How Can Trees Really Cool Our Cities Down?

How Can Trees Really Cool Our Cities Down?

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% of the sun’s radiation, and increasing the amount of heat that we lose to our surroundings by cooling the ground beneath us.

How Can Trees Really Cool Our Cities Down? Shade cools the ground. Roland Ennos, 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%.

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.

How Can Trees Really Cool Our Cities Down? Evapotranspiration in action. Roland Ennos, 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.

About The Author

ennos rolandRoland Ennos, Professor of Biomechanics, University of Hull. He is interested in the ways in which organisms interact with the physical world, particularly in their structural engineering. He has investigated the mechanical design of insect wings and plant root systems and the mechanical defences of grasses, but recently

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

Related Books

enafarzh-CNzh-TWnltlfrdehiiditjakomsfaptruesswsvthtrurvi

LATEST VIDEOS

Allergens Are On The Rise In Canada's Urban Centres
by CBC News: The National
Canadians across the country say their allergies are getting worse.
Bill Nye And The Climate Crisis
by MSNBC
On a special show before a live studio audience, Bill Nye the science guy discusses the climate crisis with Chris Hayes.
How Greenland's Massive Ice Melt Will Totally Transform The World
by Channel 4 News
Remember that heatwave back in August? Well, the Arctic remembers it too. Record rates of ice melt have been recorded…
China Is Positioned To Lead On Climate Change As The US Rolls Back Its Policies
China Is Positioned To Lead On Climate Change As The US Rolls Back Its Policies
by Kelly Sims Gallagher and Fang Zhang
As the effects of climate change become more widespread and alarming, U.N. Secretary-General António Guterres has…
What Happens When The Permafrost Thaws?
by Official W5
Almost half of Canada sits on permanently frozen land called permafrost, but climate change is causing it to thaw and…
We Are Striking to Disrupt the System: An Hour with 16-Year-Old Climate Activist Greta Thunberg
by Democracy Now!
In her first extended broadcast interview in the United States, we spend the hour with Greta Thunberg, the 16-year-old…
Annual Assessment of North Cascades Glaciers Finds Shocking Loss of Volume
Annual Assessment of North Cascades Glaciers Finds Shocking Loss of Volume
by Mauri Pelto
The summer of 2019 found the North Cascade Glacier Climate Project in the field for the 36th consecutive summer…
Breakdown In Coral Reef Iconic Spawning Puts Species At Risk Of Extinction
Breakdown In Coral Reef Iconic Spawning Puts Species At Risk Of Extinction
by Heidi Burdett
Breakdown In Coral Reef Iconic Spawning Puts Species At Risk Of Extinction

LATEST ARTICLES

Allergens Are On The Rise In Canada's Urban Centres
by CBC News: The National
Canadians across the country say their allergies are getting worse.
High-tech Weather Forecasting Aims To Bring New Hope To Indian Farmers Facing The Devastation Of Climate Change
High-tech Weather Forecasting Aims To Bring New Hope To Indian Farmers Facing The Devastation Of Climate Change
by Sam Relph
The world’s largest network of automated weather stations, created to predict droughts and floods, is helping tackle…
Regenerative Agriculture Can Make Farmers Stewards Of The Land Again
Regenerative Agriculture Can Make Farmers Stewards Of The Land Again
by Stephanie Anderson
For years, “sustainable” has been the buzzword in conversations about agriculture.
How Do You Live Off-grid?
by BBC News
The UK government proposes reducing UK greenhouse gas emissions to almost zero by 2050, under a new plan to tackle…
Requiem Or Renewal? This Is How A Tropical City Like Darwin Can Regain Its Cool
Requiem Or Renewal? This Is How A Tropical City Like Darwin Can Regain Its Cool
by Lawrence Nield
On my way to work, I walk across the intersection of Knuckey and Wood streets. Most days a homeless man on his haunches…
City Temperatures And City Economics, A Hidden Relationship Between Sun And Wind And Profits
City Temperatures And City Economics, A Hidden Relationship Between Sun And Wind And Profits
by Silvia Tavares and Taha Chaiechi
A simple thing like designing an area to make it more walkable can boost local business profits. This can also increase…
Seaweed Farming Could Really Help Fight Climate Change
Seaweed Farming Could Really Help Fight Climate Change
by Sonia Fernandez
Seaweed is a lot more than marine debris you find on the beach. It may play a big role in the effort to mitigate…
How Climate Change Could Threaten The World’s Traditional Food Dishes
How Climate Change Could Threaten The World’s Traditional Food Dishes
by Daisy Dunne
Climate change is likely to alter the way that the world grows, trades and enjoys food.