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Hey, look, it’s the polar vortex!
It’s easy to say whenever there’s extreme winter weather. In the age of social media, any small talk about a cold snap quickly devolves into “the polar vortex is back” and “this is climate change somehow, right?” I’m guilty of it myself. Mostly because I suck at small talk, but still.
Winter Storm Fern hit at the end of January 2026, stretching from Northern Mexico through the United States and Canada. The storm brought over a foot of snow to much of the country, caused 80 mph winds, a low temperature of -43°F in Minnesota, five tornadoes, and 153 known fatalities. Most importantly, it meant I had to go home on the evening of the 24th and be apart from my partner on our one-year anniversary on the 25th, though we did have the interesting experience of going to the spa in single-digit temperatures on the 24th.
Fun fact: if you get your hair wet in an outdoor hot tub, it will turn into ice. I know that now.
I often encounter a public sentiment suggesting scientists think they know everything and refuse to accept that they might be wrong. The truth couldn’t be more different. Scientists are some of the most curious people I’ve ever met, and are far more interested in sharing what they don’t know than what they do know. If they knew everything, they wouldn’t have a job. And it is precisely their scientific method of asking questions, conducting experiments, gathering data, having peers analyze it, having other scientists replicate the experiment, and very gradually and systematically building knowledge that makes science so remarkably trustworthy. Only after scientists all over the world work together to rule out every possible alternate explanation for a phenomenon will they make a definitive statement, such as “human carbon emissions are the primary driver of climate change” or “climate change poses a serious risk to human well-being.”
When people point to a storm like Fern and say “wow, we could really use some global warming right now,” we can easily refute that statement. Global warming does not eliminate cold winter weather altogether (which is part of the reason why climate change is a more befitting term in the first place). But the opposite mistake is just as tempting: grabbing the catchiest science-y phrase we’ve got — polar vortex — and slapping it on a headline or social media post as shorthand for “climate change made this happen.”
Extreme cold events like Fern are exactly the kind of event people use to decide whether they trust climate science at all. Quick, sensational reactions that lack scientific clarity can do more harm than good. While it’s not wrong to bring up the polar vortex, it’s often the least understood part of the winter climate story. The polar vortex is way up in the stratosphere, and the more straightforward climate links with a storm like Fern are the ones we can measure far closer to the Earth’s surface — a warmer atmosphere and ocean providing more moisture for heavy snow, and a jet stream disruption caused by marine heatwaves that steered bitter cold air south. The polar vortex likely played a role too, but it entails far more uncertainty and complexity than these other links. Climate communicators ought to lead with the clearest and simplest climate connections first, not overcomplicate from the get go.
To be clear, even though climate change can shape a storm, climate change doesn’t create storms out of thin air. It can make extreme weather more likely or more intense. I’ve had the pleasure of co-facilitating trainings on climate attribution science alongside scientists at Climate Central, and will be leading another such session at the Society of Environmental Journalists conference in April in Chicago (shameless plug!). On Climate Central’s website, you can look on any day in any city to see how much more likely climate change made that day’s temperature. This is done through sophisticated peer-reviewed computer models, where scientists model the real world and a pretend world without human carbon emissions, and can compare the range of likely temperatures in each scenario. So while we can’t say “climate change caused a hot day,” we can say “climate change made today’s temperature five times more likely,” for example.
For a winter storm, there’s some more moving parts. I was under the false impression until these past few weeks that it was all about the polar vortex (which I could not have given a definition for) — that changes in the vortex were caused by Arctic warming and driving extreme winter cold snaps. While parts of that were correct, it was far from the whole story. On Wednesday, through my role at Metcalf Institute, I helped host a virtual panel with three leading scientists discussing climate change’s impact on winter storms like Fern (check back here for a recording to be posted soon). The bad news is that the polar vortex is way more complicated than I realized. The good news is that you don’t actually need the polar vortex to explain Fern.
Climate change and snow
The clearest climate connection as Fern is concerned is actually not temperature, but rather precipitation. The basic idea is simple: warmer air can hold more water vapor. When the atmosphere is warmer, it has a bigger “capacity” for moisture, which means storms have more water to work with, leading to heavy precipitation. That doesn’t mean climate change leads to more winter snowfall — in fact, quite the opposite — but under very specific conditions, it can be the difference between an extreme cold snap with no snow and a slightly warmer cold snap with lots of snow.
Though it is technically never too cold to snow, snow is likeliest when temperatures are near freezing. Too cold and the air can’t hold as much water vapor. Too warm and the water will come down as rain. So as the climate warms, “more precipitation” can translate into different outcomes depending on where the storm sits on the thermometer. If it’s a shift from, say, 10°F to 15°F, extra moisture can fall as heavier snowfall. If it’s a shift from 35°F to 40°F, precipitation that would have been snow might flip to rain. Given that the latter scenario is far more common around the world, climate change does lead to less snowfall on the aggregate. However, there are plenty of extreme cold cases where more snow is the result.
How does this apply to Fern? A study from ClimaMeter released shortly after Fern compared storms like Fern (similar pressure systems and meteorological conditions) from the present (1988–2025) to storms like Fern in the past (1950–1987). It’s essentially a way of holding the storm’s overall setup roughly constant and seeing how the background climate changes the outcomes. Their study found that Fern-like storms in the modern era are associated with higher precipitation than Fern-like storms in the earlier era — up to about a 20% increase. It is due to this shift that the authors conclude human caused climate change likely influenced the storm’s very rare intensity. Other sources note that marine heatwaves in the northern Atlantic and Pacific Oceans contributed extra moisture to Fern, which would certainly back up the results of the ClimaMeter research. In the days leading up to and during Fern, sea-surface temperatures across much of the Caribbean (and along the entire route Christopher Columbus took from Spain to the New World) were so unusually warm that Climate Central’s Ocean Climate Shift Index found they were made over 100 times more likely by human-caused climate change, with some pockets over 500 times more likely.
In other words, scientists can draw a link from human climate change to increased sea-surface temperatures on those days, and a link from that warmer water and extreme precipitation during Fern. That’s perhaps the easiest one.
What about the temperatures? On average, climate change does lead to warmer winters, so that’s no help. In fact, the western United States and much of the rest of the world was experiencing unusually warm temperatures while Fern hit a comparatively small subset of the world population. However, there are mechanisms by which climate change could worsen cold snaps like Fern, even if it goes against the average trend.
Marine heatwaves and the jet stream
Let’s start with the polar jet stream — NOT to be confused with the polar vortex. The jet stream is a fast narrow river of air blowing from west to east about 5 to 9 miles above us (the same altitude where jets fly), encircling the Arctic and separating cold Arctic air from warmer midlatitude air. It forms due to a few phenomena, beginning with air pressure. In the Arctic, the air is colder, which means the molecules are moving more slowly and can pack closer together. That makes the air denser. In the midlatitudes, the air is warmer, which means the molecules are moving faster and spread farther apart, making the air less dense.
Pressure at any height is basically the weight of the air above you. When the air is dense, each “slice” of the atmosphere contains more mass, so as you climb upward you’re shedding the weight of heavier slices, and the pressure drops more quickly with height. When the air is less dense, each slice contains less mass, so you’re shedding lighter slices as you go up, and pressure drops more gradually with height. And that means several miles up where the jet stream lives, the warm midlatitude atmosphere tends to have higher pressure at that height than the cold Arctic atmosphere.
Where there is a difference in pressure, there will be wind. Wind blows from the high pressure area to the low pressure area. In this case, the wind — living several miles above the Earth’s surface — would be blowing from the midlatitudes in the south to the Arctic in the north. And it would be that simple except for one thing: the Earth’s rotation. As any dedicated Sweaty Penguin listeners may remember, my hottest take ever is that Earth is a sphere, and that sphere rotates from west to east. So if I’m a gust of wind above Texas, one day or one rotation of the Earth means I will have to go a really long distance. But if I’m a gust of wind above Ontario, that rotation is a much shorter distance since it’s closer to the top of the sphere. If I’m a gust of wind above the North Pole, the rotation won’t cause me to move at all. What all that means is wind and people and all the other stuff are actually moving through space faster in the midlatitudes than stuff in the Arctic up north.
So as a gust of wind in the midlatitudes, I’m going to be trying to go north, into that Arctic low pressure. But because I, down at the midlatitudes, am moving through space faster than the Arctic air, I’m going to end up overshooting. My momentum will swing me east. And when all over the world, midlatitude air keeps trying to blow toward the Arctic but keeps getting nudged sideways, the wind ends up racing mostly from west to east along the boundary between cold and warm air. That high-altitude, west-to-east river is the polar jet stream.
And that’s where the “narrow” part comes from. The jet stream isn’t a uniform band of wind across half the continent. It’s concentrated where the pressure difference changes fastest over a short distance — where the transition from cold Arctic air to warmer midlatitude air is sharpest. That sharp transition zone is like a steep hill in pressure terms: air wants to roll downhill quickly, and this deflection — called the Coriolis effect — keeps pushing that downhill roll into fast sideways motion. The steeper the gradient, the faster the jet.
The jet stream is not a perfect circle, though. It wiggles and meanders constantly because the Earth is not a smooth, uniform planet. Mountains deflect flow, oceans and continents heat differently, snow cover changes the surface temperature, and tropical patterns like El Niño shift the entire circulation. Even without any human influence, we’d expect some winters to have dramatic dips and bulges in the jet stream. Scientists refer to a dip northward as a “ridge” and a dip southward as a “trough.” When a trough appears over the United States, that’s when cold Arctic air dips into our region and causes a cold snap.
So what caused a trough during Fern? As mentioned earlier, we do know that there was a marine heatwave in the northern Pacific. Climate Central’s index found sea-surface temperatures across parts of the Bering Sea were so warm that they were made over 20x more likely due to human climate change. Encountering this weather system, the jet stream would have been diverted north, creating a ridge to get around this region. As it continued east, it would then have created a trough to balance out, dipping into most of the United States. So while we can’t say “climate change caused Fern,” we can say that a marine heatwave in the Pacific caused a disruption in the jet stream that caused Fern, and climate change made that marine heatwave significantly more likely. Not as catchy, but a much cleaner cut explanation.
Arctic warming and ice loss
Climate change plausibly influenced Fern in some other ways as well, and this is where the links get a little harder to make. Here’s the part we do know: the Arctic is the fastest warming region on Earth, warming about four times faster than the global average. The primary cause of this rapid warming is called the albedo effect. Think about when you wear a black shirt on a hot summer day. Black absorbs sunlight, so you end up feeling even hotter than you otherwise would. But if you wear a white shirt, since white reflects sunlight, you can remain a little cooler. The same logic applies to the Arctic. An Arctic covered in sea ice is going to be very light and reflective, leading sunlight to bounce off of it. But due to climate change, sea ice is warming, and as ice warms, it gets darker. This causes it to absorb more of the sunlight it was once reflecting, causing an even faster warming trend — which then drives more ice melt, and the cycle continues.
Of course, the Arctic is and remains significantly colder than anything to the south. This difference intensifies in the winter, when the Arctic receives little to no direct sunlight for months on end. Because the Arctic is warming faster than the midlatitudes (the band of the Northern Hemisphere encompassing the United States, the southern half of Europe, and much of central Asia), the temperature difference between the two regions is shrinking. Because the polar jet stream is driven by this contrast in temperature, scientists believe that climate change is weakening the jet stream, and if it gets weaker, it must get wavier. And if it gets wavier, it must be easier for Arctic air to slosh south and cause extreme cold snaps.
It’s a challenging hypothesis to test. We have to collect data, analyze, and have peers replicate the experiment. We have to find out if there actually has been a measurable change in extreme cold snaps caused by waves in the jet stream. And if there are, we have to rule out every single other explanation in order to accept this one. So while it’s very plausible that this mechanism made an event like Fern more likely, it’s a lot harder to put a number to it. We understand the physics and should communicate that, but it might be better served later in Fern’s climate story.
The polar vortex
The polar vortex is in a similar situation. Like the jet stream, the polar vortex is a wind pattern circling the Arctic fueled by low pressure and the Coriolis effect. However, the polar vortex sits at a far higher altitude, about 10 to 30 miles above the Earth’s surface, putting it in the stratosphere above most weather systems. The polar vortex also forms further north than the jet stream, only appears in the winter, and is a much tighter circle than the jet stream, less influenced by mountains and surface-level disruptions.
The reason it shows up only in winter is the same reason the jet stream strengthens in the winter: winter is when the high latitudes lose their sunlight. When the Arctic goes dark for months, the air above the pole cools dramatically, and it cools more than the air farther south. That creates a sharper north–south contrast up in the stratosphere, and that contrast sets up a pressure difference at those heights. And where there’s a pressure difference, there’s wind, and when we add the Coriolis effect, that wind organizes itself into another west-to-east stream.
Most of the time, the polar vortex sits up there, swirls around, keeps all that cold air to itself, and doesn’t affect our weather very much. In fact, when the polar vortex is very strong, we tend to have milder winters. The vortex should be doing its job as a lid, keeping the coldest air pooled up high and far north.
There are a few ways the vortex can get disrupted. First, the vortex can “stretch” and then “split,” actually breaking into two separate swirls. When that happens, those swirls can migrate southward, and those southward-moving swirls emphasize the dips in the jet stream that are already there, making them more intense and persistent. A second kind of disruption is a sudden stratospheric warming event, when the stratosphere over the pole becomes much, much warmer than normal. These occur every few years, and when they happen, we can see more extreme winter weather.
And the third, which may be the most plausible to link to Fern, is where the polar vortex changes shape. It goes from round to more of an elongated oval. And climate change could play a role in that. While the Arctic as a whole is the fastest warming region on Earth, just north of western Russia, in the Barents–Kara Sea region, lies the fastest warming region of the Arctic, sometimes warming up to seven times faster than the global average. This isn’t just a one-off marine heatwave, but a permanent change causing a persistent ridge in the jet stream. This new persistent wavy pattern in the atmosphere can create “wave energy” that gets transferred upward toward the polar vortex. And if conditions are right, that upward wave energy gets reflected by the polar vortex to the exact opposite side of the world, that being North America. And when that happens, it can trickle back down to the jet stream here, sending cold air south.
That’s a lot of steps and variables, but even if all of that played out during Fern, the polar vortex was not the first domino. It’s certainly not the thing to start with if you’re trying to explain Fern’s climate link quickly. And that’s exactly why “polar vortex” as a buzzword is risky. It’s the winter climate change connection we all think of first, but it’s the hardest to demonstrate and hardest to explain. Honestly, I did my best, but I’m not 100% confident I got it right. And worse yet, I’ve seen many cases where headlines simply refer to the jet stream as the polar vortex, missing that they are related but completely different systems. I get it, it’s a catchy name, but if climate communicators refer to things by the wrong name, it causes confusion at best and distrust at worst. Besides, jet stream is a pretty cool name too.
Too few news stories mentioned climate change at all during or after Winter Storm Fern, and that’s priority one. But from my perception, the ones that did seemed to consistently lead with the polar vortex as they often do, while the far simpler marine heatwave connection gets buried or left out. In my experience, people find the scientific process fascinating, and when the process is clear, it’s remarkably persuasive. The more we ground our explanations in the simplest, most demonstrable pieces — warmer oceans, more moisture, and the jet stream waviness that steers the cold — the less we have to try to explain the polar vortex right out of the gate, and the more trust climate communicators earn when the next storm hits.