Heat stress from a combination of high temperatures and humidity will affect an estimated 1.2 billion people by 2100 if greenhouse gas emissions aren’t slashed, a new study reports.

That’s over four times more people than today, the authors explain, and over 12 times as many as would be affected without industrial-era global warming. The study is the first to incorporate humidity into its analysis, which makes heatwaves harder to bear as high humidity prevents the evaporation (and cooling effect) of sweat. Heat stress is dangerous to human health as well as agriculture, the environment, and economies at large, the team adds.

Too hot for comfort

“When we look at the risks of a warmer planet, we need to pay particular attention to combined extremes of heat and humidity, which are especially dangerous to human health,” said senior author Robert E. Kopp, director of the Rutgers Institute of Earth, Ocean, and Atmospheric Sciences.

“Every bit of global warming makes hot, humid days more frequent and intense. In New York City, for example, the hottest, most humid day in a typical year already occurs about 11 times more frequently than it would have in the 19th century,” said lead author Dawei Li, a former Rutgers postdoctoral associate now at the University of Massachusetts.

Heat stress accumulates when the body cannot properly cool itself down during hot conditions. Higher than normal internal temperatures can cause heat rashes, cramps, exhaustion, or even damage the brain and other vital organs, being potentially fatal.

In order to prevent this from happening, our bodies sweat. But, if the ambient humidity is high enough (as happens in a rainforest, for example), sweat stops evaporating, so it stops cooling you down.

The study looked at how combined extreme heat and humidity would evolve in the future on a warming Earth through a series of 40 climate simulations. The authors focused on a measure of heat stress that accounts for temperature, humidity and other environmental factors, including wind speed, the angle of incoming light, and the overall level of solar and infrared radiation.

They report that under a 1.5 degree Celsius (2.7 degrees Fahrenheit) heating scenario, exposure to extreme heat and humidity in excess of safety guidelines will affect areas that are currently housing around 500 million people. Under a 2 degree Celsius warming scenario, around 800 million people would be exposed to such conditions. Finally, a 3 degrees Celsius (5.4 degrees Fahrenheit) warming scenario – the one we’re headed to currently — would put a whopping 1.2 billion people at risk from heat stress.

A resident of New York City would experience the worst heat and humidity seen in a typical year today for 4 days, 8 days, or 24 days a year under these different warming scenarios, the team explains.

The paper “Escalating global exposure to compound heat-humidity extremes with warming” has been published in the journal Environmental Research Letters.