Global warming caused by human activity is warming the seas and remains the biggest threat to the reef, according to an Australian government report.
It says actions to save the reef, which stretches more than 1,400 miles and is a World Heritage Site because of its scientific value, “have never been more time critical”.
Unesco, which grant’s world heritage site status, is due to consider whether the reef should be on its list of endangered sites.
Imogen Zethoven, director of strategy for the Australian Marine Conservation Society, told the BBC the country’s government must step up efforts to save the reef.
“Saving it means being a leader here and internationally to bring greenhouse gasses down,” she said.
Under Australian law, the Great Barrier Reef Marine Park Authority must produce a report on its state every year.
In 2009, they said the reef was “at a crossroads between a positive, well-managed future and a less certain one".
The Greater Barrier Reef, off the coast of Queensland, is the world’s largest coral reef system composed of more than 2,900 individual reefs and 900 islands.
Rising sea temperatures caused “mass bleaching events” in 2016 and 2017 that wiped out coral and destroyed habitats for other sea life.
The number of new corals dropped by 89 per cent on a huge stretch of reef.
“Threats to the reef are multiple, cumulative and increasing,” the report says. “The window of opportunity to improve the Reef’s long-term future is now.”
The marine park authority’s chief scientist David Wachenfeld told a press conference in Sydney the reef’s problems were “largely driven by climate change”.
“Despite that, with the right mix of local actions to improve the resilience of the system and global actions to tackle climate change in the strongest and fastest way possible, we can turn that around,” he said.