Winter storm Uri this week scattered bitter cold, snow and ice over a vast portion of the United States, including places rarely seen as cold as this.
States like Texas with milder winters were surprised by the cold, causing massive spikes in energy demands and a huge drop in available electricity as natural gas, coal, nuclear, and wind infrastructure froze. Tuesday was the coldest day in North Texas in 72 years. The Dallas-Fort Worth area hit a record temperature of minus 2 degrees Fahrenheit this week.
The cold weather has sparked a wave of power outages across Texas, leaving millions of residents shivering in the dark. In some places, Texans have been without electricity for days.
Officials and utility customers want to know why the Texas power grid wasn’t better prepared. And in order to be one step ahead of the next extreme event like this, it is also important to ask how high the probability is that something like this will happen again.
With average temperatures rising worldwide due to greenhouse gas emissions, there is more heat in the global climate system. This already has some predictable effects, such as an increase in the frequency and intensity of heat waves.
But it can also have some counter-intuitive effects, especially in colder times of the year. How climate change will transform winters, as scientists like to say, is an area of active research.
There are some competing ideas about how increased warming will change the likelihood of extreme cold spells, like the cold weather that is currently sweeping across much of the United States. One group of researchers says that warming makes such events less likely, while another says that warming in the Arctic increases the likelihood that cold polar air will spill further south, leading to more periods of extreme cold in the short term.
“I’d say it’s still pretty controversial and has been for a while,” said Walt Meier, senior scientist at the National Snow and Ice Data Center at the University of Colorado Boulder.
But as Texas shows, failing to prepare for extreme winter weather can be devastating. It is therefore important to find out what scenarios are ahead and how often they will occur.
How warming in the stratosphere over the Arctic resulted in cooling in Texas
Just like heat waves in summer, a prolonged drop in temperature in winter is a pronounced meteorological phenomenon. The American Meteorological Society defines a “cold spell” as “a rapid drop in temperature within 24 hours to temperatures that require significantly increased protection for agriculture, industry, commerce and social activities”.
And as with heat waves, what counts as a cold wave depends on the local climate. The temperature threshold for a cold snap in Alaska is much lower than that for Arkansas.
Several factors must be coordinated to cause a temperature drop like the one around winter storm Uri, and some of them can gain momentum months in advance and from a distance. Karin Gleason, a climate researcher at NOAA’s National Centers for Environmental Information, said the warning signs of the recent common cold in the US surfaced weeks ago. “Of course, for this particular event, we can say that January has some origins,” she said.
Usually, the icy Arctic air stays above the North Pole, surrounded by a fast-moving, narrow band of wind between 5 and 9 miles above the Earth’s surface. These winds, which can reach 250 miles per hour, are known as the jet stream. The beam acts as a barrier between the rotating cold air in the north, known as the polar vortex, and the warmer air in the south.
Meier compared the beam to an edge around a bowl and held the cold, dense air of the polar vortex in place. “Cold air is heavier, so it kind of gets stuck in that bowl,” he said. “It can slosh a bit, but it can’t get over the rim of the bowl.”
But occasionally the Arctic experiences sudden warming of the stratosphere, where the layer in the atmosphere begins to warm between 5 and 30 miles above the ground. This happens about six times per decade as part of a pattern of typical climate variability. Scientists saw this warming set in earlier this year.
Remember, by arctic standards, “warm” air is still cold compared to usual temperatures closer to the planet’s equator. The average temperatures in February in the Arctic are minus 4 degrees Fahrenheit or minus 20 degrees Celsius.
As the arctic air warms up, it becomes less dense and more energetic. “As you warm the Arctic, the cold air is now less cold and closer to the edge of the bowl,” said Meier. “It can break out of the bowl more easily and cause these kinks in the jet stream.”
Another factor that played a role in the recent US cold snap is the Arctic Oscillation. This is a pattern of the variability in the winds that surround the Arctic. The cycle is currently in its negative phase, resulting in a weaker jet with slower air currents. Sometimes the jet stream can even reverse direction from its normal west-east flow. The typical circuit route becomes shaky, with lobes extending further south across the planet.
As this cool air spreads further south, it brings severe winter weather with it. Where exactly the lobes of the jet stream reach down and bring cold air can vary worldwide. It just so happened that the United States was on the path of the current disruption this week.
What does climate change mean for extreme cold events in the future?
It is clear that a complicated set of variables had to be tuned this week to create the severe cold in the US, and such alignments are not very common, making them difficult to study and even more difficult to predict for the future .
This future is being further obscured by climate change. In general, scientists expect winters in the US to warm faster than summers. In the coming years, more heat records will be broken than cold records.
At the same time, the Arctic is warming about twice as fast as the rest of the planet, a phenomenon known as Arctic amplification. Since the region is an integral part of climate patterns, warming at the North Pole could have a significant impact on the rest of the planet.
Do warmer winters mean extreme cold events are less likely, or does a warmer Arctic mean that a wobbly jet stream is more likely to sweep lower latitudes in cool air? And is what we’ve seen so far due to natural variability, or is it part of a larger, human-driven trend?
Jennifer Francis, a senior scientist at Woodwell Climate Research Center, co-authored a 2018 study that found that warming in the Arctic was related to more frequent extreme winter weather in the United States. In a 2020 study, Francis and her co-authors also found that observations and computational models seem to come to different conclusions about the role of Arctic reinforcement in extreme winter weather closer to the equator.
In an email, she said the bitter cold in the US right now is a sign of what is to come. “The large, sustained collapse of the jet stream south in the jet stream that caused this cold invasion is likely to be more common in a warming climate, as are the warmer-than-normal spells that lie alongside that collapse,” said Francis.
Both extreme heat and extreme cold can occur side by side, with the meander-shaped beam acting as a barrier in between. This is what this week’s global temperature anomalies map shows, which shows areas with temperatures below normal (blue) can appear alongside regions with temperatures above normal (red):
While much of the US is unusually cold, the Arctic is unusually warm. Climate Reanalyzer / University of Maine
“That is exactly what is happening now, and these undulating jet streams are expected to be more common as we continue to warm the planet, especially melting the Arctic,” Francis said.
However, other researchers are more skeptical about the connection between climate change and increasing extreme cold events.
James Screen, Associate Professor of Climate Science at the University of Exeter, co-authored a 2020 paper in which he argued that “the impact of the Arctic on mid-latitudes is small compared to other aspects of climate variability”.
“I don’t think we can make a sure link between climate change and increasing cold events,” Screen said in an email. “IMHO the strongest statement we can make is that climate change is reducing the severity of cold weather events.”
That is not to say that there is definitely no link between warming in the Arctic and cold weather extremes closer to the equator. Rather, according to Screen, the findings so far do not show a sufficiently large effect to overcome the warming caused by increased greenhouse gas emissions.
At the same time, continued warming in winter due to climate change and an increase in polar vortex eruptions at lower latitudes are not necessarily in contradiction, as Stefan Rahmstorf, head of Earth system analysis at the Potsdam Institute for Climate Impact Research, emphasized on Twitter. The winters can warm up in the long term, while the polar eddy overflows more frequently in the short term.
Scientists expect to have a better understanding of what to expect in extreme cold conditions as they collect more data. “Maybe we can’t say for sure if there is a connection, but it’s about how quickly this connection becomes clear and how big this connection will be,” said Meier.
Even if the planet warms up, there will still be cold days, even extremely cold days. This year’s winter in the US is likely just a decline in the general upward trend in winter warming. “You will still have a winter here and there that will be an outlier for these other trends,” Gleason said. It’s just that the coldest days get shorter and shorter at some point.