The world's oceans are enduring the longest-lasting and most widespread coral bleaching event ever recorded, according to the National Oceanographic and Atmospheric Administration (NOAA).

And the massive bleaching event is predicted to get even worse—extending into a third, unprecedented year.

The current bleaching event has already all but destroyed Australia's Great Barrier Reef, and NOAA's experts say that warm ocean temperatures will soon trigger bleaching in the northern hemisphere, "including around Hawaii, Micronesia, the Florida Keys and Puerto Rico," the Guardian reports.

Bleaching occurs when overly warm ocean waters cause coral to expel the algae living inside of it, which turns the coral white and erodes its structures, as Common Dreams has reported.

NOAA's Coral Reef Watch has described the growing swaths of bleached and dying coral as "ghostly underwater graveyards."

"It's time to shift this conversation to what we can and are doing to conserve these amazing organisms in the face of this unprecedented global bleaching event," said the director of NOAA's coral reef conservation program, Jennifer Koss, to the Guardian.

"Coral in every major reef region has already experienced severe bleaching," the newspaper adds.

"Climate change has caused global sea surface temperatures to rise by about 1C over the past century, pushing corals closer to their bleaching threshold. A strong El Niño, as well as other weather phenomena, raised the temperature further this year," the Guardian notes.

Extended bleaching events eventually kill a reef—as has already been happening to large stretches of the Great Barrier Reef.

And as with many effects of climate change, the poorest and smallest nations will likely suffer the most from the catastrophic bleaching event. Mark Eakin, director of NOAA's Coral Reef Watch, told the Guardian, "The biggest bleaching threat over the next six months is to the reefs in two U.S. freely associated states: Palau and the Federated States of Micronesia. Islanders there are very dependent on their coral reefs and diving tourism is a major contributor to their economies."

"This event may have major ecological and economic impacts on those islands," Eakin said.

Dead coral reefs "are perhaps the starkest reminders—like the melting Arctic—that a thickening blanket of greenhouse gases is irrevocably changing the face of the Earth," as Inside Climate News recently wrote.