World's big cities see 26 per cent rise in extreme hot days: Study

The world’s largest cities are enduring significantly more extremely hot days than they did 30 years ago, with the frequency rising by about a quarter, according to new research released on Tuesday.

The UK-based International Institute for Environment and Development (IIED) analysed temperature records from 43 major cities, including the 40 most populous capitals, dating back to 1994.

The study found that the number of days exceeding 35°C in these urban centres increased by 26 per cent over the past three decades — from an average of 1,062 very hot days a year between 1994 and 2003 to 1,335 between 2015 and 2024.

Delhi, where the population has grown by more than half since 2013, is among the cities under increasing heat stress. The report cautions that residents of informal settlements in Delhi and similar cities face acute risks from prolonged extreme heat due to poor housing and weak infrastructure.

The findings show that 2024 registered the highest number of very hot days across the selected cities — 1,612 in total. This was 196 more than the previous peak in 2019 and 52 per cent higher than in 1994. All three of the hottest years on record — 2024, 2023 and 2019 — have occurred within the last six years.

Cities recording unprecedented numbers of very hot days in 2024 include Antananarivo (Madagascar), Cairo (Egypt), Johannesburg (South Africa), Kinshasa (Democratic Republic of Congo), Manila (Philippines), Rome (Italy), Tokyo (Japan), Washington DC (US) and Yaoundé (Cameroon).

Brazil, which is due to host COP30, has also seen striking changes. In Brasilia, there were only three days above 35°C in the decade to 2003, but this climbed to 40 in the most recent ten years. São Paulo, traditionally regarded as relatively cool, recorded 120 days above 30°C in 2024 — the highest total in the study period.

Europe is not immune. Rome averaged 11 days above 35°C between 1994 and 2003, rising to 24 in the past decade. Madrid saw an increase from 25 such days to 47, while Berlin has also experienced more very hot days.

“Global temperatures are rising faster than governments probably expected and definitely faster than they seem to be reacting,” said Anna Walnycki, a researcher at IIED. She warned that without adaptation measures, “millions of city dwellers” would face worsening conditions due to the urban heat island effect.

She added that although poorer residents everywhere are vulnerable, the impact will be most severe in low-income or unplanned settlements in the Global South, where housing and infrastructure are weakest.

The IIED said cities urgently need financial support to make buildings more resilient, improve ventilation and insulation, create shaded areas and prepare heat action plans, while ensuring all new construction is designed for higher temperatures.

The report notes that nearly one-third of the world’s urban population lives in informal settlements, leaving them disproportionately exposed to deadly heatwaves. The 43 cities covered in the analysis are home to around 470 million people — a population expected to grow substantially in the coming decades.

With PTI inputs