Increasing tree coverage to 30% in European cities could reduce deaths linked to urban heat island effect

Global warming and the expansion of the built environment are expected to intensify the urban heat island (UHI) effect, and related adverse health impacts. These impacts occur not only in extreme heatwaves, but also during more frequent, moderate increases in air temperature¹. In this study, researchers analysed data on 93 European cities to estimate the number of lives that trees could save.

Green infrastructure – i.e. vegetation incorporated into urban environments – provides a cooling effect that can mitigate the UHI effect. A previous study² estimated the cooling capacity of trees in more than 600 European cities at between 1.1°C on average and up to 2.9°C. Meanwhile, the Nature Restoration Law recommends a minimum of 10% tree canopy cover for European cities, while other studies have recommended that urban neighbourhoods should aim for 30% tree coverage to improve microclimate, air quality and health³. Urban trees are also relatively simple and cost-effective to implement⁴, say researchers behind a new study on the benefits trees could confer on summer mortality.

To quantify the potential benefit of increasing urban tree cover, the researchers estimated the annual summer mortality burden (in adults over 20 years old) attributable to UHIs, and the burden that could be prevented by increasing cover to the recommended 30%, in 93 European cities. The cities were selected based on available Urban Climate model⁵ temperature data⁶. They also retrieved high resolution population data and age distribution data from European resources, including Eurostat data and the ‘Global Human Settlement Layer’ resource, using data from 2015 as this was the most recent available at high resolution. Taking weekly all-cause mortality data, they estimated deaths per age group, in 250m x 250m areas, each day over the course of summer (1 June–31 August 2015). Comparing this to a scenario simulating no UHI effects, they estimated the effect of exposure to UHIs on mortality.

They then estimated the temperature reductions that would result from increasing tree coverage to 30% in each city, and the number of deaths that this could potentially prevent. This type of comparative risk assessment serves to provide evidence for policymakers on the health-promoting effects of interventions – in this case urban planning decisions.

The 93 cities included northern European cities such as Glasgow, Stockholm and Tallinn, and southerly cities such as Athens, Palermo and Seville. Nearly 58 million people aged over 20 years resided in the cities in the study year. In this population, about 128 000 deaths occurred in summer – just under a quarter of the yearly total. Mean city tree coverage was 11% (weighted to take account of population differences).

The researchers found that average temperatures in cities, weighted by population, were 1.5°C higher due to the UHI effect – and this was associated with 6 700 premature deaths in summer, or 4.3% of all summer deaths. They estimated that 78% of the total population of the European countries in the study would experience a summer UHI effect greater than 1°C, and 20% more than 2°C.

Increasing tree coverage to 30% would lower temperatures by an average 0.4°C in the cities involved in the study, with a maximum effect of 5.9°C in some areas. This decrease could avoid 2 644 premature deaths, about 1.8% of all summer deaths in these European cities, and nearly 40% of deaths attributable to UHI effects.

The number of deaths attributable to the UHI effect was highest in central and southern European cities, for instance in Spain, Italy, Hungary, Croatia and Romania – and the benefits of increased tree coverage on mortality were most evident in these cities, as well. For example, UHI was not associated with any premature summer deaths in Gothenburg, Sweden, but 32 per 100 000 age-standardised inhabitants in Cluj-Napoca, Romania. Similarly, increasing tree cover in Oslo would not prevent any premature UHI-related deaths, but could prevent 22 per 100 000 age-standardised inhabitants in Palma de Mallorca.

Some cities already had close to 30% tree coverage, note the researchers, so they would benefit less from more plantings, but they also found that tree distribution within cities was often uneven. This might point to environmental injustice, where tree cover was lacking in socioeconomically deprived areas, and planners should look to redress this imbalance. The researchers acknowledge, however, that a 30% target will be difficult to achieve in cities with little available open public space – in which case planners could aim for lower targets, or encourage planting on private land. Finally, while trees are crucial in creating climate-resilient cities, they should be combined with other cooling strategies such as replacing asphalt with vegetated surfaces or less impermeable materials such as granite.

The researchers acknowledge some limitations to the study, such as building the predictive model based on a US dataset. Although a European dataset would have been preferable, the European weather station data do not provide sufficient coverage or a wide enough range of variables.

Footnotes:

1. Masselot P. et al. (2023) Excess mortality attributed to heat and cold in 801 cities in Europe. Lancet Planet Health 7 (4): E271–E281.
2. Marando, F., Heris, M.P., Zulian, G., Udías, A., Mentaschi, L., Chrysoulakis, N., Parastatidis, D. and Maes, J. (2022) Urban heat island mitigation by green infrastructure in European Functional Urban Areas. Sustainable Cities and Society, 77: 103564.
3. Konijnendijk, C.C. (2022) Evidence-based guidelines for greener, healthier, more resilient neighbourhoods: Introducing the 3–30–300 rule. Journal of Forestry Research, 1–10.
4. US Environmental Protection Agency. Reducing urban heat islands: compendium of strategies (2008). https://www.epa.gov/heat-islands/heat-island-compendium
5. The Urban Climate Model combines large-scale meteorological data for surface, sea, precipitation, soil, and vertical profile and includes a description of the terrain with information about land use, vegetation (e.g. the normalised difference vegetation index), and soil sealing. Temperature series were created by averaging the 100 m grid cells with centroids within the spatial boundaries of each 250 m grid cell.
6. Copernicus (2018) Climate variables for cities in Europe from 2008 to 2017 https://cds.climate.copernicus.eu/cdsapp#!/dataset/sis-urban-climate-cities?tab=overview [Accessed 20th Oct 2021].

Source:

Iungman, T., Cirach, M., Marando, F., Barboza, E.P., Khomenko, S., Masselot, P., Quijal-Zamorano, M., Mueller, N., Gasparrini, A., Urquiza, J. and Heris, M. (2023) Cooling cities through urban green infrastructure: a health impact assessment of European cities. The Lancet, 401(10376): 577–589.

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“Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by the Science Communication Unit, The University of the West of England, Bristol.

Notes on content:

The contents and views included in Science for Environment Policy are based on independent, peer reviewed research and do not necessarily reflect the position of the European Commission. Please note that this article is a summary of only one study. Other studies may come to other conclusions.