The occurrence of record-cold weather can seem puzzling during an era of global warming. After all, given that the world is getting warmer, how can it also be colder than usual in your backyard?
Cold where you are, but warm elsewhere
Temperature records show that the Earth has warmed a little more than 1 degree Celsius (2 degrees Fahrenheit) since 1880. Yet short-term variations in weather, such as cold snaps — rapid drops in air temperature that result in consecutive days of colder-than-average weather — are still occurring.
To understand how record-cold events can exist in a warmer world, consider an event from winter 2018-2019.
In January 2019, a cold air outbreak swept across portions of the Northern Plains and Midwestern United States. Temperatures plunged below minus-40 degrees Fahrenheit, with wind chills in the neighborhood of 60 degrees below zero. However, despite this bitter cold snap, the nation’s average temperature for the month was nearly three degrees Fahrenheit warmer than normal. January 2019 was also the globe’s third-warmest January on record.
How is that possible? Well, while a few U.S. regions were experiencing record-breaking cold, above-average warmth was occurring in other parts of the country and the rest of the world. For example, January 2019 temperatures across the western United States ranged from three to nine degrees above the normal January average, while in Australia and Asia, temperatures were seven degrees or more above normal.
Such situations exemplify how cold snaps and global warming can and do coexist: Cold extremes are occurring over a smaller fraction of the global surface area than above-average temperatures. In other words, what happens locally, or over short periods of time, is not necessarily representative of what’s happening nationally and globally.
Speaking to the Washington Post, Jason Furtado, assistant professor of meteorology at the University of Oklahoma, explains it using this widely used analogy: “One down day on the Dow Jones doesn’t mean the economy is going to trash. (Likewise) one cold day doesn’t suddenly mean that the general trend in global climate change is suddenly going in the opposite direction.”
Carl Schreck, atmospheric scientist at North Carolina State University’s North Carolina Institute for Climate Studies, agrees. “Cold snaps don’t disprove global warming,” he says, “they just mean that weather and seasons still happen.”
Despite climate change, winter still exists
Another point to keep in mind that the warming climate hasn’t eradicated winter altogether.
As such, it is still possible to experience a range of cold weather conditions, including extremes such as a week of high temperatures in the teens or a brief cold snap in May. And variations in weather patterns caused by naturally occurring phenomena, such as El Niño and La Niña, can influence cold air outbreaks in the U.S.
Although winter persists, global climate change has made winters less harsh overall, say Schreck and other scientists. This phenomenon is evident from wintertime minimum temperature data, as shown in the graph below. During the period between 1910 and the 1980s, the land mass of the United States frequently experienced cold extremes during winter, according to NOAA’s National Centers for Environmental Information. However, since 1990, few parts of the nation — typically no more than 10% of its area — have experienced extremely cold winters, a sign that bitter U.S. winters have become less widespread.
What is a polar vortex and how is it affected by climate change?
A growing body of research suggests that Arctic warming might actually be contributing to periodic cold snaps across the United States, as counterintuitive as that sounds. When unusually warm temperatures are present in the Arctic, the Arctic Oscillation — the back-and-forth movement of a large area of low pressure and cold air between the Arctic and the mid-latitudes — is said to be in a “negative phase.” Scientists are finding that during a negative phase of the Arctic Oscillation, the polar jet stream, which encircles the cold air mass called the polar vortex, becomes more wobbly.
Schreck says this extra wobble has two effects. For one, it allows colder Arctic air masses within the polar vortex to dip farther southward than usual into North America and the Northern Hemisphere. It also makes weather patterns more slow-moving and “sticky,” meaning cold air outbreaks might last longer.
In short, yes, the Earth is warming, but don’t throw away your snow shovel or your winter boots just yet. We’ll still have bouts of cold and ice weather even as global warming continues.