Climate scientists often warn that rising CO2 levels in the atmosphere will cause an increase in the number and intensity of heat waves in many regions of the world. But a new study is cautioning that climate change will also significantly increase humidity, magnifying the effects of these heat waves and making it more difficult for humans to safely work or be outside.

The study analyzed projected “wet bulb” temperatures — a measure of heat stress that combines the effect of heat and humidity — for the next century. (Wet bulb readings are taken by literally draping a water-saturated cloth over the bulb of a conventional thermometer.) Today in the southeastern United States, wet bulb temperatures of 84 degrees Fahrenheit are rare, but by the 2070s and 2080s, these conditions could happen 25 to 40 days each year, the report found.

The situation would be even worse in northern India, central and western Africa, eastern China, and South America. The study found that by 2080, extreme wet bulb conditions could become 100 to 250 times more frequent in the tropics.

Lab experiments have shown that a wet bulb reading of 90 degrees F is the threshold beyond which people have trouble doing anything outside. The report, published in the journal Environmental Research Letters, found such conditions could occur three to five days a year in parts of the tropics. By late this century, climate change could even occasionally cause wet bulb conditions of 95 degrees Fahrenheit — a level equivalent to nearly 170 degrees of “dry” heat, which would make it difficult to survive without artificial cooling.

The study authors, all from Columbia University, warn that the intense humidity could significantly impact the economy, agriculture, and the military.

“The conditions we’re talking about basically never occur now — people in most places have never experienced them,” the study’s lead author, Ethan Coffel — a graduate student at Columbia University’s Lamont-Doherty Earth Observatory — said in a statement.