Globe and Mail: The amount of thick sea ice in the Arctic has shrunk sharply, according to new U.S. surveillance data, adding urgency to Canada’s push to exert sovereignty in the North. But Foreign Minister Lawrence Cannon said yesterday it could be decades before the Northwest Passage is open for shipping. “Some experts predict that the entire Arctic could be ice free by 2013, others say that this will happen by 2050,” Mr. Cannon told the Centre for Strategic and International Studies, a …