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Climate Change Is Making Cold Snaps More Rare, On Average

A new study questions models predicting more Arctic weather finding its way to the continental US.
Image: Central Park/Wiki

Last winter's succession of cold snaps across the United States led some scientists to postulate that periods of brutal cold might actually be a global warming-related phenomenon, and will likely increase in frequency as climate change progresses. The idea, essentially, is that warm water is becoming more and more exposed to Arctic air as ice coverage diminishes, leading to a convection effect as warming air circulates with typically cold polar air, and the pushing of that cold Arctic air toward more southerly temperate latitudes. Makes sense.

Unfortunately (for the theory), according to a paper out today in Nature Climate Change, the overall observational data doesn't quite back this notion up. What it shows is an overall decrease in cold weather extremes across the US and Europe over the past several decades. "Autumn and winter days are becoming warmer on average, and less variable from day-to-day," said study author Dr. James Screen, a mathematician at the University of Exeter, in a statement. "Both factors reduce the chance of extremely cold days."

The critical piece of Screen's stidy is the net change in the temperatures of Arctic winds blowing south, which typically cause cold snaps like last winter's. The wind arriving from the north is on average warmer, and more consistently so. "Cold days tend to occur when the wind is blowing from the north, bringing Arctic air south into the mid-latitudes," Screen said. "Because the Arctic air is warming so rapidly these cold days are now less cold than they were in the past."

As for the models popularized during last winter's brutal deep freezes, Screen's paper offers this: "Previous hypotheses linking Arctic amplification to increased weather extremes invoke dynamical changes in atmospheric circulation, which are hard to detect in present observations, and highly uncertain in the future. In contrast, decreases in subseasonal cold-season temperature variability, in accordance with the mechanism proposed here, are detectable in the observational record and are highly robust in twenty-first-century climate model simulations."

One possible answer might be that changes in atmospheric circulation, of the sort that drive polar vortexes south, occur with a lag. Ice melt hasn't been a steady process through the decades considered in Screen's study, and is in fact accelerating rapidly. If convection effects are the result of increasingly exposed Arctic waters, we might find the cold snap effect bunched up in just the past several years, rather than nicely spread out over the past decades. That is, migratory polar vortexes might be too new to show up in a long-term analysis.

The safer interpretation is just that overall Arctic warming trends wash out new convection effects. They're there, sure, but put against the overall warming trend described by Screen, it's still not enough to nullify the larger changes.