Why is U.S. in deep freeze while much of the world is extra toasty?

Much of the United States is shivering through brutal cold as most of the rest of the world is feeling unusually warm weather. However strange it sounds, that contradiction fits snugly into explanations of what climate change is doing to Earth, scientists said.

In a map of global temperatures, the last several days, big chunks of the world – the Arctic, Asia, parts of Africa, the Middle East and South America – show as dark red, signifying more than a dozen degrees Fahrenheit (7 degrees Celsius) warmer than the late 20th-century average. But the United States stands out like a cold thumb – a deep bluish-purple that is just as out of whack but on the frigid side.

The way the cold is invading is through a weather phrase that is becoming increasingly familiar to Americans: the polar vortex. It's a weather term that goes back to 1853 but has only been frequently used in the past decade or so.

That could be because the icy stabs are happening more often, said winter weather expert Judah Cohen of Atmospheric Environmental Research, a commercial firm outside of Boston.

The polar vortex is strong, icy weather that usually stays over the top of the planet, penned in by strong winds that whip around it, Cohen said.

"What we found is when the polar vortex stretches like a rubber band, severe extreme winter weather is much more likely in the United States. That's where it tends to be focused and in January we have an extreme case of that stretching of the polar vortex."

This one is stronger and may last longer than most, Cohen said.

Cohen and others have done studies that show the polar vortex outbreaks have become more frequent in recent decades.

The idea is the jet stream – the upper air circulation that drives weather – is wavier in amplified global warming, said University of Wisconsin-Madison climate scientist Steve Vavrus. And those wave changes in the upper air knock the polar vortex out of its place and toward the United States, Cohen said.

It's a theory still debated by climate scientists but growing in acceptance. Initially, Vavrus and Jennifer Francis, a Woodwell Research Center climate scientist, theorized it was due to melting Arctic sea ice leading to barometric pressure changes. Now several scientists say it's more complicated, yet still connected to climate change and the supercharged warming in the Arctic, with other factors like Siberian snow cover and other atmospheric waves also playing a role.

"The key takeaway for me right now is that Arctic Amplification is happening and has complex interactions within our climate system. Winter will always bring us cold weather, but like the warm season it may be changing ways that we understand and ways that we are still learning about," said University of Georgia meteorology professor Marshall Shepherd. 

Think of what's happening as an orchestra making one symphony, and "what's driving all those orchestra instruments is a warming planet," Northern Illinois University meteorology professor Victor Gensini said.

Despite the U.S. cold, Earth's global average temperature keeps flirting with daily, weekly and monthly records, as it has for more than seven months. That's because the United States is only 2 percent of Earth's surface, scientists said.

(Cover: Central Park after New York City's biggest snowfall in two years, New York City, U.S., January 16, 2024. /CFP)