Why the Midwest’s deep freeze may be a consequence of climate change

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• Judy Woodruff:

  More than 80 million Americans, about a quarter of the U.S. population, are expected to deal with subzero temperatures this week.

  It's especially brutal in places that are long used to the deep freeze of winter. Minnesota, for example, could break temperature records dating back to the 1800s. This blast is tied to a huge pocket of cold air, now known as the polar vortex.

  There are lots of questions about that, and Amna Nawaz is here to try to get some answers tonight.

• Amna Nawaz:

  Judy, windchills in several states have already dropped, or soon will drop, to the negative 40s or 50s, which leaves people trying to understand what is behind this weather phenomenon.

  Is there a connection between climate change and this cold front? And for some skeptics of climate change, it's providing new fuel for their disbelief. That includes President Trump, a skeptic not only of climate change, but also the scientific consensus behind it.

  As the polar vortex set in earlier this week, the president tweeted: "What the hell is going on with global warming? Please come back fast. We need you."

  Let's get a better understanding of what's behind the polar vortex, and any connection to climate change, with Jennifer Francis. She's a senior scientist at Woods Hole Research Center, well known for its work on the impact of climate change.

  Dr. Francis, welcome to the "NewsHour." Thanks for making the time.

  The polar vortex is now a trending hashtag on Twitter. A lot of people have a lot of questions.

  Let's start with that. What exactly is the polar vortex, and why are we dealing with it right now?

• Jennifer Francis:

  Yes, so the polar vortex is a new word in the lexicon of Americans, just starting a few years ago, and it gets used wrongly often.

  What the polar vortex truly is, is way up in the atmosphere over the North Pole, about 30 miles up, is a ring of winds blowing in the counterclockwise direction that keep the cold air bottled up over the Arctic, way high up in the atmosphere. So this is called the stratospheric polar vortex.

  That's the real one. And what it often gets used wrongly for is to the talk about the jet stream, which is much lower in the atmosphere. It is really what creates all of our weather that we feel down here on the surface. It is also a river of wind that flows around the Northern Hemisphere, but at a much lower level.

  So there are these two spinning rivers of wind up over the Northern Hemisphere that control our weather. And right now, the true polar vortex has actually split into two, which doesn't happen very often. And one of those lobes of cold air that is normally is bottled up over the North Pole has drifted down over North America and brought all that cold air with it.

  And that's why this particular cold front or cold air mass is just so severe.

• Amna Nawaz:

  So, that's high we are dealing with it right now, but you mentioned it split into two. Explain that to me. Why did that happen?

• Jennifer Francis:

  What we think is happening that connects back to climate change is that, back in the summer, we lost a lot of ice in a region just north of Western Alaska in the Arctic Ocean.

  That allowed a lot of extra heat to get absorbed in the water there, and, in fact, the ice still hasn't grown back. And that heat then gets reemitted back to the atmosphere during the fall and winter, when the cold air comes back, and it makes kind of a bulge in the atmosphere.

  And if that bulge gets big enough, it can actually make the jet stream take a northward swing right there. And if that northward swing is big enough, it will send wave energy up into the stratosphere, where the polar vortex is, and it can kind of knock it off its rocker, if you will.

  If you can think of like a top spinning up there, it can bump into this top and get it to wobble, and sometimes it wobbles so much, that it actually creates this split in the polar vortex.

• Amna Nawaz:

  So, just to summarize, you think that heat there, as a result of climate change, is basically causing a disruption in the polar vortex that already exists; that's what is causing it to shape-shift and move, and that's why we are experiencing it?

  Now, that's the link between client change and this weather phenomenon; is that right?

• Jennifer Francis:

  That's what we think is going on. It is a very new topic of research. It is certainly not settled. There are only a handful of papers that have come out so far that are supporting this hypothesis.

  But it certainly looks like it. This year is a classic example. Last year was too, and we think that this is a robust connection.

• Amna Nawaz:

  Well, so let me ask you about that, though, because if you don't know, if this is what we believe is actually happening, then are people right to be casting doubt on this?

• Jennifer Francis:

  Well, you know, that is how science works.

  Somebody has a hypothesis or an idea of how something is connected, they do a bunch of experiments, they look at the real world, and analyze the real atmosphere, and look at how things are connected to the real atmosphere, and then sometimes they use numerical climate models or sort of like our weather forecast models to try and simulate those connections.

  And, by doing that, that gains credibility and gives us some confidence that these sorts of connections exist. But the problem is, the atmosphere is a very complex beast, so there is still a lot of research to do. But I think this concept has gained a lot of traction in the last few years. And there is really — there is really no alternative explanation, other than this has just happened by random chance.

• Amna Nawaz:

  Dr. Francis, I would like you to ask your help in making another distinction, if you can for us, between weather and climate, the difference between the events that we're seeing and talking about and this overall trend of climate change.

• Jennifer Francis:

  Right.

  So, more generally, climate is just the long-term average of the weather conditions that happen in a given area. So, for example, where I am right now in Florida, typically, it is a warmer climate than it is up in Boston, where I usually live. So that would be climate.

  But weather is the day-to-day variations. Some days, it is colder than normal, some days, it is warmer than normal, some days, it rains harder than others. So those day-to-day fluctuations are the weather.

  Sometimes, they are referred to as the difference between your personality, which is kind of how you are most of the time, and your mood on any given day. So, it is that kind of a relationship.

  But, as we know, the climate is gradually changing. Actually, it is changing rapidly, compared to changes in the past. But that's a gradual change that we're observing happening. And we know why. It's all because of human activities increasing the greenhouse gases in the atmosphere that trap a lot more heat down by the surface.

• Amna Nawaz:

  That's an important explanation and distinction.

  Dr. Jennifer Francis, thank you so much for making the time.

• Jennifer Francis:

  You're welcome.