JUDY WOODRUFF: New concerns about warming in the Antarctic and its consequences. Ray Suarez has our Science Unit update.
RAY SUAREZ: A floating plane of ice roughly six times the size of Manhattan Island has broken free and crumbled into the sea off the southwest coast of Antarctica. That collapse threatens the Wilkins Ice Shelf, a much larger sheet of ice, with break-up and melting.
Scientists say the prime culprit is global warming. This video, shot by a British expedition, shows how the collapse, which began in late February, has progressed.
The western Antarctic is the fastest-warming place on Earth. It's warmed at a rate of nearly one degree Fahrenheit per decade.
Satellite imagery shows the shelf's disintegration. The sky-blue coloring comes from the deep glacial ice, now exposed. The shelf is thought to have been in place for hundreds of years.
For more, we turn to Stephen Schneider, a climatologist at Stanford University, and founder and editor of the journal Climatic Change. Schneider was also one of the lead authors on a serious of reports from the United Nations Panel on Climate Change. That panel shared the 2007 Nobel Peace Prize.
Professor Schneider, a lot of attention has been turned to the Arctic Sea and Greenland. Is there loss of polar ice that's just as serious going on in the Antarctic region?
STEPHEN SCHNEIDER, Climatologist, Stanford University: Well, the Antarctic is getting more strongly in the act and not leaving the whole show up to the north. What we're seeing is a systematic pattern of bits of pieces of the shelf in Antarctica coming apart.
What do we mean by a shelf? Ice starts up on the land, on the continent, and then it flows out into the ocean. And it can be as tall as a big building. And then it gets stuck on risers or islands, and what that shelf does is it's floating. So if it melts, it's not going to raise the sea level, because anymore than melting the ice cube in your Coke would melt it.
But what it does do is it holds the ice on the land back. And now, when the shelf breaks up, it allows the ice on the land to flow more rapidly in the water. That could threaten several feet of sea level rise in the time frame of many decades, up to centuries.
That's what has people concerned. And the fact that the Antarctica is beginning to look more like it's part of the story, as well.
RAY SUAREZ: Antarctic scientists say they've been watching this happen since the 1990s, but is it happening faster than anybody thought?
STEPHEN SCHNEIDER: I've heard it said from scientists who study it who said, "Well, I kind of predicted 30 years ago, if it gets another degree or two warmer, we've got to expect this to happen."
I've heard them say, "Well, it's happening faster than we expected," which is the same language you hear from people studying Greenland, where the margins of Greenland, the sides, have been disintegrating much faster than people expected.
The big problem is, how much of this can we attribute to us and human activities which dump our smokestack and tailpipe waste in the atmosphere? And how much can we attribute this to Mother Nature just giving us a perverse cycle?
That's a much tougher question, though the fact that it's happening in both hemispheres and it's happening at wider scale has led people to think it's pretty unlikely that human activities aren't at least part of the story.
RAY SUAREZ: Well, you say it's a much tougher question. Doesn't determining the answer to that tougher question give us some guidance on how humankind should respond to this or whether it should respond at all?
STEPHEN SCHNEIDER: Yes, you're absolutely right, Ray. We really want to know: How much have we caused this problem?
Unfortunately, scientists are stuck in this belief that we tell people probabilities, not absolute answers. So what we're going to say is that we think it's likely -- meaning more than two out of three -- that Greenland is going to melt significantly in the next century, and that that's going to raise sea levels anywhere from, if we're lucky, three or four feet, up to, if we're unluckily, 10 or more feet on the time frame of a few centuries.
And the same is true also down in the west Antarctic ice sheet. That can raise sea level up a foot or two.
But there are other parts of Antarctic that have even more potential sea level rise further south, and they're starting to show signs of going. That's where the real big worries are.
So the problem, unfortunately, is not one that we can say, "It's certain. This is going to happen." We're really into what's called risk management. It's the way the military operates or people buy insurance or investment bankers, which have up- and downside risk.
It's, how much chance do we want to take with the planetary life support system when there's a good case for concern, but that case has not proved beyond any and all doubt?
RAY SUAREZ: Back to the Wilkins Ice Shelf in Antarctica, you mentioned that the sea levels wouldn't rise because that ice is already on the ocean. But it's freshwater. And when it melts, doesn't it change the salinity, doesn't it change the chemistry of the oceans? And do we have to worry about that?
STEPHEN SCHNEIDER: Well, it's going to change the chemistry of the ocean locally a lot. It will have a significant impact on ecosystems.
There are penguins of various types there. There are microbiological entities that live on the bottom of the ice. I think that would be significant. The freshwater can affect even beyond that local area.
The main thing that humans worry about in that part of the world is, even though the ice shelf itself isn't going to raise sea level, it takes away a support, like a buttress on an old cathedral, that holds up the wall. This is holding up the landed ice.
Now, if that landed ice can flow more rapidly into the ocean, that will have significant sea level rises on the order of many feet on the time frame of centuries.
RAY SUAREZ: Does more standing water on Antarctica itself also threaten the ice that's there? Does it make it somehow looser?
STEPHEN SCHNEIDER: Science usually operates in sort of three modes, things that are well established, we know what we're talking about, more highly confident. There are competing explanations, we have a pretty good idea, but we're not sure. And then things are speculative.
When you start looking at the disintegration of west Antarctic, we're somewhere in the competing explanations. And there's a big argument about whether melting water that goes down lubricates the bottom of the ice sheet. When that happens, it starts to slip.
When it starts to slip, it actually generates a tremendous amount of heat, like when you rub your hands together, the same way. That melts the ice even further and causes it to accelerate. We're worried about that a lot in Greenland, by the way.
So those processes, which are very plausible -- they're not absolutely certain yet at the scale where we would find totally alarming, but we're certainly worried about them.
And what we're most worried about is, by the time we know for sure, is it possible we'll have created an irreversible commitment toward the deglaciation of west Antarctic and parts of Greenland? And, unfortunately, I don't think we could rule that out at the odds of a coin flip.
JEFFREY BROWN: Professor Stephen Schneider, thanks for joining us.
STEPHEN SCHNEIDER: Thank you, Ray.JUDY WOODRUFF: To learn more about polar ice caps and rising sea temperatures, visit our Web site at PBS.org.