Trees Can Help Cities Beat the Heat

A leafy canopy creates shelter from the sun’s rays

Prof. Bob Brown

Prof. Bob Brown

Have you hugged a tree lately? More importantly, have you planted a tree lately? Not only do they absorb carbon dioxide and produce oxygen, but they can also help keep urban areas cool. Incorporating more greenery into urban landscapes is key to staying cool as the mercury rises.

“There are things that landscape architects and urban designers can do to make cities cool, particularly during heat waves,” says landscape architecture professor Bob Brown in the School of Environmental Design and Rural Development. “For some reason, that information isn’t being used very much.” He says countries such as Germany incorporate climatic factors into urban design to a greater extent than Canada does.

Although the cause of climate change remains a hot topic among scientists and environmentalists, Brown says city planning needs to reflect rising temperatures, a trend that is likely to continue. From backyards to big cities, urban planning needs to work with nature, not against it, says Brown. “We think that architects, landscape architects and urban designers should be considering climate any time they do any design work.”

Replacing green spaces with hard surfaces such as roads and parking lots creates “heat islands.” Grass and vegetation stay cooler than artificial turf, for example, which can reach temperatures of up to 60 C. “It’s as hot as an asphalt parking lot,” he says. “You can actually cook an egg on it.”

Cities often lack sufficient tree coverage, which exposes hard surfaces to the brunt of the sun’s solar radiation. “These surfaces stay really hot all night,” and they continue to give off heat into the morning. As the surface temperature increases, so does the air temperature.

Some types of hard surfaces overheat more than others. Asphalt absorbs heat because of its dark colour and releases heat into the air. Light-coloured concrete deflects solar radiation and doesn’t get as hot as asphalt. “It doesn’t heat up very much at all.” Porous hard surfaces are preferable to solid ones, because they allow rainwater to flow into the ground, where it later evaporates when exposed to the sun.

One of the easiest things that homeowners can do to keep their properties cool is to plant trees on the west side of their homes to create shade during the hottest time of the day: late afternoon. Mature trees also increase property values. But, he adds, “If you’re going to put in trees, do it right so they’ll survive.” Young trees need to be watered regularly until their roots become established.

Brown says people often mistakenly believe that the air temperature in the shade of a tree is cooler than the surrounding air, but in fact the temperatures are almost the same. The coolness you feel under a tree’s leafy branches is due to the shade, which prevents the sun’s rays from hitting – and heating – your skin. Water can also help absorb heat. When the sun’s rays hit a wet surface or vegetation, most of the energy goes into evaporation instead of heating the air.

Replacing your lawn with heat-tolerant ground cover can cut down on your water bill and yard work. “I really support the idea of getting rid of lawns,” says Brown, who has planted an evergreen ground cover at home. “I have very little lawn in my yard.” He says some homeowners prefer the curb appeal of a well-manicured lawn over a naturalized yard that may attract the ire of neighbours.

On a larger scale, city parks with lots of green space have been shown to exhibit a “park cool island” effect, says Brown. Parks are often several degrees cooler than the surrounding concrete jungle. As air passes through the park, it cools the neighbourhood.

In one study, Brown and his colleagues looked at census data and how canopy cover affected the health of neighbourhood residents during a heat wave. “There was a very clear relationship: the more canopy cover in a neighbourhood, the fewer emergency response calls.” And the opposite was true: fewer trees resulted in more 911 calls.

[slideshow gallery_id=”11″]