A significant new analysis released on Tuesday reveals a stark reality: the world’s largest cities are enduring a substantial increase in extremely hot days each year. On average, they now face 26% more scorching days compared to thirty years ago. Researchers from the UK-based International Institute for Environment and Development (IIED) meticulously studied temperature data from 43 prominent cities, encompassing the 40 most populous capitals, tracking trends since 1994.
Specifically, the study pinpointed that the count of ‘very hot days’ (exceeding 35 degrees Celsius) in these urban centers surged by an alarming 26% over 31 years. The annual average climbed from 1,062 days between 1994 and 2003 to 1,335 days from 2015 to 2024. Cities like Delhi, which has seen its population boom by at least half since 2013, are experiencing intensified heat stress. The report highlights that residents in informal settlements within these cities are especially at risk due to inadequate housing and infrastructure that offer little protection from relentless high temperatures.
The year 2024 stands out as the hottest on record within the surveyed cities, registering a staggering 1,612 very hot days. This figure represents an increase of 196 days compared to 2019 (the second highest year) and a striking 52% jump from the 1994 total. Notably, the three years with the most extreme heat have all occurred within the last six years, with 2024, 2023, and 2019 leading the trend.
Several global capitals set new records for 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).
Even in Brazil, the host nation for the upcoming COP30 climate conference, the impact is evident. Its capital, Brasilia, experienced only three days above 35 degrees Celsius between 1994 and 2003, a number that skyrocketed to 40 in the most recent decade. Sao Paulo, traditionally known for its milder weather, recorded an unprecedented 120 days above 30 degrees Celsius in 2024 alone.
Europe is not immune to this escalating heat. In Rome, Italy, the average number of days surpassing 35 degrees Celsius more than doubled, climbing from 11 (1994-2003) to 24 in the last decade (2015-2024). Madrid, Spain, witnessed an even sharper increase, from 25 such days to 47 over the same periods. Berlin, Germany, also reported a rise in these extreme heat occurrences.
Anna Walnycki, a leading researcher at IIED, voiced a critical observation: “Global temperatures are escalating more rapidly than governments anticipated, and certainly faster than their current responses suggest.” She further warned that “a failure to adapt will subject millions of urban residents to increasingly uncomfortable and perilous living conditions, exacerbated by the urban heat island effect.” Walnycki emphasized that while the most vulnerable globally—from London to Luanda or Lima—will bear the brunt, the consequences will be far more severe in the low-income and unplanned communities of the Global South due to inferior housing and infrastructure.
The IIED stresses the urgent need for cities to secure funding to enhance building insulation and ventilation, implement comprehensive heat action plans, and establish more shaded areas. Crucially, new construction must be designed to withstand a warming climate. The analysis underscores that almost a third of the world’s urban dwellers reside in informal settlements, making them disproportionately vulnerable to life-threatening heatwaves. The 43 cities examined in this study collectively house approximately 470 million people, a number projected to grow significantly in the coming decades, intensifying the challenge.