Still not 100% sure

Another study indicating that the warming Arctic region is helping cause freak cold winter spells in North America, due to the polar vortex breaking down and causing cold air to spread south:

In their study, published this week in Science¹, Cohen and his colleagues compared 40 years of satellite observations of atmospheric conditions over the Arctic with experiments based on computational climate models. The models probed how a decline in Arctic sea ice and snow cover would affect airstreams in the region. Since ice and snow reflect a large fraction of incoming sunlight back into space, whereas the darker ocean and land surface absorb more radiation, this decline is known to drive Arctic warming.

The researchers found that episodes of polar-vortex stretching have markedly increased in the past few decades, and that their models reproduced this behaviour well when they included the effects of Arctic warming.

But some climate scientists are still not sure:

...the idea that Arctic warming might be responsible for cold spells in mid-latitude regions is still hotly debated among climate scientists. At first glance, it might seem obvious that winters will generally tend to get milder in a warming world. But climate models that are commonly used to study complex links between the different components of the climate system diverge on the issue of how strongly Arctic warming might influence mid-latitude winters, and state-of-the-art models do not accurately replicate observed trends in the behaviour of the polar vortex. It remains to be seen whether the models are missing something, or whether the observations of polar-vortex stretching merely reflect natural climate variability, says Daniela Matei, a climate modeller at the Max Planck Institute of Meteorology in Hamburg, Germany, who was not involved in the study.

I don't know:  seems a bit of an odd co-incidence for it to be "natural climate variability" at a time of clear and rapid Arctic warming...