Since 2015, all tropical coral reefs have seen above-normal temperatures, and more than 70 percent experienced prolonged high temperatures that can cause bleaching.

US coral reefs were hit hardest, with two years of severe bleaching in Florida and Hawaii, three in the Mariana Islands, and four in Guam, according to the NOAA.

"This global coral bleaching event has been the most widespread, longest and perhaps the most damaging on record," said Mark Eakin, the NOAA's Coral Reef Watch coordinator.

Healthy coral reefs protect shores from storms and offer habitats for fish and other marine life, including ecologically and economically important species.

However, after corals die, reefs quickly degrade and the structures that corals build erode. While corals can recover from mild bleaching, severe or long-term bleaching is often lethal, experts say.

Early in 2017, the rise in water temperature caused significant bleaching in the Great Barrier Reef in Australia for the second consecutive year and also in American Samoa, which was severely affected in 2015.