More than one-third of the world's coral reefs this year are at risk from some level of bleaching — an effect that can whiten and kill coral as ocean temperatures rise — researchers from groups including the National Oceanic and Atmospheric Administration said Thursday.

Reefs around Hawaii, Haiti, the U.S. Virgin Islands and Puerto Rico are among those that stand to suffer as ocean waters warm, NOAA said, with as many as 95 percent of U.S. corals exposed to possible bleaching conditions by the end of this year. The findings have caused NOAA to declare the third global coral bleaching event ever on record — the others having occurred in 1998 and 2010.

In all, the scientists expect more than 4,600 square miles of reef to be killed off by bleaching in 2015.

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