It’s Not Just the Extreme Heat. It’s the Extreme Humidity

This story originally appeared in The Guardian and is part of the Climate Desk collaboration.


Intolerable bouts of extreme humidity and heat that could threaten human survival are on the rise across the world, suggesting that worst-case-scenario warnings about the consequences of global heating are already occurring, a new study has revealed.

Scientists have identified thousands of previously undetected outbreaks of the deadly weather combination in parts of Asia, Africa, Australia, South America, and North America, including several hot spots along the US Gulf Coast.

Humidity is more dangerous than dry heat alone, because it impairs sweating—the body’s lifesaving natural cooling system.

The number of potentially fatal humidity and heat events doubled from 1979 to 2017 and are increasing in both frequency and intensity, according to the study published in Science Advances.

In the US, the southeastern coastal corner from eastern Texas to the Florida Panhandle experienced such extreme conditions dozens of times, with New Orleans and Biloxi, Mississippi, the hardest hit.

The most extreme incidents occurred along the Persian Gulf, where the heat and humidity combination surpassed the theoretical human survivability limit on 14 occasions. Doha, the capital of Qatar, where the 2022 World Cup is scheduled, was among the places to suffer—albeit briefly—these potentially fatal weather events.

The ominous findings come as something of a surprise to scientists, as previous studies had projected such extreme weather events would occur later in the century, mostly in parts of the tropics and subtropics where humidity is already a problem.

“Previous studies projected that this would happen several decades from now, but this shows it’s happening right now,” said lead author Colin Raymond from Columbia University’s Lamont-Doherty Earth Observatory. “The times these events last will increase, and the areas they affect will grow in direct correlation with global warming.”

Earlier studies relied on average heat and humidity recorded over several hours in large areas, whereas the Columbia team analyzed hourly data from 7,877 individual weather stations, allowing them to pinpoint short, localized incidents.

In dry conditions, the body cools itself by sweating, which evaporates away. Humidity impedes evaporation and can even halt it completely in extreme conditions. If the body’s core overheats, organs can quickly begin to fail, leading to death within hours.

Meteorologists measure the heat/humidity effect on the so-called wet-bulb Centigrade scale, known as the heat index, or “real-feel” Fahrenheit readings in the US.

Even the strongest, well-adapted people cannot carry out ordinary outdoor activities like walking or digging once the wet bulb hits 32 °C (89.6 °F), though most would struggle well before that. In theory, humans cannot survive above 35 °C (95 °F) on the wet-bulb scale—the peak suffered in small areas of Saudi Arabia, Qatar, and United Arab Emirates, according to the study.

Slightly less extreme but more frequent outbreaks were detected across India, Bangladesh, Pakistan, northwestern Australia, and coastal regions along the Red Sea and Mexico’s Gulf of California.

“We may be closer to a real tipping point than we think,” said coauthor Radley Horton.

Air-conditioning should help mitigate the impact for some people in rich countries such as the US and Qatar, but longer enforced periods indoors could have devastating economic consequences, according to Horton. Nor is air-conditioning an option for most people in the poorer, high-risk countries where subsistence farming remains common.

Kristina Dahl, a climatologist at the Union of Concerned Scientists in the US, said the new paper shows “how close communities around the world are to the limits.”


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