SYDNEY, Australia — Scientists have again sounded the alarm about Australia’s imperiled Great Barrier Reef, saying that by the 2030s it could see devastating mass bleachings as often as every two years unless greenhouse gas emissions are drastically reduced.

“This would effectively sign the death certificate of one of the world’s largest living marine structures,” said Martin Rice, acting chief executive of the Climate Council, a publicly funded Australian research institute.

The Great Barrier Reef, the largest coral reef system on earth, was struck in 2016 and 2017 by massive die-offs of coral — caused by extreme ocean temperatures — that erased much of its dazzling color. Scientists said that while the reef would partly recover, it would never look the same again.

Nearly a third of the reef’s coral were killed, and the damage radically altered its mix of coral species, scientists said.