Related Articles
Last month was the second-hottest September ever recorded; it came after the world’s warmest summer ever, in a year that is on track to be the most searing in recorded history.
There’s only so much the human body can take. Heat killed 60,000 people in Europe alone in 2022, and at least 55,000 people in Russia in 2010. Now, growing research suggests that humans may be more vulnerable to rising temperatures than scientists had previously believed.
“It’s scary as hell,” said Matthew Huber, director of the Institute for a Sustainable Future at Purdue University.
In 2010, Dr. Huber and Steven Sherwood, a climate scientist at the University of New South Wales in Australia, first proposed a limit to how much heat the body could handle.
They knew that humidity impedes evaporation and reduces the body’s ability to cool itself by sweating. And sweat is critical: It’s responsible for up to 80 percent of heat loss from the body. So the researchers turned to a measurement that accounts for this effect, called wet bulb temperature, or Tw.
A wet bulb thermometer is essentially a thermometer wrapped in a damp wad of cotton. As water evaporates it cools the bulb, which makes it a convenient proxy for the way that sweating cools the body.
Reasoning from basic physiological principles, they concluded that a Tw of 35 degrees Celsius would be the limit of human heat tolerance. That figure is more extreme than it sounds, because it accounts for humidity as well as heat. It translates to about 103 degrees Fahrenheit with 75 percent humidity, or 115 Fahrenheit with 50 percent humidity.
These conditions are rare. Because air carries more water vapor as it warms, it is unusual to reach such high levels of humidity at those temperatures. Still, beyond this threshold, the body’s cooling mechanisms begin to fail, and core temperature starts to rise. The onset of heat stroke becomes a matter of time.
Heat is dangerous at far lower wet bulb temperatures, too. But that 35 Celsius level was absolute, Dr. Huber and Dr. Sherwood concluded — the point at which even a healthy young person, in the shade, with unlimited water, would succumb in about six hours.
At the time, sustained heat waves so severe were thought to have never occurred. Dr. Huber and Dr. Sherwood predicted that these conditions could emerge with five to seven degrees Celsius of global warming, compared with preindustrial levels, “calling the habitability of some regions into question.”
Although the chances of that much warming are remote, the conclusion was still alarming. The New Scientist called it Thermogeddon.
In 2018, researchers using updated climate models found that China, without large reductions in emissions, could regularly be exposed to the human limit by 2070. By the end of the century, so, too, might the Persian Gulf, India and Southeast Asia.
All told, nearly two billion people might face temperatures that the body cannot tolerate for long. Mortality from extreme heat could surpass that of all infectious diseases combined, and rival that of cancer and heart disease.
A wet bulb temperature of 35 degrees Celsius quickly became an understood guardrail among scientists, an edge that humanity must not cross.
Then, in 2020, researchers discovered signs that a few regions in the Middle East had already crossed the threshold several times since 2005, albeit briefly. One author of the paper noted that “we may be closer to a real tipping point on this than we think.”
And there’s a bigger problem with that accepted limit. “It’s not real,” said Larry Kenney, a professor of physiology at Penn State.
The wet bulb limit was always theoretical, rather than experimental, he added: “Humans don’t sweat like a thermometer.” In fact, our physical tolerance for heat may be lower.
In 2021, Dr. Kenney recruited 24 volunteers to swallow a capsule thermometer and climb into a heat chamber as his graduate students dialed up the temperature inside. What they found was surprising.
The threshold for maximum heat tolerance actually was about a wet bulb temperature of 31 Celsius. “That’s the limit for all comers,” he said. Above that, “core temperature does this,” he said, tracing a line with his finger toward the ceiling.
I took a spin through the heat chamber myself. Afterward, a graduate student showed me a graph of my temperature, measured by the tiny thermometer I had swallowed that morning in Dr. Kenney’s office.
After a few minutes in the chamber, my temperature rose gently before plateauing. This reflected my body’s adaptive mechanisms kicking in. But then, after about 45 minutes at Tw 31 Celsius, the line on the graph kicked up again and kept rising.
Slowly but surely, I was accumulating more heat than I could get rid of. The line was still rising when the grad students killed the heat and brought me out of the chamber.
“This is a best-case scenario, with young healthy people,” Dan Vecellio, one of Dr. Kenney’s colleagues, said of my results. “We’ve also done this work with older people, and the threshold is lower.”
During the heat wave in Europe in 2022, nearly 60 percent of deaths occurred among people over 80. Since Dr. Kenney published his initial results, researchers at the University of Sydney have issued corroboration: They found that the limit may be as low as a wet bulb temperature of 22 Celsius among older participants.
If the actual limit of human heat tolerance is less than Tw 35 Celsius, then the threat of lethal heat may be greater than believed. Last October, Dr. Kenney’s team collaborated with Dr. Huber to publish a paper mapping the new, lower threshold onto climate projections.
They found that sustained exposure to a wet bulb temperature of 31 Celsius was limited below 2 degrees Celsius of warming. Global surface temperatures have so far increased by about 1.4 Celsius (roughly 2.5 Fahrenheit) since the preindustrial era.
But the pace of warming is expected to accelerate, with the Intergovernmental Panel on Climate Change projecting a rise of three degrees Celsius by 2100 under current climate policies. Above two degrees, lethal heat waves might rapidly emerge in some of the world’s most densely populated regions, including eastern China, India and sub-Saharan Africa, according to Dr. Kenney and his colleagues.
If warming were to exceed four degrees, these conditions would begin to affect Southeast Asia, Australia, the Middle East, South America, and several American cities, including Chicago, Houston and Washington.
At that level of warming, three billion people could be exposed to at least a week of temperatures at the human limit every year, Dr. Kenney said. Some 1.5 billion could experience a month of such conditions, and several hundred million could experience “an entire season” of “life-altering extreme heat.”
“These results indicate that a significant portion of the world’s population will experience — for the first time in human history — prolonged exposures to uncompensable extreme moist heat,” he and his colleagues wrote in the study.
Some uncertainty remains — the human research is still in an early stage, and the amount of warming the planet will experience remains unknown.
“We have to remember that people are already dying in the heat well before we reach these thresholds,” Dr. Vecellio said. Over a thousand people died this year during the hajj in Saudi Arabia, he pointed out.
Dr. Vecellio noted that holding global warming to two degrees Celsius largely avoids the truly catastrophic heat waves predicted by their models. “We obviously want to not even get to two degrees Celsius,” he said. “But two degrees Celsius is, like — that’s it.”
<
