January 31, 2023

expert reaction to study modelling the impact of urban trees on heatwave deaths

A study published in The Lancet looks at cooling European cities through urban green infrastructure.

Dr Radhika Khosla, Co-Director of the Future of Cooling Programme, University of Oxford Martin School, said:

“This research adds to growing calls for countries to take seriously the threat of extreme heat and consider how we can cool down our cities without resorting to energy intensive air conditioning which risks causing even more energy demand, worsening climate change.

“The largest impact of trees on cooling will be localised to the area they are planted, where they disrupt urban hot spots. Therefore, the location and particular conditions of tree planting would likely impact the number of avoided deaths and would need to be very carefully considered by policy makers on the ground.

“While it is important to be cautious about exactly how many heat-related deaths would be avoided by tree planting, the multiple benefits of urban trees are numerous – from much-needed carbon dioxide removal to biodiversity, shade, comfort and mental wellbeing.”

Dr Laurence Wainwright, Departmental Lecturer at the Smith School of Enterprise and the Environment, University of Oxford, said:

“This is an important piece of research that extends our understanding of the intricate interplay between cities, human health, ambient temperature and urban green infrastructure – especially trees. The authors are to be commended for their efforts in pulling together considerable quantities of data and robustly modelling it in order to make a novel contribution to the field.

“Things in the real world seldom work out as succinctly and tidily as they do in journal article graphs and equations. That said, the authors have done a fairly good job here in capturing and accounting for many of these factors. Their modelling – and the assumptions beneath these models – were both comprehensive and cautious and made a solid attempt to capture many of these nuances. Moreover, the authors were very forthcoming about the limitations of their impact assessment. They did not bite off more than they could chew in terms of scope; nor did they overpromise in terms of the benefits of increasing urban tree coverage. 

“The interactions between urban areas, temperature, humans and nature are highly complex and intricate, and trying to find causal relationships between phenomena is challenging due to the multiple, and often synergistic, variables at play. While we cannot be certain, we can say with a reasonably high level of confidence based on this research that urban tree planting – on the right scale, in the right places, and under certain other conditions – likely leads to a modest-yet-real reduction in heat-related deaths in many urban areas.

“For policymakers, urban planners, and city mayors, the findings of this research should be seen with a level of optimism: the urban heat island effect can be combatted. We already know that increasing green spaces and planting more trees in urban areas carries a large number of benefits and offers a disproportionally high number of benefits vs effort: improved mental health, promotion of leisure and exercise, a reduction in noise pollution and increased urban biodiversity. This research shows that the ‘bang for buck’ is even better that we’d first thought: we can now add a likely reduction in heat-related deaths under certain circumstances to the list.”

Prof Yadvinder Malhi, Professor of Ecosystem Science at the University of Oxford, said:

“There is a lot of news focus on trees as carbon sinks, and in terms of climate mitigation, so it is very welcome to see this paper about the importance of trees in terms of climate adaptation and resilience, where they have a very real role to play.

“The study appears robust in its analysis and credible in its conclusions. The finding makes a lot of intuitive sense: trees will have a beneficial impact in lowering the mean temperature in cities. But trees also provide other climate adaptation benefits, crucially providing shade during hot weather.

“More than half of the world’s people live in towns and cities, so trees are going to be critical in making urban areas resilient to climate change and improve urban environments. Urban trees bring many co-benefits beyond climate change adaptation: many studies show just seeing and smelling trees benefit health and well-being, as well as enhancing urban biodiversity. But most tree cover is found in wealthy towns and neighbourhoods, so enhancing urban tree cover can reduce this inequity and particularly reduce the high vulnerability of poorer neighbourhoods to climate change.”

‘Cooling cities through urban green infrastructure: a health impact assessment of European cities’ by Tamara Iungman et al. was published in The Lancet at 23.30 UK time on Tuesday 31^st January 2023.

DOI: https://doi.org/10.1016/S0140-6736(22)02585-5

Declared interests

Dr Laurence Wainwright has no interests to declare. 

Prof Yadvinder Malhi: “no interests to declare.”

For all other experts, no reply to our request for DOIs was received.