So, If Global Warming Is Real, Why Is It So Cold?

By Dr. Don Wuebbles

Any time we get a cold-weather event, people start asking this question. That is particularly true in an event like we just went through with extreme cold and lots of snow in late November and Early December 2025 in the Midwest and Northeast.

What people don’t seem to realize, although it did receive some admittedly minor discussion in the media, is that we just experienced a very unusual event that has driven this weather in our part of the world, a Sudden Stratospheric Warming.

To have a Sudden Stratospheric Warming this early in the winter is extremely unusual,and it is affecting the jet stream and the transport of cold polar air into North America. Below, through a series of questions, we examine what is happening.

People often confuse the weather and climate. What is the difference?

Weather is the state of the atmosphere at a given time; for example, today is about 25°F in Illinois and cloudy. If it is an unusually cold day in your location in early December, it has nothing to do at all with whether the climate in your area is changing.

Climate is the averages and statistics of weather over long time periods, typically 20-30 years. Mark Twain is attributed with saying, “Climate is what you expect, weather is what you get.” So, when we discuss climate change, we are referring to long-term trends and changes in the temperature, precipitation, severe storms, and other parameters we generally associate with weather. Climate change refers to changes happening over one or more decades.

What is a Sudden Stratospheric Warming, and why does it matter?

First of all, it is important to recognize that there is a stratospheric polar vortex that is different but can interact with the tropospheric polar vortex.

The stratospheric polar vortex is a low-pressure weather system that sits primarily in the lower stratosphere above the Arctic. The main feature of the stratospheric polar vortex is the strong west-to-east winds that usually encircle the North Pole, especially during the dark Arctic winter.

A Sudden Stratospheric Warming (SSW) is a phenomenon where temperatures in the stratosphere, roughly 10 to 30 miles above the Earth's surface, increase rapidly by tens of degrees Celsius in just a few days.

These events typically happen once a year, usually in January or February, so it is extremely unusual for one to occur in November (we had signs of a disruption beginning to occur in the stratosphere in September, but the actual SSW did not occur until mid-November).

An SSW is a significant disruption of the stratospheric polar vortex that begins with large-scale atmospheric waves (called Rossby waves) getting pushed higher into the atmosphere. These waves can “break” (like waves in the ocean) on top of the polar vortex and weaken it. If waves are strong enough, the winds of the polar vortex can weaken so much that they can reverse from being westerly to easterly. This leads to cold air descending and warming rapidly in the lower stratosphere. In turn, this change in the stratospheric polar vortex can then affect the tropospheric polar vortex.

What is the polar tropospheric vortex, and how does it affect us?

The polar vortex in the troposphere is broader in extent and occurs year-round (often referred to as the jet stream), while the polar vortex in the lower stratosphere occurs from fall until spring.

The tropospheric polar vortex is a normal atmospheric feature that affects our weather every winter. Cold-air outbreaks are fundamentally tropospheric events (see discussion about the waviness of the jet stream below), but can also be greatly affected by changes in the stratospheric vortex.

While the tropospheric and stratospheric polar vortices are clearly distinct, they can interact. Both vortices can be influenced by a Sudden Stratospheric Warming, like the one that led to the most recent extreme cold event in the Midwest. The changes occurring in the stratosphere during an SSW event disrupt the tropospheric polar vortex, causing it to weaken, wobble, or even split. The disruption can allow cold Arctic air to move south into the mid-latitudes, leading to colder weather on the ground several weeks later.

How does global warming affect the polar jet stream?

The polar jet stream is a year-round feature of midlatitude weather; it is a river of high winds in the upper troposphere that circulates around the globe, separating Arctic air from midlatitude air. Its strength depends on the difference in temperature between the cold air over the Arctic and the milder air at midlatitudes.

With the Arctic warming at twice the rate of the rest of the world, the temperature difference is declining between the Arctic and midlatitudes, which leads to a weaker jet stream. The jet stream generally used to have a small S-shaped oscillation around the planet, but now we are getting a much wavier pattern than before.

This can lead to situations where warm air can penetrate much further northward than previously, and where cold air can penetrate much further southward, like this past January. This can also lead slower moving weather systems. The combination of increased waviness in the jet stream and slower-moving fronts can worsen severe weather at a given location. An SSW throws another wrench into these effects.

So, why is this cold event happening now?

Scientists don’t know if the current unusual SSW-driven cold event affects in the United States is just a natural event or if it is related in some way to the changing climate. We do know this is an event that may affect our weather for a month or possibly a few months. But this event does not affect long-term projections in our climate.

The bottom line is that even as our climate warms overall, extreme cold weather events can and will still happen.

Photo credit: Laura Adai via Unsplash