The world is heating up. How much can our bodies handle?

In the summer of 2023, a dozen people willingly walked into a steel chamber at the University of Ottawa designed to test the limits of human survival. Outfitted with heart rate monitors and temperature probes, they waited in 42 degrees Celsius, or 107 degrees Fahrenheit, while the humidity steadily climbed, coating their bodies in sweat and condensation. After several hours, their internal body temperatures began ratcheting upward, as the heat cooked them from the outside in.

“Few people...