A recent report by the World Meteorological Organisation spoke of heat waves as the "deadliest meteorological hazard” mankind has faced within the last decade, affecting people living on all continents and setting new national heat records in many regions.
Canada’s top weather phenomenon in 2021 was the record-breaking heat waves that swept through British Columbia, according to Environment and Climate Change Canada.
The temperature in Lytton, British Columbia, hit 49.6 C on June 29. The following day, a wildfire destroyed 90 percent of the town, killing two people and displacing over a thousand others.
Lingshan Li, a PhD researcher at Concordia University, said in an article for the World Economic Forum that heat waves also exacerbate existing health issues, including cardiovascular and respiratory disease. They’re associated with increased hospital admissions, psychological stress, and aggressive behaviour, as well as excess mortality.
“During heat waves, the highest temperatures are often found in urbanised areas. Urbanisation is almost always associated with an increase in paved, impervious areas, and often a decrease in greenery,” said Li.
Concrete, asphalt roads, and other built materials readily absorb, store, and release heat, which results in rising city temperatures, a phenomenon called the urban heat island.
Numerous studies have found that small pockets of urban forest and greenery can reduce the urban heat island effect. Small green spaces, such as yards, rooftops, and small parcels of undeveloped land, can make significant contributions to lowering urban heat, but it is often overlooked when developing strategies for urban cooling.
The effect of small green spaces
Existing cities rarely have the opportunity to add large green spaces due to a general lack of space. Smaller green spaces, however, can still meaningfully decrease local surface temperatures.
Small green spaces such as yards, rooftops and small areas of undeveloped land can significantly decrease ambient urban temperatures.
A recent study in Adelaide, Australia, found that tree cover and, to a lesser extent, grass cover decreased local daytime surface temperatures by up to 6C during the extreme summer heat.
On a small scale, tens of square metres, trees reduced daytime surface temperatures twice as much as grass cover. But grass and other small plants, which grow relatively quickly compared to trees, can be used as ground cover while trees grow.
Li suggested that cities should adopt both short-term and long-term strategies to respond to extreme heat, “including the replacement of paved and impervious surfaces with grasses and turf, and increasing tree plantings to boost canopy coverage.”
Furthermore, the report suggested that when managing small green spaces, city planners and foresters can select tree species based on their ability to cool the environment. Green spaces with a high diversity of tree species have a greater cooling effect in spring, summer, and fall.
For example, tree canopies with large leaves and high transpiration rates, the evaporation of water from plants occurring at the leaves, provide an increased cooling effect.
The report said that the layout of green spaces is another factor that city planners should be concerned about. When green spaces are broken up into smaller pieces, spread further apart or unevenly distributed, their cooling contribution is lower. Basically, the denser green space is, the greater the cooling effect it has.
In addition to the cooling benefits of green spaces, they provide micro-ecosystems for insects, birds, reptiles, and small mammals.
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