Small Green Spaces Can Help Keep Cities Cool During Heat Waves

A recent World Meteorological Organization report identified heat waves as the “deadliest meteorological hazard” from 2015 to 2019, impacting people across all continents and setting new national heat records in many regions. In Canada, the top weather event in 2021 was British Columbia’s record-breaking heat, according to Environment and Climate Change Canada. On June 29, the temperature in Lytton, B.C., soared to 49.6°C. The following day, a wildfire devastated 90% of the town, resulting in two fatalities and displacing 1,200 people.

The Health Impacts of Heat Waves

Heat waves exacerbate existing health issues, including cardiovascular and respiratory diseases. They are associated with increased hospital admissions, psychological stress, aggressive behavior, and excess mortality. Urban areas often experience the highest temperatures during heat waves due to the urban heat island effect, where concrete and asphalt absorb and release heat, raising city temperatures.

Urban Heat Islands and Green Spaces

Urbanization typically increases paved, impervious areas and decreases greenery, contributing to the urban heat island effect. Numerous studies have shown that urban forests can mitigate this effect, and many policies focus on large green spaces. However, small green spaces, such as yards, rooftops, and undeveloped land parcels, can also significantly reduce urban heat but are often overlooked in urban cooling strategies.

The Effect of Small Green Spaces

Large green spaces are rare in cities, but smaller vegetated areas can still substantially lower local temperatures. A recent study in Adelaide, Australia, found that tree canopy cover and, to a lesser extent, grass cover, reduced local daytime surface temperatures by up to 6°C during extreme summer heat. Suburban yards and gardens further inland can decrease local surface temperatures by up to 5°C.

On a smaller scale, trees reduce daytime surface temperatures twice as much as grass cover. While trees provide substantial cooling, grass and other low-lying plants grow relatively quickly and can be an effective short-term solution.

Strategies for Urban Cooling

Cities should adopt both short-term and long-term strategies to combat extreme heat. This includes replacing paved and impervious surfaces with grasses and turf and increasing tree plantings to enhance canopy coverage. Additionally, city planners and foresters can select tree species based on their cooling capabilities. Green spaces with diverse tree species provide greater cooling effects throughout the year and have a larger maximum temperature drop in summer compared to less diverse spaces.

Enhancing the Cooling Effect

Trees with large leaves and high transpiration rates (the evaporation of water from plant leaves) provide more cooling. The structure of green spaces also influences cooling efficiency. In summer, a plant community with multiple layers of trees, shrubs, and herbs can reduce air temperature by 1°C on a sunny day and 0.5°C on a cloudy day, compared to areas dominated solely by tall trees.

Optimal Layout for Green Spaces

The spatial configuration of green spaces is crucial for maximizing their cooling capacity. Heavily fragmented green spaces, spread further apart, or unevenly distributed, offer lower cooling contributions. A study on the spatial configuration of green spaces in Baltimore, MD, and Sacramento, CA, revealed varying results. Researchers measured the cooling effect based on the total perimeter of green patches per square kilometer (edge density). In Baltimore, higher edge density was associated with less cooling, whereas in Sacramento, it was associated with more cooling. These discrepancies likely result from local conditions, such as the amount of shade cast by trees and the fragmentation of green patches.

Group Planting for Maximum Cooling

Overall, trees generally have a stronger cooling effect than grass. Planting trees in groups, rather than individually or in lines, is recommended for better microclimate regulation. Group plantings create more significant cooling effects through combined shade and transpiration.

Small green spaces offer substantial cooling benefits during summer in cities. By optimizing the management and configuration of these spaces, cities can enhance their cooling effects and minimize trade-offs. This approach not only helps mitigate the urban heat island effect but also contributes to healthier and more comfortable urban environments.

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