How trees keep us and our communities cool

Hilary Duff

Welcome to the dog days of summer! Those of us who are fortunate enough to live near a green space are likely finding some relief by taking shelter beneath the shade of a giant tree.

Trees are nature’s air conditioner, and nowhere is this truer than in the urban centres that more than 80% of Canadians call home. While trees bring countless benefits to our communities, we thought we’d take advantage of the steamy summer climes to focus specifically on the cooling benefits of trees.

How exactly do trees moderate temperatures?

Great question! Trees improve the livability of our urban communities in two key ways.

First, shade is the most obvious factor. Leafy canopies prevent sun rays from reaching our skin and the ground. This is especially important in cities, where asphalt, concrete buildings, the metal of cars and buses, and other man-made environments absorb more heat than natural surfaces.

These built environments retain heat and radiate it throughout the day and night, creating urban heat islands that are often several degrees warmer than nearby rural areas. This difference is most evident at night. Though the sun has set, cities can experience air temperatures that are up to 12 degrees Celsius warmer due to the slow release of heat from sidewalks and roads. Trees slow this heat absorption, reduce the intensity of heat islands and, when combined with other vegetation and green space, contribute to all-around cooler communities.

Second, though trees (and other plants) don’t breathe per se, they do something similar called transpiration. Root systems serve as a series of intricate drinking straws, absorbing water that travels up through the tree. Transpiration is the process in which some of that water is released back into the air through small pores in the tree’s leaves. Evaporated from a liquid as a result of the sun’s rays, the released water vapour cools the surrounding air temperature and accounts for about 10% of the moisture in the Earth’s atmosphere.

Trees also keep our buildings cool

With COVID-19 we’re all spending a bit more time indoors than usual. Fortunately, you don’t need to go outside to experience the beneficial cooling effects of trees.

Consider one study from central Montreal that simulated the effect of increased tree cover on air temperature. While the study confirmed that streets lined with a dense urban canopy experienced a lower ground level temperature, researchers also noted that roadside tree planting contributed to cooler air temperatures up to 20 storeys above the ground.

Trees planted on the south and west-facing sides of buildings are particularly effective, sheltering air conditioning units and windows from the most intense hours of sunlight. Numerous studies support the reduction in building energy use and cost savings offered by trees, though figures and air temperature effects vary from community to community.

Let’s continue up to the very top of buildings. Green rooftops are becoming increasingly popular around the world as a way to increase energy efficiency in cities. While green roofs are typically planted with an assortment of shrubs and grasses, trees are also appropriate under certain conditions. Natural Resources Canada estimates green roofs can reduce daily energy demand by up to 75% in the summer and spring, naturally keeping buildings cool and saving residents money.

Trees: Critical infrastructure in fighting climate change

Canada’s summer temperatures aren’t expected to lower anytime soon. In fact, scientists say our country is warming at twice the rate of the rest of the world. Exceeding historical temperature records has become an alarming annual norm. Urban greening is one way to keep community temperatures in check and prevent the negative human health effects that can come with too much heat exposure.

Beyond providing relief on hot days, trees also have a critical role to play in creating climate change resilient communities. Planting, protecting, and maintaining urban forests will maximize the absorption and storage of carbon dioxide, which helps mitigate the root cause of our climate crisis. Experts say tree planting is one of the most inexpensive and accessible climate solutions, and it’s a reason why the Canadian government has committed to planting two billion trees over the next decade.

Our Canadian summers can be hot and intense, yet during these fleeting months, having a refreshing appreciation for our urban trees and the role they play in our communities will inspire us to keep them as green as possible, while staying cool.


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