Hot cities: researching extreme heat in the world's biggest capital cities

Globally, average temperatures have been on the rise for the past 30 years. We have taken a deep dive into the rapidly rising temperatures across the world's 20 largest capital cities.

Article, 23 July 2024
Cars stopped on the motorway. A digital sign reads "Extreme heat. Save power 4-9PM. Stay cool"

Cars stopped on the highway during a heatwave (Photo: Chris Yarzab, via Flickr, CC BY 2.0)

We have analysed rising temperatures in the 20 most populous capital cities, which are home to more than 300 million people globally. Our research shows the world’s biggest capital cities have experienced a 52% increase in the number of days reaching 35°C over the past three decades.

There were 16,586 days where the temperature hit at least 35°C during the last 30 years, and decade on decade the number has increased with 6,488 days above 35°C from 2014-23.

This is an issue affecting cities in both the global North and South. For example, the Bangladesh capital, Dhaka, has seen a 97% rise in the number of days reaching at least 35°C during the past 30 years, whereas in the last three decades London, UK, has seen 116 days above 30°C, with more than half of these occurring in the last 10 years.

This ‘hot cities’ research highlights the urgent need for policymakers to do more to help communities adapt to the new reality of these hotter temperatures.

The research also aims to raise the mainstream media profile of the impacts of climate change; showing concrete examples of how people, health, services, infrastructure and livelihoods in cities in the global North and South are suffering from extreme heat – a consequence of inaction on climate change.

We have created factsheets for each of the cities (see below) that show individual data and key findings, as well as making the data for all cities available.

Printscreens of media coverage of the IIED's press release on extreme heat in the world's biggest capital cities

Media coverage from Reuters, CNN, The Dong-A Ilbo, The Daily Star and Sky of IIED's research on extreme heat in the world's biggest capital cities (Photo: IIED)

In August 2024 IIED produced further research showing how all of the United States' 50 most-populated cities have got hotter over the past 50 years, with all but three experiencing more 'extremely hot' days above 95 degrees Fahrenheit.

The study also showed summers in the US are starting earlier and finishing later. In the 10 years from 1974-83, CNN reported the first extremely hot day of the season, on average, was on 22 June, and the final one on 4 September. "But over the past decade, that period is starting more than a week earlier and is running about a week longer than it used to."

Watch this short video highlighting the increase in temperatures in some of the world's 20 biggest cities, or see a playlist of individual city videos via IIED's YouTube channel

Download all the research data (xlsx file) compiled under this work

Note: To allow for comparability of data across multiple cities, airport site data was used. Airports are often used as official sites for measuring temperatures. According to the UK Met Office, monitoring equipment is set at an internationally-agreed distance from the runway to ensure no external factors can influence readings.

Contact

Simon Cullen ([email protected]), senior press and PR manager, IIED