The world’s largest capitals are experiencing more extremely hot days than ever before, according to a new study, revealing the dangerous trend is being driven by scorching temperatures in 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% increase over the past three decades in the number of days when temperatures rise above 35 degrees Celsius (95 degrees Fahrenheit), according to a analysis from the International Institute for Environment and Development (IIED) published on Friday.
From Buenos Aires, Argentina to the French capital Paris and Cairo, Egypt, the study finds that every decade, as human-caused greenhouse gas emissions increase, major capital cities are recording more and more extremely hot days – threatening human health, economies and infrastructure.
“Climate change is not just a threat to the future – it is already happening and getting worse,” said Tucker Landesman, senior researcher at IIED, in a press release.
“In just one generation, there has been an alarming increase in the number of extreme heat days in some of the world’s largest capital cities – exacerbated by the urban heat island effect,” which occurs when cities replace natural land with roads and buildings that trap more heat.
Asian cities, which account for about half of the world’s most populous capitals, have seen some of the largest temperature increases – a trend evident in recent heatwaves across the continent, from Southeast Asia to China and India. Asia is particularly vulnerable to climate risks, due to its high population density, poverty and the number of people living in low-lying areas that are susceptible to flooding, sea-level rise and other natural disasters.
New Delhi topped the list of hottest cities, with 4,222 days above 35 degrees Celsius in the last thirty years – more than any other city analyzed. Between 2014 and 2023, just under half (44%) of days in the Indian capital met this threshold, compared to 35% between 1994 and 2003 and 37% between 2004 and 2013.
The capital region is only getting hotter. In late May, parts of Delhi reached 49.9 degrees Celsius (121.8 degrees Fahrenheit) – the highest temperature ever recorded in the city, straining India’s electricity grid and power supply. The heat continued into the night, leaving residents with little respite.
“We have been living in this neighborhood for 40 years, but we have never experienced a summer like this,” Kalyani Saha, a 60-year-old resident of Lajpat Nagar in southeast Delhi, told CNN.
“We only get water once a day, and it is scalding hot. Unless you fill a bucket and let it cool all day before using it, you cannot bathe in this water.”
A rickshaw driver from Delhi told CNN that he is getting fewer passengers as people prefer air-conditioned taxis to open-air transportation.
“My body can’t handle it, but I have to keep cycling,” says Sagar Mandal, 39. “We are used to physical labour, we don’t complain about it. But this heat is not normal, something has to change.”
In the Indonesian capital Jakarta, the number of days with temperatures above 35 degrees Celsius has seen one of the largest increases in the past 30 years: from 28 days between 1994 and 2003 to 167 days between 2014 and 2023.
Seoul, South Korea, and Beijing, China have also seen a significant increase in extremely hot days. In 2018, Seoul saw 21 days above 35 degrees Celsius — more than the previous 10 years combined. The number of days above 35 degrees in Beijing has increased by 309% since 1994.
Cities are also facing extended periods of high temperatures as governments fail to meet climate targets and reduce emissions sufficiently. In October 2023, Jakarta had 30 consecutive days of temperatures above 35 degrees Celsius – more days than in the entire period between 1994 and 2003.
Extreme heat can be deadly, especially for vulnerable groups who may not have access to cool areas. Between June 11 and June 19, there were 192 heatwave deaths among the homeless in Delhi, a record high compared to the same period in the past five years, said a report by the NGO Center for Health Development India.
Young children, the elderly and pregnant women are at greater risk during heat waves. This can also have a devastating impact on casual and hourly workers. They may face work stoppages or have the choice between staying home without pay or working under dangerous conditions.
Heat also hurts economies, damaging crops and livestock and reducing labor productivity, especially in places without air conditioning, as workers need more breaks to rest and rehydrate.
Extreme heat also stresses infrastructure such as highways, roads, power lines and railways, disrupting the supply chain and causing power outages and illnesses.
According to a 2022 study by Dartmouth College, extreme heat has cost the global economy trillions of dollars since the early 1990s. The world’s poorest countries and those with the lowest emissions are bearing the brunt of the consequences.
“Facing the challenge of extreme heat will require bold action from policymakers, including serious investments to adapt to this new reality,” said IIED’s Landesman.
“For many cities, it is not a lack of knowledge, capacity or resources that prevents large-scale action to tackle climate change, but rather a lack of political will and governance tools.”
CNN’s Esha Mitra and Kathleen Magramo contributed reporting.
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