Climate crisis

Look: Map reveals what cities are at risk of deadly urban heat

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Climate change threatens everyone on Earth, but the effects won’t be felt equally everywhere.

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When it comes to rising temperatures, some of the most popular places to live are also now among the most dangerous places to live on Earth.

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Heard of the urban heat island effect? Dark, hard surfaces like asphalt trap heat, so cities tend to run much hotter than rural areas surrounding them.

A study published in the Proceedings of the National Academy of Sciences shows that the incidence of exposure to potentially deadly heat in cities has tripled since 1983.Adapted from Tuholske et al., PNAS, 2021

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The researchers looked at a value called the wet-bulb temperature, which uses heat and humidity to measure the effectiveness of evaporation as a means to keep people cool.

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Above a wet-bulb temperature of 30 degrees Celsius, evaporation no longer keeps you cool, making it dangerous to be in the heat.

According to the study, the number of person-days spent above a 30-degree wet-bulb temperature grew from 40 billion per year in 1983 to 119 billion per year in 2016.Adapted from Tuholske et al., PNAS, 2021

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The person-day value means the number of people multiplied by the number of days. So a city of 1,000 people would experience 1,000 person-days of exposure in a 24-hour period.

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The massive increase in exposure isn’t only due to rising heat. Some of the hottest cities in the world are also experiencing a population boom, so more people are at risk.

According to the study, two-thirds of the increase is caused by rising urban populations, with just one-third stemming directly from higher temperatures.

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Dhaka, the capital of Bangladesh, had the highest extreme heat exposure of any city in the study. Its population nearly quintupled between 1983 and 2016.

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Increased heat exposure in Shanghai and Guangzhou, China is likewise being driven by population growth in both cities. It’s the same story in the growing cities of Bangkok, Dubai, and Hanoi.

An interactive map released with the study highlights cities with the largest population changes (like Dhaka, seen here), or the largest changes in temperature.

Adapted from Tuholske et al., PNAS, 2021

In places like Baton Rouge, Louisiana, rising heat is driving an uptick in exposure rates, as the population remains relatively static.

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Cities with stable populations like Baghdad, Cairo, and Mumbai have also seen an uptick in heat exposure driven by rising temperatures.

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The trend toward urbanization could be making climate change even more dangerous, the study shows.

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The authors say urban planners can use the data to build cooling into cities — by planting more trees or using less heat-trapping materials — and mitigate the heat island effect.

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Even with smarter heat management, climate change and urbanization are not slowing down. As the 2021 United Nations climate report shows, slowing climate change will take a lot more than one city’s actions.