“That corresponds to three times the volume of all the glaciers in the Alps,” Dr. Boggild said. “If you lose that much volume you’d definitely see new islands appear.”

He discovered an island himself a year ago while flying over northwestern Greenland. “Suddenly I saw an island with glacial ice on it,” he said. “I looked at the map and it should have been a nunatak, but the present ice margin was about 10 kilometers away. So I can say that within the last five years the ice margin had retreated at least 10 kilometers.”

The abrupt acceleration of melting in Greenland has taken climate scientists by surprise. Tidewater glaciers, which discharge ice into the oceans as they break up in the process called calving, have doubled and tripled in speed all over Greenland. Ice shelves are breaking up, and summertime “glacial earthquakes” have been detected within the ice sheet.

“The general thinking until very recently was that ice sheets don’t react very quickly to climate,” said Martin Truffer, a glaciologist at the University of Alaska at Fairbanks. “But that thinking is changing right now, because we’re seeing things that people have thought are impossible.”

A study in The Journal of Climate last June observed that Greenland had become the single largest contributor to global sea-level rise.

Until recently, the consensus of climate scientists was that the impact of melting polar ice sheets would be negligible over the next 100 years. Ice sheets were thought to be extremely slow in reacting to atmospheric warming. The 2001 report by the Intergovernmental Panel on Climate Change, widely considered to be an authoritative scientific statement on the potential impacts of global warming, based its conclusions about sea-level rise on a computer model that predicted a slow onset of melting in Greenland.

Image Dennis Schmitt, a 60-year-old explorer, discovered an island in Greenland that had been bound to the mainland. Credit... Jeff Shea for The New York Times

“When you look at the ice sheet, the models didn’t work, which puts us on shaky ground,” said Richard Alley, a geosciences professor at Pennsylvania State University.