Extreme and potentially hazardous heat and humidity conditions, including some surpassing the theoretical human survivability limit, have begun occurring around the world, according to a study published last week in the journal Science Advances.

"Previous studies projected that this would happen several decades from now," said the study's lead author, Colin Raymond, "but this shows it's happening right now. The times these events last will increase, and the areas they affect will grow in direct correlation with global warming."

Raymond, now a postdoctoral researcher at NASA's Jet Propulsion Laboratory, researched the paper as a doctoral student at Columbia University's Lamont-Doherty Earth Observatory. He was quoted in a Columbia news release.

High humidity worsens the effects of heat because it inhibits the body's cooling mechanism—the evaporation of sweat from the skin. Meteorologists use a heat index scale, taking into account both heat and humidity, to estimate the "how it really feels" temperature.

The researchers found that the number of readings approaching or exceeding a heat index of 115°F doubled from 1979 to 2017. Heat index readings of 125°F, previously considered rare, totaled about 1,000. More than a dozen times along the Persian Gulf, heat index readings briefly surpassed the theoretical human survivability limit of 160°F.

The dots show occurrences of heat index readings that reached at least 125°F in the US Southeast from 1979 to 2017. Green dots show 1 occurrence, orange dots 3, and red dots 10. Map by Colin Raymond

Previous studies have indicated that nobody, no matter how fit or well-adapted, can carry out normal outdoor activities when the heat index reaches 132°F. For any heat index readings of 115°F or higher, Raymond said, "It's hard to exaggerate the effects."

Earlier studies had not found such high peaks because researchers typically use averages of heat and humidity measured over large areas and several-hour periods. The new study used hourly data from 7,877 individual weather stations around the world, enabling the researchers to pinpoint short-lived heat/humidity peaks affecting small areas.

"These measurements imply that some areas of Earth are much closer than expected to attaining sustained intolerable heat," said Steven Sherwood, a climatologist at the University of New South Wales Sydney in Australia. He was not involved in the study.

"It was previously believed we had a much larger margin of safety."