Petite green spaces can assist keep cities nippy during heatwaves

Recent World Meteorological Organization The report called heatwaves the “deadliest meteorological hazard” from 2015 to 2019, affecting people on every continent and setting fresh national heat records in many regions. The most significant weather event in Canada in 2021 was record-breaking heat in British Columbiaaccording to Environment and Climate Change Canada. Temperatures in Lytton, BC, reached 49.6 C on June 29. The next day, a fire destroyed 90 per cent of the town, killing two people and forcing 1,200 others from their homes.

Heatwaves also exacerbate existing health problems, including cardiovascular and respiratory diseases. These are associated with increased hospital admissions, mental stress and aggressive behaviorbut also excess mortality.

During heat waves, the highest temperatures often occur in urban areas. Urbanization is almost always associated with an escalate in the number of paved, impermeable areas and often with a decrease in greenery. Concrete and asphalt roads and other building materials readily absorb, store, and release heat, raising city temperatures, a phenomenon called the urban heat island.

Many studies have shown that urban forests can reduce the urban heat island, and many policies focus on enormous green spaces. Petite green spaces such as backyards, rooftops, and miniature plots of undeveloped land can make an impressive contribution to reducing urban heat, but are often overlooked when developing urban cooling strategies.

The Impact of Petite Green Spaces

Cities rarely have the opportunity to add enormous green spaces to assist counteract the effects of heat waves. Smaller spaces of vegetation, however, can still significantly reduce local land temperatures.

A recent study conducted in Adelaide, Australia, found that Tree canopy cover and, to a lesser extent, grass cover reduced local daily surface temperatures by up to 6 degrees Celsius during extreme summer heat. Further inland, suburban backyards and gardens can lower local surface temperatures by as much as 5 C.

On a fairly miniature scale, on the order of tens of square meters, trees lowered daily surface temperatures by twice as much as grass cover. But grass and other miniature, low-growing plants grow relatively quickly compared to trees.

Cities should adopt short- and long-term strategies to respond to extreme heat, including replacing paved and impervious surfaces with grass and turf and increasing tree plantings to escalate canopy cover.

Strengthening the cooling effect

Additionally, when managing miniature green spaces, urban planners and foresters can select tree species based on their ability to nippy the environment. Green areas with a large variety of tree species have a greater cooling effect in spring, summer and autumn.They also have a greater maximum temperature drop in summer compared to spaces that are less diverse.

For example, tree canopies with enormous leaves and high rates of transpiration — the evaporation of water from plants through leaves — can provide better cooling.

The structure of a green space can also affect its cooling efficiency. In summer, a plant community consisting of many layers of trees, shrubs and herbs can additionally lower the air temperature by 1 C on a sunny day and by 0.5 C on a cloudy day, compared to an area dominated only by tall trees.

Tree Grouping

The layout of green spaces—their spatial configuration—is another factor that city planners should consider when thinking about the cooling capacity of miniature green spaces. When green spaces are highly fragmented—broken into smaller pieces—scattered farther apart or unevenly distributed, their cooling contribution is reduced.

A study examining the spatial configuration of green areas in two citiesBaltimore, Md., and Sacramento, Calif., showed different results, for example. The researchers looked at the total circumference of green patches for each square kilometer of land (a metric called edge density) and measured the cooling effect. Higher edge density was associated with less cooling in Baltimore but more cooling in Sacramento.

The discrepancy is probably due to local conditions: Trees may cast more shade on cooling surfaces, suppressing their effect. Or a green patch with higher edge density may contain a greater number of smaller and fragmented patches of trees with lower evapotranspiration (evaporation from the ground plus transpiration from the plants). For a manager, the trade-offs may be challenging to reconcile.

However, in general, trees tend to have a stronger cooling effect than grass. Planting trees in groups rather than individually or in rowsis recommended for regulating the microclimate (local climatic conditions at the Earth’s surface).

Petite green spaces can provide a lot of summer cooling in cities. And cities can learn to better manage the configuration of miniature green spaces to get more cooling benefits and minimize tradeoffs.