Some say the world will end in fire, others say ice — but it’s the humidity that’ll do us in, according to a new study.

As the earth warms, temperatures and seas aren’t all that will rise. Very humid days will occur at about double the rate of high-temperature days alone, according to researchers at Columbia University’s Earth Institute in a study published Friday. That, they think, will make parts of the Earth essentially uninhabitable for at least part of the year.

It’s often been suggested (or, at least, hoped) that no matter how much the climate warms, that humans might be able to adapt. But there’s a hard upper limit on that: If the “wet-bulb temperature” — measured by wrapping the bulb of a thermometer in a wet cloth and taking the temperature of the air — exceeds 95 degrees Fahrenheit (or, 35ºC), humans are pretty much toast. It might seem like an odd way to measure the upper limit of human survivability, but that’s the simplest way to measure how much a human body could theoretically cool itself, assuming a perfectly healthy body.

“You rapidly approach a situation where it’s thermodynamically impossible to keep your body cool,” Radley Horton, an associate research professor at Columbia University’s Earth Institute and the Lamont-Doherty Earth Observatory, a co-author of the study, told VICE News.

As humidity increases, so does the wet-bulb temperature. Because we never hit a wet-bulb temperature of 95 degrees Fahrenheit in today’s climate, it’s hard to say what the societal effects would be. But wet-bulb temperatures between 84 degrees and 88 degrees Fahrenheit (29–31ºC) have been responsible for tens of thousands of deaths around the world. A wet-bulb temperature of 86 degrees Fahrenheit (30ºC) was recorded during a heat wave in 2015 in the southeastern coastal Indian state of Andhra Pradesh that killed at least 2,500 people.

The risks are starkest for places that already see high heat and humidity, like the Persian Gulf and the Tropics. And some of the regions most at risk of these spikes are the most densely populated — places like Northeast India, East China, West Africa, and the Southeastern U.S. Many of those places don’t currently have good access to medical care, and the population flight from these regions could make it even harder to help the most vulnerable.

“This hazard will emerge within the next generation, but it really peaks in two generations, around the 2070s,” Horton said.

This no doubt will be catastrophic, and have social and economic effects that are hard to fathom. Access to air conditioning — which, paradoxically, is currently largely powered by fossil fuels — will become a life-and-death issue, and it’ll happen quickly.