Exposure to deadly urban heat has tripled since the 1980s, and now affects nearly a quarter of the world’s population, a study has found.
Scientists put the worrying trend down to the combination of rising temperatures and growing numbers of people living in urban areas, and warned of its potentially fatal impact.
In recent decades, hundreds of millions of people have moved from rural areas to cities, which are now home to more than half the world’s population. Amid surfaces such as concrete and asphalt, which trap and concentrate heat, and little vegetation, temperatures are generally higher in urban areas.
“This has broad effects,” said Cascade Tuholske, the lead author of the study published in the journal PNAS and 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.”
The study used infrared satellite imagery and maximum daily heat and humidity readings from more than 13,000 cities from 1983 to 2016 to determine the number of people exposed to the days a year that exceeded 30C (86F) on the wet-bulb globe temperature scale (which takes into account the multiplier effect of high humidity) in an area. They matched the findings with the cities’ populations over the same period.
The study found that the number of person-days (the cumulative population exposed to cumulative heat in a given year for a particular place) soared from 40bn a year in 1983 to 119bn in 2016, representing a threefold increase. In 2016, 1.7 billion people were subjected to extreme heat conditions on multiple days.
Although it varied between cities and regions, scientists attributed two-thirds of the overall rise in exposure to increased urban populations and a third of it to global heating.
The worst affected city was Dhaka. Between 1983 and 2016, during which time the city’s population rose dramatically, Bangladesh’s capital experienced an increase of 575 million person-days of extreme heat. Other cities that underwent rapid population growth include Shanghai and Guangzhou in China, Yangon in Myanmar, Bangkok in Thailand and Dubai in the United Arab Emirates.
Cities that had at least half of their heat exposure caused by global heating include Baghdad in Iraq, Cairo in Egypt and Mumbai in India.
Of the cities studied, 17% experienced an additional month of extreme-heat days during the period, which spanned just over three decades.
Tuholske said: “A lot of these cities show the pattern of how human civilization has evolved over the past 15,000 years. The Nile, the Tigris-Euphrates, the Ganges … there is a pattern to the places where we wanted to be. Now, those areas may become uninhabitable. Are people really going to want to live there?”
Meanwhile in the US about 40 large cities have had rapidly growing exposure to heat, including Houston, Dallas-Fort Worth, San Antonio and Austin in Texas, Pensacola in Florida, Las Vegas in Nevada, Baton Rouge and Lake Charles in Louisiana and Providence in Rhode Island.
The PNAS paper is one of a number of newly published studies examining the impact of extreme heat.
A Brazilian study analysing the impact of forest loss on human health found that by 2100 as many as 12 million Brazilians could be exposed to extreme risk of heat stress as a result of large-scale deforestation of the Amazon and climate change.
The research, published in the Communications Earth & Environment journal on Friday, found there was a deforestation threshold in the Amazon that could threaten human survival if breached.
Meanwhile, a European study released on Monday predicts the economic costs of heatwaves could increase by nearly five times by 2060.
Scientists from the European Commission’s Joint Research Centre and other institutions estimate that recent heatwaves have led to an annual loss of 0.3-0.5% of European gross domestic product, losses they project will grow steadily over the next 40 years.
By the 2060s, they predict heatwaves will increase to an annual average of 1.14% and southern European countries will face the highest economic losses.
