JUDY WOODRUFF: Now: a scientific puzzle over the melting of one of planet Earth's largest sheets of ice.
Margaret Warner has our story.
MARGARET WARNER: This week, NASA announced a surprising finding. Earlier this month, the surface of the ice sheet covering Greenland melted more widely than has been seen in 33 years of satellite imagery.
Typically, about half of Greenland's surface ice thaws each summer. Most of it usually refreezes in place, while some flows into the ocean. But this month was different.
On July 8, satellite imagery showed about 40 percent of Greenland's top ice layer, shown here in shades of pink, had thawed. The white area was still solid ice.
Just four days later, 97 percent of the ice -- again shown -- again shown in pink -- had thawed.
It coincided with another striking development in Greenland. A major glacier in the northwest known as the Petermann Glacier lost a major chunk of ice. The iceberg that broke off, as shown here in time-lapse photos, was more than twice the size of Manhattan. An even bigger piece of the glacier separated in 2010.
For more on all this, I'm joined by Thomas Wagner of NASA. He directs the agency's programs for glaciers, sea ice and polar regions.
And welcome, Mr. Wagner. Thanks for coming in.
THOMAS WAGNER, NASA: Thanks for having me.
MARGARET WARNER: So, first of all, how surprising -- let's start with the thaw, the melt that occurred on Greenland itself. How surprising was this?
THOMAS WAGNER: Complete surprise, so much of a surprise that one of the scientists who studied this thought there must be something wrong with his instrument.
MARGARET WARNER: So, then you determined that this was for real?
THOMAS WAGNER: Right.
And what happened was this, was we got some reports that there was melt going on all around Greenland, literally like so much water running off that it was washing out bridges and things, that there were runways that were on the snow that were having problems.
And so what we did was look at the satellite records, which are great because they cover the whole ice sheet. And what we found pretty quickly was that it had melted, and it had melted in places that we had never seen melt before.
MARGARET WARNER: And so is this -- this is not unprecedented, though. How unusual is this?
THOMAS WAGNER: It's unusual for us, because the thing you have got to understand is the top parts of the Greenland ice sheet are 12,000 feet above sea level.
It's very, very cold there. It's never above freezing. And what happened was, we had temperatures go up to almost 42 degrees in places. When we look back, though, kind of in deep time, which we can get from ice cores around Greenland, we found out melting like this probably does happen maybe about on average every 150 years. So, this is really unusual. Maybe the last time it happened was 1889.
MARGARET WARNER: But not unprecedented?
THOMAS WAGNER: Not unprecedented in that we see a deep-time example that this has happened before.
But, you know, and this is a case where we actually have observations and we know what went on and we can correlate it with weather patterns.
MARGARET WARNER: Now, what explains it?
THOMAS WAGNER: Right.
Well, the simplest explanation is this, is we had some pockets of warm air really form around Greenland that literally washed up over the entire ice sheet. And a lot of that is related. If you look at -- there are some indices people use to describe the state of the atmosphere.
One of them is the North Atlantic oscillation. And this year, that happened to have a really strong high-pressure system formed over Iceland that allowed this warm air sit around and move over Greenland.
MARGARET WARNER: So, you're basically talking about the weather?
THOMAS WAGNER: Yes, and that is one way to thing about it. Climate is sort of long-term weather. But short term, you can get extreme variations. And we're seeing an extreme variation in Greenland.
MARGARET WARNER: So, you're saying you can't really attribute this to climate change?
THOMAS WAGNER: No.
And that's one of the things. We spent a long time trying to word the document that we put out describing it. And we said, look, there is evidence that this has happened before. Now, that doesn't mean -- we really don't know the explanation for this one. If it happens again, if it starts to happen repeatedly, then we have an indication that there might be a real shift going on in the Arctic system there.
MARGARET WARNER: All right. So, now, it has been about a week since you -- since the end of that -- those photos that we showed. What's happening now on the ground? Do you even know?
THOMAS WAGNER: Well, up high on the Greenland ice sheet, the instruments that we have there are saying that things are cooling off already.
MARGARET WARNER: So they're starting -- starting to refreeze?
THOMAS WAGNER: Right, but at the lower elevations, it's still really warm.
And you can almost think of it -- at high elevations, think of like good packing snow. You have got a lot of water in between the snow particles. But at lower elevations of Greenland, there's so much melt that had coalesced into lakes and ponds and rivers, and that stuff is still running. We have actually got scientists down on the ground right now studying that.
MARGARET WARNER: Now, let's move on to the Petermann Glacier, and that rather dramatic picture of the -- I guess it's in the sort of fjord, but it broke off.
THOMAS WAGNER: Exactly.
MARGARET WARNER: Well, first of all, that isn't -- that happened two years ago as well. Look into deep time. How unusual is this kind of event?
THOMAS WAGNER: So, the Greenland ice sheet works is that you -- almost like if I poured honey in the middle of the table.
Snow builds up in the middle of Greenland, compacts the ice, and then it flows out to the sides, flows out in outlet glaciers, like you say, out the fjord. When it hits the ocean, it begins to float, and then that forms an ice shelf, which normally breaks off and it's continuously fed by snow building up in the middle.
In the case of Petermann, there's a debate going on right now as to what's going on. It could be normal that this calving occurs, or it could be that ocean warming, which has also been observed in that area, caused this.
MARGARET WARNER: And so nobody knows what caused this either?
THOMAS WAGNER: Right, but we can't lose sight of the bigger picture, which is this. The Greenland ice sheet has been losing tremendous amounts of ice for decades, on average 150 gigatons a year. You're talking so much ice that this is contributing about 0.3 millimeters of sea level rise a year around the globe.
MARGARET WARNER: And how much of the sea level rise is that?
THOMAS WAGNER: Yes. Well, it's about three millimeters a year overall.
But the problem, too, with Greenland is that it looks like it's been accelerating in recent years. And so depending on what time range you look at it, now it could be contributing a half-a-millimeter, maybe even more.
MARGARET WARNER: So, big picture, what are the consequences of these two events, whatever the cause? Because I take your point that it's going to take subsequent years to know. What are the consequences?
THOMAS WAGNER: There's a couple of things.
Number one, for us, this is a phenomenal natural experiment that has been run. So this is a chance for us to go and look at the effects are of water on the ice sheet. And if we are in a warming world, we can look at -- does that water make it to the bottom of the ice and lubricate the glaciers so they form faster?
There is also a little bit of a debate right now as to how does Greenland lose all of its ice? Is it melting or is that the flow of glaciers out to the ocean? And it's about 50/50. But -- well, we think it's about 50/50. After this year, we may have some new ideas on that.
MARGARET WARNER: Does this affect, say, the broader picture of the whole water system in the Arctic?
THOMAS WAGNER: Oh, sure.
And one of the things, this then also gives us a chance to highlight what's going on in the Arctic. This year, we're losing sea ice overall. We're down close to another record year. And we're probably at a record year. The sea ice is thinner now than it's ever been. We're seeing warming going around all around in the Arctic. Permafrost is thawing and those kinds of things. So it really does look like the Arctic has shifted in state.
MARGARET WARNER: But is it fair to say that at least some of this, both the breaking off of the glacier, but, more importantly, the melt, is going to add both warmth -- is it going to add warmth and also volume to the oceans, in very basic terms?
THOMAS WAGNER: Oh, definitely, yes. Melting -- that water, most of it goes down and flows out. A lot of it refreezes in place, but if it's at lower elevations, it flows out into the ocean.
And these glaciers that break off, that is the mechanism -- both of these things are the mechanism by which we raise sea level from the ice sheets.
MARGARET WARNER: Well, Tom Wagner from NASA, thank you so much.
And you can find the time-lapse satellite images of Greenland's ice melt on our Science page.