The world’s biggest capital cities have experienced a 52% increase in the number of days reaching 35°C over the past three decades, according to new analysis by IIED.

The analysis looked at the 20 most populous capital cities – home to more than 300 million people – including Dhaka (PDF), London (PDF), Cairo (PDF), Manila (PDF), Kinshasa (PDF), Tokyo (PDF) and Paris (PDF).

It shows an overall rise in the number of days of extreme heat, meaning millions of people are living under increasing levels of heat stress with risks to personal health and infrastructure.

Key findings include:

·  Cumulatively, there were 16,586 days where the temperature hit at least 35°C during the 30-year period. However, with each decade, the number has been steadily rising:

·  1994-2003: 4,755 days

·  2004-2013: 5,343 days

·  2014-2023: 6,488 days

·  New Delhi (PDF) recorded the highest number of days over 35°C out of any capital city analysed, with 4,222 days reaching that threshold. During the past decade alone, 44% of days in the city reached 35°C – a significantly higher percentage than the previous two decades (37% in 2004-2013; 35% in 1994-2003).

·  Jakarta (PDF) experienced one of the most significant jumps in the number of days over 35°C in the past 30 years. Between 1994-2003 there were just 28 days over 35°C in Jakarta. By the following decade (2004-2013) this had increased to 153 days, and in the most recent decade (2014-2023) it had reached 167 days.

·  Other cities with sharp increases in the number of hot days (more than 35°C) over the past 30 years included Seoul (PDF), Buenos Aires (PDF) and Beijing (PDF). These were calculated based on the linear trendline across 30 years.

·  The number of consecutive days of extreme heat in some cities is also increasing. For example, Jakarta experienced 30 consecutive days over 35°C in October 2023 – more days than the entire 10-year period between 1994-2003.

·  In Paris, there’s been a 57% rise in the number of days reaching the 30°C threshold over the past 30 years.

Many countries have already declared several extreme heat waves this year, recognizing the severe impact climate change is putting on people’s health and community infrastructure.

There’s growing consensus that heat waves should be officially categorized as disasters. This research highlights the urgent need for policymakers to do more to help communities adapt to the new reality of these hotter temperatures.

IIED senior researcher Tucker Landesman said: “Climate change is not just a future threat – it’s already happening and getting worse.

“In just one generation, there’s been an alarming increase in the number of days of extreme heat affecting some of the world’s biggest capital cities - made worse by the urban heat island effect.

“Millions more people are experiencing heat stress as temperatures reach dangerously high levels in some cities, which is having a profound impact on people’s health.

“Some groups are more vulnerable to the negative health effects of extreme heat – the young, the elderly, those living in substandard housing, and those who don’t have access to air conditioning, shade and cooler places such as green space.

“We know that hot weather is not felt evenly across cities. Pockets of extreme heat are more likely in certain types of neighborhoods and commercial districts. This is tied to inequality and how we design buildings and public infrastructure. In many ways, city planning codes and regulations are failing to protect people from the effects of climate change.

“Responding to the challenge of extreme heat will require bold action from policymakers, including serious investment to adapt to this new reality.

“For many cities, it’s not a lack of knowledge or capacity or resources that’s preventing large-scale action to address climate change, rather it’s a lack of political will and governing tools.

“To make change happen, there needs to be strategic coordination between health, finance, environment and transport policy experts, along with civil society groups and frontline communities. Without a properly resourced response to the climate crisis, millions of people will continue to suffer the worst effects of climate change.”