Reading: Can Trees Cool a City?

A The urban heat island effect is the extra warmth of a city compared with the countryside around it. In a study published in The Lancet in 2023, Iungman and colleagues set out to estimate two things for 93 European cities: how many deaths could be attributed to urban heat islands, and how many deaths a rise in tree coverage would prevent. They examined one summer, from June to August 2015, and adults aged 20 or older. Every analysis was carried out for areas of 250 meters by 250 meters.

B Weighted by population, the average temperature increase caused by the heat island effect was 1.5°C, within a range of 0.5°C to 3.0°C. The researchers estimated that 6,700 premature deaths could be attributed to this extra heat, with a 95% confidence interval from 5,254 to 8,162. That is around 4.33% of all summer deaths. To handle uncertainty, the team ran Monte Carlo simulations, which produced the point estimates and confidence intervals, and they also carried out sensitivity analyses to test how robust the estimates were.

C The team also estimated what would happen if tree coverage in each city rose to 30%. In that scenario the cities would be cooler by a mean of 0.4°C, although the change ranged from 0.0°C to 1.3°C. The researchers estimated that 2,644 premature deaths could be prevented, with a confidence interval from 2,444 to 2,824. This corresponds to 1.84% of all summer deaths, a smaller share than the 4.33% linked to the heat island itself.

D In a press release from the institute that led the study, Nieuwenhuijsen, one of the researchers, said that the number of trees is not the only question: how the trees are distributed across a city matters as well. Nieuwenhuijsen also said that existing trees should be preserved, because new trees take a long time to grow. The release reported that southern and eastern European cities had the highest rates of extra heat-related deaths and would benefit most from more tree cover. It also reported that, according to the authors, planting can be difficult in some cities because of their design and should be combined with other measures such as green roofs.

E According to a European Commission news item on the research, the researchers used data from 2015 because it was the most recent year available at high resolution. Iungman said in the press release that the study looked only at the cooling effect of trees, not at the other health benefits of greener cities. The news item says that in some cities tree coverage was already near 30%, so planting more trees would help less there. The researchers also accept that a 30% target may be difficult to reach in cities with little open public space, and the news item adds that planners in such cities could set a lower target or encourage planting on private land. The figures are estimates for a single summer, 2015.