The world’s major cities are now experiencing a quarter more very hot days every year on average than they did three decades ago, according to a new analysis published on Tuesday. 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 (classified as very hot days) in these cities rose by 26 percent over the 31-year period. This means an increase from an average of 1,062 annually between 1994 and 2003 to 1,335 between 2015 and 2024.
Delhi, a city whose population has grown by at least half since 2013, is particularly vulnerable to worsening heat stress. The analysis warns that residents of informal settlements in cities like Delhi are especially susceptible to sustained high temperatures due to inadequate housing and infrastructure.
The study highlights that 2024 recorded the highest number of very hot days, with 1,612 across the selected cities. This figure is 196 days more than in 2019, which was previously the second highest, and represents a 52 percent increase compared to 1994. The top three years for extreme heat have all occurred in the past six years, with 2024, 2023, and 2019 leading the trend.
Several global cities experienced record numbers of very hot days in 2024, including Antananarivo (Madagascar), Cairo (Egypt), Johannesburg (South Africa), Kinshasa (Democratic Republic of Congo), Manila (Philippines), Rome (Italy), Tokyo (Japan), Washington D.C. (U.S.), and Yaounde (Cameroon).
In Brazil, set to host COP30, the capital Brasilia saw a dramatic increase from only three days above 35 degrees Celsius between 1994 and 2003, to 40 days in the most recent decade. Even Sao Paulo, known for its milder climate, recorded 120 days above 30 degrees Celsius in 2024, the highest in the study period.
The research also indicated rising temperatures across Europe. Rome, Italy, experienced an increase in average days above 35 degrees from 11 (1994-2003) to 24 (last decade). Madrid, Spain, saw its average jump from 25 to 47 such days between the earlier period and 2015-2024. Berlin, Germany, also registered a rise in very hot days.
Anna Walnycki, an IIED researcher, commented, “Global temperatures are rising faster than governments probably expected and definitely faster than they seem to be reacting.” She emphasized that “failing to adapt will condemn millions of city dwellers to increasingly uncomfortable and even dangerous conditions because of the urban heat island effect.” Walnycki further explained that the poorest populations, whether in London, Luanda, or Lima, will bear the brunt of these impacts, with conditions significantly worse in low-income or unplanned communities in the Global South due to inferior housing and infrastructure.
The IIED stresses that cities urgently require funding to improve building insulation and ventilation, develop comprehensive heat action plans, and establish more shade cover. This is crucial for ensuring that new construction is adequately prepared for a warming world. The analysis concludes by noting that nearly a third of the global urban population resides in informal settlements, making them the most exposed to deadly heatwaves. The 43 cities examined in the study have a combined population of approximately 470 million, a figure projected to increase substantially in the coming decades.