Coral reefs are crucial to ocean ecosystems — providing food and shelter to a quarter of all marine life. They also support food stocks that feed more than a billion people. This year, reefs around the world are threatened like never before: At Christmas Island near Australia, scientists estimate more than 80 percent of all the coral is now dead. And at the Great Barrier Reef, a similar story is unfolding: More than half of the reef’s coral has died. Scientists point to warming ocean temperatures and successive El Nino events as causes. For this month’s Environmental Outlook, the threat to coral reefs worldwide and what it will take to save them.