Global warming is causing trillions of dollars of damage to coral reefs, aggravating risks to tropical small island states threatened by rising sea levels, a U.N. report said on Thursday. The rise in sea levels off some islands in the Western Pacific was four times the global average, with gains of 1.2 cms (0.5 inch) a year from 1993 to 2012, due to shifts in winds and currents, said the United Nations' Environment Programme (UNEP).