The Great Barrier Reef has experienced its greatest annual loss of live coral across most of its expanse in four decades of record-keeping, Australian authorities say.

Also Read | Australia’s Great Barrier Reef off UNESCO danger list, still under ‘serious threat’

But due to increasing coral cover since 2017, the coral deaths — caused mainly by bleaching last year associated with climate change — have left the area of living coral across the iconic reef system close to its long-term average, the Australian Institute of Marine Science said in its annual survey on Wednesday (August 6, 2025).

The change underscores a new level of volatility on the UNESCO World Heritage Site, the report said.

Mike Emslie, who heads the tropical marine research agency’s long-term monitoring program, said the live coral cover measured in 2024 was the largest recorded in 39 years of surveys.

The losses from such a high base of coral cover had partially cushioned the serious climate impacts on the world’s largest reef ecosystem, which covers 344,000 square kilometers (133,000 square miles) off the northeast Australian coast, he said.

“These are substantial impacts and evidence that the increasing frequency of coral bleaching is really starting to have detrimental effects on the Great Barrier Reef,” Emslie said on Thursday.

“While there’s still a lot of coral cover out there, these are record declines that we have seen in any one year of monitoring,” he added.

Mr. Emslie’s agency divides the Great Barrier Reef, which extends 1,500 kilometres (900 miles) along the Queensland state coast, into three similarly-sized regions: northern, central and southern.

Living coral cover shrunk by almost a third in the south in a year, a quarter in the north and by 14% in the central region, the report said.

Because of record global heat in 2023 and 2024, the world is still going through its biggest — and fourth ever recorded — mass coral bleaching event on record, with heat stress hurting nearly 84% of the world’s coral reef area, including the Great Barrier Reef, according to the U.S. National Oceanic and Atmospheric Administration’s coral reef watch. So far at least 83 countries have been impacted.

This bleaching event started in January 2023 and was declared a global crisis in April 2024. It easily eclipsed the previous biggest global coral bleaching event, from 2014 to 2017, when 68.2% had bleaching from heat stress.

Large areas around Australia — but not the Great Barrier Reef — hit the maximum or near maximum of bleaching alert status during this latest event. Australia in March this year started aerial surveys of 281 reefs across the Torres Strait and the entire northern Great Barrier Reef and found widespread coral bleaching. Of the 281 reefs, 78 were more than 30% bleached.

Coral has a hard time thriving and at times even surviving in prolonged hot water. They can survive short bursts, but once certain thresholds of weeks and high temperatures are passed, the coral is bleached, which means it turns white because it expels the algae that live in the tissue and give them their colors. Bleached corals are not dead, but they are weaker and more vulnerable to disease.

Coral reefs often bounce back from these mass global bleaching events, but often they are not as strong as they were before.

Coral reefs are considered a “unique and threatened system” due to climate change and are especially vulnerable to global warming beyond 1.5 degrees Celsius (2.7 degrees Fahrenheit) since pre-industrial times, the United Nations Intergovernmental Panel on Climate Change proclaimed in 2018. The world has now warmed 1.3 degrees Celsius since pre-industrial times. That report said “tropical corals may be even more vulnerable to climate change than indicated in assessments made in 2014.”

The report said back-to-back big bleaching events at the Great Barrier Reef in the mid 2010s “suggest that the research community may have underestimated climate risks for coral reefs.”

“Warm water (tropical) coral reefs are projected to reach a very high risk of impact at 1.2°C, with most available evidence suggesting that coral-dominated ecosystems will be non-existent at this temperature or higher. At this point, coral abundance will be near zero at many locations,” the report said.

Published – August 07, 2025 10:23 am IST

The Great Barrier Reef will continue to deteriorate, largely to climate change, and the window to secure its future is rapidly closing. That is the sobering conclusion of a major new report into the state of the reef.

(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)

Water temperatures in and around the Great Barrier Reef, Australia, in the past decade have been the warmest in the past 400 years. The results were published recently in Nature. These periods of warming increase the risk of mass coral bleaching and mortality and are likely driven by human-induced climate change. The Great Barrier Reef has undergone a sequence of mass bleaching events in recent years, with the events increasing in frequency since some of the first recorded episodes occurred in the 1980s. Mass coral bleaching can be spurred by warming water temperatures linked to global warming. Analysis of sea surface temperatures in the Coral Sea, which contains the Great Barrier Reef, have until now mainly been limited to recent instrumental observations.

Researchers now reconstructed sea surface temperature data from 1618 to 1995 using coral skeleton samples from within and surrounding the Coral Sea and coupled this dataset with recorded sea surface temperature data from 1900 to 2024. They identified relatively stable temperatures prior to 1900. From 1960–2024, however, they observed an average annual warming for January to March of 0.12 degree C per decade. The average sea surface temperatures for January and March in the mass coral bleaching years of 2016, 2017, 2020, 2022, and 2024 were considerably warmer than in any year in the reconstruction prior to 1900 and were five of the six warmest the region has experienced in the past four centuries. Further modelling suggests that this rate of heating post-1900 can be attributed to human influence.

The authors note there are remaining uncertainties in reconstructed sea surface temperature data due to some of the chemical proportions in the coral that are used to model temperatures being influenced by other variables such as salinity. However, these uncertainties could be reduced with additional sampling of coral cores from the region. The researchers note that even if global warming is kept under the Paris Agreement’s goal of 1.5 degree C above pre-industrial levels, 70% to 90% of corals across the globe could be lost, and that future coral reefs will likely feature a different community structure with less diversity in coral species.

An undated photo received from ARC Centre of Excellence for Coral Reef Studies shows a ‘mass bleaching event’ of coral on Australia’s Great Barrier Reef. File
| Photo Credit: AFP

A “mass bleaching event” is unfolding on Australia’s famed Great Barrier Reef, authorities said on March 8, as warming seas threaten the spectacular home to thousands of marine species.

Often dubbed as the world’s largest living structure, the Great Barrier Reef is a 2,300 km (1,400 mile) expanse of tropical corals that house a stunning array of biodiversity.

But repeated mass bleaching events have threatened to rob the tourist drawcard of its wonder, turning banks of once-vibrant corals into a sickly shade of white.

“We know the biggest threat to coral reefs worldwide is climate change. The Great Barrier Reef is no exception,” Environment Minister Tanya Plibersek said in a statement.

“We need to act on climate change. We need to protect our special places and the plants and animals that call them home.” The damaging mass bleaching event — the seventh since 1998 — was confirmed by government scientists following aerial surveys of 300 shallow reefs.

The Australian Reef Authority said it would now need to conduct further surveys to assess the severity and extent of bleaching. Coral bleaching occurs when underwater temperatures are more than one degree warmer than the long-term average.

As corals come under heat stress, they expel algae living within their tissues — draining them of their vibrant colours. “Ocean temperatures along the Great Barrier Reef have approached record levels in the past few weeks,” according to official monitoring.

Richard Leck, head of oceans at World Wildlife Fund Australia, said it was likely that masses of coral would die if ocean temperatures did not cool rapidly in the coming weeks. “This bleaching event is unfolding in an area where corals have not been previously exposed to these extreme temperatures,” he said.

Mr. Leck said climate change was “putting tremendous pressure” on the Great Barrier Reef. “The current bleaching event followed similar setbacks in the Northern Hemisphere last year,” Mr. Leck added, which caused major coral mortality in Florida and the Caribbean.

Some species of bleached coral have proven remarkably resilient and can recover if ocean temperatures cool. But professor Terry Hughes, one of Australia’s foremost coral reef scientists, said bleaching events were now happening so frequently that reefs were struggling to recover.

Recovery in danger

“The reef is no longer capable of recovering to the mix of coral species and the sizes of corals that were there 20 years ago,” he told AFP.

“The irony is that the corals that are now prevalent on most parts of the Great Barrier Reef are fast growing and rapidly regain cover, but the kicker is that they are heat sensitive and are less tolerant to the next inevitable beaching event.”

Mr. Hughes said the heat stress had increased in the past few days and would likely worsen in the coming two weeks. The fate of the reef has been a recurrent source of tension between the Australian government and the United Nations’ World Heritage Committee.

The World Heritage Committee has threatened to put the reef on a list of “in danger” global heritage sites, a move that would likely damage its allure for international tourists. Behind-the-scenes diplomacy and fierce lobbying from Australia has so far kept it off the list.

Before this event, Australia’s Great Barrier Reef suffered mass coral bleaching in 1998, 2002, 2016, 2017, 2020 and 2022.

A school of fish swim above a finger coral colony as it grows on the Great Barrier Reef off the coast of Cairns, Australia October 25, 2019.
| Photo Credit: Reuters

It might sound like science fiction, but “marine cloud brightening” is being seriously considered as a way to shield parts of the ocean from extreme heat.

We’re using water cannons to spray seawater into the sky. This causes brighter, whiter clouds to form. These low marine clouds reflect sunlight away from the ocean’s surface, protecting the marine life below from the worst of climate change.

Australia’s Reef Restoration and Adaptation Program – a collaboration between several universities, CSIRO and the Australian Institute of Marine Science – is exploring whether cloud brightening could reduce coral bleaching. As an oceanographer and engineer I lead the program’s research into cooling and shading techniques.

We started exploring cloud brightening after the mass bleaching event in 2016. First, we needed to develop and test the underlying technologies in the lab. Then we began pilot testing in the central Great Barrier Reef near Townsville during January 2020. After several iterations we have now moved beyond “proof of concept” to investigating the response of the clouds themselves.

A bright idea

British cloud physicist John Latham originally proposed cloud brightening in 1990 as a way to control global warming by altering Earth’s energy balance. He calculated that brightening clouds across the most susceptible regions of the world’s oceans could counteract the global warming caused by a doubling of preindustrial atmospheric carbon dioxide. That’s a level likely to be reached by the year 2060.

Recently, scientists have begun to consider regional rather than global application of cloud brightening. Could brightening clouds directly over the Great Barrier Reef for a few months reduce coral bleaching during a marine heat wave?

Modelling studies are encouraging and suggest it could delay the expected decline in coral cover. This could buy valuable time for the reef while the world transitions away from fossil fuels.

Lowering the heat stress on the ecosystem would produce other benefits when combined with other reef interventions – such as improved control of invasive crown of thorns starfish and planting of corals with increased heat tolerance.

But these studies also show there’s a limit to what can be achieved. Long-term benefits are only possible if the cloud brightening activity occurs alongside aggressive emissions reductions.

Cloud brightening does have risks as well as benefits, but the prospect of intermittent regional use is very different to large-scale “solar geo-engineering” proposals for shading and cooling the whole planet.

We expect the regional effect will be short-lived and reversible, which is reassuring. The technology must be operated continuously to modify clouds and could be stopped at any time. The sea salt particles sprayed in the process typically only persist in the atmosphere for one to several days.

How do you brighten a cloud?

A warm cloud (as opposed to an ice cloud) is a collection of small water droplets floating in the air.

A cloud of many small droplets is brighter than one with fewer large droplets – even if both clouds contain the same amount of water overall.

Every droplet begins with the condensation of water vapour around a nucleus, which can be almost any kind of tiny particle suspended in air.

Typically, in the lower atmosphere over land there are thousands to tens of thousands of these tiny particles suspended in every cubic centimetre of air. We call these airborne particles “aerosols”.

Aerosols may be natural such as dust, sea salt, pollen, ash and sulphates. Or they may come from human activity such as burning fossil fuels or vegetation, manufacturing, vehicle exhaust and aerosol spray cans.

In very clean maritime air, the aerosols available to form clouds are mainly sulphates and sea salt crystals. And they are few and far between, only a few hundred per cubic centimetre.

When a cloud forms under these conditions, water vapour is forced to condense around fewer nuclei, creating larger droplets and fewer of them. Large droplets reflect less light for the same volume of cloud water.

To brighten such clouds, we can spray large quantities of microscopic seawater droplets into the air. This process of atomising seawater mimics the generation of sea salt aerosols by wind and waves in the ocean. If these are incorporated into a cloud and create extra droplets, the cloud will be brightened.

Sea salt also provides additional shade by direct scattering of light.

Testing the theory

Although scientists have researched cloud brightening for more than 30 years, no one had ever directly tested the theory. In Australia, we have now developed technology to a point where we are starting to measure the response of the clouds.

We are beginning such tests with the support and permission of Traditional Owners, who have sustainably managed their Sea Country for tens of thousands of years.

Our research program involves more than 15 research institutions and has multiple levels of governance and oversight.

Not so far-fetched

Most people probably don’t realise we are already inadvertently brightening the clouds. The Intergovernmental Panel on Climate Change estimates humanity’s unintentional release of aerosols offsets around 30% of the warming effect due to greenhouse gases.

Sulphates in ship exhaust are such a potent source of aerosols for droplet formation, the passage of ships leaves cloud trails called ship tracks.

When the International Maritime Organisation introduced new rules limiting the sulphur content of marine fuels, the number and extent of ship tracks drastically reduced, especially in the Northern Hemisphere. A recent study even suggests the devastating heat wave that swept the Northern Hemisphere earlier this year was worsened by the absence of ship tracks.

The world-first research we are conducting in Australia aims to determine if we could harness the clouds in an effective, environmentally responsible and socially acceptable manner for the future conservation of one of our most precious ecosystems.