Exposure to deadly urban heat has tripled worldwide since the 1980s, a new study has warned.
Researchers at Columbia University's Earth Institute said a quarter of the world's population is now affected as a result of both rising temperatures and booming urban population growth.
They studied more than 13,000 cities across the globe and based their calculations on the number of 'person-days' that inhabitants were exposed to extreme combinations of heat and humidity.
These went from 40 billion per year in 1983 to 119 billion in 2016 — a threefold increase. By 2016, 1.7 billion people were being subjected to such conditions on multiple days, the researchers said.
Researchers at Columbia University's Earth Institute said a quarter of the world's population is now being exposed to deadly urban heat as a result of both rising temperatures and booming urban population growth. This graphic shows the annual increases in the rate of urban population exposure to extreme heat in more than 13,000 cities worldwide from 1983 to 2016
Experts studied more than 13,000 cities across the globe and based their calculations on the number of 'person-days' that inhabitants were exposed to extreme combinations of heat and humidity. Pictured is the sun beating down on downtown Phoenix in Arizona
Urban population growth accounted for two-thirds of the exposure spike, while rising temperatures caused by global warming contributed a third.
The worst-hit city was Dhaka, the fast-growing capital of Bangladesh, which has seen an increase of 575 million person-days of extreme heat over the past 40 years, mainly due to its ballooning population from 4 million in 1983 to 22 million today.
This caused 80 per cent of the increased exposure, the researchers said, although they cautioned that it doesn't mean it hasn't seen rising temperatures as well, it's just this has been overshadowed by the population increase.
Over recent decades, hundreds of millions of people have moved from rural areas to cities, which now hold more than half the world's population.
There, temperatures are generally higher than in the countryside because of sparse vegetation and abundant concrete and asphalt that tend to trap and heat—the so-called urban heat island effect.
'This has broad effects,' said the study's lead author, Cascade Tuholske, a postdoctoral researcher at Columbia University's Earth Institute.
'It increases morbidity and mortality. It impacts people's ability to work, and results in lower economic output. It exacerbates pre-existing health conditions.'
Other big cities showing similar population-heavy trends include Shanghai and Guangzhou, China; Yangon, Myanmar; Bangkok; Dubai; Hanoi; Khartoum; and various cities in Pakistan, India and the Arabian Peninsula.
On the other hand, some other major cities saw close to half or more of their exposure caused by a warming climate alone rather than population growth.
Among them were Baghdad, Cairo, Kuwait City, Lagos, Kolkata, Mumbai, and other big cities in India and Bangladesh.
The populations of European cities have been relatively static, so increases in exposure there were driven almost exclusively by increased warmth.
'A lot of these cities show the pattern of how human civilisation has evolved over the past 15,000 years,' said Tuholske, pointing out that many are located in warm climates where humidity is
