Remember the polar vortex?
The term entered popular awareness during the winter of 2013-14, which brought biting cold to much of the country and saw record low temperatures around the Great Lakes region.
Now climate scientists at Environment and Climate Change Canada who have been studying the phenomenon to see whether something unusual was behind it have an answer: Such an extreme winter is within the range of normal for Canada.
The result has broad implications because it speaks to the apparent slowdown in the rate at which the Earth is heating up. The slowdown, or global warming "hiatus" as it is sometimes called, is a smaller-than-expected increase in global average surface temperature since about the year 2000. Observations suggest the planet is getting warmer, but not as quickly as predicted.
The Canadian study, published Monday in the journal Nature Climate Change, reinforces the idea that natural variability is behind the hiatus, too, since very cold winters in one region tend to drag down the global average. It implies that things could just as easily swing the other way in the future, causing temperatures to rise even faster than climate models predict. Such a reverse swing may be behind the searing heat that people are currently experiencing in the American southwest.
"Our findings don't undermine the influence that humans have been having on the climate," said John Fyfe, a senior research scientist at the Canadian Centre for Climate Modelling and Analysis in Victoria. "But we shouldn't be surprised to find that human-induced climate change is overlaid on top of a level of internal variability that can be quite large in magnitude and persistent."
According to Dr. Fyfe, that persistence helps explain why a cluster of cold winters can crop up in close succession, as occurred with three out of the five winters between 2009 and 2014.
The study challenges a theory, put forward in 2014, that the cold winter of 2013-14 was triggered by an unusual warming in the western tropical Pacific that year.
Using the federal government's own climate-system model, which runs on a dedicated supercomputer, Dr. Fyfe and a collaborator ran 200 detailed simulations of the Earth's atmosphere, looking at the kinds of fluctuations that arise from year to year. They then zeroed in on four of those that most closely reproduced the kind of extreme winter conditions that occurred in North America during 2013-14.
What they discovered is that such conditions can arise naturally without the need for a trigger in the tropical Pacific or anywhere else apart from normal regional fluctuations. Most significantly for Canadians living in the eastern half of the country, the excessively cold winters do not appear to be some sort of cruel byproduct of climate change, which means they are not destined to become more frequent, despite the recent spate of cold snaps.
Not everyone is convinced. Kevin Trenberth, a senior scientist at the U.S. National Center for Atmospheric Research in Boulder, Colo., took issue with the way the Canadian study deals with climate anomalies and noted that excessive precipitation over the tropical Pacific in 2013-14 had "major consequences for the atmosphere," setting up effects that rippled across North America.
Climate-change skeptics have often seized upon such disputes to cast doubt on whether scientists can be certain about the role of humans in escalating global warming. Researchers are adamant that this misrepresents what they are arguing about.
The new study will likely fuel the debate about whether the apparent hiatus in warming is real. Last year, an analysis led by Thomas Karl at the U.S. National Oceanic and Atmospheric Administration suggested that the hiatus simply reflects biases in temperature data. But both Dr. Fyfe and Dr. Trenberth say the hiatus is real. If so, it may only be a temporary and partial brake on global warming, and a warning that temperatures could climb much more steeply in the future.