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Ten years ago, a piece of ice the size of Rhode Island disintegrated into the waters of Antarctica. That's after two other massive ice shelves along the Antarctic Peninsula suffered similar fates a few years before. The events became poster children for the effects of global warming, but a new study finds the story isn't quite so simple, as NPR's Richard Harris reports.

RICHARD HARRIS, BYLINE: There's no question that the collapsing ice shelves were dramatic, and the evidence is clear: Unusually warm air triggered the final demise of these huge chunks of ice. But a lingering question is whether these events can be attributed to human-induced global warming. Robert Mulvaney and colleagues at the British Antarctic Survey have new insights into that question. They drilled down 1,000 feet and collected an ice core that tells the story of heating and cooling along the Antarctic Peninsula over the past 50,000 years, and Mulvaney says the sharp warming trend we're seeing now is quite unusual but not entirely unique.

ROBERT MULVANEY: In the last 2,000 years, there was another period round about the 1500s where we also saw a warming about similar to the rate of today, but then, if you like, it reversed and cooled again. And then at about 400 A.D., we also saw not quite as great a warming there as well, but, again, it cooled again.

HARRIS: That finding, published in Nature magazine, raises an obvious question. Since there's a history of extreme temperature changes in this part of Antarctica, what can we say about the cause of the latest strong warming trend?

MULVANEY: I still think the jury must be out, whether you can attribute it one way or the other. I think it neither proves nor denies that this is a human-induced change.

HARRIS: And that makes this finding interesting not just for scientific reasons but for political ones. There's a very broad scientific consensus based on many lines of evidence that human changes to the planet are having a real impact on air temperature, sea temperature, droughts and melting ice. But Mulvaney knows that this more ambiguous finding is likely to be picked up by skeptics who are looking for evidence to argue that natural variability rather than human activities explain the Earth's rapid warming. So be it.

MULVANEY: I'm a scientist. I'm just being honest and reporting what we see. I'm not trying to be political here. I'm not trying to either prove or disprove human-induced climate change.

HARRIS: In fact, Mulvaney says when he started studying Antarctica 20-plus years ago, he was skeptical that humans were already affecting the climate and the ice.

MULVANEY: I think as the years have gone by, I think I've moved considerably away from that early skeptical stance and I - as I stand here now, you can't keep going year after year with a lot of the evidence, most of the evidence all pointing in the same direction and not come to the conclusion that the climate is warming.

HARRIS: Another Antarctica expert, Gene Domack from Hamilton College, says it is hard to distinguish natural cycles from human-caused trends in something as dynamic as ice, but he says these changes in Antarctica are mirrored with similar changes now in ice-sheet disintegration in the Canadian Arctic.

GENE DOMACK: So when you begin to see different regions that naturally have different comings and goings of their ice mass, when you see them behaving in a coherent faction, then that tells you something is up that's more widespread than a local phenomenon.

HARRIS: And global changes are more likely due to human activity. This question is obviously important because melting ice has historically raised and lowered sea levels dramatically, so knowing its fate matters a lot to the hundreds of millions of people who live near the coast. Richard Harris, NPR News.

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