Researchers at the UK-based International Institute for Environment and Development (IIED) examined temperature data from 43 key cities, including the 40 most populous capitals, dating back to 1994.
They found that the number of days above 35 degrees Celsius (very hot days) in these cities rose by 26 per cent over the 31-year period, climbing from an average of 1,062 annually between 1994 and 2003 to 1,335 between 2015 and 2024.
Delhi, one of the cities where the population has grown by at least half since 2013, is among those facing worsening heat stress.
The analysis warns that residents of informal settlements in cities such as Delhi are particularly vulnerable to sustained high temperatures because of poor housing and infrastructure.
The study shows 2024 recorded the highest number of very hot days, 1,612 across the selected cities. This was 196 more than in 2019, the second highest, and 52 per cent higher than the tally in 1994. The top three years for extreme heat have all come in the past six years, with 2024 followed by 2023 and 2019. Antananarivo (Madagascar), Cairo (Egypt), Johannesburg (South Africa), Kinshasa (Democratic Republic of Congo), Manila (Philippines), Rome (Italy), Tokyo (Japan), Washington DC (US), and Yaounde (Cameroon) notched a record number of very hot days in 2024.
In Brazil, which will host COP30, the capital Brasilia saw only three days above 35 degrees between 1994 and 2003, compared to 40 in the most recent decade. Sao Paulo, known for its cooler climate, had 120 days above 30 degrees in 2024, the most in the study period.
The study found temperatures are rising in Europe too.
In Rome, Italy, the average number of days above 35 degrees went up from 11 between 1994 and 2003 to 24 in the last decade.
People in Madrid (Spain) faced 25 such days on average in the earlier period, which increased to 47 between 2015 and 2024. Berlin (Germany) also recorded more very hot days.
Anna Walnycki, a researcher at IIED, said: “Global temperatures are rising faster than governments probably expected and definitely faster than they seem to be reacting.”
She added, “Failing to adapt will condemn millions of city dwellers to increasingly uncomfortable and even dangerous conditions because of the urban heat island effect.”
The poorest will likely suffer the most whether they are in London, Luanda, or Lima, but the impacts will be significantly worse in low-income or unplanned communities in the Global South thanks to lower-quality housing and infrastructure, she added.
IIED said cities urgently need funding to improve insulation and ventilation in buildings, develop heat action plans and create shade cover, while ensuring new construction is fit for a warming world.
The analysis notes that nearly a third of the global urban population lives in informal settlements, leaving them most exposed to deadly heatwaves.
The 43 cities studied have a combined population of about 470 million, a figure that is expected to rise sharply in the coming decades.
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