Urban exposure to extreme heat has tripled since the ‘80s

Global warming is rapidly heating up the planet, and the issue is becoming increasingly problematic in urban environments. A new study of more than 13,000 cities has concluded that the number of days people are being exposed to extreme heat and humidity has tripled since the ‘80s.

The researchers, based at Columbia University’s Earth Institute, have concluded that the trend is the result of both rising temperatures and urban population growth. Vegetation and nature helps cool the countryside, while the abundance of impermeable surfaces in urban environments makes it difficult for heat to escape.  

“This has broad effects,” explained study lead author Cascade Tuholske. “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 researchers found that the number of person-days that city dwellers were exposed to extreme heat and humidity rose by 79 billion between 1983 and 2016. This data was achieved by using infrared satellite imagery, heat, and humidity readings across 13,115 cities.

Using these instruments, the researchers counted days of extreme heat based on “real feel” temperatures on the “wet-bulb globe temperature” that exceeded 106 degrees Fahrenheit.

Across this data, two-thirds were accounted for by urban population growth. The cities that were the most affected were clustered at low altitudes. The Bangladesh capital of Dhaka topped the list at 575 person-days, with other cities such as Shanghai, Bangkok, and Dubai suffering similar rates of extreme heat.

“A lot of these cities show the pattern of how human civilization has evolved over the past 15,000 years,” stated Tuholske, pointing out that many cities are located in warm climates where humidity is delivered by big river systems such as The Nile and 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?”

Many other major cities saw over half of their extreme heat and humidity exposure because of climate change alone, with cities such as Baghdad and Mumbai taking the hardest hit. European cities have seen relatively static population growth, so exposure increases in the continent have been almost entirely caused by global warming. On the other hand, cities in the U.S. – particularly in Texas and across the Gulf Coast – have seen exposure increases due to both population increase and climate change.

Kristina Dahl, a climate researcher at the Union of Concerned Scientists, explained that the study could serve as a starting point for identifying ways to address local heat issues, such as planting trees and other mitigation methods. “This study shows that it will take considerable, conscientious investments to ensure that cities remain liveable in the face of a warming climate,” said Dahl.

The research has been published in the Proceedings of the National Academy of Sciences.

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By Calum Vaughan, Earth.com Staff Writer