Green skylines offer urban re-leaf

Vine-covered façades and lush rooftop gardens offer more than a green oasis in the concrete jungle. Studies show that they can help urban areas adapt to climate change and also decrease greenhouse gas emissions by reducing the energy spent on heating and cooling.

According to scientists, climate change will result in more frequent heat waves and several climate models indicate an increase in precipitation intensity, suggesting the possibility of more extreme rainfall events. These effects will be exacerbated in urban areas, where concrete and pavement reflect heat and prevent stormwater from being absorbed into the ground. In addition, urban areas must also cope with air quality problems, which may worsen in coming years.

Proven to greatly mitigate these impacts, vertical and rooftop gardening has seen a widespread renaissance in Europe in recent years, but is still little used in North America. To investigate its application in Canada, Environment Canada and several private sector partners recently completed a report on the benefits of rooftop and vertical gardens, titled Greenbacks from Green Roofs: Forging a New Industry in Canada, for the Canada Mortgage and Housing Corporation.

According to the report, one of the chief benefits of planting vegetation on buildings is to reduce energy usage and therefore greenhouse gas emissions. By protecting buildings from wind, plants can reduce heating in winter by 25 per cent and, through direct shading and evaporative cooling, air conditioning in summer can be reduced by 50 to 75 per cent. A 16-centimetre thick blanket of plants can increase the R-value of a wall by as much as 30 per cent.

Wall and rooftop gardens also regulate the "urban heat island," a phenomenon that causes cities to be up to 8°C warmer than the surrounding countryside due to re-radiated heat. Through evapotranspiration, a layer of vegetation can reduce the amount of re-radiated heat on a hot summer day by up to 90 per cent, thereby reducing the urban heat island by 3-4°.

One of the most tangible effects of green roofs is their ability to retain stormwater. In urban areas, most runoff flows into stormsewers, picking up contaminants such as oil, grease and heavy metals on the way, and depositing them into lakes, rivers or groundwater aquifers. According to European studies, rooftop gardens retain 70 to 100 per cent of precipitation that falls on them in summer and about half that in winter—storing it until it is taken up by the plants and returned to the atmosphere through evapotranspiration. Studies also show that plants act as a natural filter for runoff—removing up to 95 per cent of heavy metals such as cadmium, copper and lead.

Rooftop gardens also improve air quality, filtering out gaseous pollutants and particles. They protect building membranes from ultraviolet radiation and physical damage, and can be used to grow food, serve as habitat for wildlife, and even to foster well being.

Case studies show that the handful of major rooftop gardens in Canada—which run the gamut from a subsidized apartment in Toronto where tenants grow their own rooftop produce to a parking garage in Quebec City where a rooftop meadow has solved a rainwater leakage problem—are successful. However, the report suggests that demonstration projects, awareness campaigns and economic incentives are needed if green skylines are to become widespread in Canada.

With their data collection and review complete, EC scientists and partners from the National Research Council and the roofing industry are planning to embark on a five-year monitoring project to compare differences in water quality, energy usage and other environmental factors between two similar buildings in Ottawa—one with and one without a rooftop garden. By using climate data to simulate the effects of these technologies in other cities, they hope to raise awareness of the potential rooftop and vertical gardens hold in helping Canadians meet the climate change challenges of the new millennium.

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