Large parts of the United States and Canada are in the grip of frigid winter weather, with a surge of polar air pushing subzero temperatures and dumping snow down over much of North America.

The US National Weather Service has warned of "life-threatening" low temperatures in several states for much of this week, with "significant snow and ice" expected across wide swaths of the country. 

For the northern states of Montana, North Dakota and Minnesota, on the border with Canada, meteorologists like Ben Noll of The Washington Post have forecast temperatures below minus 40 degrees Fahrenheit (minus 40 degrees Celsius), with wind chill making it feel even colder.

It's not the first bout of extreme cold to hit the US this winter. In January, freezing temperatures and storms spread heavy snow, sleet and freezing rain across southern states from Texas down into Florida, areas unaccustomed to such winter weather.

Just one month earlier, the world marked the end of the warmest year on record, according to the World Meteorological Organization, the EU's Copernicus Climate Change Service and other researchers. Climate scientists have linked this global heating trend to the buildup of greenhouse gas emissions in the Earth's atmosphere — gases created by transportation, agriculture, heating and other human activities. 

So how can these warming temperatures result in extreme cold?

Understanding a polar vortex

The impact of climate change is unpredictable. Some parts of the world have been warming more quickly than others — the northern Arctic region up to four times faster than the rest of the world, according to a 2022 report in Nature.

This extreme warming, known as Arctic amplification, has been linked by some researchers to the weakening of the two polar vortices, the jet stream at lower altitudes and the stratospheric polar vortex, higher up.

These whirling bands of wind circle the planet west to east, created by the difference in pressure between the cold northern air and warmer air further south. Usually, these strong winds help keep the bone-numbing temperatures centered over the North Pole.

But in a 2022 report, the UK-based academic initiative World Weather Attribution said the weakening of these vortices could be "linked with extreme winter weather across Eurasia and North America." Scientists in the organization use real-world data and climate models to explore the connection between climate change and events such as heat waves, floods and storms.

Though research in this field is still relatively recent, as it depends in part on satellite data, some scientists think the rapid warming of the Arctic could be contributing to the more unpredictable, meandering winds — and as a result, a more disorganized polar vortex leading to sudden cold snaps fueled by icy northern air pushed toward the equator.

"As the Earth warms, the temperature difference between the Arctic and mid-latitudes is getting smaller. This makes the polar jet stream slower and weaker," wrote Wanying Kang, an assistant professor in atmospheric sciences, on the Massachusetts Institute of Technology's Climate Portal.

"That slower jet stream has less eastward momentum and is more likely to bend north and south as it encounters small variations in temperature and pressure. If it bends far enough, the barrier between Arctic and mid-latitude air can plunge as far south as Mexico, bringing Arctic temperatures with it."

That meandering jet stream was illustrated by the US National Oceanic and Atmospheric Administration during the January cold snap, which forced the US presidential inauguration indoors for the first time since 1985.

Climate change can bring intense cold, more snow

Judah Cohen, a winter storm expert at the private US-based research firm Atmospheric and Environmental Research, partially attributed this latest intense blast of winter to a "stretched polar vortex" event, which has dipped down into North America. A 2021 study published in Science, in which Cohen took part, found that "climate change in general, but Arctic change in particular, is favorable for forcing these events."

Writing on his weekly blog on Monday, he said this 10th polar vortex disruption of the winter could bring "severe winter weather" to North America and East Asia, and "record cold temperatures" to the eastern US.

Ryan Maue, a meteorologist and former chief scientist at the US scientific agency National Oceanic and Atmospheric Administration, said on X late Monday that this latest polar vortex event was likely to be the "most intense of the winter."

WWA has cautioned, however, that despite "some evidence of a weakening jet stream and [stratospheric polar vortex], it is not yet conclusive that this is outside the realm of natural climate variations."

WWA researchers have also noted that a warmer climate could, paradoxically, contribute to heavier snowfall in parts of North America, North and East Asia and Greenland. Warmer air, due in part to climate change, can hold more moisture — and if it gets cold enough, that moisture can fall as snow.

"In these places, snowfall is occurring over a shorter period of the year and less often, but sometimes with greater intensity," they wrote.

The unsteady polar vortex means the latest cold snap will be severe, but likely won't last long. As Noll of The Washington Post noted on Sunday, warmer temperatures are expected across the US as soon as next week.

Edited by: Tamsin Walker