The heatwave continues. The Met Office has issued heat and health warnings. Many modern houses, designed to keep heat in, are proving unpleasant habitations. Researchers are seeking ways to cool houses. And yet, nature provides us with a solution – trees.
Trees work wonders by reducing temperatures on hot days. Heat maps show that across five major English cities, areas with more trees and green spaces are up to 5 degrees cooler. In US cities, where running domestic air conditioning is commonplace, the presence of urban trees reduces residents’ electricity usage, thus saving billions of dollars in cooling costs.
The cooling power of a leaf canopy
With warmer conditions becoming a regular feature of British summers, consider foregoing the beach and instead, going for a walk or picnic (but not a barbecue) in a wood. That can be just as much fun, and you’ll be surprised that you feel dramatically cooler. But why?
The cooling power of trees starts with shade. Forests and woodlands, with their dense tree canopies, as well as being beautiful to look up into, help keep temperatures down. A parasol may provide you with a degree of shade but trees are permanent and shade the ground all day. So, the ground in the shadow of the trees will be cooler than the ground in the shadow of a moving umbrella.
Photos by Kate Moore. Used with permission.
The second mechanism is evapotranspiration. This is the process where trees draw water from the soil through their roots and release it as water vapour through their leaves. The effect is a natural cooling mechanism. A single, large, mature tree might transpire up to 400kg of water daily – providing cooling power equivalent to about four standard air conditioning units.
The urban heat cycle
Across the country, humans have created hard surfaces – pavements, roads, driveways, patios, car parks, buildings and rooftops. Each of them is exposed to direct sunlight, absorbing and storing heat, releasing it back into the air long after the sun goes down. The net effect is that in densely populated areas, the temperature can be up to 12°C warmer than the surrounding countryside. This localised warming within cities is known as the urban heat island effect.
Trees interrupt this process, preventing surfaces from heating up in the first place. A 2023 modelling study found that almost 40% of the 6,700 deaths attributable to urban heat in Europe in 2015 could have been prevented if 30% of the land area of the cities had had tree cover.
So-called ‘tree huggers’ have often been derided in the media as thwarting human progress. And yet, these champions, working to safeguard trees as far back as 1730 and prevent their axing, were perhaps visionary. If we look at modern building sites or industrial parks, the lack of trees, or worse, the cutting down of mature trees to build more hard surfaces, is foolish at best. And this is at a time when a Wildlife BCN survey reports that across 21 different woodland sites, a concerning 29% of nearly 6,000 trees are found to be suffering with tree diseases.
Addressing the loss of trees
There is action in recognition of the importance of trees and vegetation to break the urban heat cycle. Greener Ipswich is planting trees in the town centre and waterfront area to cool the town in summer, as well as improve air quality and increase biodiversity.
But planting is just the start of a long and, not always successful journey. National Highways planted over a million trees and shrubs along the A14 following its upgrade. Their aim was to provide environmental screening, reduce noise and provide habitat.
While losses of about 10% might be expected, up to 30% of the first tranche of saplings have already died. Hot summers, drought, inadequate ground preparation, and not planting the right type of tree in the right location are blamed. The Arboricultural Association advises that for the first three years after planting, newly-planted trees should be given 50 litres (11 gallons) of water per week during the summer months.
It takes many years for a tree to reach maturity. A birch or rowan may be considered mature after 15-20 years old, whereas a slow-growing oak might take 80-120 years to reach full maturity. By 2030, the UK is projected to experience summer temperatures frequently exceeding 37°–40°C. By 2050, temperatures could reach up into the mid 40s.
The sooner we plant trees – ideally in groups, as solitary trees are more vulnerable – and nurture them through the early years, the cooler we’ll feel. So, whether your garden is small or large, it’s time to plant trees to give you your own shaded, and naturally air-conditioned space.
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