With another winter storm barreling across the United States, a lot of people are asking what it all means. Does this have anything to do with climate change?
The broadest answer is that climate is the backdrop for all weather, so climate change must, by definition, play some role. But the day-to-day, weather related details of that role are still sketchy. It's also still difficult to point to a very broad thing like global climate and tie it directly to a single weather event -- or the frequency and intensity of the storms that have hit the eastern United States this winter.
"This is a speculative and genuinely controversial area of the science," said climate scientist Michael Mann of Penn State University. "There are some leading climate scientists who have provided evidence that climate change may be leading toward more persistent weather anomalies which can, for example, give the sort of extended periods of cold seen in the eastern and central U.S. this year, but at the expense in this case of a very warm western U.S. and unprecedented winter warmth in Alaska, and record warmth in many parts of Europe."
Perhaps one of the most under-reported weather facts this winter is that almost every place except the central and eastern United States has been abnormally warm this winter.
"California has had a drought and Alaska in January was 15 degrees F above normal," said Kevin Trenberth of the National Center for Atmospheric Research. "It's only the East Coast that's cold and that's a relatively small area."
And that frigid air, itself, is always around in the winter. Every winter in the northern hemisphere the Arctic goes dark and the air there gets very cold, explained Trenberth. Whether areas to the south get extreme cold weather depends on how well that frigid polar air is kept in place.
"Either it sits there or somebody opens the refrigerator door and it gets out," Trenberth said.
In recent years the fridge was left open over Europe. This year it's over North America. The flip side to this escape of polar air is that it allows the Arctic to warm up this winter, which is not good for the sea ice that needs to recover from some extreme years of summer time melting.
As for how the fridge door gets left open, one popular scientific hypothesis is that the warming of the Arctic has caused the jet stream to slow and become more meandering. This idea, advanced by Rutgers University climate scientist Jennifer Francis, could be the reason that the loopy patterns form in the polar vortex, which brought deathly cold air to so many parts of the eastern United States. If Francis is right, then there is a rather clear climate change connection to the severe winter weather.