If heat and humidity cross a certain extreme threshold—a “wet bulb” temperature of 35 degrees Celsius—the human body can’t survive long outside. It’s a scenario that some researchers had predicted becoming common later in the century, when climate change may make some regions essentially unlivable. But a new study suggests that dangerous, previously unprecedented levels of heat and humidity are already beginning to occur.
“At certain points, for short periods of time, these dangerous heat-humidity thresholds are already happening,” says Radley Horton, a research scientist at Columbia University’s Lamont-Doherty Earth Observatory and one of the authors of the new paper, published in Science Advances. “They’re not just a risk for later this century.”
Extreme heat is already deadly. In the U.S., it kills more people than any other weather-related event. These deaths mostly affect the elderly, but when simultaneous extreme heat and extreme humidity cross the “survivability limit,” even someone who’s young and healthy won’t be able to live long if they can’t retreat into air-conditioning. “If you took the healthiest person and put them in the shade with an endless supply of water and either no clothing or clothing where it was easy to sweat, there is actually a threshold for heat and humidity where you can’t sweat fast enough to avoid overheating,” Horton says.
Researchers started studying the combination of heat and humidity over the past several years, realizing that looking at temperature alone “isn’t telling the whole story in terms of how people and critical systems are affected by heat,” he says. But until now, scientists expected impacts at a later date.
Past studies didn’t show that critical thresholds are already starting to be crossed in some locations. In the past, other studies looked at data over larger areas, over longer periods of time; the new study looked at hourly data from thousands of weather stations and discovered thousands of instances where the combined heat and humidity temporarily soared to dangerous levels. In the southeastern United States, extreme conditions occurred in cities such as New Orleans and Biloxi, Mississippi. India, Bangladesh, and Pakistan saw repeated waves of extreme heat and humidity. In the Persian Gulf, a handful of heatwaves went past the survivability limit.
The most extreme cases lasted just hours, which is why there haven’t yet been reports of mass deaths. The literature suggests that a healthy person might be able to survive for six hours outside in the worst conditions. Most people living in the Persian Gulf also have access to air-conditioning; more research is needed on the details, including whether some deaths may not have been reported (of migrant workers, for example). But what’s clear is that the situation is likely to get worse.
Since 1979, the number of instances of extreme heat and humidity has more than doubled. As climate change progresses, the frequency will increase—although limiting emissions will help dramatically. A study last year found that in the U.S., more than 200 cities would face at least a week of “off the charts” extreme heat by the middle of the century, but even with slow climate action, that would drop to seven cities. (That study predicted days that would surpass a heat index of 127 degrees Fahrenheit, a related measure of heat and humidity; a wet bulb temperature of 35 degrees Celsius is equivalent to a heat index of roughly 160 degrees Fahrenheit, but the heat index tops out at 127.)
Those who have to work outdoors, such as farmers and construction workers, will be hardest hit. In regions where air-conditioning is widespread, infrastructure will be strained. In places like India, for example, where air-conditioning is still fairly rare, the number of air conditioners may grow from 30 million to 1 billion by the middle of the century, using vast amounts of electricity. If that power isn’t all renewable, it will add to the problem, ultimately making heat waves even worse.
“This is a classic example of a sort of subtle, hidden threat that could mean for more and more regions, having air-conditioning or being sure that the power won’t go out is truly a life or death issue,” Horton says. “So it’s going to put more strain on electric networks for electricity at precisely those times when you can’t afford to have the power go out.”