COPENHAGEN, Nov 12 (Reuters) - Iceland has designated the potential collapse of a major Atlantic Ocean current system a national security concern and an existential threat, enabling its government to strategize for worst-case scenarios, the country's climate minister told Reuters. The Atlantic Meridional Overturning Circulation, or AMOC, current brings warm water from the tropics northward toward the Arctic, and the flow of warm water helps keep Europe’s winters mild. But as warming temperatures speed the thaw of Arctic ice and cause meltwater from Greenland’s ice sheet to pour into the ocean, scientists warn the cold freshwater could disrupt the current’s...