Fact Check-Mass bleaching events in the Great Barrier Reef kill coral and risk the future of the reef

The Australian Government’s Great Barrier Reef Marine Park Authority Summer Snapshot 2021-22 document made headlines recently after it revealed over 90% of the Great Barrier Reef’s coral “exhibited some bleaching,” based on an aerial survey.

But one scientist, Peter Ridd, sought to downplay the significance of the bleaching visible from the air in a social media post, insisting that "Most bleached coral fully recovers" and that a separate report shows “the [Great Barrier] reef presently has record high, or near record high, coral cover in 2020/21” (here).

The government document -- which notes that areas of the reef showing bleaching were consistent with previously documented areas of ocean warming -- can be seen here .

Ridd does not dispute the government’s finding of visible coral bleaching but claims bleached coral does not die, it mostly makes a full recovery. He also claims air surveys only capture the top part of the reef, so cannot reflect reef health deeper down.

He says he decided to test the government’s findings, “By diving – and we found spectacular coral… Certainly, there was some bleaching but almost no mortality.”

Other social media posts have echoed Ridd’s claims that "the reef is fine" underwater (here and here), and that coral recovers from bleaching (here).

Ridd is a former academic who was terminated by James Cook University in Australia, where he had been head of the Physics department, for remarks he made about a colleague’s work investigating the impact of climate change on the Great Barrier Reef (here).

Reuters contacted Ridd for comment, and he said: “After all these mass bleaching events, which were supposedly unprecedented, the reef is doing well enough. How can that happen without there being some exaggeration about damage to the reef?”

Reef Recovery

Ridd took the Australian Institute of Marine Science (AIMS) long-term monitoring findings as his basis for asserting that the health of the Great Barrier Reef (GBR) is not under threat (here).

Although the AIMS study does show some recovery to the reef following previous stressors, it does not indicate an overall picture of reef health, which marine biologists say is increasingly vulnerable to the effects of climate change.

The study’s summary says, “hard coral cover increased across all three regions … in the last two years, and most reefs surveyed had moderate or high coral cover.” It also says that the “GBR remains exposed to the predicted consequences of climate change” and that “the observed recovery has been seen previously and can be reversed in a short amount of time.”

Professor of Marine Sciences at Stanford University, Steve Palumbi, says claiming that the Great Barrier Reef is seeing record coral cover is misleading. “The GBR is showing strong recovery this year from a record double-whammy of bleaching,” he says.

Palumbi added: “Coral cover is actually slightly lower than normal levels in the north, central and southern parts of the huge reef. But coral diversity hasn’t recovered - the increase of cover is due to a set of very fast-growing species. Like a forest of fast-growing trees, not the normal oaks.”

There are many varieties of coral on the reef, with hard coral cover of huge significance to marine scientists.

Pointing to a study (here), President of the International Coral Reef Society ( Andrea Grottoli, says, “Australia is at record low of hard coral cover (see Ch 2, page 8), most of this decline due to declines on the GBR.”

Ridd, the original social media poster, is familiar to marine biologists who claim he is known for spreading misinformation about the Great Barrier Reef.

Selina Ward, senior lecturer at Queensland University specializing in coral reef ecology and physiology, told Reuters via email that: “[Ridd] usually cherry picks small sections of data, ignoring everything around it.”

She added, “Reefs are good at recovering when there are not additional stresses. The trouble now is the frequency of stress events – bleaching events, cyclones and Crown of thorns outbreaks that damage the reef.”

Scientific fact-checkers Climate Feedback debunked previous claims by Ridd about the Great Barrier Reef (here).

Bleaching = Damage

In the original social media post, Ridd claims that “Bleaching is a common, but not very damaging, phenomenon.”

Terry Hughes, professor of marine biology at James Cook University in Australia, described this claim as “utterly false.”

He pointed to a study he co-authored, which concludes, “The increasing prevalence of post-bleaching mass mortality of corals represents a radical shift in the disturbance regimes of tropical reefs, both adding to and far exceeding the influence of recurrent cyclones and other local pulse events, presenting a fundamental challenge to the long-term future of these iconic ecosystems”(here).

That coral bleaching is common and becoming ever more common should not be taken as a reassurance that everything is fine with the reef, says Grottoli.

“It is incredibly damaging as it can lead to mass mortality events that can take years or decades to recover from -- provided there are no additional bleaching events,” she told Reuters via email.

Coral can survive bleaching events, but the frequency of bleaching – as a direct consequence of climate change – is threatening the future of the Great Barrier Reef.

Grottoli adds, “Over the last decade, 14% of the world’s coral reefs have been lost due to climate change. Even if corals don’t die, they are physiologically impacted and work in my own lab has shown that it can take more than a year to recover and that some species of corals may survive the first bleaching event, but not fare as well following bleaching the following year.”

Ward backs up her sentiments: “Even if mortality is not high, corals that have bleached take time to get their strength back, may become diseased, out-competed and take time to reproduce again. Our biggest concern of course is the decreasing interval between bleaching events, diminishing the time available for recovery.”

More information on the impact of climate extremes on coral reefs can be seen (here), (here) and (here).


False. While coral recovery on the Great Barrier Reef was high in 2020/2021 after previous stressors, this finding does not paint a picture of overall reef health or mean the reef is no longer at risk. Bleaching continues to be potentially deadly, and to threaten coral cover and diversity on the reef.

This article was produced by the Reuters Fact Check team. Read more about our work to fact-check social media posts  here .