The future looks sweltering. That is if nothing is done about fossil fuel emissions, according to a new study from the National Center for Atmospheric Research and the Lawrence Berkeley National Laboratory.

Published in Climatic Change, researchers Claudia Tebaldi and Michael Wehner found that if human-produced greenhouse gas emissions remain unrestrained, 60% of the planet’s land surface will be subject to annual heat waves, the likes of which are seen today only once every 20 years.

In places like the Middle East, heat waves have already been a problem. In July 2015, The Washington Post reported that the Iranian city Bandar Mahshahr reached a heat index level of 165 degrees F. The temperature was second only to Dhahran, Saudi Arabia’s level on July 8, 2003, which was 178 degrees F.

“It’s the extreme weather that impacts human health; this week could be 2 degrees Celsius (C) hotter than last week, and that doesn’t matter,” said Wehner, of the Lawrence Berkeley National Laboratory, in a statement. “Now, imagine the hottest day that you can remember and instead of 42 degrees C (107.6 degrees F) it’s now 45 degrees C (113 degrees F). That’s going to have a dangerous impact on the poor, the old and the very young, who are typically the ones dying in heat waves.”

In 2004, a heat wave that struck the Chinese megacity Guangzhou left 39 people dead, 27 of them elderly.  

Not only will heat waves be more frequent, but they may also be more intense in the future, the researchers found. Without any greenhouse gas mitigation, a heat wave with a 1-in-20 chance of occurring in 2050 would be 5.4 degrees F hotter for 60 percent of the Earth’s landmass.

The situation only gets increasingly dire in the succeeding years. According to the National Center for Atmospheric Research, by 2075, “the percent of land areas subject to 20-year events that are at least (9 degrees F) hotter swells from 10 to 54 percent.”

But if human-produced greenhouse gases are reduced, the researchers claim the heating effects can be abated. Instead of over half the planet’s landmass being subject to heat waves on the order of 9 degrees F hotter than today’s, only about a quarter will face such extreme weather.

The research was funded by the U.S. Dept. of Energy.