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The Himalayas are sometimes called the world's third pole because they're covered with glaciers. Water from those sheets of ice helps feed important rivers, including the Ganges and the Indus. As the glaciers melt, they will contribute to rising sea levels. And what scientists don't know is how global warming really affects all that ice. Here's NPR's Richard Harris.

RICHARD HARRIS, BYLINE: Just a few years ago, it seemed that the Himalayas were on the brink of disaster. The U.N.'s Intergovernmental Panel on Climate Change made alarming claims about the fate of all that ice. You can almost see Jeffrey Kargel, at the University of Arizona, cringe as he describes it.

JEFFREY KARGEL: One page had the most the most egregious errors you could imagine - just one after another - including the claim that the glaciers would disappear by 2035.

HARRIS: That was dead wrong. The error put a lot of egg on the face of the U.N. Climate Change Panel. But it also sent glacier scientists scrambling. They knew very little about the state and fate of those glaciers, even the basics. It's true that a billion people live downstream from those glaciers. But even that requires an asterisk.

KARGEL: That sort of statement can be exaggerated to imply that somehow, if the glaciers disappear, the taps are going to run dry for a billion people. And that's just patently not the case.

HARRIS: Various estimates suggest that the glaciers provide 1 or 4 or 10 percent of the water for people downstream, with most of the rest coming from the abundant monsoon rains. But Kargel says some specific valleys may run dry, and others may flood. It's wrong to think of the Himalaya as a monolithic block of ice.

KARGEL: Some areas of the Himalaya - and the nearby ranges, such as the Karakoram - will see shrinking ice masses, shrinking glaciers. But some, actually, are projected to increase in mass, up to a point.

HARRIS: In fact, scientists are seeing some glaciers gaining mass today, thanks to increased snowfall. Tobias Bolch, at the University of Zurich, says consider the Karakoram, home to more than 40 percent of the region's glaciers.

TOBIAS BOLCH: And there, the glaciers - at least, during the last 10 years - are gaining mass, while in most of the other regions in the Himalaya, they are losing mass.

HARRIS: Bolch and Kargel are two authors of a paper in "Science" that reviews the current state of knowledge about the Himalayan glaciers. Bolch says scientists are starting to get a much better handle on what's happening to the glaciers right now, but he says they struggled mightily over a larger question.

BOLCH: What can we really state about the fate of the glaciers - because there's so many uncertainties that it's really hard to predict the future of the glaciers.

HARRIS: Right now, the glaciers that are melting are losing one or two feet of thickness every year. If they continue at that pace, some of those glaciers could be gone by the end of the century.

But that's too simple a way to look at the problem. That's because our climate is also expected to get wetter as it gets warmer. And Michael Bishop, at the University of Nebraska in Omaha, says that's a critical wildcard.

MICHAEL BISHOP: If we have increasing precipitation, that might feed the glaciers and keep them around longer. However, if we had higher temperatures and decreasing precipitation, that might lead to their demise.

HARRIS: Scientists all around the world can agree that the glaciers in the Himalayas aren't going to melt away by 2035. But ask them how long they will be around, and you will hear many variations of this answer from Michael Bishop.

BISHOP: I think it's a bit premature to be able to say we can accurately predict this or accurately predict that, due to the complexity of the situation. And I think a lot of people forget about that. They think that we just know everything there is to know about these systems. They're extremely complicated systems.

HARRIS: One of the biggest unknowns is about human nature, rather than ice. What are we going to do to slow global warming in the coming decades?

Richard Harris, NPR News.

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