heatwave, there were 70,000 more deaths recorded, compared with equivalent cool periods. We urgently need more research to find out how much shade from trees could cool down the terraced houses and apartment blocks, where so many less well-off people live.

Beating the heat

Trees can also be used to tackle a bigger problem: the urban heat island. During periods of calm, sunny weather, the air temperature of cities can be raised above that of the surrounding countryside by up to 7°C, especially at night. In cities, the hard, dark asphalt and brick surfaces absorb almost all the incoming short-wave radiation from the sun, heating up to between 40°C and 60°C, and storing energy which is then released into the air during the still of night, when it can be trapped in the narrow street canyons.

Evapotranspiration in action. Image: Author provided.

Urban trees can counter this process by intercepting the radiation before it reaches the ground, and using the energy for evapotranspiration. Evapotranspiration occurs when the sun’s rays hit the trees' canopy, causing water to evaporate from the leaves. This cools them down – just as sweating cools our skin – thereby reducing the amount of energy left to warm the air.

The effects of evapotranspiration can be quantified in two ways. First, you can measure the temperature of the tree canopy, which is typically much cooler than built surfaces – only 2°C to 3°C above air temperature. Unfortunately, we can’t really claim that this temperature difference is evidence of cooling capacity; leaves would be cooler than built surfaces even if they weren’t losing water, because they are cooled more effectively by convection.

A better method is to calculate the cooling effect of a tree directly, by measuring how much water it is losing. You can do this by measuring the sap flow up its trunk, or the water loss from single leaves. These methods show that tree canopies can divert over 60% of the incoming radiation to evapotranspiration. Even a small (4m high) Callery pear tree – a commonly planted species in Northern Europe – can provide around 6kW of cooling: the equivalent of two small air-conditioning units.


But there’s a catch: trees only provide this cooling effect when they are growing well. By measuring water loss from individual leaves, we showed that sparser, slower-growing plum and crab apple trees provided only a quarter of the cooling effect of the Callery pears.

What’s more, the effectiveness of trees can be greatly reduced if the growing conditions are poor. We found that the transpiration of Callery pears could be reduced by a factor of five, if the roots were growing through compacted or poorly aerated soil. Much more research is needed on the relative performance of large and small trees, whether they’re planted on streets or in parks.

One final difficulty in working out the cooling power of trees is to determine how much a given tree’s evapotranspiration will actually reduce the air temperature. As so often in science, a modelling approach is needed, with physicists, engineers and biologists working together. We need to put realistic trees into detailed regional climate models, which can mimic the complex daily movements of air and energy through the city. Only then can we determine the regional benefits of the urban forest, and work out how to use trees to make our cities cooler and more pleasant places to live in.

Roland Ennos is professor of biomechanics at the University of Hull. The Conversation

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

 
 
 
 

So why is Peterborough growing so quickly?

Peterborough Cathedral. Image: Jules & Jenny/Flickr/creative commons.

The latest instalment of our series, in which we use the Centre for Cities’ data tools to crunch some of the numbers on Britain’s cities.  

The 2001 census put the population of Peterborough at 156,000. Some time before next spring, it’s projected to pass 200,000. That, for those keeping score, is an increase of about 28 per cent. Whether this makes it the fastest growing city in Britain or merely the second or the fourth – the vagueness of Britain’s boundaries means that different reports reach different conclusions – doesn’t really matter. This is a staggering rate of growth.

Oh, and since austerity kicked in, the city council has had its grant from central government cut by 80 percent.

Expansion on this scale and at this rate is the sort of thing that’d have a lot of councils in our NIMBY-ish political culture breaking out in hives; that seems to go double for Tory-run ones in Leave-voting areas. This lot, though, seem to be thriving on it. “I think the opportunity in Peterborough is fantastic,” says Dave Anderson, the city’s interim planning director. “We’re looking at growing to 235,000 by the mid-2030s.”

More striking still is that the Conservative council leader John Holdich agrees. “I’m a believer in ‘WIMBY’: what in my back yard?” he says. He’s responsible, he says, not just to his electorate, but “to our future kids, and grandkids” too – plus, at that rate of growth, a lot of incomers, too.

All this raises two questions. Why is Peterborough growing so quickly? And what can it do to prepare itself?

If you’re a little uncertain exactly where Peterborough is, don’t worry, you’re in good company. Until 1889, the “Soke of Peterborough” was an unlikely east-ward extrusion from Northamptonshire, far to its south west. Then it was a county in its own right; then part of the now-defunct Huntingdonshire. Today it’s in Cambridgeshire, with which it shares a metro mayor, the Conservative James Palmer. When I ask Holdich, who’s giving me a whistlestop tour of the city’s cathedral quarter, to explain all this, he just shrugs. “They keep moving us about.”

Sitting on the edge of the Fens, Peterborough is, officially, a part of the East of England region; but it’s just up the road from East Midlands cities including Leicester and Nottingham. I’d mentally pigeonholed it as a London-commuter town, albeit a far flung one; but when I actually looked it up, I was surprised to discover it was closer to Birmingham (70 miles) than London (75), and halfway up to Hull (81).


The more flattering interpretation of all this is that it’s on a bit of a crossroads: between capital and north, East Anglia and the Midlands. On the road network, that’s literally true – it’s where the A1 meets the A47, the main east-west road at this latitude – and railway lines extend in all directions, too.

All of which makes Peterborough a pretty nifty place to be if you’re, say, a large logistics firm.

This has clearly contributed to the city’s growth. “It has access to lots of land and cheaper labour than anywhere else in the Greater South East,” says Paul Swinney, director of policy at the Centre for Cities. “Those attributes appeal to land hungry, low-skilled business as opposed to higher-skilled more knowledge-based ones.”

That alone would point to a similar economy to a lot of northern cities – but there’s another thing driving Peterborough’s development. Despite being 70 miles from the capital, the East Coast Main Line means it’s well under an hour away by train.

In 1967, what’s more, the ancient cathedral city was designated a new town, to house London’s overspill population. The development corporation which owned the land and built the new town upon it, evolved into a development agency; today the same role is played by bodies like Opportunity Peterborough and the Peterborough Investment Partnership.

The city also offers relatively cheap housing: you can get a four-bed family home for not much over £200,000. That’s fuelled growth further as London-based workers scratch around for the increasingly tiny pool of places that are both commutable and affordable.

The housing affordability ratio shows average house prices as a multiple of average incomes. Peterborough is notably more affordable than Cambridge, London and the national average. Image: Centre for Cities data tool.

It’s made it attractive to service businesses, too. “London has probably played quite a big role in the city’s development,” says Swinney. “If you don’t want to move too far out, it’s probably one of the cheapest places to move to.”

The result of all this is that it has an unusually mixed economy. There’s light industry and logistics, in the office and warehouse parks that line the dual-carriageways (“parkways”) of the city. But there are also financial services and digital media companies moving in, bringing better paying jobs. In a country where most city economies are built on either high value services or land-hungry warehousing businesses, Peterborough has somehow managed to create a mixed economy.

Peterborough’s industrial profile: more services and less manufacturing, and more private and fewer public sector jobs, than the national average. Image: Centre for Cities.

At the moment, if people think of Peterborough at all, they’re likely to imagine a large town, rather than the fair-size regional city it’s on course to become. Its glorious 12th century cathedral – the hallmark of an ancient city, and at 44m still by far the highest spot on the horizon for miles around – is stunning. But it’s barely known to outsiders, and at least twice on my tour, the council’s communications officer proudly announces that the Telegraph named her patch as one of the best towns to live in within an hour of London, before adding, “even though we’re a city”. 

So part of the council’s current mission is to ensure that Peterborough has all the amenities people would expect from a settlement on this scale. “What the city needs to do is to adopt the mind-set of a slightly larger city,” says Anderson. Slightly smaller Swansea is developing a new music arena, of the sort Peterborough doesn’t have and needs. He frets, too, about retail spend “leaking” to Cambridge or Leicester. “Retail is now seen as a leisure activity: in the core of the city it’s important that offer is there.”

To that end, the early 1980s Queensgate shopping centre is being redeveloped, with John Lewis giving up a chunk of space to provide a new city centre cinema. (At present, the area only has road-side suburban multiplexes.) There’s major office, retail and housing development underway at North Westgate, as well as work to improve the walking route between the station and the commercial centre, in a similar manner to Coventry.

Fletton Quays. Image: Peterborough Investment Partnership.

Then there’s the city’s underused riverside. The council recently moved to new digs, in Fletton Quays, on the far bank of the River Nene from the centre. Across the river from the Embankment, the city centre’s largest green space, it’s a pretty lovely spot, of the sort where one might expect riverside pubs or restaurants with outdoor seating – but at the moment the space is largely empty. The Fletton Quays development will change all that, bringing more retail space and yes, new homes, too.

Jobs in Peterborough are unusually distributed around town: in many cities, most jobs are in the central business district. Image: Centre for Cities.

The big thing everyone agrees is missing, though, is a university. It already has the University Centre Peterborough, where degrees are provided by Anglia Ruskin University. The plan is for the site – a joint venture between ARU and Peterborough Regional College – to go its own way as an independent institution, the University of Peterborough, in autumn 2022. That should help provide the skills that the city needs to grow. A growing student population should also bring life and cash to the city centre. 

How big could Peterborough get? Could its enviable combination of good location and cheap housing and grand ambitions combine to make it the modern equivalent of Manchester or Liverpool – one of the great cities of the 21st century?

Well, probably not: “I think the optimum size for a city is probably about 250,000,” says Holdich. But that’s still a whole quarter bigger than now, and the council leader even discusses the possibility of refitting his dual-carriageway-based-city with some kind of light rail network to service that growing population. Peterborough’s not done growing yet.

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

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