Record-breaking warm waters have bleached large parts of Australia’s Great Barrier Reef this year, as they did in 2016 and 2017, scientists reported on Thursday — the latest sign that global warming threatens the health of one of the world’s most important marine ecosystems.

“We can confirm that the Great Barrier Reef is experiencing its third mass bleaching event in five years,” David Wachenfeld, chief scientist of the Great Barrier Reef Marine Park Authority, said in a video posted on its website.

Water that is warmer than normal stresses the corals that create a reef, causing them to lose color and even become white. Corals that experience minor or moderate bleaching usually recover, but those that are severely bleached often die.

Scientists say reefs around the world have been dying at an alarming rate for several years because of global warming. Reef corals grow very slowly, and while most of them can live only in warm water, they are highly sensitive to above-normal temperatures.