The 25.5 square-mile Ayles ice shelf broke free from the northern coast of Ellesmere Island in August 2005, National Geographic said. Scientists waited until now to announce the event because they wanted to reconstruct what happened.

When it occurred, the break registered as a small earthquake on instruments stationed 150 miles away, observers said.

"We've spent the last year reconstructing exactly what happened," said Luke Copland, a geographer with the University of Ottawa in Ontario.

Copland and colleagues concluded that the disintegration was caused by several factors, mostly related to global warming, he said.

Ice shelves are floating glacial tongues attached to land, filling bays in the Arctic and Antarctic. The Ayles shelf, one of six in the Canadian Arctic, was believed to be between 3,000 years and 4,500 years old, scientists said.

Scientists earlier said global warming may mean the Arctic Ocean's summer sea ice will be gone by 2040. Copland said ice shelves are valuable because they offer historical and ecological insights.