A new analysis released by the UK-based International Institute for Environment and Development (IIED) paints a stark picture of escalating urban heat, showing that the world's major cities are enduring 26% more very hot days annually compared to three decades ago. By examining temperature data from 43 key cities—including the 40 most populous capitals—from 1994 onward, researchers found the average number of days exceeding 35 degrees Celsius climbed from 1,062 between 1994 and 2003 to 1,335 between 2015 and 2024. This trend is particularly alarming for densely populated urban areas, where the urban heat island effect amplifies temperatures, exacerbating health risks and straining infrastructure for the 470 million residents in these cities alone.
The study highlights 2024 as a record-breaking year, with 1,612 very hot days across the selected cities—52% higher than in 1994 and 196 more than the previous peak in 2019. The top three hottest years have all occurred within the last six years: 2024, 2023, and 2019. Cities like Delhi, where the population has surged by at least 50% since 2013, are among the hardest hit, with informal settlements facing acute vulnerability due to substandard housing and limited green spaces. Other global hotspots recording unprecedented heat 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 Yaounde (Cameroon), underscoring the worldwide scope of this crisis.
Regional disparities are evident, with dramatic increases in even traditionally milder climates. In Brazil, set to host COP30, Brasília's very hot days jumped from just three in the 1994-2003 period to 40 in the latest decade, while São Paulo logged 120 days above 30 degrees Celsius in 2024—the highest in the study. Europe is not spared; Rome saw its average very hot days double from 11 to 24, Madrid's rose from 25 to 47, and Berlin experienced a notable uptick. These shifts reflect accelerating global warming, outpacing governmental responses and leaving urban dwellers exposed to prolonged heat stress that can lead to heatstroke, dehydration, and exacerbated respiratory issues.
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IIED researcher Anna Walnycki warned that "global temperatures are rising faster than governments probably expected and definitely faster than they seem to be reacting," emphasizing the disproportionate impact on low-income communities in the Global South. Nearly a third of the world's urban population resides in informal settlements, where poor insulation, inadequate ventilation, and lack of shade heighten dangers from deadly heatwaves. The urban heat island effect—where concrete and asphalt trap heat—further intensifies these risks, condemning millions to uncomfortable and hazardous conditions, particularly in cities like London, Luanda, or Lima.
To combat this escalating threat, IIED urges immediate investment in urban adaptation measures, including enhanced building insulation, comprehensive heat action plans, expanded green shading, and climate-resilient new constructions. As urban populations are projected to swell dramatically in the coming decades, these interventions are not just essential but urgent to safeguard vulnerable residents and mitigate the human and economic toll of extreme heat in our warming world.
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