Canwest News Service: One of Canada’s five remaining Arctic ice shelves – the 4,500-year-old, 50-sq.-km. Markham Ice Shelf – has broken completely away from Ellesmere Island and drifted into the Arctic Ocean, the most dramatic sign yet of how rising temperatures and retreating sea ice are creating “irreversible” changes to the country’s polar frontier. Just days after Prime Minister Stephen Harper returned from a northern tour that highlighted an emerging national debate over the economic and ecological …