The study suggests that planting more trees could mean fewer people die from high summer temperatures in cities.
Increasing tree cover from the European average of 14.9% to 30% can lower the temperature in cities by 0.4 C, which could reduce heat-related deaths by 39.5%, according to the first-of-its-kind modelling of 93 European cities by an international team of researchers.
The lead author, Tamara Iungman, from the Barcelona Institute for Global Health, said: This is becoming increasingly urgent as Europe experiences more extreme temperature fluctuations caused by climate change.
We already know that high temperatures in urban environments are associated with negative health outcomes, such as cardiorespiratory failure, hospital admission, and premature death. She said that her team wants to influence policymakers to make cities greener, more sustainable, resilient and healthy, as well as to mitigate climate breakdown, because heat-related illness and death is expected to present an even bigger burden to health services over the next decade than cold temperatures.
The researchers used mortality data to estimate the potential reduction in deaths from lower temperatures as a result of increased tree coverage. According to data from 2015, 2,644 could have been prevented had tree cover been increased, compared to the 6,700 premature deaths that year attributed to higher urban temperatures.
The cities most likely to benefit from the increase in tree coverage are in south and eastern Europe, where summer temperatures are highest and tree coverage tends to be lower.
In Cluj-Napoca in Romania, which had the highest number of premature deaths due to heat in 2015, the tree coverage of 32 per 100,000 people is just 7%. It is as low as 3.6% in Lisbon, Portugal and 8.4% in Barcelona. That compares with 15.5% in London and 34% in Oslo.
The study co-author Mark Nieuwenhuijsen, a researcher at the Barcelona Institute for Global Health, said the team picked 30% as that is a target that many cities are currently working on.
He said that there was no need for buildings to be razed and replaced with parks, since there is enough space to plant more trees in all cities the team looked at. He noted that policymakers must ensure trees are evenly distributed between richer and poor neighbourhoods, as well as the EU's 3 billion trees plan and the UK government's proposal to ensure every home is within a 15 minute walk from green space.
He said that cities that are too car-dominated should replace asphalt roads with trees, which absorb heat.
He said that planting more trees in cities should be prioritized because it brings a huge range of health benefits beyond reducing heat-related deaths, including reducing cardiovascular disease, dementia and poor mental health.
Prof Yadvinder Malhi, professor of ecosystem science at the University of Oxford, said: More than half of the world s people live in cities and cities, so trees are going to be critical in making urban areas resilient to climate change and improving urban environments. Urban trees bring many co-benefits beyond climate change adaptation: many studies show that just seeing and smelling trees benefits health and wellbeing, as well as enhancing urban biodiversity. Most tree cover is found in wealthy towns and neighbourhoods, so enhancing urban tree cover can reduce this inequity and reduce the vulnerability of poorer neighbourhoods to climate change.