The 21st century will be the urban century, as more than two billion additional people arrive in cities globally. As a result of this unprecedented rapid urbanisation, one study forecast that by 2050, fine particulate matter could kill 6.2 million people per year. Another issue is weather-related disasters that are estimated to kill 12,000 people around the world annually.
A report by Nature Conservancy, Planting Healthy Air, suggests that planting urban gardens could reduce address these problems. Trees provide shade, reduce atmospheric temperatures, dampen the effects of global warming, beautify the city and are uplifting. Research has also shown that greenery is good for immune system health and keeps cancer at bay.
However, scientists specify that targeting specific city areas is also important as it has an almost 100-fold greater return on investment (ROI) in tree planting compared to the least suitable neighbourhoods. Generally, neighbourhoods that are characterised by higher population density are ideal, and thus more people who will benefit from cleaner air, and by higher concentrations of PM2.5 that can be removed by trees.
The new report sifts through global statistics to give some very fascinating results:
1. Reducing atmospheric temperatures
In terms of PM—or temperature—reduction benefits, trees are currently providing on average 1.3 million people at least a 10 µg/m³ reduction in PM2.5, 10.2 million people at least a 5 µg/m³ reduction, and 52.1 million people at least a 1 µg/m³ reduction.
Similarly, trees are also providing 68.3 million people with a roughly 0.5 to 2.0° C (0.9 to 3.6° F) reduction in summer maximum air temperatures.
Urban street tree planting and canopy enhancement can also be a cost-effective way to make the air healthier. A well-targeted tree-planting campaign is about the same price as getting diesel vehicles off the road.
2. Reducing heatwave-related deaths
The maximum possible tree planting in cities would reduce high temperature-related mortality by 2.4 – 5.6 per cent, saving between 200 and 700 lives annually.
3. Reducing energy consumption and increasing carbon sequestration
Electrical use in 245 cities could be reduced by 0.9 to 4.8 per cent annually (9.8 Bn to 48 Bn KWhr). Under the maximum street tree-planting scenario, net carbon sequestration would increase by 2.7 Mn to 13 Mn tonnes CO2.
So what’s stopping many cities from going on a tree-planting binge? Rob McDonald says it’s space constraints, water availability, maintenance and the perception of trees as an aesthetic luxury. Many urban planners also do not think trees qualify as a public health expenditure— they often fall under the purview of parks and recreation departments, which leads to under-investment in urban planting.
He concludes that while trees cannot and should not replace other strategies to make air healthier, trees can be used in conjunction with these other strategies to help clean and cool the air. In the right places, they give us a breath of fresh air and transform our cities into liveable ones.