SYDNEY, Australia: Australia's spectacular Great Barrier Reef is experiencing its most widespread bleaching on record, with 73 percent of surveyed reefs damaged.

Often dubbed the world's largest living structure, the Great Barrier Reef is a 2,300-kilometer (1,400-mile) long expanse, home to a stunning array of biodiversity, including more than 600 types of coral and 1,625 fish species.

But aerial surveys conducted by scientists show that about 730 out of more than 1,000 reefs spanning the reef have bleached, government reef authorities said on Wednesday.

For the first time, extreme bleaching — which is when more than 90 percent of coral cover has whitened — has occurred everywhere on the Great Barrier Reef.

"The cumulative impacts experienced across the reef this summer have been higher than previous summers," the federally funded Marine Park Authority said in a statement.

A separate government report into this summer's event said that up to 46 percent of reefs had experienced record heat stress, compared to 2016, when only 20 percent were exposed.

This event is the fifth mass bleaching on the reef in the past eight years.

Bleaching occurs when water temperatures rise and corals expel microscopic algae, known as zooxanthellae, to survive. If high temperatures persist, the coral can eventually turn white and die.

Richard Leck, World Wide Fund for Nature Australia's head of oceans, told Agence France-Presse (AFP) that the bleaching had impacted an "unprecedented" amount of the reef, particularly in areas which had previously escaped major bleaching events.

"This is the worst event the southern reef has experienced," he said. "This scary thing about this year's event is that is it more widespread than in 2016, but we won't know for a few months what the mortality of corals looks like."

Roger Beeden, the Reef Authority's chief scientist, said climate change posed the biggest threat to reefs globally.

"The Great Barrier Reef is an incredible ecosystem, and while it has shown its resilience time and time again, this summer has been particularly challenging," he said.

Experts have warned that urgent action is needed to reduce the impacts of coral bleaching, including reducing global emissions and increasing local restoration projects.

Great Barrier Reef Foundation Managing Director Anna Marsden said the reef had " suffered its worst summer on record."

"If we do not take immediate action to reduce global emissions and implement effective conservation measures, we risk losing these irreplaceable ecosystems forever," she added.

Recovery in doubtAFP journalists visited one of the reef's worst-impacted areas this month.

Lizard Island, a small slice of tropical paradise off Australia's northeast tip that is usually teaming with vibrant coral life, resembled a watery graveyard.

Marine biologist Anne Hoggett, who has lived and worked on Lizard Island for 33 years, said when she first arrived, coral bleaching only occurred every decade or so.

Now, it is happening every year, she added, with about 80 percent of vulnerable Acropora corals on the island reef suffering bleaching this summer.

"We don't know yet if they've already sustained too much damage to recover or not," Hoggett told AFP.

Australia has invested about AU$5 billion ($3.2 billion) into improving water quality, reducing the effects of climate change, and protecting threatened species.

The country is one of the world's largest gas and coal exporters and has only recently set targets to become carbon neutral.

Whether these efforts will be enough for the reef to keep its World Heritage Status will be examined by the United Nations Educational, Scientific and Cultural Organization later this year.