Air conditioners will have to change to deal with future humidity, experts say

“It’s not the heat; it’s the humidity.”

That’s what people say in the summer, when stepping outside feels like being enveloped in a thick, puréed soup. When sweat — the body’s natural cooling mechanism — can’t evaporate off the body and pools on the skin.

For humans, the saying is true: It’s humidity that can take simply “hot” temperatures and tip them over the edge into “unsurvivable.”

But it also applies to humanity’s lifeline on a warming planet: air conditioners.

It’s well known that many air conditioners are energy guzzlers that are filled with planet-warming refrigerants. Air conditioning generates about 4% of global greenhouse gas emissions, twice as much as the entire aviation industry. But what many people don’t realize is that most air conditioners put an enormous amount of their energy just toward removing humidity from the air.

Removing that humidity is critical for human health and comfort. But a group of experts is warning the current generation of air conditioners isn’t ready for the hot and humid future.

Modern air conditioners do two things: They cool the air and, in turn, they also reduce humidity. (Cooler air holds less water vapor, so a change in air temperature means that some water vapor will have to be condensed into water.) But most air conditioners today have been designed to do the former, not the latter; they aren’t necessarily equipped or tested for very humid areas.

In humid conditions, humans feel hotter — even at the same temperature. That’s why experts measuring heat stress like to use “wet-bulb temperature,” a measure of heat and humidity combined. Seventy degrees Fahrenheit with 40% humidity might feel relatively comfortable, while 70 degrees and 70% humidity would feel uncomfortably hot.

People tend to set their air conditioners to lower temperatures on humid days to compensate for the sticky discomfort.

That, in turn, uses a ton of energy. “Air conditioners tend to overcool in order to remove humidity from the air,” said Nihar Shah, director of the Global Cooling Efficiency Program at Lawrence Berkeley National Laboratory. “The air conditioners consume more energy than they need to.”