How climate change is making parts of the world too hot and humid for humans

When it comes to heat, the human body is remarkably resilient — it’s the humidity that makes it harder to cool down. And humidity, driven in part by climate change, is increasing.

A measurement of the combination of heat and humidity is called a “wet-bulb temperature,” which is determined by wrapping a completely wet wick around the bulb of a thermometer. Scientists are using this metric to figure out which regions of the world may become too dangerous for humans.

A term we rarely hear about, the wet-bulb temperature reflects not only heat, but also how much water is in the air. The higher that number is, the harder it is for sweat to evaporate and for bodies to cool down.

[Wet-bulb temperature is important, climate experts say. So what is it?]

At a certain threshold of heat and humidity, “it’s no longer possible to be able to sweat fast enough to prevent overheating,” said Radley Horton a professor at Columbia University’s Lamont-Doherty Earth Observatory.

Scientists have found that Mexico and Central America, the Persian Gulf, India, Pakistan and Southeast Asia are all careening toward this threshold before the end of the century.

“Humid heat risks are grossly underestimated today and will increase dramatically in the future,” Horton said. “As locations around the world experience previously rare or unprecedented extremes with increasing frequency, we run the risk that our previous messaging about extreme heat risk — already woefully inadequate — will fall further short of the mark.”

You might think that being closer to the beach would be a great way to catch that ocean breeze and cool off. But Horton said proximity to water in extreme conditions could make things worse. As warming temperatures cause the water to evaporate, it adds humidity to the air.

“If you’re sitting in a city along the Persian Gulf, the sea breeze could be a deadly breeze,” he said.

To better understand why these places are becoming too hot and humid for humans to endure, you have to first understand how the body cools itself.

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