Australia's spectacular Great Barrier Reef is experiencing the 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-kilometre (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 about 730 out of more than 1,000 reefs spanning the Great Barrier 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 bleached -- 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 of reefs were exposed.

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

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

Richard Leck, WWF Australia head of oceans, told AFP 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."

The Reef Authority's chief scientist Roger Beeden 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 urgent action is needed to reduce the impacts of coral bleaching, including reducing global emissions and increased 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."

- Recovery in doubt -

 

AFP journalists visited one of the worst-impacted areas of the Great Barrier Reef 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 said, 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 Aus$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 UNESCO later this year.