Bike down a city street on a hot summer night until you pass a park and be rewarded with a blast of cool air.

This, in miniature and in reverse, is an easy way to understand what geographers and climatologists call the Urban Heat Island effect: even if a city and its rural surroundings receive the same amount of solar radiation, the city will be hotter.

But if the Urban Heat Island sounds simple, it masks a host of social thorny ramifications.

In Toronto, there is “almost a perfect overlay between poor areas and hot areas,” says Kevin Behan, deputy director of the Clean Air Partnership, an environmental group.

Mitigating the Urban Heat Island effect — which can be as easy as switching roof colours — is a matter of social justice, many experts say. And as climate change continues to amplify weather extremes, that task is increasingly urgent.

“During the summer, cities are getting a lot warmer,” says Hashem Akbari, an Urban Heat Island specialist at Concordia University in Montreal. “People need to have air conditioning, if they can afford it. If they cannot afford it, they pay for it in different ways” — with their health and sometimes with their lives.

Scientists began noticing the Urban Heat Island effect almost as soon as cities were first industrialized. In London, a chemist and amateur meteorologist named Luke Howard showed in the early 1800s that the heart of the city was an average of 0.9 C hotter than the leafy suburbs of Tottenham, Stratford and Plaistow.

Modern scientists have confirmed that the average temperature difference between an urban heat island and its rural belt is usually 1 C to 2 C but can reach as much as 12 C in extreme cases.

Cities, with their smokestacks and exhaust pipes, produce more heat than the countryside, but they also retain more heat. Concrete, brick and asphalt absorb energy from the sun and radiate it back into the urban atmosphere at night — that’s when the urban heat island effect is most pronounced.

Trees, soil and water, on the other hand, release cooling moisture into their surroundings through the process known as evapotranspiration.

“The areas of Toronto that we know are highly treed — the central areas, like Rosedale, Forest Hill and the ravines — come out looking much cooler than parts of the city that are much more built up,” says Stephanie Gower, a research consultant at Toronto Public Health.

Gower described a heat “doughnut” that rings the city, including those areas of Scarborough and Etobicoke where there is less mature tree canopy.

Even within a small section of the city, the contrast can be extreme. The leafy streets of High Park are separated from the highrise apartment buildings of nearby Parkdale by a mere railway line, but heat-wise the two neighbourhoods are “chalk and cheese,” says Behan.

The Urban Heat Island problem “doesn’t affect everyone equally,” he says.

People who are more vulnerable to heat tend to live where the effect is most amplified or in buildings with no air conditioning.

“Children under 5, people over 65, those without English as a first language, immigrants, even from hot countries, are part of the vulnerable populations in Toronto,” says Behan.

For that reason, city officials take a keen interest in the Urban Heat Island effect. For instance, after a formal recommendation from Toronto Public Health, the city’s Urban Forestry department began using heat vulnerability maps to determine where new trees should be planted first.

In Toronto, it is estimated that heat contributes to an average of 120 premature deaths a year. But that average can mask extremes — deaths spike during very hot summers.

Last year, Concordia’s Akbari wrote a study that examined the effect of changing roofs from black, which absorbs sunlight, to white, which reflects it back into space.

In simulations, Akbari found that every one percentage point increase in a square metre of urban reflectivity could offset seven kilograms of carbon dioxide.

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If every city in hot and temperate climates changed its roofs and road surfaces to reflective colours, Akbari calculated the effect would be the equivalent of taking every car off the road for the next 50 years.

Green roofs also significantly offset the urban heat island effect, turning a heat-absorbing surface into one that participates in evapotranspiration.

“I’m not only interested in the science,” says Akbari. “I would measure my success by the amount of energy and number of peoples’ lives saved.”