Winters get warmer with climate change. So what explains Texas’ cold snap in 2021?

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It was the coldest February Texas had seen in more than four decades, and the sustained blast of arctic air knocked out much of the state’s power grid for several days, causing hundreds of deaths and billions of dollars in damage.

Yet 2021 also brought the planet’s 16th-warmest February since records began. On average, winters are getting more mild because climate change has increased temperatures worldwide. How could a warmer world bring such a severe cold snap to Texas?

Scientists say they are still working to understand the relationship between climate change and extreme winter weather patterns. Many factors can influence localized cold snaps, and evidence suggests that climate change is affecting longstanding climate patterns in new ways.

“The way those kinds of events occur involve a lot more complicated atmospheric processes,” said Jennifer Francis, a senior scientist and acting deputy director for Woodwell Climate Research Center in Falmouth, Massachusetts.

Francis and other scientists said there’s a significant body of research that can help explain why Texas — and other areas of the U.S. — may still experience extreme cold from time to time amid an overall warming of the planet.

“Maybe there was some nuance that was missed when people started talking about winters disappearing and how we’re never going to see snow again,” said Judah Cohen, a leading scholar on winter weather and climate change and the director of seasonal forecasting at the climate analytics company Atmospheric and Environmental Research. “People say, ‘I was told one thing and I’m experiencing something else.’”

Several factors impact the frequency and severity of cold spells in Texas, from the strength of the polar vortex — a seasonal, swirling mass of cold air that circles high above the Arctic — to whether we’re in an El Niño or a La Niña year, which influences whether Texas has a wet or dry winter, to the natural patterns that influence the position and strength of the jet stream, which can determine the path and duration of weather systems.

Here’s what factors scientists say can cause an extreme cold snap to hit Texas — and how such storms may be influenced by climate change.

Is Texas likely to see another major freeze this winter?

Scientists say it’s unlikely that another cold spell like the one that swept across the state in February will happen this winter. The National Oceanic and Atmospheric Administration forecasts a warmer-than-normal winter for Texas due to continued dry La Niña conditions — although last winter was also during a La Niña year.

In addition to La Niña, the stratospheric polar vortex, a swirling mass of cold air that circles 10-30 miles above the arctic, is currently very strong, trapping cold air up north.

More sea ice than during recent years has also formed in the Bering Sea near Alaska, which Francis said can keep the jet stream from dipping down toward Texas. The jet stream forms where temperature differences cause strong winds, swooping storms along its path.

“We’ve got these other differences that are happening that, all combined, probably create a very different winter this year from the winter last year,” Francis said. “I think it’s unlikely that we’re going to see a big cold air outbreak like we saw last year.”

How the polar vortex causes cold spells

The stratospheric polar vortex, high above in the Earth’s stratosphere, forms in the fall, when the northern hemisphere gets dark and cold.

The February storm happened after the polar vortex broke down, sending very cold air much farther south than normal.

Just as ice skaters spin faster and tighter by pulling in their arms, a strong polar vortex keeps cold air spinning fast and tight in the northern part of the planet.

“That keeps a lot of the Lower 48 at least fairly mild,” said Jason Furtado, an associate professor in the School of Meteorology at the University of Oklahoma. “It’s locking up the cold air at the high latitudes because it’s encased in this circulation.”

But when the polar vortex is weak, it’s like an ice skater losing their balance: It begins to wobble, and that can push arctic air onto the jet stream, or what’s called the tropospheric polar vortex, a mass of fast-moving winds that flow 5-9 miles above the Earth’s surface in a wavy path around the Northern Hemisphere.

That’s precisely what happened in February: The stratospheric polar vortex weakened and divided in January, contributing to the jet stream’s big swing south. A few weeks later, it brought a big mass of cold air down low to the Central and South United States.

Climate change and the Arctic 

Several factors influence whether the polar vortex is strong or weak. For example, the winds’ strength can be affected by a pattern called arctic oscillation.

“When the arctic oscillation is positive, it strengthens the jet stream, which can reduce the likelihood of getting cooler air outbreaks,” said Deepti Singh, a climate change researcher and an assistant professor at Washington State University Vancouver. “When it’s negative, the jet stream is weaker and more wavy, and that allows the arctic air to flow to the lower latitudes.”

The arctic oscillation was in a strongly negative phase during the February winter storm.

Emerging science has suggested that climate change may play a role, too.

Cohen, the director of seasonal forecasting at Atmospheric and Environmental Research, published a study in the journal Science in September that found the polar vortex is breaking down more frequently in recent decades than it did in the past, forming patterns that have the potential to bring those big blasts of cold air deeper into the U.S.

Climate change has also hit the Arctic harder than anywhere else: Since 2000, Arctic temperatures have risen about twice as fast as global temperatures. Global warming is causing more of the Arctic’s sea ice to melt than in the past, leaving open water that absorbs more of the sun’s heat, accelerating warming in the Arctic.

Whether a warmer Arctic is directly related to a weakened polar vortex is still an active area of research. Cohen’s research suggests that one “hot spot” of sea ice melt in the Barents-Kara Sea, northwest of Russia, could be to blame: The heating there is so far north that the added heat can cause the jet stream to bulge north and interfere with the polar vortex.

“My argument is that arctic change [from global warming] is forcing the jet stream in a configuration that, at least partially, favors a weaker polar vortex,” said Cohen, who is also a visiting scientist at MIT.

Francis’ research has suggested that when the Arctic is abnormally warm and has seen significant ice loss, the jet stream tends to weaken. Certain extreme weather events — droughts, heat waves, cold spells and heavy precipitation — are often associated with “persistent” weather patterns brought by the jet stream’s weakened state, Francis wrote in a 2012 study published in Geophysical Research Letters.

But other scientists have argued that the disruptions in the polar vortex could simply be a natural variation. And the emerging science on how a melting Arctic could bring severe winter weather to lower latitudes has yet to be clearly replicated in climate models, which would help provide definitive links by isolating and eliminating other potential causes.

“There’s so many pieces to the puzzle, and there’s still a lot of research going on to understand how they’re all connected,” said Francis, a leading researcher on how the dramatic warming of the Arctic may be linked to extreme weather patterns in the U.S. and Europe.

Still, she added, “The Arctic is warming unbelievably fast, and it’s a tremendous change in a really important part of the climate system.”

Disclosure: Washington State University has been a financial supporter of The Texas Tribune, a nonprofit, nonpartisan news organization that is funded in part by donations from members, foundations and corporate sponsors. Financial supporters play no role in the Tribune’s journalism. Find a complete list of them here.