We went to Antarctica to understand how changes to its vast ice sheet might affect the world.

Glaciers in certain areas have been undercut by warmer ocean waters, and the flow of ice is getting faster and faster.

THE ACCELERATION is making some scientists fear that Antarctica’s ice sheet may have entered the early stages of an unstoppable disintegration.

Because the collapse of vulnerable parts of the ice sheet could raise the sea level dramatically, the continued existence of the world’s great coastal cities--Miami, New York, London, Shanghai and many more--is tied to Antarctica’s fate.

Four New York Times journalists joined a Columbia University team in Antarctica late last year to fly across the world’s largest chunk of floating ice in an American military cargo plane loaded with the latest scientific gear.

Inside the cargo hold, an engineer with a shock of white hair directed younger scientists as they threw switches. Gravity meters jumped to life. Radar pulses and laser beams fired toward the ice below.

On computer screens inside the plane, in ghostly traces of data, the broad white surface of the Ross Ice Shelf began to yield the secrets hiding beneath.

“We are 9,000 miles from New York,” said the white-haired engineer, Nicholas Frearson of Columbia. “But we are connected by the ocean.”

A rapid disintegration of Antarctica might, in the worst case, cause the sea to rise so fast that tens of millions of coastal refugees would have to flee inland, potentially straining societies to the breaking point. Climate scientists used to regard that scenario as fit only for Hollywood disaster scripts. But these days, they cannot rule it out with any great confidence.

Yet as they try to determine how serious the situation is, the scientists confront a frustrating lack of information.

Recent computer forecasts suggest that if greenhouse gas emissions continue at a high level, parts of Antarctica could break up rapidly, causing the ocean to rise six feet or more by the end of this century. That is double the maximum increase that an international climate panel projected only four years ago.

But those computer forecasts were described as crude even by the researchers who created them. “We could be decades too fast, or decades too slow,” said one of them, Robert M. DeConto of the University of Massachusetts, Amherst. “There are still some really big question marks about the trajectory of future climate around Antarctica.”

Alarmed by the warning signs that parts of the West Antarctic ice sheet are becoming unstable, American and British scientific agencies are joining forces to get better measurements in the main trouble spots. The effort could cost more than $25 million and might not produce clearer answers about the fate of the ice until the early 2020s.

For scientists working in Antarctica, the situation has become a race against time.

Even as the threat from global warming comes into sharper focus, these scientists understand that political leaders--and cities already feeling the effects of a rising sea--need clearer forecasts about the consequences of emissions. That urgent need for insight has led scientists from Columbia to spend the past two Antarctic summers flying over the Ross Ice Shelf, a floating chunk of ice larger than California.

The Ross shelf helps to slow the flow of land ice from Antarctica into the ocean. Compared with other parts of Antarctica, the shelf seems stable now, but computer forecasts suggest that it might be vulnerable to rapid collapse in the next few decades.

If the world is allowed to heat up enough, scientists have no doubt that large parts of Antarctica will melt into the sea.

But they do not know exactly what the trigger temperature might be, or whether the recent acceleration of the ice means that Earth has already reached it. The question confronting society, said Richard B. Alley, a climate scientist at Pennsylvania State University, is easier to ask than to answer:

“How hot is too hot?”