The world is heating up, and cities are bearing the brunt of it. A new study of more than 13,000 cities has found that exposure to a deadly combination of heat and humidity has tripled since the 1980s.
The trend affects nearly a quarter of the world’s population, according to the research, which appears in the Proceedings of the National Academy of Sciences.
The tripling effect is driven by two main factors: firstly, rising temperatures from human-induced climate change, and secondly, a boom in urban population.
This is a deadly combination. Cities are hit hard by the urban heat island effect: they can be up to 10 degrees Celsius warmer than the surrounding countryside due to lack of vegetation and abundant concrete.
“This has broad effects,” says lead author Cascade Tuholske from Columbia University in the US. “It increases morbidity and mortality. It impacts people’s ability to work, and results in lower economic output. It exacerbates pre-existing health conditions.”
To see just how much these effects have increased as the climate changes, Tuholske and team combined infrared satellite imagery and on-the-ground monitoring from 13,115 cities.
Specifically, they looked at maximum daily heat and humidity readings from 1983 to 2016, to find out how many days people were exposed to extreme heat. This was defined as 30 degrees Celsius on the wet-bulb globe temperature scale, which takes into account high humidity – this is the point where even healthy people find it almost impossible to function outside.
Then, the team paired the weather data with stats about the cities’ populations, with grim results.
In 1983, city dwellers were exposed to 40 billion days of extreme heat. In 2016, this number soared to 119 billion days.
The increase in exposure was largely caused by ballooning urban populations, as hundreds of millions of people flocked to cities from rural areas in recent decades. Global warming accounted for one-third of the increase.
The worst-hit city was Dhaka, the capital of Bangladesh. It saw an increase of 575 million days of extreme heat from 1983 to 2016, largely because its population increased from four million in 1983 to 22 million today. Other hard-hit cities – mainly because of population growth – include Shanghai, Guangzhou, Yangon, Bangkok, Dubai, and Hanoi.
But some cities were more influenced by the climate than population growth, including Baghdad, Cairo, Kuwait City, Lagos, Kolkata and Mumbai, as well as European cities where populations have remained relatively static.
“A lot of these cities show the pattern of how human civilisation has evolved over the past 15,000 years – the Nile, the Tigris-Euphrates, the Ganges,” says Tuholske.
Many of these cities are located in warm climates, where river systems deliver humidity – attractive for farming and urbanisation.
“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?”
It is clear that as the climate warms, our cities will need to adapt to reduce the impact of the urban heat island effect.
Studies like this can help inform ways to address local heat issues – from increasing urban greening by planting trees to painting rooftops to reflect heat.
“This study shows that it will take considerable, conscientious investments to ensure that cities remain livable in the face of a warming climate,” says Kristina Dahl, a climate researcher at the Union of Concerned Scientists.
You can explore the results for yourself in this interactive map of heat in 13,000 cities around the world.
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