Why This Summer Is So Hot—And Why the Future Will Be So Much Hotter

A massive heat wave stretching from sea to shining sea is hitting the U.S. this week, promising temperatures well above 100°F in large swaths of the country. Meteorologists say heat waves like this one hit the northern hemisphere every year—even if they do not typically cover such a broad area—but in the coming decades Americans should expect stronger and more frequent hot spells.

This particular heat wave is caused by hot air traveling up from west to east, bringing warm temperatures throughout the week. A high-pressure system—known colloquially as a “heat dome”—will hold the heat on the Earth’s surface. (Meteorologists disagree on whether the colloquial description really fits the phenomenon). Temperatures will likely hit 110°F in the Phoenix and 105°F in parts of the Midwest, according to National Weather Service projections. They could hit 100°F in some East Coast cities over the coming weekend, but it remains too early to know for sure.

“There’s a very large bubble that’s working its way across the nation,” said Sean Sublette, a meteorologist at Climate Central. “The atmosphere travels in waves—waves that are up and waves that are down. Up waves are allowing a lot of hot air to come up through the equator.”

The coming warm spell is just a taste of future summers when heat waves will be stronger and more frequent. Recent research has shown that average summer temperatures post-2050 will regularly top today’s records, unless there are efforts to curb greenhouse gas emissions. “Extremely hot summers always pose a challenge to society,” said Flavio Lehner, a researcher at National Center for Atmospheric Research, following the release of a study on summer heat. “Such summers are a true test of our adaptability to rising temperatures.”

Read More: These Fuel Economy Regulations Mean You May Soon Be Buying an Electric Car

In the coming decades, humans should expect an uptick in heat waves due to global warming, though the change will be more subtle than later this century, Sublette said.

Climate change “is going to make heat waves more common and stronger,” he said. “It’s going to be slow in terms of the human lifespan, but you are going to have heat waves ratcheting up more rapidly.”

The heat wave contributes to the likelihood that this July will be the hottest on record. Scientist already anticipate that 2016 will be the hottest year on record—due to climate change and a strong occurrence of El Niño.