The Business Of Business: Green infrastructure to cool cities

Researchers from University of Toronto’s Climate Lab say that increasing multiple types of green infrastructure in urban areas may be a viable solution to cool cities.

A controlled field study published in the International Journal of Biometeorology found that the widespread use of urban vegetation and forestry, green roofs, green walls, and urban agriculture systems can be beneficial in improving urban climate and regulating temperatures.

According to Emily Amon, the Director of Green Infrastructure at Green Communities Canada, implementing green infrastructure in urban areas is crucial as global warming continues to intensify.

“We’re in a crisis situation,” she says. “Our planet is warming quite quickly; probably a lot faster than we’re even aware of.”

Canada is warming at twice the global average, according to a report by Green Communities Canada.

And as temperatures rise, extreme heat continues to contribute to deaths from cardiovascular and respiratory disease, especially among vulnerable populations, such as the elderly, the chronically-ill and the socially disadvantaged, the researchers in the study say.

Laurie-Maude Drapeau, a scientist at the Institut national de santé publique du Québec, explains that the number of days that Quebec experiences extreme heat could triple by 2040-2070.

“We’d go from a dozen days to about 42 days of 30° temperatures or more in the next few years,” she says. “It’s clear that if we don’t act now, the repercussions could be irreversible at a certain point.”

Fortunately, the potential of green infrastructure to mitigate extreme heat is “significant,” according to the researchers.

This is because trees and vegetation cool the environment through evaporation and transpiration, also known as evapotranspiration.

Here’s how the process works, according to the United States Environmental Protection Agency: When water evaporates – which occurs when it turns into vapor – it absorbs heat from its surroundings; and, when plants transpire – which occurs when they release water vapor through their leaves – they add moisture into the air.

Drapeau explains that a mature tree that transpires 450 litres of water is equivalent to about five air conditioners operating for 20 hours a day.

“It’s really an important effect in terms of cooling, so we shouldn’t overlook it,” she says.

Urban vegetation, forestry and green walls can also regulate temperatures through shading, say the researchers.

The controlled field experiment – which used climate monitors at fixed locations in Toronto over two summers – showed an average monthly temperature reduction between 0.3 and 1.3 °C where green infrastructure was implemented.

While these results are promising, there are limitations to this solution.

According to Guillaume Grégoire, an assistant professor in the department of Agricultural and food sciences at Université Laval, a lack of space in urban areas often poses a challenge.

“It’s very difficult to do greening in downtown Montreal, for example, because there’s not much space, so we’re often stuck with [green] rooftops as the only option,” he says.

But green roofs have their own limitations, as well. Drapeau explains that, in order for a building to support a green roof, it must have a specific structure and be strong enough.

“If the structure isn’t adapted, you have to renovate [and] sometimes that can be very very costly,” she says.

Despite these limitations, Canadian cities are beginning to integrate green infrastructure, but Amon hopes for a more cohesive strategy and vision across the country in the future.

“I think that there’s a real place for provincial and federal oversight, and encouragement and enabling of greater adoption for green infrastructure,” she says.

– Melissa Migueis