The world’s largest capital cities are seeing more extremely hot days than ever, according to a new study, which says the dangerous trend is being driven by scorching temperatures across Asia as the climate crisis worsens.

The world’s 20 most populous cities – together home to more than 300 million people – have seen a 52% jump in the number of days exceeding 35 degrees Celsius (95 degrees Fahrenheit) over the past three decades, according to an analysis by the International Institute for Environment and Development (IIED) published on Friday.

From Buenos Aires, Argentina to the French capital Paris and Egypt’s Cairo, the study found with each passing decade, as human-caused greenhouse gas emissions rise, major capital cities are recording more and more extremely hot days – posing threats to human health, economies and infrastructure.

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

“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,” which occurs when cities replace natural land with roads and buildings that retain more heat.

Asian cities, which make up about half of the world’s most populous capitals, saw some of the biggest temperature increases – a trend that’s evident in recent heat waves across the continent, from Southeast Asia to China and India. Asia is uniquely vulnerable to climate risks, due to its high population, poverty, and proportion of people living in low-lying areas, prone to flooding, sea level rise and other natural disasters.

New Delhi topped the list of hottest cities, recording 4,222 days above 35 degrees Celsius in the past three decades – more than any other city analyzed. Between 2014 and 2023, just under half (44%) of days in the Indian capital met that threshold, compared to 35% from 1994 to 2003, and 37% from 2004 to 2013.

The capital region is only getting hotter. In late May, one part of Delhi reached 49.9 degrees Celsius (121.8 degrees Fahrenheit) – the city’s highest temperature on record, straining India’s electricity grid and power supply. The heat also persisted into nighttime, leaving little respite for residents.

“We’ve been living in this neighborhood for 40 years, but we have never seen a summer like this,” Kalyani Saha, a 60-year-old resident of Lajpat Nagar in South East Delhi.

“We get water only once a day, and it’s scalding hot, unless you fill up a bucket and let it cool off all day before using it, you can’t bathe in this water.”

A Delhi rickshaw driver told he’d been getting fewer passengers, as people opted for air-conditioned taxis over open-air transport.

“My body can’t take it, but I have to keep cycling,” said Sagar Mandal, 39. “We are used to physical labor, we aren’t complaining about that. But this heat is not normal, something has to change.”

Indonesia’s capital Jakarta saw one of the biggest jumps in the number of days over 35 degrees Celsius in the past 30 years, from 28 days between 1994 to 2003 to 167 days from 2014 to 2023.

Seoul, South Korea and Beijing, China have also experienced significant rises in extremely hot days. In 2018, Seoul saw 21 days over 35 degrees Celsius — more than the previous 10 years combined. Beijing’s number of days over 35 degrees has increased by 309% since 1994.

Cities are also seeing longer stretches of high temperatures, as governments fail to meet climate targets and sufficiently curb emissions. In October 2023, Jakarta experienced 30 consecutive days over 35 degrees Celsius — more days than during the entire period between 1994 and 2003.

Extreme heat can be deadly, especially for vulnerable groups who may not have access to cool spaces. Between June 11 and 19, Delhi saw 192 heat wave-related deaths among its homeless population, a record high compared with the same period in the past five years, according to a report from the NGO Centre for Health Development India.

Young children, the elderly and pregnant people are at higher risk during heat waves, which can also have a devastating impact on informal and hourly workers, who may face work stoppages or a choice between staying home unpaid or working in dangerous conditions.

Heat also hurts economies, damaging crops and livestock and reducing labor productivity, especially in places without air conditioning, as workers require more breaks to rest and rehydrate.

And extreme heat stresses infrastructure, including highways, roads, electrical wires and railways, causing supply chain disruptions and blackouts and disease.

According to a 2022 Dartmouth College study, extreme heat has cost the global economy trillions of dollars since the early 1990s, with the world’s poorest and lowest-emitting countries bearing the brunt of the impact.

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

“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.”