In the summer of 2023, dozens of people were happy to walk into the University of Ottawa steel chamber, designed to test the limits of human survival. Equipped with a heart rate monitor and temperature probe, it waited at a temperature of 42 degrees Celsius, or 107 degrees Fahrenheit, and the humidity steadily rose, coating my body with sweat and condensation. A few hours later, the internal temperature began to ratchet upwards as the heat cooked them from outside.
“There are few people on Earth who have actually experienced temperatures like this,” said Robert Meade, a postdoctoral researcher in epidemiology at the Harvard School of Public Health, who led the study. “Imagine the moisture condenses into your skin like water on a hot day. It’s how hot it was compared to the temperature of your skin.”
In their experiments, participants tested their body’s ability to cope with extreme heat by exposing them to temperatures that would no longer cool themselves. Their studyThis risk threshold, published last week in the Science Journal PNAS, confirmed it was much lower than scientists had previously thought. The so-called wet bulb temperature is 26-31°C, which accounts for heat and humidity.
Scientists call this limitation the point of “uncompensable” heat stress. “When climate change drives heat waves, there was a lot of interest in defining these upper limits.”
When studying the health risks of heat, scientists often refer to wet bulb temperatures. This is because moisture in the air can make heat waves far more deadly by blocking the ability to sweat effectively.
For over 10 years, the maximum wet bulb temperature that the body can handle is 35 c – It is unlikely that global warming will become a common occurrence until it reaches an astounding 7 C over pre-industrial temperatures.
It was not until 2022 that a group of researchers tested this limit with human subjects. 26 C. This threshold means that a vast area of ​​the planet can be at risk of living with 2 c global warming. As early as 2045 If greenhouse gas emissions do not decrease.
“In warm climates, these thresholds are expected to begin to cross more frequently,” said Tony Wolf, an assistant professor at the University of Georgia, who studied heat stress and co-authored the 2022 study. “The heat waves are large and last a long time.”
Other studies like Wolf have tested this low heat tolerance for several hours. But Wolf said the latest research is the first thing to do so over nine hours, and it’s close to what a person actually experiences during a heat wave. Researchers estimated that heat dissipation would occur after 10 hours, but only a few participants were unable to complete the nine hours completely while exposed to temperatures with “uncompensable” heat restrictions. At slightly lower temperatures, participants were on track to experience heat strokes within 35 hours.
“It’s very rare that a high light bulb temperature is wet for more than a day,” Mead said. “But when you consider what it is for someone who is actually exposed to these temperatures, that limit shows what point the central temperature of this crazy train is and rises.”
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A variety of factors can increase the likelihood of increasing heat stress even at low temperatures. Working outdoors, having existing health conditions and inability to access air conditioning can result in a moderate heat wave. And Mead’s study tested the study of Wolf, a young and healthy adult. the study It was found that older people experience heat stress at low temperatures.
“The circulation system for older people is not that good at dispersing heat,” said Radley Horton, a professor at the Columbia School of Climate. “When the temperature starts to really get extreme, your body has to start making some difficult choices,” he said.
In February, Horton published a Learn in nature With 2c warming discovered, more than a third of the land on Earth could be too hot for people over 60 years old. This is a presumed risk zone five times more than a younger adult. The study found that regions with particularly hot and humid climates, such as the Middle East, West Africa and Southeast Asia, are hit hardest. Cities such as Karachi in Pakistan can bake at high temperatures for 20% of seniors a year.
A study from Pennsylvania State University predicts that global warming will be below 2 degrees Eliminate the risk of uncompensable heat. However, over the past year, global temperatures have skyrocketed beyond scientists’ predictions, marking 2024 as the first year to violate 1.5c warming.
The rise in heat has already taken a serious cost. Fever deaths in the US between 1999 and 2023 It’s more than doubledrising from about 1,000 deaths a year to over 2,000. Over the same time, nearly 15 million people have died in the heat around the world. In 2023 alone Over 47,000 Europeans The Mediterranean countries (20% faster and warmer than the rest of the globe) have been the most intense.
“People are already dying from the heat wave,” Wolf said. “So, what are these thresholds for now that have begun to increase the risk of fever-related illnesses and death, regardless of what happens to the future climate?”
