THE LANCET: Planting more trees could decrease deaths from higher summer temperatures in cities by a third, modelling study suggests
Peer-reviewed / Simulation or modelling / People
- Modelling of 93 European cities finds that increasing tree cover up to 30% can help lower the temperature of urban environments by an average of 0.4 degrees and prevent heat-related deaths. (Average city tree coverage in Europe is currently at 14.9%.)
- Of the 6,700 premature deaths attributed to higher temperatures in cities during 2015, one third of these (2,644) could have been prevented by increasing urban tree cover up to 30%.
- The authors note that these findings highlight the need for more sustainable and climate-resilient strategies to be integrated into local policy decisions to aid climate change adaptation and improve population health.
One third of premature deaths attributable to higher temperatures in European cities during summer 2015 could have been prevented by increasing urban tree cover to 30%, reveals a modelling study published in The Lancet. The study also found that tree cover reduced urban temperatures by an average of 0.4 degrees during the summer.
“We already know that high temperatures in urban environments are associated with negative health outcomes, such as cardiorespiratory failure, hospital admission, and premature death. This study is the largest of its kind, and the first to specifically look at premature mortality caused by higher temperatures in cities and the number of deaths that could be prevented by increasing tree cover,” says lead author, Tamar Iungman, Barcelona Institute for Global Health. [1]
“Our ultimate goal is to inform local policy and decision-makers about the benefits of strategically integrating green infrastructure into urban planning in order to promote more sustainable, resilient and healthy urban environments and contribute to climate change adaptation and mitigation. This is becoming increasingly urgent as Europe experiences more extreme temperature fluctuations caused by climate change; despite cold conditions currently causing more deaths in Europe, predictions based on current emissions reveal that heat-related illness and death will present a bigger burden to our health services over the next decade.”
Urban environments record higher temperatures than the surrounding countryside generally referred to as “urban heat islands”. This temperature difference is caused by human modification of landscapes, such as removal of vegetation, the presence of asphalt and use of building materials that absorb and trap heat. As emissions continue to exacerbate climate change and global heating, increased temperatures in cities are predicted to become more intense, resulting in an increasingly urgent need for cities to adapt to improve health outcomes.
The researchers estimated mortality rates of residents over 20 years old in 93 European cities (listed in appendix) between June and August 2015, accounting for 57 million inhabitants in total. Mortality data from this period was analysed with daily average city temperatures in two modelling scenarios: the first comparing the city temperature without urban heat islands to city temperature with urban heat islands, and the second simulating the temperature reduction as a consequence of increasing the tree cover to 30%. Exposure response functions were used to estimate the number of deaths attributable to urban heat as well as the number of deaths that could be prevented through increasing the tree cover.
The population-weighted city average daily temperature difference between cities and countryside from June - August 2015 was 1.5 degrees warmer than the surrounding countryside, with the maximum temperature difference measured at 4.1 degrees hotter in Cluj-Napoca, Romania. Across all cities, 75% of the total population lived in areas with an average summer city temperature difference greater than one degree, and 20% with an average summer temperature difference greater than two degrees, compared to the surrounding countryside.
In total, 6,700 premature deaths could be attributed to hotter urban temperatures during the summer months, accounting for 4.3% of summer mortality and 1.8% of year-round mortality. One in three of these deaths (2,644 total) could have been prevented by increasing tree cover up to 30%, and therefore reducing temperatures. This corresponds to 39.5% of all deaths attributable to hotter urban temperatures, 1.8% of all summer deaths, and 0.4% of year-round deaths.
There was a large variability in temperature-related mortality rates between cities, from no premature deaths attributable to hotter urban temperatures in Goteborg, Sweden, to 32 premature deaths per 100,000 people in Cluj-Napoca, Romania. Overall, cities with highest temperature mortality rates were in Southern and Eastern Europe where the highest temperatures were reached, with these cities benefitting the most from an increase in tree coverage.
The results of this study support the idea that urban trees provide substantial public health and environmental benefits, however the authors acknowledge that increasing tree coverage should be combined with other interventions to maximise urban temperature reduction (for example, changing ground surface materials to reduce night-time temperatures such as replacing asphalt with trees). Meeting the target of 30% tree coverage can be very challenging for some cities due to urban design, with city average tree cover in Europe currently at 14.9%.
“Our results suggest large impacts on mortality due to hotter temperatures in cities, and that these impacts could be partially reduced by increasing the tree coverage to help cool urban environments. We encourage city planners and decision-makers to incorporate the urban green infrastructure adapted to each local setting whilst combining with other interventions to maximise the health benefits while promoting more sustainable and resilient cities, especially as we already know that green spaces can have additional health benefits such as reducing cardiovascular disease, dementia and poor mental health, improving cognitive functioning of children and the elderly, and improving the health of babies,” says study co-author Mark Nieuwenhuijsen, Director of Urban Planning, Environment and Health at the Barcelona Institute for Global Health. [1]
The authors acknowledge some limitations with this study. The study could not be conducted for a more recent year than 2015, due to unavailability of population data. In addition, a US dataset was used for building the cooling model in the study as opposed to a European dataset. Finally, this study focused on the health impacts of high temperatures but did not look at cold temperature. Whilst cold temperatures currently have greater impacts on health in Europe, health impacts due to heat are projected to exceed those caused by cold under current emission scenarios, highlighting the importance of adapting our cities now.
Writing in a Linked Comment, Kristie Ebi, University of Washington, USA, who was not involved in the research, said: “Essentially all heatwave-related deaths are preventable; no one needs to die from the heat. With climate change projected to increase the frequency, intensity, and duration of extreme heat events, communities need to understand the most effective interventions, particularly developing and deploying heatwave early warning and response systems. Equally important are Heat Action Plans that explicitly incorporate the consequences of a changing climate into longer-term urban planning. Heat Action Plans detail how to modify urban form and infrastructure to increase the resilience and sustainability of our communities as we face an even warmer future… Encouraging and enabling decision-makers and local communities to develop and implement Heat Action Plan is an effective way to promote climate resilience as soaring temperatures continue to be felt globally. The tools and guidelines are available; the gaps are in human and financial resources for implementation. The time to start is now.”
NOTES TO EDITORS
This study was funded by GoGreenRoutes, Spanish Ministry of Science and Innovation, Internal ISGlobal fund Medical Research Council-UK, and European Union’s Horizon 2020 Project Exhaustion. A full list of researcher institutes can be found in the paper.
[1] Quote direct from author and cannot be found in the text of the Article.
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JOURNAL
The Lancet
METHOD OF RESEARCH
Computational simulation/modeling
SUBJECT OF RESEARCH
People
ARTICLE TITLE
Cooling cities through urban green infrastructure: a health impact assessment of European cities
ARTICLE PUBLICATION DATE
31-Jan-2023
Over 4% of summer mortality in European cities is attributable to urban heat islands
A new study conducted with data from 93 European cities estimates that one third of deaths attributable to heat islands could be avoided if trees covered 30% of urban space
Peer-Reviewed PublicationIMAGE: KEY RESULTS view more
CREDIT: ISGLOBAL
Over four percent of deaths in cities during the summer months are due to urban heat islands, and one third of these deaths could be prevented by reaching a tree cover of 30%, according to a modelling study published in The Lancet and led by the Barcelona Institute for Global Health (ISGlobal), an institution supported by “la Caixa” Foundation. The study results, obtained with data from 93 European cities, highlight the substantial benefits of planting more trees in cities to attenuate the impact of climate change.
Exposure to heat has been associated with premature mortality, cardiorespiratory disease and hospital admissions. This is particularly true for heat waves, but also occurs with moderately high temperatures in summer. Cities are especially vulnerable to higher temperatures. Less vegetation, higher population density, and impermeable surfaces for buildings and roads, including asphalt, lead to a temperature difference between the city and surrounding areas – a phenomenon called urban heat island. Given the ongoing global warming and urban growth, this effect is expected to worsen over the next decades.
“Predictions based on current emissions reveal that heat-related illness and death will become a bigger burden to our health services over the next decades,” says ISGlobal researcher Tamara Iungman, first author of the study.
An international team led by Mark Nieuwenhuijsen, director of the Urban Planning, Environment and Health Initiative at ISGlobal, estimated mortality rates of residents aged over 20 in 93 European cities (a total of 57 million inhabitants), between June and August 2015, and collected data on daily rural and urban temperatures for each city. The analyses were performed at a high-resolution level (areas of 250m x 250m). First, they estimated the premature mortality by simulating a hypothetical scenario without urban heat island. Second, they estimated the temperature reduction that would be obtained by increasing tree cover to 30% and the associated mortality that could be avoided.
“Our goal is to inform local decision-makers about the benefits of integrating green areas into all neighborhoods in order to promote more sustainable, resilient and healthy urban environments,” explains Nieuwenhuijsen.
The protective effect of trees
The results show that, from June to August 2015, cities were on average 1.5oC warmer than the surrounding countryside. In total, 6,700 premature deaths could be attributed to hotter urban temperatures, which represents 4.3% of total mortality during the summer months and 1.8% of year-round mortality. One third of these deaths (2,644) could have been prevented by increasing tree cover up to 30%, thereby reducing temperatures. Overall, cities with the highest excess heat-mortality rates were in Southern and Eastern Europe, with these cities benefiting the most from an increase in tree cover.
The study highlights the substantial benefits of planting more trees in cities, although the authors acknowledge that this can be challenging in some cities due to their design, and that tree planting should be combined with other interventions such as green roofs or other temperature-reducing alternatives.
“Our results also show the need to preserve and maintain the trees that we already have because they are a valuable resource and it takes a long time to grow new trees. It is not only about increasing trees in the city, it is also about how they are distributed,” says Nieuwenhuijsen.
The analyses were done for 2015 because population data were not available for later years, but, as Iungman points out, the study provides valuable information for adapting our cities and making them more resilient to the health impact of climate change. “Here we only looked at the cooling effect of trees, but making cities greener has many other health benefits, including longer life expectancy, fewer mental health problems and better cognitive functioning”, she adds.
“Vulnerability to heat changes from city to city depending on several factors. Understanding the benefits of policies such as increasing tree cover can help inform action to reduce risks and prevent avoidable deaths, especially with climate change", says Antonio Gasparrini, Professor of Biostatistics and Epidemiology at the London School of Hygiene & Tropical Medicine (LSHTM) and one of the authors of the study.
Supplementary material
Dataset: Check the resulting data for the 93 European cities.
Videos: Quotes Mark Nieuwenhuijsen; Quotes Tamara Iungman
Reference
Iungman T, Cirach M, Marando F., Pereira-Barboza E., Khomenko S., Masselot P., Quijal-Zamorano M., Mueller N., Gasparrini A., Urquiza J., Heri M., Thondoo M., Nieuwenhuijsen M. Cooling cities for health through urban green infrastructure: a health impact assessment for European cities. The Lancet, 2023. https://doi.org/10.1016/S0140-6736(22)02585-5
JOURNAL
The Lancet
METHOD OF RESEARCH
Computational simulation/modeling
SUBJECT OF RESEARCH
People
ARTICLE TITLE
Cooling cities for health through urban green infrastructure: a health impact assessment for European cities
ARTICLE PUBLICATION DATE
31-Jan-2023
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