Greening cities: what’s the point?

illustration of buildings and trees on a blue background

From flood mitigation to mental health: the scientific case summarized

Claim

Evidence

Numerous studies suggest that infrastructure such as sustainable drainage systems, parks and swales can significantly reduce water run-off rates, making floods less severe and protecting water treatment facilities. A 2013 study commissioned by the UK government suggested that a 10% increase in tree cover reduces run-off by nearly 6%. Studies in the US have calculated the flood prevention “service” provided by green spaces saves public authorities millions of dollars a year. There is also evidence to suggest soil and plants filter pollutants out of water. Persuaded by the evidence, in 2011 Philadelphia embarked on a 25-year US$2.4bn strategy to create a city-wide network of storm-water infrastructure such as street trees and parklets. Five years in, it estimated this was already reducing run-off by 1.5 billion gallons annually.

Claim

Evidence

A 2018 report for the UK government estimated that the country’s vegetation removes 1,354 kilotonnes of air pollutants annually; a 2006 study from the US credited its urban trees with the annual removal of 711,000 tonnes. Other studies have shown that green roofs can remove significant volumes of particulates. The WHO estimates that one in every 40 deaths in developed countries is due to air pollution. In Kentucky, researchers are attempting to make explicit the link between greening and health with the Green Heart Louisville programme, run by the city’s medical school, to plant 8,000 trees and measure the effect on inhabitants. “It’s the biggest ever medical trial where nature is the pharmaceutical,” says Pascal Mittermaier at project partner TNC.

Claim

Evidence

The “urban heat island” effect can raise the night-time temperature in large cities by up to 9ºC. Overheating has been blamed for excess deaths, with a recent study suggesting climate change will result in a five-fold increase in heat-related deaths in Australia by the 2030s. Trees transpire water, with a 1999 study suggesting a large tree can take out 1000 mega joules of heat energy each day. This effect is heightened by the shade trees provide: a 2010 study found that urban parks were on average 1ºC cooler than their surroundings. A 2007 study in Manchester, UK, found a 10% increase in tree cover could reduce climate change-driven temperature increases by nearly 4ºC.

Claim

Evidence

Urban greenery is claimed to have a wide range of psychological benefits. A UK government-commissioned report concluded in 2013 there was “strong evidence from a large number of high-quality studies spanning several years” that green space alleviates stress, fatigue and other mental health issues, delivering positive effects on mood, concentration and self-discipline, with more marked effects in urban areas. A study referenced in Tim Beatley’s book Biophilic Cities has found that people immersed in nature even behave more generously.

Claim

Evidence

The most obvious economic benefit is the impact on property prices of proximity to green space. Various studies summarized in a 2008 piece of research identified a price premium of up to 19%, with most suggesting between 5-10%. (New York’s High Line park is estimated to have delivered a US$4bn boost to nearby real estate.) Studies suggest that offices and shops overlooking green space are let more quickly, and with lower tenant turnover. Green spaces also play a part in making cities more attractive, thereby helping bring in visitors and investment. In addition, a huge range of indirect economic benefits are claimed from ecological “services”, from reducing noise pollution to controlling pests.

Claim

Evidence

Evidence suggests that cities, far from being ecological deserts, are capable of supporting a rich array of wildlife, given the right conditions. A study by Bristol University in the UK, for example, found little statistical difference in the diversity and abundance of bee species in cities compared to nature reserves. A 30-year UK study by zoologist Jennifer Owen of her own garden, meanwhile, found more than 2,500 plant and animal species — one in 10 of all UK species. Academic Menno Schilthuizen highlighted, in his 2018 book Darwin Comes to Town, how plant and animal species are already exhibiting successful evolutionary adaptations to life in cities — with bird songs, for example, becoming higher pitched to overcome city noise. A study on urban biodiversity by a group of landscape academics in 2013, “Patterns and Trends in Urban Biodiversity and Landscape Design”, found that cities are “novel places teaming with unique plant and animal communities” which can play “an essential role” in stemming biodiversity losses.

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