LONDON, October 1, 2025: The number of extremely hot days in the world’s most populous capital cities has increased by 25 percent compared to the 1990s, according to new research released by the International Institute for Environment and Development (IIED). The study analyzed temperature data from 43 major global capitals and found a significant rise in the frequency of days reaching or exceeding 35 degrees Celsius over the past three decades. The IIED assessment, which compared data from two ten-year periods, 1994 to 2003 and 2015 to 2024, showed that the annual number of days above 35°C rose from an average of 1,062 to 1,335.
The findings highlight the growing impact of extreme heat on urban populations, particularly in cities experiencing rapid population growth and limited infrastructure resilience. Cities across Asia, Africa, Europe and the Americas recorded sharp increases. In Manila, the number of days over 35°C tripled. In Rome and Beijing, the total doubled. Madrid saw an increase from 25 to 47 days annually, while in London, which typically experiences milder conditions, the number of days exceeding 30°C also doubled.
The year 2024 registered the highest number of extremely hot days since records began, with 1,612 such days logged across the 43 cities. Nine cities recorded their highest annual tallies of very hot days, including Washington DC, Tokyo, Johannesburg, Cairo, Yaoundé, Antananarivo and Manila. The rise in extreme temperatures aligns with global climate data identifying 2024 as the hottest year ever recorded. IIED researcher Anna Walnycki said the trend poses severe risks to urban populations, especially in low-income areas where housing is often substandard and ill-equipped to handle extreme heat.
2024 marked as the hottest year with record-breaking city heat
Around one-third of urban dwellers worldwide live in informal settlements or slums, where access to cooling, clean water and medical services is limited. The urban heat island effect, in which built-up areas retain more heat than surrounding regions, further amplifies temperature extremes in cities. This effect disproportionately affects densely populated neighborhoods and contributes to higher rates of heat-related illness and mortality. The IIED report also highlights technical challenges in how urban heat is measured.
Much of the available data relies on satellite-based land surface temperature readings, which may not accurately reflect the air temperatures experienced by people on the ground. Researchers note that these discrepancies could affect the effectiveness of heat adaptation policies if not properly addressed. In response to the findings, the IIED is urging governments and city planners to prioritize investment in heat-resilient infrastructure, including improved building design, increased urban vegetation, enhanced ventilation systems and targeted heat action plans.
Heatwave risks increase as informal housing struggles to cope
The report emphasizes that urban planning must evolve quickly to meet the needs of growing city populations amid rising temperatures. Despite global pledges under the Paris Agreement, emissions linked to fossil fuel combustion remain elevated. Scientists have stated that emissions must fall by 45 percent by 2030 to maintain a realistic chance of limiting global warming to 1.5°C above preindustrial levels. The IIED’s updated analysis expands on a previous 2024 report that studied 20 capitals.
The latest findings include 43 cities and use temperature data through the end of 2024, offering one of the most detailed assessments to date of how extreme heat is impacting urban centers. With more than half of the global population now living in cities, the acceleration of heat extremes in these areas is being recognized as a critical public health and infrastructure challenge. The report concludes that immediate action is required to ensure cities remain livable as the climate continues to warm. – By Content Syndication Services.