Last week Greenland's Petermann Glacier shed a 100-square-mile chunk of ice, releasing a huge iceberg. Although the glacier has been regularly shedding smaller chunks, this was nearly one-quarter of the entire ice shelf of the glacier. It is the largest piece of ice to detach from an Arctic glacier since 1962 and follows the six warmest months on record. The chunk of ice is four times the size of Manhattan island, and is a possible danger to ships as it drifts into North Atlantic shipping lan...