In October 2016, the American magazine Outside published an obituary for the Great Barrier Reef (GBR). The piece was widely criticized at the time for being premature.
Recently, a panel of scientists at the 6th International EcoSummit in Gold Coast, Australia, in June 2023, concluded that prospects for the GBR 50 years from now are bleak. Experts believe that the biggest threat to corals is heat stress from climate change. If this is is left unaddressed, coral bleaching could become more frequent.
In coral bleaching events, algae living in coral tissues, which gives them their effervescent colors, are expelled by the corals, turning them completely white. Corals can survive bleaching events, but are at greater risk of death. Experts believe that while some corals have the ability to gradually adapt to increased temperatures, and improving water quality could mitigate some damage, the corals are destined to go extinct in 50 years if carbon emissions aren't controlled.
Australian ecologist Morgan Pratchett said that the GBR is one of the best-managed coral reef systems in the world, but a sustained decline was seen in one of the key metrics of coral health—live coral cover. This metric measures the proportion of reef surface covered by live stony corals to other organisms. Live coral cover has declined from 40% in the 1960s to close to 10% in the 2010s.
Pratchett, who works at the ARC Centre of Excellence for Coral Reef Studies at Townsville, said critics often tell him that reefs with high coral cover still exist (in the GBR).
“But, I argue that there has been a major reduction in the reefs with high cover. Many reefs are falling into the situation where they have less than 20 or 10% coral cover.”
Clothilde Langlais, a physical oceanographer at the Commonwealth Scientific and Industrial Research Organisation in Hobart, said that ecosystem models on the GBR predict coral cover of 5% by 2070.
Models that included the impact of interventions like cloud brightening did show an effect on the decline of coral cover. Cloud brightening increases the cloud cover in a region to reduce the heat and light stress on a reef. “But not a big one, I would say. It will delay just a bit, but the drop will still happen. So it’s not very positive,” she added.
Pratchett listed the reasons for the decline in cover: outbreak of crown-of-thorns starfish, climate-induced coral bleaching, and perennial threats associated with cyclones and major storm events.
The most pressing threat is climate change, which manifests itself as coral bleaching. The last mass bleaching event in 2016 saw up to 75% of corals die in the GBR.
The problem is not restricted to the GBR, said Pratchett. Some studies estimate that more than 50% of corals alive in our lifetime have died due to coral bleaching, he added.
Like the GBR, the greatest threat to India’s corals is coral bleaching due to heat stress. Three mass bleaching events occurred in 1998, 2010 and 2016, impacting all major Indian coral reef regions.
Indian coral reefs are perhaps at a greater threat. According to the IPCC’s latest climate change report, sea surface temperatures in the Indian Ocean are rising faster than the global average.
India has five main reef structures—Lakshadweep, Andaman and Nicobar Islands, the Gulf of Mannar in Tamil Nadu, the coast of Malvan in Maharashtra and parts of Goa, and Gulf of Kutch in Gujarat. While Lakshadweep has atoll reefs, with ring-like features around the island, the remaining ones are fringing reefs. In India, most corals are listed under Schedule One of the Wildlife Protection Act, giving them the highest protection.
It’s not just coral deaths that are worrying, but also the lack of replacement, Pratchett said. His study looked at how many new corals were formed on sampled reef sites along the GBR a year after successive ocean heatwaves in 2016 and 2017.
“In the aftermath of the major bleaching event that occurred in 2016, the amount of coral settlement on reefs, on pretty much the entire length of the GBR, is a fraction of what it was before the bleaching.”
The study found that population of some species like the branching coral Acropora tumbled by 93%.
When coral cover reaches less than 10%, it can have a massive effect on the ecosystem. Pratchett said that at less than 10% of coral cover, up to 70% of the fish disappear. He added that they go locally extinct if not majorly decline in abundance. Presenting data from a paper he co-authored, which looked at changes in the abundance of coral fishes in the GBR following climate induced coral bleaching, Pratchett said that 45 of 116 species exhibited significant changes. “Overwhelmingly, fishes decline in abundance. It doesn’t matter if we are talking about top predators, which should have no apparent reliance on corals, or the coral dwelling damselfish.”
Pratchett said that this has a major impact not only on biodiversity but also on half a billion people worldwide who derive their protein daily from coral reef ecosystems.
India has also reported lower fish catch in coral reef areas. Fishermen in Lakshadweep, where according to the non-profit Nature Conservation Foundation (NCF), coral cover has fallen from 51.6 % in 1998 to 11% in 2017, have reported declining catch for grouper fishes. A study documenting changes after the 2010 bleaching event in Lakshadweep, found that grouper fishes migrated to the relatively more stable eastern reefs from the more vulnerable western reefs.
Emily Howells, a coral biologist from Southern Cross University, Australia, said that two factors would determine the future trajectory of corals. The first is how fast warming happens, which depends on emissions. And the second is the evolutionary potential of corals to adapt to heat stress.
She said several corals live for decades and some for hundreds of years, and adapt to warming conditions over their lifespan. Some coral species can even switch out their algae partners to a more heat-tolerant species.
“So, it’s not whether corals have the ability to adapt to heat. It’s whether they have the ability to adapt fast enough,” she said.
Recent studies have shown that there is genetic variability in corals. During a bleaching event, you’ll see individuals of the same species living side by side on the reef. One may be bone white — bleaching on the pathway to death, and the other faring a little better, said Howells.
“We can identify which corals have the most heat-tolerant genetic variance. And we can target those for breeding programs. That is what is happening now, and showing encouraging findings.”
The hope Howells projected came with a caveat. “But there is an upper ceiling for physiological acclimatization,” she added.
In 2022, scientists from the Zoological Survey of India (ZSI), studying the massive coral bleaching even in 2016 in the Andaman sea, found that heat tolerance of different coral species in the Andamans varied greatly. Some species showing good tolerance to heat stress and bounced back immediately after a bleaching event. Others that were more susceptible to bleaching were found to be slower to recover.
In 2016, coral bleaching up to 83.6% had been recorded in the Andaman sea coastal areas, with a maximum of 91.5% in the South Andaman region. Almost one-fourth of live coral cover was lost due to the mass bleaching. The ZSI study concluded that the coral bleaching event in 2016 could be attributed to climate change.
Mass coral bleaching events have occurred at least once a decade in India. In 2010, 70% of live coral cover was lost in some of the Andaman Islands. In 2004, the tsunami caused loss of 30% coral cover in North Andaman, and 20% in South Andaman.
While tackling climate change is critical, Pratchett said that other anthropogenic pressures, such as sedimentation, pollution, and eutrophication on the reefs, are starting to constrain corals’ recovery. “If we can allow coral reefs to be more resilient to bounce back from these disturbances, then we might have higher coral cover, at least in the short term, while we get on top of the mission.”
For instance, there is a debate on whether improving water quality could stop outbreaks of crown-of-thorns starfish. Pratchett said a peak outbreak could see anywhere between 5 to 13 million starfishes on the GBR, with each starfish capable of eating five square meters of live coral a year. So improving water quality and undertaking innovative restoration projects are part of the toolkit, he suggested.
Howell said that other options include both biological and engineering solutions. According to Howell, one of the promising ones is the capture of coral spawns. Only 1% of coral spawns make it back to the reef. “We can capture that and rear them to the larval stage, and put them back on the reef. We can harness a lot of pawns that were not going forward to make a coral in the reef.”
There are other ongoing programs designed to maximize coral survival on the reefs with predator exclusion devices and cloud brightening, which increases cloud cover in a particular area to reduce heat and light stress on the reef.
India has taken up coral restoration and regeneration projects at a few reef sites. Coral transplantation has been carried out in all five reef regions in India, in varying capacities. India’s first experiment with coral transplantation to restore the degraded coral reefs was in 2002 in the Gulf of Mannar in Tamil Nadu. In 2008, a coral recovery program was launched in the Gulf of Kutch in Gujarat in partnership with the local community. Restoration using Mineral Accretion Method device have been attempted in Andamans—this method uses metal frames to help coral grow around them.
Improving water quality leaving the river systems and feeding into the GBR lagoon is a significant area of interest for the government. Andrew Brooks of Griffith University, who works in the GBR catchment area, said the Queensland government has committed over a billion (Australian) dollars to manage coral reef stressors. “At the moment, 50% is in water quality management, and 50% is on direct intervention on the reef.”
However, measuring improvement in water quality is very difficult, Brooks said.
“We are dealing with problems up in the catchment area and measuring the response down at the end of the system. It’s a difficult thing to pin down.”
The Great Barrier Reef is the world’s largest connected coral reef system. It’s larger than most countries in the world. Pratchett said that the big question is if interventions can be done at a scale that makes a difference.
Howell added that these interventions would hardly make a blip on the future trajectories of corals, ultimately.
“Our restoration targets are going to get harder and harder with the further delay in emissions reduction,” she said. “We have been trying to make corals cope with 1.5 degrees of warming. We don’t want to be doing this with 3 degrees of warming. Because that would be in the realm of impossible.”
Coral reefs in India are also facing other human-caused threats. For instance, the 72,000 crore mega-infrastructure project on the Great Nicobar island involves clearing forest areas and coral reef areas for the project. A report by the ZSI that is part of the Environmental Impact Assessment (EIA) for the project recommends transplanting the coral beds to an alternate suitable offshore habitat. Many conservationists have denounced these moves as some of these corals are century-old big boulders, and could nearly kill them.
This story was supported by Internews' Earth Journalism Network. It was first published in The Wire on 18 July 2023 and has been lightly edited for length and clarity.
Banner Image: The Great Barrier Reef / Credit: Lachlan Rogers via Flickr (CC BY-SA 2.0).