An investment of just $4 (£3.20) per resident in some of the world's largest cities could improve the health of tens of millions of people by reducing air pollution and cooling city streets, according to a new study by influential US conservation body The Nature Conservancy (TNC).
Developed with the C40 Cities Climate Leadership group, the Planting Healthy Air study applies established research at a global scale to identify where an investment in tree planting can make the biggest impact. Its primary author and TNC lead scientist for global cities Rob McDonald said: "Urban trees can save lives and are just as cost-effective as more traditional solutions like putting scrubbers on smokestacks or painting roofs white."
The report puts the number of deaths globally from the effects of fine particulate matter at more than three million a year, through ailments such as asthma, heart disease and stroke, but says trees can remove up to a quarter of particulate matter pollution within a few hundred metres.
Trees can also cool their immediate vicinity by as much as 2 degsC, alleviating the impact of a changing climate, it adds, and calculates that an annual global investment of $100m (£80m) in tree planting could provide 77 million people with cooler cities and 68 million people with measurable reductions in particulate pollution.
Crunching data for 245 cities worldwide, the study found that those with high population density, high levels of pollution and heat and a low cost of planting trees showed the highest return on investment (ROI), and that these were concentrated in South Asia. By comparison London showed "moderate median ROI for tree planting", with the highest return likely to be achieved in the city centre. "For an additional annual investment of $18m (£14m) in street tree planting, we estimate that 1.5 million people could have a >1(mu)g/cu m reduction in particulate matter," it says.
"London and south-east England have by far the worst air in Britain, largely due to traffic levels," it notes. "In London, in addition to death attributable to particulate pollution, more than 40,000 'life years' were lost in 2010."
It acknowledges London's "aggressive steps to improve its air quality", including the current Low Emission Zone and even stricter Ultra Low Emission Zone planned for 2020, the requirement for new black cabs to be electrically powered from 2018 and the city's recently introduced electric double-decker buses. "Trees alone cannot solve all of the world's urban air and heat challenges, but they're an important piece of the solution," McDonald added.
London's mayor Sadiq Khan has described the city as "the most polluted in Europe" but last month backtracked on his earlier pledge to plant two-million trees in the capital (HW, 28 October). The city's administration still aims to increase London's tree coverage by five per cent by 2025.
Last week Defra was rebuked by the High Court for knowingly adopting an "optimistic forecast" in its air pollution modelling. This was then used to justify deferring the creation of several proposed low-emission zones in urban areas intended to bring the UK in line with environmental law. Currently London exceeds World Health Organization limits for particulate matter and nitrogen dioxide.