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 an analysis by the think-tank the International Institute for Environment and Development (IIED).
Researchers collated temperature data for the 40 most populous capital cities and three extra cities with high political significance this year, dating from 1994. This expanded 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, it found. Nine places saw their highest ever number of extremely hot days in 2024, including Washington DC.
One of the key takeaways from the LUCI Cities & Lighting Summit in London in April was that our accelerating climate crisis – especially hotter cities – was going to mean lighting professionals, municipalities and urban planners are all going to have to work together to rethink, and reimagine, how cities work at night, including urban lighting at night.
As cities heat up, citizens will be more likely to be wanting to use urban and public realm spaces at night, when it is cooler. Therefore, how spaces are used and lit – and thought about – will need to adjust, as Lighting Journal reported in June (‘City heat’, vol 90 no 6).
Andreina Seijas, founder and principal of consultancy and think-tank Night Tank, highlighted how lighting professionals will need to grasp the realities of our changing climate in the context of cities at night, including thinking about how the night – and the city at night – can work to become a climate adaptation tool.
“As temperatures go higher, we will see that in many parts of the world – and it is already happening – people will spend more time out at night. So, it is really important to start unpacking those behaviours. What kinds of activities are changing, what kinds of habits are changing, and how should we reflect those changes in policies?” she pointed out.
The IIED report, equally, highlighted the importance of adapting cities to the effects of climate change while cutting greenhouse gas emissions.
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,” she added.
Key points highlighted in the report were:
- 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.
Image: a heatwave in Washington DC in 2019, with the city being one of nine that notched up the hottest days during 2024. Shutterstock
