Reuters: The West Antarctic ice sheet may start to collapse if sea temperatures rise by 5 degrees Celsius (9 degrees Fahrenheit), triggering a thaw that would raise world ocean levels by 5 meters (16 ft), U.S. scientists said. Such a rise in sea levels — taking thousands of years — would swamp many coasts and cities and wipe some low-lying Pacific islands off the map. West Antarctica, the part of the frozen continent most vulnerable to climate change, has thawed several times in the …