New study: Upper bound for wet bulb temps lower than 35 C

Prior to the experiment, each participant swallowed a tiny radio telemetry device encased in a capsule that would then measure their core temperature throughout the experiment.

Then, the participant entered a specialized environmental chamber that had adjustable temperature and humidity levels. While the participant performed light physical activity like light cycling or walking slowly on a treadmill, the chamber either gradually increased in temperature or in humidity until the participant reached a point at which their body could no longer maintain its core temperature.

Heat waves can already be deadly for the most vulnerable people — but in a warming world, temperatures and humidity will, for growing stretches of every year, surpass a threshold that even young and healthy people could struggle to survive, according to new research published Monday.

Lahore, Pakistan, already an epicenter of human ills linked to climate change, could surpass that survivability threshold for two or three weeks out of the year by the middle of the century, for example, the study in the Proceedings of the National Academy of Sciences found. Under the most dramatic global warming scenarios, it could last for months.

In the Red Sea port of Al Hudaydah, Yemen, such oppressive conditions are expected to last a month or two — or, at the highest levels of global warming projections, would endure for most of the year, scientists found.

The research is the latest to build upon the idea that there is a limit to how much heat and humidity the human body can withstand, that it is likely lower than once thought, and that exposure to it will increase dramatically in the coming decades. The hottest parts of the planet have already surpassed it for brief periods, at least.

That doesn’t mean those places are already “unlivable” for humans, said Daniel Vecellio, the study’s lead author. But they could soon be, if their changing climates mean long stretches without respite from intense heat and humidity, he said.

“It’s when you see these accumulations of weeks or months of this at a time that things become ‘too hot for humans,’” said Vecellio, a postdoctoral researcher at George Mason University’s Virginia Climate Center.

Research has already found increasing likelihood of heat waves that could overwhelm the body’s ability to cool itself. A similar study published in September found that some 200 weather stations around the world have already at times surpassed the threshold. In places like Europe and North America, where people aren’t acclimated to intense heat, temperatures and humidity could surpass the survivability threshold a couple of times a decade even for the most conservative warming estimates.

Past research found that transfer of heat could no longer occur at 35 degrees Celsius (95 Fahrenheit) on what is known as the wet bulb temperature scale, which factors in both temperature and humidity. (Unlike the heat index, which also factors in both heat and humidity, the wet bulb temperature is not designed to be interpreted as a measure of how hot it feels outside.)

But, in a study published last year, the Penn State researchers found that threshold to be closer to a wet bulb temperature of 31 degrees Celsius (88 Fahrenheit) for a sample of young and healthy research subjects who were not accustomed to such muggy conditions. Hours of exposure to such heat could trigger illness or death if people are unable to seek shade or air conditioning, drink water or otherwise find a way to cool themselves down, the researchers found.

In their latest study, those researchers explored where and how often that heat threshold might be surpassed in future climate scenarios, in which average planetary temperatures rise above preindustrial levels by anywhere from 1.5 degrees Celsius (2.7 Fahrenheit) to 4 degrees Celsius (7.2 Fahrenheit). They project hundreds — if not thousands — of hours a year with such extreme conditions across parts of Asia, the Middle East and Africa.

Lahore, for example, would surpass the heat and humidity threshold for 69.5 hours a year under the most conservative warming scenario — nearly nine days if the heat lasted eight hours a day, or more than two weeks if it lasted four hours a day. At 4 degrees Celsius of global warming, the wet bulb temperature would remain above the threshold for more than 1,000 hours a year.

In Al Hudaydah, where the study found the most enduring heat and humidity, conditions could remain at an unsurvivable extreme for more than 2,400 hours a year at the highest levels of global warming — equivalent to 100 straight days.

But if warming was limited to 1.5 degrees Celsius above preindustrial levels, a target global leaders agreed upon in Paris in 2015, the study projects the Yemeni city would experience less than 100 hours of such heat and humidity.

Vecellio said the data shows the planet can save countless lives by taking efforts to limit global warming.

“We can stave off some of the worst impacts,” he said.

Other places the study estimates would endure days or weeks above the heat threshold [at 4 C]:

• Delhi, with 39 hours at 2 degrees of warming and 556.9 hours at 4 degrees of warming.
• Hanoi, with 37.7 hours at 2 degrees of warming and 602.1 hours at 4 degrees.
• Dammam, Saudi Arabia, with 223.6 hours at 2 degrees of warming and 804.7 hours at 4 degrees.
• Dubai, with 117.7 hours at 2 degrees of warming and 783.9 hours at 4 degrees.
• Bandar Abbas, Iran, with 175.5 hours at 2 degrees of warming and 958.6 at 4 degrees.

The study does not project such extreme heat and humidity in Europe, and only relatively brief stretches in North America, where widespread air conditioning also reduces the impact of heat. About a day’s worth of it could occur each year, on average, in New York City and Chicago under the most aggressive global warming scenarios, the researchers found.