World’s major cities hit by a quarter more hot days than in the 1990s
Adapting to climate change a must as urban populations rise.
Display on building showing 36°C in São Paulo, Brazil (Photo: Gildásio Filho, via Unsplash)
The world’s major cities now experience a quarter more very hot days per year on average than they did three decades ago, according to new analysis by IIED.
Researchers collated temperature data (PDF) for the 40 most populous capital cities and three extra cities with high political significance this year, dating from 1994. This expands on research in 2024 that examined the 20 largest capitals.
The number of days over 35°C in these cities has risen by 26% in the 31-year period studied, from an average of 1,062 a year between 1994 and 2003 to 1,335 between 2015 and 2024. Nine places saw their highest ever number of extremely hot days in 2024, including Washington DC.
These results highlight the importance of adapting cities to the effects of climate change while cutting greenhouse gas emissions. It comes as the deadline for countries to submit their Nationally Determined Contributions looms ahead of COP30.
Anna Walnycki, an IIED principal researcher, said: “Many of us know what it’s like to lie awake at night dripping sweat during a heatwave.
“And the knock-on effects to other areas of our lives are obvious – worse sleep means worse performance at work and less energy for the things we enjoy doing.
“This isn’t a problem we can simply air-condition our way out of. Fixing it requires comprehensive changes to how neighbourhoods and individual buildings are designed, as well as bringing nature back into our cities in the form of trees and other plants.
“Climate change is the new reality. Governments can’t keep their heads buried in the sand anymore.”
This graph shows the number of days over 35°C in 43 major world cities over the last 31 years. To see the data for each individual city, alongside 10-year averages, download the full dataset (Excel, 19MB)
Key points
- 2024 saw the highest total of very hot days (1,612) in the selected cities during the period analysed. That was 196 more than the second-highest total in 2019 (1,416 days), and 52% more than in 1994 (1,058 days).
- The top three years for extreme heat have occurred in the past six years (2024, 2023, 2019). Nine cities notched a record number of very hot days in 2024: Antananarivo, Cairo, Johannesburg, Kinshasa, Manila, Rome, Tokyo, Washington DC and Yaoundé.
- COP30 host Brazil provided some noteworthy findings. The capital Brasilia had only three days over 35°C in total between 1994 and 2003, compared to 40 between 2015 and 2024. In São Paulo, Brazil’s largest city and one which has a cooler climate, 2024 saw 120 days reach 30°C – the highest tally in the period studied.
- In South Africa, the current G20 president, residents of Pretoria and Johannesburg are really feeling the heat. The capital now averages 11 days a year over 35°C, up from only three between 1994 and 2003. In Johannesburg only three days in total reached 35°C between 1994 and 2021, but between 2022 and 2024, 26 did so.
- The populations of 16 cities have grown by at least half since 2013: Addis Ababa, Antananarivo, Bangkok, Cairo, Dar es Salaam, Delhi, Dhaka, Hanoi, Johannesburg, Kampala, Kinshasa, Luanda, Pretoria, Nairobi, Riyadh, and Yaoundé.
This graph shows the number of days over 35°C in Africa's major cities over the last 31 years. To see the data for each individual city, alongside the 10-year average, download the full dataset (Excel, 19MB)
Regional analyses, including cities newly added this year, provide illustrative examples of the problem.
In southeast Asia there are steep upward temperature trends, with all locations except Bangkok experiencing more very hot days. Hanoi’s 10-year average has nearly doubled since the decade of 1994 to 2003, and Kuala Lumpur’s has almost tripled.
After Europe experienced a blistering summer this year, IIED’s analysis shows temperatures climbing there too. Rome saw its average annual number of 35°C days rise from 11 between 1994 and 2003 to 24 in the most recent decade. Residents of Madrid sweated through 25 such days on average between 1994 and 2003, rising to 47 between 2015 and 2024. Berlin also saw an increase.
Anna Walnycki added: “Global temperatures are rising faster than governments probably expected – and definitely faster than they seem to be reacting.
“Failing to adapt will condemn millions of city dwellers to increasingly uncomfortable and even dangerous conditions because of the urban heat island effect.
“The poorest people will likely suffer the most whether they’re in London, Luanda or Lima, but the impacts will be significantly worse in low-income or unplanned communities in the global South thanks to lower-quality housing and infrastructure.
“Cities need an immediate funding boost to improve insulation and ventilation of buildings, develop heat plans and create shade cover where possible.
“Planners should also make sure new and redeveloped buildings are fit for a warming planet.”
This graph shows the number of days over 35°C in Europe's most populous capital cities over the last 31 years. To see the data for each individual city, alongside 10-year averages, download the full dataset (Excel, 19MB)
Hot weather can be deadly for the young and elderly, but no one escapes its reach, with workers in outdoor or manual jobs particularly at risk. Sustained high temperatures inside poorly designed homes are also a problem, in rich countries as well as low-income ones.
Globally, about a third of urban residents live in slums or informal settlements – the neighbourhoods most vulnerable to extreme heat.
The cities in this analysis have a combined population of nearly 470 million, and urban populations are predicted to grow significantly in the coming decades. This will be a problem only if cities, planners and service providers fail to work with communities to accommodate it.
Notes to editors
- Cities studied: Abidjan, Addis Ababa, Ankara, Antananarivo, Baghdad, Bangkok, Beijing, Berlin, Bogota, Brasilia, Buenos Aires, Cairo, Dar Es Salaam, Delhi, Dhaka, Hanoi, Jakarta, Johannesburg, Kabul, Kampala, Kinshasa, Kuala Lumpur, Lima, London, Luanda, Madrid, Manila, Mexico City, Moscow, Nairobi, Paris, Pretoria, Riyadh, Rome, Santiago, São Paulo, Seoul, Singapore, Tehran, Tokyo, Washington, D.C., Yangon, Yaoundé.
- Data for each individual city, alongside 10-year averages, is available by downloading the full dataset (PDF)
- Cities were selected based on population size using the CIA World Factbook and supplementary sources. Lack of quality temperature data for Khartoum and Abuja meant they were replaced by Kampala and Berlin. Yangon, the former capital of Myanmar, would have been inside the top 40 by population. Due to its significance, it is included here even though Naypyidaw is now the official administrative capital. Population data for Naypyidaw is unreliable.
- Cities averaging three 35°C days or fewer per year were selected for additional analysis on days reaching 30°C.
- This analysis uses Custom Weather’s historical data and analytics, which aggregates and presents daily weather data collected from a single airport location in each city. Some airports were slightly outside of the cities analysed so we have, where possible, used the one nearest the centre.
For more information or to request an interview, contact Jon Sharman:
+44 7407 727 886, or [email protected]
