Green spaces can save lives
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Against the backdrop of global climate change, extreme heat events are becoming hotter, longer, and more frequent. Such sustained extreme heat has already severely impacted people's health all over the world.
It is generally acknowledged that some urban landscape features, such as green spaces, can offer respite from high temperatures. However, few studies have shown how changing the landscape of a city affects its residents' ability to cope with extreme heat and the effect on mortality.
In two recently published papers, researchers from China, South Africa, and the UK found that green spaces, such as trees lining streets, can alleviate extreme heat's negative impacts on human health.
They also observed a higher risk of heatwave-associated mortality when buildings are more densely packed together and when the land surface temperature is higher at night.
The research findings provide scientific evidence for policymakers and urban designers to form and implement strategies and health interventions for extreme heat conditions, such as incorporating different types of greenery into cities.
The researchers chose to study Hong Kong due to its hot and humid summers as well as its high building and population density. They analyzed mortality, meteorological, and land use data collected between 2005 and 2018.
They found that green spaces have a remarkable ability to protect people from heat-related illnesses, especially under longer and more intense heatwaves, said the lead author of both papers.
What the researchers say: "Vegetation tends to release more water vapor as the surrounding temperatures rise, thus leading to a better cooling effect of the air around it. Trees, in particular, can also provide shade for residents and pedestrians," he said. "In contrast, areas with a high density of buildings are hotter as the concrete structures absorb more solar energy and block air from passing between them, thus trapping heat. During prolonged heat events, buildings may not be able to completely cool off at night, which increases nighttime temperatures."
"Higher nighttime land surface temperatures can contribute to a more intensive urban heat island effect, a phenomenon where urban areas are hotter than nearby countryside. This can lead to temperatures staying higher for longer," she added.
The researchers also found that, during heatwaves, older adults (over 65) and men are more likely to be affected by changes made to the urban landscape.
"Older adults tend to engage in fewer heat-adaptive behaviors like turning on fans or air conditioners, taking a shower, or leaving the house," the researchers said. “Elderly people's daily activities are often within or close to their residential areas. Therefore, they're more susceptible to the surrounding urban landscape. Also, due to different gender distributions in industrial sectors, men may work outdoors for longer and are more likely than women to be affected by heatwaves in certain situations."
Urban greenery is usually measured using satellite imaging, however, this often misses aspects people see on the ground. Previous studies may, therefore, have underestimated the health benefits of eye-level urban greenery against extreme heat events.
To overcome these problems, the researchers assessed urban greenery by using data from satellite imagery and Google Street View images. Compared with the assessment of overhead-view urban greenery using satellite images, the eye-level greenery measurement is more accurate in reflecting what people see and access when they walk in the city.
“It's difficult to use the satellite images to detect urban greenery like vertical green walls or shrubs and lawns covered by trees. This is why we used panoramic streetscape images from Google Street View to have a more precise understanding of how street-level greenery affects heat mortality," the lead author said.
"Green spaces and foliage along walking streets can improve a pedestrian's thermal comfort in hot weather. It also reduces people's exposure to unhealthy factors like air pollution and traffic noise,” she continued. "The design of our cities will become increasingly important in mitigating the risk of heat-associated death in the future as extreme heat events are projected to become more frequent, longer and more intensified."
So, what? More and more studies are showing the value of greenery, parks, and water views in contributing to better mental and physical health.
As an aside, it’s worth noting that as office and apartment buildings grow taller and taller they may be adding to our mental health problems. Prior research that showed that we are simply not designed to live or work in buildings of more than 5 stories (55 feet from the ground, roughly the average height of the tallest climbable savanna trees). Working or living in taller structures increases our stress level and leaves us prone to a number of potentially serious psychological or physical health problems.
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