Researchers estimate that if countries are able to cap warming at 1.5 degrees Celsius above pre-industrial levels, the tropics will be spared temperatures that surpass the "survival limit." But life in the world's hottest latitudes could become intolerable if those controls aren't met.
The study focused on a measure called wet-bulb temperature, which accounts for heat and humidity, and is similar to what weather watchers know as the heat index.
"The general idea is, the body doesn't just respond to temperature, it responds to humidity," said Kristina Dahl, a climate scientist who was not involved in the study.
The body cools itself primarily through sweating and the evaporation of sweat from the skin, Dahl explained. At a certain heat-humidity point, she said, it becomes "thermodynamically difficult" for that to happen.
Scientists believe that a wet-bulb temperature of 35 degrees C is the upper limit of human tolerance. It's akin to a heat index of 160 degrees Fahrenheit.
People vary in how much heat they can stand. But at a wet-bulb temperature of 35 degrees C, anyone lingering outdoors would be in trouble.
The body normally maintains a fairly stable internal temperature of 37 degrees C (98.6 degrees F). Skin temperature has to be a little lower, to allow core heat to flow to the skin. If it's not, a person's internal temperature could quickly rise, explained Yi Zhang, the lead researcher on the new study.
"High core temperatures are dangerous or even lethal," said Zhang, a graduate student in atmospheric and oceanic sciences at Princeton University in New Jersey.
For their study, Zhang and her colleagues made projections as to how global warming could affect wet-bulb temperatures in the tropics (between 20 degrees north and south of the equator). That includes the Amazon rain forest, a large share of Africa, the Indian peninsula and parts of Southeast Asia.
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