But the temperature there does sometimes rise above freezing in the summer, and the new research raises the possibility that it might begin to happen more often, potentially weakening the ice sheet through surface melting. The ice sheet is already under attack at the edges by warmer ocean water, and scientists are on alert for any new threat.

A potential collapse of the West Antarctic ice sheet is one of the long-term hazards that have led experts to worry about global warming. The base of the ice sheet sits below sea level, in a configuration that makes it especially vulnerable. Scientists say a breakup of the ice sheet, over a period that would presumably last at least several hundred years, could raise global sea levels by 10 feet, possibly more.

The new research is an attempt to resolve a scientific controversy that erupted several years ago about exactly how fast West Antarctica is warming. With few automated weather stations and even fewer human observers in the region, scientists have had to use statistical techniques to infer long-term climate trends from sparse data.

A nearby area called the Antarctic Peninsula, which juts north from West Antarctica and for which fairly good records are available, was already known to be warming rapidly. A 2009 paper found extensive warming in the main part of West Antarctica, but those results were challenged by a group that included climate change contrarians.

To try to get to the bottom of the question, David H. Bromwich of Ohio State University pulled together a team that focused on a single temperature record. At a lonely outpost called Byrd Station, in central West Antarctica, people and automated equipment have been keeping track of temperature and other weather variables since the late 1950s.