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File photo taken in October 2016 shows coral bleaching at the Great Barrier Reef in Australia, a World Heritage Site.
CNN  — 

The southern Great Barrier Reef is suffering from extensive coral bleaching due to heat stress, the reef’s managers said Wednesday, raising fears that a seventh mass bleaching event could be unfolding across the vast, ecologically important site.

Aerial surveys conducted last weekend by the Great Barrier Reef Marine Park Authority and Australian Institute of Marine Science found bleaching was “extensive and fairly uniform across all surveyed reefs.”

The teams flew over 27 inshore reefs in the Keppel Islands and Gladstone region and 21 offshore reefs in the Capricorn Bunkers off the coast of southern Queensland last weekend.

Dr. Mark Read, the authority’s director for reef health, said most coral surveyed “displayed some level of bleaching with white and fluorescent colonies observed in shallow reef areas.”

Covering nearly 133,000 square miles (345,000 square kilometers), the Great Barrier Reef is the world’s largest coral reef, home to more than 1,500 species of fish and 411 species of hard corals. It contributes billions of dollars to the Australian economy each year and is promoted heavily to foreign tourists as one of the country’s – and the world’s – greatest natural wonders.

But soaring ocean temperatures are fueling destructive bleaching of the reef, as the world continues to burn planet-heating fossil fuels. Ocean temperatures are also becoming even hotter under the current El Niño — a natural climate pattern that brings warmer-than-average sea-surface temperatures — which is one of the strongest on record.

Bleaching occurs when stressed coral ejects algae from within its tissue, depriving it of a food source. If the water temperature remains higher than normal for too long, coral can starve and die, turning white as its carbonate skeleton is exposed.

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Dead coral found at lady Elliot island, off the coast of Queensland, Australia on October 10, 2019.

The Great Barrier Reef’s managers plan to extend aerial and in-water surveys across the entire reef over the coming weeks. While the southern part of the reef is the most affected, the reef authority has received reports of bleaching from all other regions of the marine park.

“Aerial surveys are an ideal tool to assess the spatial extent of bleaching, but we need to go under the water to understand more about the severity of bleaching and how deep the bleaching extends,” said Dr. Neal Cantin, senior research scientist with the Australian Institute of Marine Science.

A CNN team that visited the Great Barrier Reef two weeks ago observed bleaching at the southern-most coral cay, Lady Elliot Island and on four different outer reefs off Cairns, in the middle section of the reef.

And a separate report from a team at James Cook University reported areas of moderate to severe coral bleaching around the Keppel Islands, where water temperatures were well above the summer average.

“I have been working on these reefs for nearly 20 years and I have never felt the water as warm as this,” said Dr. Maya Srinivasan, a scientist at the university’s center for tropical water and aquatic ecosystem research, in a statement last week.

“Once we were in the water, we could instantly see parts of the reef that were completely white from severe bleaching. Some corals were already dying.”

Fears of seventh mass-bleaching

Scientists say corals can recover if ocean temperatures stabilize.

“We have seen fish abundance decline as coral cover declines in this region following past impacts like this. But we have also seen the recovery of coral and fish communities on many areas of reef – there just needs to be enough time between impacts to allow this recovery to occur,” Srinivasan said.

Hotter ocean temperatures caused severe mass bleaching at the Great Barrier Reef in 2016, 2017 and 2020. Previous bleaching occurred in 1998 and 2002.

Another bleaching event in 2022 – the first during a La Niña event, El Niño’s counterpart, which tends to have a cooling influence – raised serious concerns about the reef’s outlook.

There are concerns that 2024 will see a seventh mass bleaching event.

“While we must wait for official confirmation from the Marine Park Authority, it certainly looks like the seventh mass bleaching event is unfolding on the Great Barrier Reef, with reports of severe bleaching occurring along its length,” David Ritter, CEO of Greenpeace Australia Pacific, told CNN.

“We know that the climate crisis is driving marine heatwaves and leading to these bleaching events, but the frequency and scale at which they’re now occurring is frightening — every summer we’re holding our breath.”

A record marine heat wave “decimated” coral populations around Florida and the Caribbean last year and observers in Australia fear a similar fate could be in store for the country’s reefs.

“What we’re seeing in Florida and the wider Caribbean is a lesson of what’s going to happen in the next 12 months as we see this thing rolling through now,” Ove Hoegh-Guldberg, climatologist and chief scientist at the Great Barrier Reef Foundation, recently told CNN.

Last year, the warmest year since global records began in 1850, according to the US National Oceanic and Atmospheric Administration’s (NOAA) annual climate report, saw ocean temperatures surge to 100 degrees Fahrenheit (37 degrees Celsius) in some regions and the bleaching of entire reefs.

With water temperatures climbing to unprecedented heights, NOAA earlier this month added three new levels to its alert system to account for increasingly severe coral bleaching and higher mortality rates.

Alert Level 5, the new highest level classified as “near complete mortality,” means greater than 80% of corals in the highlighted area are at risk of dying due to high, long-lasting water temperatures.

Last year, the UNESCO World Heritage Committee decided not to add the Great Barrier Reef to its list of sites “in danger,” despite scientific evidence suggesting the risk of another mass bleaching event.

Greenpeace’s Ritter said that following the decision, “the Australian government promised to do everything it can to protect the Great Barrier Reef. This has to include addressing climate change as an existential threat to the reef, and ensuring our emissions reductions plans are aligned with a 1.5 degree pathway.

He added that “claims that Australia is taking the health of the Great Barrier Reef seriously ring hollow when we continue to expand and subsidize the coal and gas industry to the tune of billions every year.”

According to the Australia Institute’s Coal Mine Tracker, the Labor government has approved four new coal mines or expansions since coming to power in May 2022.

CNN’s Rebecca Wright and Hilary Whiteman contributed reporting.