Brutal Christmas storm: Is it climate change — or just weather?

Many Americans are facing brutal cold this Christmas Eve. Freezing temperatures are plunging into Florida and Texas. Single-digit lows in the Plains and Great Lakes, and blizzard conditions are wrecking travel plans, while the same “bomb cyclone” is driving coastal flooding in the Northeast. Is climate change playing a role here?

Weird weather and wild extremes have been the calling cards of a changing climate, and it’s almost reflexive to question how much influence the warming world has in this last angry burst of 2022. Recent advances in the science of weather attribution can offer quick insight — and believe it or not, what feels today like extraordinary conditions might turn out to be surprisingly ordinary.

The Climate Shift Index compares temperatures around the world with what we would expect without human-caused warming. It gives the temperature at each location a score that shows whether a day’s temperature was made more likely (indicated by positive numbers) or less likely (indicated by negative numbers) as a result of climate change. The forecast lows for Christmas Eve — freezing across the South, frigid from the Rockies to the Northeast — might seem exceedingly rare in a warming world. But the Climate Shift Index gives these a score of zero. Weather is erratic, and we are increasingly trained to ask whether we’re seeing evidence of climate change in unusual events. But sometimes the answer is no, it’s just weather.

Sometimes, we do see conditions that run counter to what we expect in a warmer world. These are negative scores on our scale. We’ll see a few of these on Christmas Eve in the eastern part of the country. This includes some negative-2 scores in a narrow band from northern Georgia to Delaware on Christmas Eve. Cold like this should occur about half as often in a warmer world.

That doesn’t mean the fingerprints of climate change are undetectable in the Christmas Eve forecast. In fact, they’re pronounced in the Southwest, where relatively mild lows in Nevada score a 3 in the Climate Shift Index — three times more likely thanks to climate change.

If you really want to experience a climate change Christmas, head to Europe. The whole continent from Lisbon to Warsaw will experience temperatures at Climate Shift Index level 2 or higher, with many places in the Alps hitting level 4 or the maximum level 5 (more than 5x more likely due to climate change).

Weather is dynamic and variable, especially in the U.S. and especially in winter. Climate change doesn’t make weather go away. Instead, climate change is a steady pressure that shifts the odds, making warm conditions more likely. This shift is easier to see if you look over months or seasons. In 232 out of 238 U.S. cities, average temperatures in the winter have climbed over the past half-century. In most, especially east of the Rockies, winter is the fastest warming season. Attribution tools like the Climate Shift Index allow us to see the signal of climate change in daily conditions, telling us the conditions we can expect more or less of as the planet warms. And, in some cases, the answer is that even unusual temperatures are sometimes just the ordinary wild and annoying thing we call weather.

Andrew Pershing is the director of Climate Science at Climate Central. He is an expert on how climate trends and events impact ecosystems and people and recently led the Oceans and Marine Resources chapter of the fourth U.S. National Climate Assessment. Follow him on Twitter: @Sci_Officer