The coral bleaching observed in the Northwestern Hawaiian Islands over the past several weeks could be caused by global warming -- or it could just be a cycle in nature, scientists say. Either way, it offers an unprecedented opportunity to study the recovery of bleached coral reefs, said scientists who returned Tuesday from a monthlong research cruise to Hawaii's most remote islands. The researchers said they were surprised by the amount of bleached, or dying, coral they observed there, even though they had a clue they might find bleaching because of abnormally high water temperatures. Though the cause of the...