Nations must make plans to help tens of millions of "sea level refugees' if climate change continues to ravage the world's oceans, German researchers said on Thursday. Waters are rising and warming, increasing the destructive power of storms, they said, and seas are becoming more acidic, threatening to throw entire food chains into chaos. "In the long run, sea level rises are going to be the most severe impact of global warming on human society,' said professor Stefan Rahmstorf, presenting a report by German scientists at a major United Nations climate change meeting.