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.