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1.5°C Target Still Means Dramatically More Heat Stress in Global Megacities

Zouzou Wizman/wikimedia commons

Millions of people who live in the world’s megacities will be at greater risk due to extreme heat, even if average global warming is kept at the preferred threshold of 1.5°C, according to a recent study in the Proceedings of the National Academy of Sciences.

Even with that degree of success in national and global climate strategies, “the number of megacities, with populations over 10 million, in the danger zone will double from today’s figure, and the numbers of people exposed to potentially lethal heat stroke and heat exhaustion could reach 350 million by 2050,” Climate News Network reports. The root of the problem “is not just the average planetary increment of heat: it is that averages conceal extremes, and the number of episodes of extreme heat is expected to climb dramatically, even with a modest-sounding increase in a global average.”

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The study looked at 44 megacities that recorded their highest temperatures in 30 years in 2015.

Climate News Net explains that higher heat brings the added risk of humidity, since the atmosphere’s ability to retain moisture increases 7% with every 1°C increase in temperature.

“If relative humidity stays constant—and that’s what we expect with climate change— and temperatures go up, that means the amount of moisture in the atmosphere is increasing non-linearly,” said lead author Tom Matthews of Liverpool John Moores University. “That really affects how easily we cool off, because we lose heat by sweating—and the more moisture in the atmosphere, the less efficient that process becomes.”

Past studies have looked at the devastating health impacts of higher average temperature increases, in the range of 4.0°C, in what CN Net describes as “the notorious business-as-usual scenario in which humans go on burning fossil fuels and depositing ever more carbon dioxide in the atmosphere.” But Matthews warned that “heat stress starts at a much lower temperature, because we are moving around, we are generating heat internally, and we need to dissipate that heat.” The Liverpool team focused its research on how frequently megacity temperatures would exceed 40.6°C, the standard threshold where meteorologists warn of potential hazards.

The result: “Those places that already experience those conditions will experience them more frequently, and the areas that experience this extreme heat will expand,” writes correspondent Tim Radford. “The numbers of people crowded in tropical megacities, often in appalling conditions, will also increase with time.” Matthews cited Lagos, Nigeria as an example of a city that will experience 100 times more heat stress by 2100 than it sees today—even as its population grows 11-fold from its 1995 level.

With that “combination of frequency of extreme heat and swelling numbers in the urban heat traps,” Radford writes, “the risk could be multiplied a thousandfold − and that’s just a temperature rise contained at 1.5°C. A rise of 2.0°C would have even more dramatic consequences.”