Extreme heat isn't just annoying, it's hazardous, especially to vulnerable populations like seniors or those with chronic illnesses.
Climate scientists have long warned that extreme weather events are becoming more common — we just watched our neighbours in Western Canada experience record-breaking temperatures when the thermometers in Lytton, B.C. soared to 49.5°C.
“With climate change, the number of very hot days in many parts of Canada is expected to more than double by the middle of the 21st century,” said Health Canada’s David Morin, in a report on reducing urban heat islands.
Urban Heat Island Effect
Because of the heat island effect, big cities like Toronto are even more at risk of extreme temperatures. A heat island is when urban areas see much higher temperatures compared to nearby rural areas.
It happens because cities like Toronto are full of concrete, asphalt, brick, glass and steel, and, along with a high proportion of dark roofs and roads, these surfaces absorb the sun's rays and radiate it out, heating up both air and surface temperatures.
In the nearby countryside, of course, you won’t find so many sidewalks, streets or parking lots. Certainly no skyscrapers.
The difference is big — the average air temperature of a city with 1 million people or more can be 1 to 3°C warmer than its surroundings, according to a federal report. In the evening, the air temperature difference can be as high as 12°C
Reducing the Heat Island
The urban forest helps reduce the heat island effect by shading paved surfaces and reducing the heat that’s sent back up into the local atmosphere. In addition, trees slowly release water vapour — a process called transpiration — that has a cooling effect on the surrounding air.
To illustrate, take a look at the relationship between high tree canopy cover and surface temperature.
This map, shared by Green Infrastructure Ontario, shows just one neighbourhood in northwest Toronto. On the left, high surface temperatures are illustrated in dark red and low surface temperatures in blue. On the right, the higher the tree canopy cover is, the darker the shade of green in the map.
Now, imagine where you’d rather be on the next hot, humid day?
It’s a reminder that the urban forest is so much more than the city’s natural areas. The trees along your street and in your backyard are helping to protect you and your neighbours from uncomfortably high temperatures.
Something to consider the next time you’re relaxing in a cool, shady spot.
If you’re looking to increase the canopy cover in your own backyard, check out the Backyard Tree Planting Program.
Ethan Rotberg is a LEAF volunteer and a Toronto-based freelance writer and content strategist. You can reach him on Twitter or at ethanrotberg.com.