The powerful heat waves washing over North America and Europe this summer have sent urban workers and dwellers scurrying for cover. Air conditioners are being switched to high, public cooling centres opened and blinds drawn across south-facing windows.
The high heat of summer can be inescapable on the streets and can restrict outdoor activities. Since the heat trapped in concrete and asphalt pavement is released only slowly, discomfort can last well into the evening. It’s called the Urban Heat Island effect (UHI). In cities of one million or more, Statistics Canada says the air temperature difference can be as high as 12°C above that of surrounding areas.
Summers have been getting hotter. Environment Canada says extreme heat events have continually risen over the past several decades.
Aside from discomfort, heat can be deadly. Statistics Canada says in Canada’s 12 most populous cities from 2000 to 2020, “approximately 670 excess non-accidental deaths, 115 excess cardiovascular deaths, and 115 excess respiratory deaths were attributable to extreme heat events.” That may under-estimate the seriousness.
A 2024 study by the Institut National de la Recherche Scientifique based in Québec City suggests elevated summer temperatures in Québec are associated with 470 deaths annually. A 2024 Canadian Climate Institute report says British Columbia could average 1,370 heat-related deaths per year by 2030.
It’s only going to get worse. The Canadian Climate Institute report also says Canada is warming twice as fast as the global average.
What can city planners and construction project owners do to cool down the over-heated outdoor urban streetscape?
The answer might seem obvious: more shade. However, shade cast from buildings on paved street and public areas might not be the solution. In fact, it is complicated. Almost all of Canada’s municipalities have policies that restrict or control how buildings cast shadows. It’s sometimes called the “right to light,” and promotes the positive health benefits of sunlight. The result can be shorter buildings spaced further apart, which runs counter to moves towards more densification.
The answer might be more trees.
Trees slow heat absorption, writes Hillary Duff for the non-profit organization Tree Canada.
“They reduce the intensity of heat islands and, when combined with other vegetation and green space, contribute to all-around cooler communities.”
She adds the root structure of trees transpire, meaning they absorb water and send it up to the leaves where it evaporates and cools the air temperature.
Trees are remarkably effective at reducing urban temperatures.
“Because cool air settles near the ground, air temperatures directly under trees can be as much as 25°F cooler than air temperatures above nearby blacktop,” says the U.S. Department of Energy.
To help city planners, the Washington, D.C.-based non-profit American Forest has just released a high-resolution mapping tool in collaboration with the UCLA Luskin Center for Innovation. Over 100 of the largest urbanized areas in North America are included in the mapping. A highlight of the research is the claim that, “at noon, trees provide 25 times more shade than buildings.”
V. Kelly Turner of the UCLA Luskin Center says the mapping tool can be combined with a city’s own policy-mapping tools to help them establish treescapes near roads, in parks and in other open public spaces, develop maintenance strategies and work more effectively with project developers.
“Most cities aren’t armed with the data they need to know if there is shade where communities need it most. That’s where the Shade Map comes in. Never before has it been this easy for communities in the U.S. to know how much shade they have, where it is and whether the source is buildings or vegetation.”
Cities such as Austin, Texas, Phoenix, Ariz. and Detroit, Mich., are already putting the mapping tool to work, she says.
Although Toronto is part of the overall American Forest study, the city can also draw data from The Tree Equity Score Analyzer (TESA) launched in 2024. TESA was co-developed with American Forest, “with a team of dedicated Toronto stakeholders composed of city government, non-profit environmental organizations and residents.”
At the same time, urban overheating is a relatively seasonal issue in Canada. A side benefit of planting deciduous trees in Canadian cities is that their leaves provide shade in summer but are gone in autumn and winter when the sun’s light and heat are more appreciated.
Tree coverage in urban areas is a global issue requiring urgent attention, said the World Economic Forum in a 2002 bulletin.
“Unlike air and water, cities pay relatively little attention to how trees are treated and protected. This is evident from a steady decline in urban tree cover. Over a five-year period, average global urban tree cover decreased by nearly 40,000 hectares (100,000 acres) per year.”
If adopted by large municipalities, the mapping data now available can act to mitigate the impact of future high heat events, improve the urban experience and reduce the number of associated deaths
John Bleasby is a freelance writer. Send comments and Inside Innovation column ideas to [email protected].
