Exposure to dangerous levels of heat and humidity in cities across the world has exploded over the past three decades, according to new research. In a study of more than 13,000 cities, the number of people exposed to extremely hot and humid days in a given year (measured in “person-days”) tripled between 1983 and 2016.

It’s a symptom of two trends colliding: urban population growth and rising global average temperatures. Cities often reach higher temperatures than surrounding rural areas because they’re typically designed in a way that traps heat. So when people flock to urban centers, they’re also flocking to places where they could be at greater risk of heat-related illness and death. To make matters worse, extreme heat is already a leading weather-related killer, and climate change is exacerbating the problem.

Cities will have to find ways to stay cool in a warming world in order to better protect their residents, the authors argue. “In some cities, long-term adaptation is frankly bleak. But for much of the planet, we can use tools we already have,” says Cascade Tuholske, a postdoctoral researcher at Columbia University and lead author of the study published today in the journal Proceedings of the National Academy of Sciences. “Population growth and urbanization to me is not the problem. It’s a lack of planning.”

Roughly a quarter of the world’s population lives in places where exposure to extreme heat and humidity is growing, Tuholske and his colleagues found. For the study, they defined “extreme” as at least 30 degrees centigrade on the wet-bulb globe temperature scale. The wet-bulb globe scale is a detailed assessment of heat, humidity, wind speed, cloud cover, and the angle of the sun. Thirty degrees on that scale is comparable to a day that feels like 106 degrees Fahrenheit. (This isn’t a direct conversion from Celsius to Fahrenheit because factors outside of temperature are taken into consideration.)

They relied on both infrared satellite imagery and on-the-ground readings to determine weather conditions over roughly the last three decades. Using population data gathered by the European Commission and Columbia’s Center for International Earth Science Information Network, the researchers multiplied the number of extremely hot days by the number of people in each city to get a total of “person-days” in which city-dwellers experienced those extreme conditions. The number of person-days grew from 40 billion per year in 1983 to 119 billion in 2016 — showing how many more people are now suffering through extremely hot weather in the world’s cities.

Rising temperatures are responsible for about a third of the global spike in exposure, the study authors sussed out. The majority of the growth in exposure has to do with more people moving to urban areas. But that ratio can vary from city to city. The boom in population growth was the main driver of heat exposure in Delhi, India. But in Kolkata, climate change was a slightly bigger factor. The differences show that solutions likely need to be tailored to fit each city. Since cities continue to attract new residents and global average temperatures continue to rise, they’ll need to act fast.

Cities can reach temperatures several degrees hotter than surrounding areas because of a process called the Urban Heat Island Effect. Asphalt and other dark surfaces absorb heat. Exhaust from factories and tailpipes release more heat. And there are fewer trees to give shade or plants to provide the cooling effect of evapotranspiration (a process similar to how humans cool down by sweating). These effects tend to be worse in neighborhoods that have received less investment and more industrial activity over time, a phenomenon that other research has shown to take disproportionately heavy tolls on communities of color in the United States.

Despite the growing risk, heat-related deaths are largely preventable. To cool down, cities can paint rooftops and other surfaces white to reflect heat. Bringing more trees and greenery to neighborhoods also helps. There’s also more that can be done to warn people of coming heatwaves so that they can find public cooling centers or other spots to ride out the heat with air conditioning. New York City and Los Angeles are just a couple of the major metropolitan areas around the world working to do just that.

“For billions of people on Earth, our study is retrospective. This is their lived experience every day,” Tuholske says. “We really need to learn from them and work with people in really hot cities to adapt.”

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