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.
If that happened in Europe, for example, where air conditioning is rare and heat acclimatization is low, “you could have mass fatalities or casualties,” said Carter Powis, that study’s lead author and a researcher at the University of Oxford.
Both studies are based on research Vecellio and a team conducted at Pennsylvania State University testing an understanding that, at some level of heat and humidity, the human body can no longer cool itself and its internal temperature rises uncontrollably. The human body sheds heat when sweat evaporates into the air, and also by radiating warmth into the surrounding environment.
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:
- 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.
That doesn’t mean there won’t be summers where dangerous heat could hit such places for much longer stretches, though, Powis said.
September was the planet’s most anomalously hot month ever observed, about 1.7 to 1.8 degrees Celsius hotter than preindustrial levels. If such a departure from the norm occurred in July or August, “you’re looking at substantial risk” of unsurvivable heat, Powis said.
The research underscores how the most severe impacts of climate change will be felt in countries that have done the least to create it, said Fahad Saeed, a Pakistan-based climate scientist with the German think tank Climate Analytics. It shows much of the worst and most lasting heat occurring in densely populated and underdeveloped parts of southern and southeastern Asia and Africa.
And while data has shown massive death tolls from heat waves in Europe, for example, the toll of the impacts in those hardest-hit regions is likely dramatically underestimated because of a lack of research and reporting, Saeed said.
“They are the ones who are bearing the brunt,” he said.