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.

 
 
 
 

In New Zealand, climate change is driving an eco-nationalist revival

The green and pleasant land of the South Island. Image: Getty.

“Ten years ago I would have called them settler f*****g land squatters,” Mike Smith, Maori-dom’s most tenacious activist, said last November as he reflected on the agriculture industry’s central role in driving climate change. “Now I have to try and get these people on board.”

Smith is infamous for taking a chainsaw to Auckland’s most prominent tree on a damp October night in 1994 in protest of the “Pākehā” – or white European –dominated government’s fiscal envelope restraining treaty settlements. Now he’s turned his hand to another cause close-to-home for the Maori, New Zealand’s indigenous population: the environment.

“We’re super vulnerable, like we are to anything,” Smith says. “When it comes to climate change it’s like the poorest people in the world are going to be hit the hardest first, and that’s a lot of us.”

Prime Minister Jacinda Ardern appears, at least rhetorically, the most sympathetic leader to his cause in a decade. In her campaign launch speech late last year, she made the future government’s position clear: “Climate change is my generation’s nuclear free moment.”

This message should resonate with followers of her Labour party: the NZ left has long nurtured an environment-orientated “culture-of-protest”. So Ardern’s call to arms was bound to gain her loyal support among children of the 1960s and ‘70s, who led the march against nuclear ship visits, spurring on the government of the time to wriggle out from the US nuclear umbrella, and place a ban on foreign nuclear ship visits.

Now, it is to the tycoons of deep sea oil exploration they aim to close NZ’s ports.

In this, Smith is not short of support locally, with marches run by grassroots organisations and international animal welfare funds beginning to gain traction with every day New Zealanders. In this, Ardern’s prediction is correct: the Coal Action Network Aotearoa (CANA), is reminiscent of the Campaign for Nuclear Disarmament (CND), an earlier cluster of left-wing pacifists and nature lovers who drove the creation of the nuclear free zone.  

In December, 15,000 passionate protesters marched through the capital. And with the government’s recent ban of offshore oil exploration projects, Jeanette Fitzsimons, former Green party co-leader and the head of CANA, is optimistic about similar change to the energy and farming sectors.

The Labour-NZ First-Green party coalition seems focused on setting a new global precedent, weaning NZ away from a United States which has jettisoned the Paris Agreement. The move replicates another 20 years ago, when New Zealand’s anti-nuclear movement was central to an upsurge in New Zealand nationalism. Now, the same sense of going it alone on foreign policy is apparent both locally and in Parliament.

Dr. Gradon Diprose, a senior lecturer at Massey University, argues that this echoes an older expression of colonial nationalism, that saw “New Zealand as a land of natural abundance”. This: “eco-nationalism” is centered on “protecting certain visions of picturesque landscapes and unspoiled natural beauty”. The slogan “Clean, green New Zealand” is prevalent in popular culture and tourism marketing. The public seems to have latched onto it too, and ranked keeping NZ’s waterways “clean and green” top of a recent survey of of kiwis’ top concerns.

Three decades ago, it was the 10 July 1985 sinking of the Greenpeace flagship Rainbow Warrior that thrust local activists’ fears into the public eye, resulting in an almost nation-wide expression of climate-protectionism.


The bombing, a French intelligence operation sparked by Greenpeace’s calls for an end to foreign nuclear testing in the Pacific, galvanised a great deal of change to New Zealand’s overseas defence policies. A lack of censure from New Zealand’s Western allies drove Wellington to distance itself from the United States, while the shock of seeing a friendly nation violate NZ’s sovereignty left many at home seething.

Thirty years on, the foreign policy split throughout the Anglosphere, regarding Russian-Western relations, globalism, and the old international rules-based order, is becoming wider. Climate change is just the tip of the iceberg.

Most Kiwis you talk to will shake their heads in disapproval at US president Donald Trump’s scandalous outing last year in Helsinki. But US defiance of internationally brokered climate resolutions is something they can see clearly reflected in rural communities across the country.

The country saw records broken at both ends of the extreme weather spectrum last year. As 2018 kicked off, Kiwis sweltered through the hottest summer on record, while in Golden Bay, a small inlet near the northern tip of the South Island, residents endured the largest flood in 150 years. So, when President Trump tweets “So much for Global Warming”, the majority of New Zealanders look back fondly on NZ’s 1985 decision to boycott the “ANZUS” treaty, putting New Zealand at odds with its war-time ally America on defence legislation.

Public calls to take the same track on environmental regulation have become louder in the wake of Donald Trump’s election. The former US Secretary of State, Rex Tillerson, received a frosty “unwelcome” while on a flyby to the capital in 2017, with the New York Times’ Washington correspondent, Gardiner Harris remarking: “I’ve never seen so many people flip the bird at an American motorcade as I saw today”. Protests against President Trump’s stance on climate change are beginning to gain greater traction further still, with the hundred-strong “march for science” setting the tone for the new government later that year.

New Zealand certainly isn’t afraid of radicalism, and its activists are persistent. It’s already banned single use plastics in supermarkets. Plenty more is to come, Smith says.

And yes, reform is going to inhibit sometimes vital industries: “It doesn’t matter which way you spin the dice on this, whatever’s being done is going to hurt. People who are looking for a painless way of mitigating climate change, [but] I don’t think there is one.”

But among Smith’s troupe of climate agitators, the feeling is that, without drastic change, “the land”, the heart of the Maori ethos, is going to be hurt far more.

Back in Auckland, NZ’s financial hub, an electric scooter craze is gripping the city. This, too, has gained the support of local environmentalists. In New Zealand, a national sense of pride is always coupled with a certain eccentricity. In a country this size, change always starts small.