April 21, 2025

expert reaction to study assessing temperature-related deaths in urban heat islands

A study published in Nature Climate Change assesses temperature-related deaths in urban heat islands.

Dr Chloe Brimicombe, Climate Scientist and Extreme Heat Impact Researcher, University of London, The London School of Hygiene and Tropical Medicine, said:

“The paper shows how urban greening (maintaining parks and planting trees) and cool roofs, painting roofs white are intervention which reduces the rise in heat in cities and are associated with reduced heat related mortality.

“In the paper all the results are related to a general U-shaped relationship this is where we see more deaths when it’s cold and hot in comparison to when it’s just warm. This is a global trend but there are actually local differences so sometimes we see a J shape where there’s a point at which heat related mortality rises faster than cold related mortality. It’s also different for age, cause of death and for socio-economic factors like wealth, type of housing and ethnicity.

“Also, there is what we call a lagged effect, someone could take up to a month to die from cold, whereas it is 3 days to a week with heat – when we consider our body, they affect us differently – you shiver when cold and sweat when hot.

“In addition, this paper does not take into account humid heat which is more deadly than dry heat. And we have seen in other recent studies that urban greening may be affecting this and therefore heat during the night-time and mortality.

“This is a problem in UK towns and cities, there has been local research showing that green roofs are associated with reduced heat-mortality in London. We see a rise in cold and heat related mortality in UK cities, we all remember how unbearable UK urban areas were in the July 2022 heatwave. This paper gives evidence that more funding should be set aside so that local towns and cities can increase adaptation strategies including maintaining parks, planting trees and vegetation, increasing coverage of cool roofs and reflective roofing which may reduce the burden of heat related mortality.

“With all this in mind the paper demonstrates really how we need to take into account local contexts and there is a lot that local cities are doing, and we should celebrate these efforts, but adaptation alone is not enough we need to urgently prioritise transitions to net zero and beyond.”

Dr Clare Heaviside, Associate Professor (Cities, Climate and Health), University College London (UCL), said:

“Urban adaptations influence local temperatures year-round, impacting human health. These complex effects vary across populations, cities and geographic and climatic zones. Therefore, maximising benefits and minimising negative impacts in specific settings requires location-specific modelling and data (health and meteorology), rather than relying on generalised results that extrapolate sparse data to different settings.

“That is why our (and many other) modelling studies use locally derived temperature-mortality relationships, daily mortality data and detailed regional urban climate modelling to best assess the impacts of adaptations like greening and cool roofs. For example, we modelled the health impacts of the UHI in summer and winter in the West Midlands, and found a protective effect of the UHI on winter mortality as well as a (larger) negative impact of the UHI in summer.

“Cool roofs reduced summer mortality and had a negligible effect in winter, so even with cool roofs present all year round, the reduced solar radiation in winter meant that the cool roofs did not diminish the beneficial impact of the UHI during cold weather. Therefore we found an annual net positive health impact of cool roofs, due to the larger benefits in summer. Of course this is highly context specific and will not necessarily be the case everywhere, so we would need similar data and modelling in other locations to test similar impacts of adaptations.”

Dr Madeleine Thomson, Head of Climate Impacts and Adaptation at Wellcome, said:

“While the study’s modelling provides valuable insights into global trends, it assumes adaptation strategies have the same impact across seasons around the world.

“Local context matters. Effective climate adaptation must be tailored to the place — and the people — it’s meant to protect. Many cities are already doing this with evidence-based approaches that consider the impact year-round.

“Copenhagen, for example, planted deciduous trees that offer shade in summer to reduce urban heat, while shedding their leaves in winter to let sunlight in—helping to cool the city in summer without intensifying winter cold. This seasonal adaptability makes them an effective tool against the urban heat island effect.”

“Extreme heat is a rising global threat due to climate change, putting vulnerable groups — including older adults, children, and pregnant women — at serious health risks. Cities must act with urgency, using local evidence to protect lives now and into the future.”

‘Dual impact of global urban overheating on mortality’ by Shasha Wang et al. was published in Nature Climate Change at 16:00 UK time Monday 21 April 2025, which is when the embargo will lift.

DOI: 10.1038/s41558-025-02303-3

Declared interests

Dr Chloe Brimicombe: No COIs

Dr Madeleine Thomson: No interests to declare.