Is Mild Winter a Sign of Climate Change?

John Ydstie speaks to Robert Henson of the National Center for Atmospheric Research about the unseasonably warm weather and what might be behind it.

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JOHN YDSTIE, host:

To talk more about our spring-like January, we're joined now by Robert Henson. He's a science writer and editor at the National Center for Atmospheric Research in Boulder, Colorado. And he's author of "The Rough Guide to Climate Change." He joins us from member station KCFR in Denver.

Welcome.

Mr. ROBERT HENSON (Author, "The Rough Guide to Climate Change"): Thanks, John.

YDSTIE: This winter warmth is happening not only in the U.S. but also across Europe. What is going on this year?

Mr. HENSON: Well, there's several factors. You know, weather and climate are the sum total of a bunch of different factors that reverberate and sometimes constructively help each other. And we have a lot of factors joining together to create warmth. You could look at the Atlantic and the Pacific and the Arctic and each part of the globe has contributed something. The Pacific has produced El Nino this winter, and that tends to cause warming across the northern tier of the U.S. And it's certainly been manifested in the upper Midwest.

In the Artic, we've had a very low amount of sea ice; in fact, the lowest on record for December in terms of how far the ice extends, and very little snow cover, even into Southern Canada. And when you have not much snow on the surface, then it's easier for warm temperatures to extend all the way down to the ground when they flow up from the south.

And then in the Atlantic, we have something called the North Atlantic Oscillation that's been in a mode that tends to foster warmth in both the eastern U.S. and across Europe, where it's been amazingly warm.

YDSTIE: Mm-hmm. And the North Atlantic Oscillation - not quite as well known as El Nino. Just give us a brief description of what that is.

Mr. HENSON: Well, in the Pacific, when you have an El Nino going on, that's because warm waters are spread across a large part of the Equatorial Pacific, where they don't usually exist. In the North Atlantic Oscillation, it's not linked so much to the ocean; it's more an atmospheric phenomenon alone. Normally, you have high pressure to the south, toward the Azores, and low pressure to the north toward Iceland. And the North Atlantic Oscillation is defined as the pressure difference between those two spots.

And when you have what's called the positive mode of the NAO, which we have going on now, the pressure is higher to the south than usual and lower to the north. And that tends to funnel the jet stream across the ocean real effectively. And that keeps Atlantic air flowing into Europe and producing extreme and sustained warmth. In fact, for example, Berlin, Germany has not had a day that's stayed below freezing the entire winter. And normally, most days this time of year never get above freezing.

YDSTIE: Now, you mentioned three things here: El Nino in the Pacific, the North Atlantic Oscillation, and then warming in the Artic this year. How might they all be affected by global warming? Or are they being caused by global warming?

Mr. HENSON: Well, the Artic situation is the most clear. Sea ice has been shrinking and getting thinner almost literally year by year. The last several years has seen many records set there. And there seems to be a good deal of certainty that greenhouse gases - increased by humans - have a lot to with that.

The situation with El Nino in the North Atlantic Oscillation is a lot fuzzier. There isn't a real clear signal in how climate change will affect those. So one way to picture that is the Artic getting warmer and warmer, and then intersecting with these Pacific and Atlantic influences in ways that are a little hard to predict and hard to analyze. But certainly leading to winters like the kind we're having now, which are exceptionally warm. Could be next winter will be cooler.

But if you think of it as a two steps forward, one steps back process, the next time we have these factors coming together in this way, maybe it'll be even a notch warmer still.

YDSTIE: Is there any way to differentiate between the characteristics of weather caused by the cyclical phenomenon like El Nino or the North Atlantic Oscillation, and climate change?

Mr. HENSON: You know, one of the big questions right now is just what you've asked, how these cyclical things are affected by climate change and how they intersect with climate change. The way I like to think of it is climate change raises the baseline. Then you get the cyclical phenomenon on top of it, and that alone will maybe raise temperatures by a degree or two. But there maybe something more, and that's what we really don't know yet.

YDSTIE: Of course, while the East Coast is basking in springtime temperatures, in Colorado where you are, and across the Plains, there's been a record snowfall. In fact, another storm there on Friday. Any relation between the warmth in the East and the big snows in the West?

Mr. HENSON: Well, you know, it's tricky. And a lot of times climate scientists are accused of wanting to have it both ways when they say that warmth could be related to global warming and then blizzards like we've maybe related to global warming. In fact, though, we typically get our big snows in the fall and in the spring out here in Colorado. Mid-winter is not a big time for snow, at least in Denver and the High Plains, where that recent blizzard occurred.

So we've been quite taken aback by how much snow we've had in the Boulder/Denver area. At least in Boulder, we've had over 50 inches of snow in three weeks, and that's really unheard of for the middle of winter.

The warmer the atmosphere is, the more moisture it can hold, and the more it can snow. And despite the fact we've had all this snow, the temperatures have not really been all that cold. We haven't gotten below zero Fahrenheit. We've had many days above freezing, even as we try to dig out from all the snow. So it's pretty uncharacteristic. And certainly people talk about weather patterns evening out, but you average the whole country and the whole northern hemisphere, it's - there's no question that it's a warmer than usual winter.

YDSTIE: What's the potential impact of a long period of warm winters?

HENSON: You know, the amazing thing about this particular warm wave, as you might call it, hasn't been so much the single-day temperatures. There have been other days roughly as warm in previous winters. What we haven't seen very much, if at all, is the sustained warmth. For example, an entire month where every single day is above average. And that tends to cause the kinds of biological affects that we've been hearing about, you know, the cherry blossom trees blooming in parts of New York City, for example.

And those factors are going to be sensitive to sometimes small changes and small increases in temperature. If you have a place where the normal low maybe 31 and it shifts up to 33, all of a sudden you're having normal nights above freezing rather than below freezing. So the small threshold could sometimes have big impacts.

Scientists like to look at climate change as a situation of winners and losers. Certainly the ski industry in Northeast can't be very happy about the threshold warmer winters in the Northeast. If you're a wheat farmer in Canada, you might appreciate warmer climate. It may allow you to have a longer growing season.

I think one of the biggest problems is the uncertainty element, and on a larger scale we're going to have to be ready for a broader array of weathering climate impact. That takes time and energy and costs money.

YDSTIE: Robert Henson is author of the "The Rough Guide to Climate Change." He's also a science writer and editor at the National Center for Atmospheric Research in Boulder, Colorado.

Thanks very much for being with us.

Mr. HENSON: Well, thanks, John.

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