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GUY RAZ, host:

Welcome back to ALL THINGS CONSIDERED from NPR News. I'm Guy Raz.

Britain's shivering through its coldest winter weather in 30 years. The Chinese capital of Beijing had its biggest one-day snowfall since 1951 and much of the U.S. is thawing out from an almost month-long freeze. But in one unexpected place, it's downright balmy - at least relatively speaking - and that place is the North Pole.

It turns out parts of the Arctic are 10 to 15 degrees warmer than they should be at this time of year. So, what's behind the unusual weather?

Mr. MARK SERREZE (Director, National Snow and Ice Data Center, University of Colorado at Boulder): Well, I think it's all about the Arctic Oscillation.

RAZ: That's Mark Serreze. He is the director of the National Snow and Ice Data Center at the University of Colorado at Boulder.

And he joins me from Boulder. Welcome to the program.

Mr. SERREZE: Well, thank you for having me.

RAZ: And I understand that center is where the frozen are chosen.

Mr. SERREZE: That's right. The National Snow and Ice Data Center, where the frozen are chosen.

RAZ: Where the frozen are chosen, okay. So, now that we've uncovered the culprit behind all this extra cold weather, explain what exactly Arctic Oscillation is.

Mr. SERREZE: Well, if we look at the weather patterns over the Northern Hemisphere as a whole, sometimes we find that the surface pressure, the amount of atmospheric pressure we have, is a little lower in the Arctic than it is in the middle latitudes. And then sometimes it shifts, so the Arctic get a little more and the mid-latitudes get a little less.

Well, that shift in atmospheric mass or pressure is really how we define the Arctic Oscillation. But the interesting thing is, is that the shift in pressure are associated with changes in weather that really reverberate throughout the Northern Hemisphere.

RAZ: I'm sure, as you've seen, there have been some sort of climate change -let's call them skeptics - sort of suggesting that this proves climate change. Does it have anything to do with it?

Mr. SERREZE: Well, what we're seeing here has nothing to do with global warming. What we're seeing is crazy weather. At the very same time that we've seen these areas in the middle latitudes with subzero temperatures and big snow storms, the Arctic has been much, much warmer than normal.

RAZ: Now, in December, the Arctic Oscillation went into what's called an extreme negative phase. Is that rare?

Mr. SERREZE: Well, what we've seen this December was certainly exceedingly rare. It would be like if you had a deck of cards and you told someone, well, I'm going to pull the ace of spades out of here and you actually did it, right? Well, having us going through this extreme negative mode is even more rare than something like that.

RAZ: Now, if the Arctic was much warmer over the past few weeks, does that affect the sea ice? I mean, presumably it means that the sea ice will melt more quickly?

Mr. SERREZE: On the one hand, with this extreme phase of the Arctic Oscillation, the Arctic has been very, very warm. That means less ice grows in the winter. On the other hand, the changing pattern of wind means that less ice in the Arctic gets transported out of the Arctic into the North Atlantic where it would melt. So the Arctic kind of holds on to the ice that it has. So, you have these competing effects and who is going to win? We'll probably have the answers of that, say, some time in April. We don't know right now.

RAZ: I mean, do you have a sense of what this might mean for the Arctic and the summer ice melt?

Mr. SERREZE: I think it's going to have an overall negative effect. If you look at the extent of Arctic sea ice and compare it with normal, we're already about a million square kilometers in the hole. In other words, a million square kilometers below average. So, it's not like the ice is really recovering from this.

But we'll see what happens in the spring. The real fascination with climate science is that it's a lot that we do understand, but there's a lot that we still don't.

RAZ: That's Mark Serreze. He is the director of the National Snow and Ice Data Center at the University of Colorado at Boulder.

Mark Serreze, thanks so much for joining us.

Mr. SERREZE: It's my pleasure.

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