In 20 of the world’s most populous capitals, where more than 300 million people live, between 1995 and 2023 the number of days with temperatures above 35 ºC increased significantly. Putting human health and essential infrastructure at risk
The heat island effect exacerbates the extreme heat
In the last 30 years, the days of extreme heat in the most populated cities on the Planet have increased by over 50%. And with them the health risks of more than 300 million people have grown, particularly exposed to the interweaving of climate crisis and urban environment, starting from the heat island effect. But also the pressure on essential infrastructure, such as the electricity grid.
This is what emerges from the analysis conducted by the International Institute for Environment and Development (IIED), which has reconstructed the number of days with temperatures above 35 ºC in 20 of the most inhabited global capitals.
Extreme heat, global metropolises score +52% in 30 years
A look at the data immediately gives the measure of the trend. In 1995, the days of extreme heat in the 20 metropolises were, in all, 458. In 2023 they had become 738. To separate from the trend the annual variability is useful to look at the performance on a ten-year basis. Between 1994 and 2003 the days over the 35 In 2004-2013 they rose to 5,343 (+12%), in 2014-2023 they reached 6,488 (+21% over the previous decade).
“In just one generation, there has been an alarming increase in the number of days of extreme heat that have hit some of the world’s largest capitals, compounded by the urban heat island effect,” IIED’s Tucker Landesman points out. “Some groups are more vulnerable to the adverse health effects of extreme heat: young people, the elderly, those living in poor housing and those who do not have access to air conditioning, shade and cooler places such as green spaces“.
The trend is uneven from city to city, but the trend is everywhere. In London – the northernmost of the capitals evaluated – in the first decade analyzed the thermometers never reached 35°C, in the following two decades the days rose to 7 of which only 5 between 2019 and 2022. Low numbers have little statistical relevance, but the trends are confirmed by the number of days over 30 C: from 30 in 1994-2003 to 59 in the last decade. Paris also moves on the same trajectory, with +57% of days over 30 degrees.
In other continents, the phenomenon is even more marked. New Delhi has gone from having 35% of days a year with extreme heat characteristics to 44% in the last decade. Jakarta, Indonesia, shows the most significant increase: from 28 days over 35°C in the first decade to 167 in the previous 10 years. Seoul went from 9 to 58, Buenos Aires from 7 to 35, Beijing from 132 to 245.
