Has Climate Change Made Extreme Cold Even Colder?

Later this week, a major winter storm will be sweeping across the U.S. Midwest and East, bringing significant snowfall, bitterly cold temperatures, and very strong winds to hundreds of millions of people. Temperatures are forecast to bottom out in the -20s °F in the Dakotas. Even the Gulf Coast may see temperatures in the teens.

Combining the cold temperatures with the strong winds, apparent (or “feels like”) temperatures will be below zero °F for wide swaths of the country.

In years past, climate change skeptics might have foolishly charged in brandishing such forecasts as proof that the world wasn't warming. This, of course, was cherry-picking data over a particular place and time that was not representative of broader trends.

These days, most people know better. But much of the media is still making a major mistake. When events like this take place today, a major portion of the media, supported by a handful of scientists, frame the events as happening because of climate change rather than in spite of it (for example, here, here, here). Such reporting is often paired with a claim about how climate change makes all extremes—even cold temperatures—more extreme.

This is a simple and attractive idea for many climate reporters. The problem is that it goes against common sense and the scientific consensus represented by the Intergovernmental Panel on Climate Change’s 6th Assessment Report (IPCC AR6), both of which tell us that extreme winter weather should become less extreme in a warming world.

The scientific evidence can be broken down into three categories: Observations, experimentation (done with physics-based computer models in climate science), and fundamental theory.

What do long-term observations show about extreme cold?

There are a few ways to define and investigate trends in extreme cold. One of the most prominent is to look at how the single coldest temperature of every calendar year has been changing over time. Below is the IPCC’s map of the historical trend in the annual coldest temperature at any given location. It shows almost universal warming.

Looking specifically at the eastern half of the United States (and looking at a few different measures of cold), it is clear that extreme cold has been getting less extreme, less frequent, and that cold spells have been getting shorter.

Given such evidence, it is not surprising that the IPCC uses its most confident language (“virtually certain” which indicates 99-100% probability) when it discusses the warming of cold extremes:

In summary, it is virtually certain that there has been … a decrease in the number of cold days and nights on the global scale since 1950. Both the coldest extremes and hottest extremes display increasing temperatures. It is very likely that these changes have also occurred at the regional scale in Europe, Australasia, Asia, and North America.…Annual minimum temperatures on land have increased about three times more than global surface temperature since the 1960s, with particularly strong warming in the Arctic (high confidence).

What more does climate modeling add to the extreme cold data?

Unfortunately for climate scientists, we don't have duplicate earths to run experiments on. So in order to get estimates of what temperatures would have been like without human-caused climate change, researchers have to run physics-based climate model experiments, with and without increased greenhouse gas concentrations.

These show that the observed reduction in the frequency and intensity of cold extremes is indeed due to increased greenhouse gas concentrations. They also project continued warming of extreme cold and thus decreases in the frequency and intensity of cold spells under all projections of future greenhouse gas emissions.

The IPCC summarizes this again with its strongest language:

In summary, …it is virtually certain that further … decreases in the intensity and frequency of cold extremes, will occur throughout the 21st century and around the world. …The highest increase of temperature of coldest days is projected in Arctic regions, at about three times the rate of global warming (high confidence).

Fundamental theory shows why extreme cold should become less likely

Those of us living in the mid-Northern Hemisphere experience our coldest temperatures during the winter when air from the arctic moves south over us. One of the main reasons that we are so confident that extreme cold should decrease (get warmer) under global warming is that the air over the arctic (the source of cold air) is warming much faster than the air over the rest of the planet. This is a phenomenon called Arctic amplification of warming. That means that when this cold air moves to the south, it is just not nearly as cold as it used to be.

Some scientists have argued that the Arctic amplification phenomena might affect the jet stream— a river of fast-moving air in the upper atmosphere—in a way that would increase southward plunges of cold air out of the Arctic and into the mid-latitudes. The papers that make these arguments are reviewed in IPCC AR6 Box 10.1.

The idea goes like this. As you move north in the Northern Hemisphere, temperatures get colder, but they don't usually get colder at a constant rate. Rather, there tends to be a sharp contrast between cold Arctic air to the north and mild air to the south. This sharp contrast lies underneath the jet stream. This idea will be familiar to people who regularly watch weather reports that use graphics like this:

It is not a coincidence that the jet stream occurs over the boundary between cold and warm air. The sharp contrast in temperature at the earth's surface creates large air pressure differences miles up in the atmosphere, and it is pressure differences that create winds. So the larger the temperature difference at the ground, the faster the winds in the jet stream.

Arctic amplification implies that the cold arctic air is warming more than the air further to the south, reducing the average contrast in temperature. This reduced temperature contrast should, in turn, reduce the wind speeds in the jet stream.

From there, the hypothesis is that a weaker jet stream might undulate more and become wavier (larger wave amplitude) and more meandering. This change in the character of the jet stream—which is sometimes referred to as the polar vortex—is shown in the graphic below.

Since it is the meandering that brings cold air south, increases in meandering could theoretically increase extreme cold at some mid-latitudes. A related hypothesis is that the length of cold spells could also increase because a wavier jet brings more persistent weather.

However, there are problems with several aspects of these hypotheses. First, while some simple calculations suggest that a reduced temperature contrast could lead to increased weather persistence, this is not supported by more complex modeling. Second, there is no universally accepted mechanism that connects the reduced temperature contrast to a wavier jet. There are proposed mechanisms, of course, but each is contested. Third, while some papers have claimed that there has been an observed increase in jet stream waviness, others have pointed out that this trend disappears when ‘waviness’ is defined in different ways. Some studies even show that Arctic amplification of warming could actually reduce jet stream waviness.

At the very least, we can say that any climate change-induced changes in jet stream waviness are small relative to natural variability in waviness. This is a problem for those making the argument that increased waviness could actually increase cold extremes. That’s because the increase in waviness would have to be very large in order to overcome two countervailing factors. First, the entire jet stream moves north as it gets warmer, so all undulations in the jet will begin from a more northern latitude, making it more difficult for southward excursions of cold air to affect any given latitude. Second, because of Arctic amplification itself, the cold air being imported south will be much warmer than it used to be. The evidence simply does not support the idea that increases in waviness could overwhelm these factors. So the IPCC states:

Thus, global warming dominates changes in temperature extremes at the regional scale and it is very unlikely that dynamic responses to greenhouse-gas induced warming would alter the direction of these changes.

Fundamentally, winter storms that are associated with extreme cold (like the one occurring this week) derive their power from temperature contrasts. Thus, whatever else any model says, the main expectation should be that extreme winter storms should decrease as temperature contrasts wane around the world. Indeed, this is exactly what physics-based climate modeling suggests.

So, what’s the truth about extreme cold?

Common sense, historical observations, and physics-based climate modeling all tell us that extreme winter weather, especially extreme cold, should get less extreme as the climate warms. Despite this, the idea that climate change is making cold weather even colder is just too enticing for many media outlets to resist.

Rather than having to say, “this cold happened in spite of climate change,” they get to say, “this cold happened because of climate change.” When outlets publish stories like this, they should be aware that they are jettisoning consensus climate science in the name of perpetuating an attention-grabbing narrative. It is difficult to see how that’s not climate misinformation.