A new study tracking the movements of one billion mobile phone devices has exposed how wealth and age create a hidden divide in people's ability to withstand heatwaves.
Scientists analysing data from record-breaking temperatures in 2023 found that common measures to protect people living in cities - such as issuing alerts or planting trees to increase shade - often fail to help the most vulnerable.
It follows a World Meteorological Organisation warning that the last three years are now officially the warmest on record.
Experts behind the study used anonymised phone records to see whether people stayed home during extreme heat or were forced to travel for work or essentials.
They found the most vulnerable groups least likely to escape the heat were elderly people, those living in low-income areas, and outdoor or agricultural workers.
The study, published in the Sustainable Cities and Society journal , was led by University of Southampton research group WorldPop and Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai) in China.
Lead author Dr Haiyan Liu, a visiting fellow at WorldPop , said the findings show a stark divide in how different populations survive extreme temperatures.
She said: "Many heat measures do not translate well for certain groups, such as farmers or outdoor workers who may still need to work during heatwaves.
"Extreme heat is already a major killer, associated with around 500,000 deaths worldwide each year, according to the UN.
"Our results show that the most vulnerable are often forced into more movement, not less, during the most dangerous compound heatwaves.
"Heat measures require closer coordination between meteorological and public health services to combine heat warnings with guidance on hydration, cooling and being prepared for rising heatwaves."
Researchers using records from 1.1 billion phone devices, collected from 366 cities in China, looked at how people changed their routines during extreme heat.
Results showed that wealthier people were more likely to stay at home during heatwaves while people on lower incomes were forced to travel for work - other key findings showed that:
• In cities with more elderly people or low-income residents, official heat warnings often increased people's movement - likely because they worked outdoors or had less flexibility to stay home
• Public venues with air conditioning, like libraries and community centres, were the most effective lifeline during heatwaves and were used by low-income and elderly residents to cool down
• While urban greenery, such as parks and trees, helped younger, wealthier communities, it "consistently failed" to protect elderly residents or those in agricultural areas
Dr Liu added: "Ultimately, one-size-fits-all heatwave planning is ineffective.
"Public health authorities must use more precise methods to protect those whose age or economic status leaves them with no choice but to face the heat."
The data behind the study was collected by research group WorldPop, based at the University of Southampton.
Its demographers have published more than 80,000 datasets in the last decade using mobile phone data, satellite images, and census records.
WorldPop director Professor Andy Tatem said: "With heat exposure already linked to roughly half-a-million deaths a year, the frequency of deadly heatwaves could increase tenfold by the end of the century as earth's climate changes.
"We can no longer rely on average city data to plan for heatwaves. Governments must move toward more realist methods, as highlighted in this study, specifically where the socioeconomic need is greatest."
Read the study at www.sciencedirect.com/science/article/pii/S2210670725009369 .
Or for more about WorldPop go to www.worldpop.org .
