MICHELE NORRIS, Host:

Last year, a United Nations report on climate change essentially punted on one of the most important questions; that is, how much will sea level rise in the next century as a result of global warming?

The panel said there was simply not enough data to answer that question. Since then, scientists have been circulating all sorts of guesstimates, and some are extremely high.

Now a new study brings a dose of reality, as NPR's Richard Harris reports.

RICHARD HARRIS: The real problem in gauging future sea level rise is scientists don't have a good idea about how quickly the giant glaciers in Greenland and Antarctica can ooze out into the sea. They hold huge potential for sea-level rise, but ice can only flow so fast.

But without really good computer models to calculate that flow, Tad Pfeffer at the University of Colorado says scientists are often just guessing. He says he was at a scientific meeting where someone just threw out a number, that surging glaciers from Greenland alone will increase sea levels by six feet this century.

TAD PFEFFER: And I said, where did that number come from?

HARRIS: It turns out that numbers like this are just floating around, not anchored in science.

PFEFFER: And I said, okay, well let's imagine that Greenland does raise the sea level by two meters in the next century. What would have to happen if that were to occur?

HARRIS: The question really comes down to how fast ice can flow over and around the rocks that currently hold it back.

PFEFFER: It turns out, to get two meters of sea level rise, they would have to flow awfully fast, 70 times or so faster than we observe them moving today and about three times faster than we've ever seen a glacier move.

HARRIS: Even in a much warmer world, he says that's just not reasonable. So the notion that Greenland's ice will largely slide into the sea this century is far off the mark.

PFEFFER: Then we turned that whole question around and we said, okay, let's start with reasonable assumptions. You know, let them go fast, but let them go fast within the limits of what we know can actually happen.

HARRIS: He and his colleagues came up with three scenarios, now published in Science magazine. They all assume that ice will flow much faster to the sea than it's flowing today, from twice as fast to about 10 times as fast, and that gives him an upper limit of sea level rise. It's somewhere between two feet and six feet this century.

Glacier scientist Ian Joughin at the University of Washington says six feet, or two meters, would require glaciers to undergo some very dramatic and rapid shifts into conditions more extreme than anyone has ever witnessed.

IAN JOUGHIN: So even their two-meter estimate is really sort of an upper bound, I would say, on things, which is a good number to have because people have been throwing out numbers significantly higher.

HARRIS: Having an upper limit, even one that seems to be quite a stretch, is useful, Tad Pfeffer says.

PFEFFER: A policymaker who is trying to plan for, say, two, three, four meters of sea level rise is going to make a very different set of decisions than somebody who's trying to plan for 80 centimeters to two meters of sea level rise.

HARRIS: And he adds even 80 centimeters, which is two and a half feet, shouldn't merit a sigh of relief.

PFEFFER: Eighty centimeters is a big deal if you live on the mouth of the Ganges River. So there's nothing in this that is saying, oh, let's forget about this.

HARRIS: Princeton University Scientist Michael Oppenheimer agrees, and he adds that sea level is going to keep rising for centuries.

MICHAEL OPPENHEIMER: So what might seem unlikely, possible but unlikely, for this century, just might be the reality for 50 years later.

HARRIS: And he adds the pace of sea level rise in the long run will be influenced by how quickly we are able to reduce the emissions that contribute to global warming.

Richard Harris, NPR News.

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