As of last month, those sounds have quieted further. More than two-thirds of the world’s coral reef areas experienced bleaching-level heat stress over the past year, according to data from the US National Oceanic and Atmospheric Administration (NOAA).

In April, NOAA announced that the fourth global coral-bleaching event on record — and the second in a decade — is underway. This means that bleaching has occurred, or is occurring, in all the ocean basins that support warm-water coral life, in both hemispheres.

Drone footage of Australia’s Lizard Island indicates that about 97% of corals have died over the past three months. In India, NOAA has issued a red alert for the Gulf of Mannar, indicating a high likelihood of mass coral mortality. Marine heatwaves in the Lakshadweep Sea since October have destroyed reefs there.

An estimated 99% of all existing reefs will die out as the planet reaches and breaches 2 degrees Celsius of warming, likely by 2100. (The planet is currently, on average, about 1.2 degrees Celsius warmer than the pre-industrial levels of the 1850s.)

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Given that warming will likely intensify, at least in the short term, what is the outlook, and why does it matter?

First, the outlook. Coral won’t go extinct, oceanographers say; some species will die out and others will step in, so most reefs will look very different, as temperatures rise.

Why? Well, corals are unusual animals. They are hard (like rocks) and live rooted to the ground (like plants). But they spawn to reproduce, as animals do; and like other animals, cannot produce their own food.

Coral polyps get their nutrition (and their colour) from microscopic algae called zooxanthellae that live in their tissues. Changes in seawater temperature, light or nutrients can cause the algae to release toxic chemicals, forcing the corals to expel them. This leaves the polyps translucent, starving and vulnerable to disease.

As temperatures rise, corals may be replaced by algae in some regions. In others, more weedy species may persist, says Ian Enochs, head of a coral research programme at NOAA’s Atlantic Oceanographic and Meteorological Laboratory. “These species don’t create the same complex habitats that reef-building corals do, and so they don’t support all the biodiversity and ecosystem services that a healthy reef provides.”

Reefs may shift into deeper waters, adds Alexander Neufeld, science programme manager at the Florida-based non-profit Coral Restoration Foundation (CRF). “Or we may see new kinds of reefs on which corals continue to exist but where they are no longer the foundational keystone species.”

Why does this matter?

Coral reefs are often called the rainforests of the sea. They support large swathes of oceanic ecosystem.

An estimated 25% of all marine species intersect with such reefs at some point in their lifecycle, says Derek Manzello, coordinator of the NOAA online research initiative and resource Coral Reef Watch.

They also act as a natural barrier, protecting coastlines from erosive waves and violent storms. This protection helps other vital habitats, such as seagrass beds and mangrove ecosystems, flourish. These, incidentally, are carbon sinks, which means that their well-being has implications for far larger ecosystems too.

Meanwhile, many of the fish that such reefs support are filter-feeders, consuming particulate matter and other pollutants and keeping seawater clean or feeding on corals and excreting fine grains of biogenic sand that make up seafloors and beaches.

Any disturbance that kills enough corals forces reefs into a net erosional state, which means the architecturally complex reef structure gets broken down, sometimes quickly, says Manzello. “Reefs will soon be flatter and less topographically complex, which has serious ramifications for local biodiversity.”

A flattened reef will provide considerably less coastal protection from storms too.

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For now, there is some good news.

Dramatic new efforts are underway, to keep reefs from fading or flattening. The new efforts include a form of in-vitro fertilisation (IVF) in which reproduction is boosted and monitored. There are experiments underway with coral “gyms” and nurseries, and evacuation efforts that include keeping some specimens safe on land, in tanks, for a time, and later restoring them to hospitable zones in the sea.

Eventually, though, the primary driver of large-scale coral reef decline must be addressed, “and that is climate change and ocean warming,” says Manzello.

More needs to be done in India, adds Mridula Ramesh, founder of the Sundaram Climate Institute and author of The Climate Solution. “While the coral-climate story is grim, we can’t blame everything on climate change alone. Preventing effluents from being dumped into sensitive waters and promoting sustainable fishing practices are immediate practises that can help and need greater attention.”

Manzello agrees. “To give corals the best possible chance, we need to reduce every other stressor that we can control.”

How can you help? If you’d like to be more involved, the NGOs ReefWatch and Coastal Impact invite citizens to pitch in, and CRF and NOAA offer volunteering opportunities in India too.

Survivor: Marine edition

It was like a massacre. In the summer of 2023, the Sombrero Reef was almost obliterated, when sea surface temperatures — which usually hover between 23 and 29 degrees Celsius in the string of islands that make up the Florida Keys — shot up to 32.5 degrees Celsius.

The non-profit Coral Restoration Foundation (CRF) watched as the protected site they had been working to restore for a decade, was lost.

Desperate, they began to rescue still-living corals from the nearby waters of the Upper and Lower Keys. Working with the US National Oceanic and Atmospheric Administration (NOAA) and with other non-profit groups, thousands of survivor colonies were lifted out of the warming ocean and moved to temperature-controlled tanks, where they stayed until their home was habitable again.

When temperatures dropped in October, the evacuated corals were relocated to the sea. This helped preserve at least some of the region’s genetic diversity.

This was the first such rescue effort, but CRF has successfully rehabilitated corals over and over within the ocean, over the past decade. Small bits nurtured in “nurseries” are reattached to parent reefs, to keep the latter from fading.

The “nurseries” are made up of vertical structures called coral trees that stand in the sea. Here, fragments are exposed to 360-degree sunlight and high nutrient loads. In six months, a fragment the size of a finger can grow to the size of a football.

Each attempt at rehabilitation gives the scientists more metrics to test: Can they make the corals more resistant to heat; boost their immunity to certain diseases?

But all the “sciencing” is merely a stopgap, says Alexander Neufeld, science programme manager at CRF. “What we need is for pollution and climate changes to be curbed, for long-lasting impact.” That said, Neufeld adds, “there are more corals on the reefs in Florida today than there would have been if we had spent the last 20 years doing nothing.”

Inner piece: Lab-evolved algae

One of the ways to make reefs more heat-resistant is to breed more heat-resistant algae.

After all, it is the algae that respond to the temperature, releasing toxic chemicals that cause the polyps to expel them. Without the algae, the corals then lose their colour, and their source of nutrition (like all animals, they cannot make their own food).

Previous attempts to breed heat-resistant algae have failed. But, in 2012, researchers at the Australian Institute of Marine Science decided to give it a fresh shot.

They evolved 10 clonal strains of a common coral microalgae, over four years, in a laboratory, with the key focus being resilience to high temperatures. Findings published in journals such as Science Advances in 2020 showed some of these species proved beneficial to keeping corals alive in temperatures as high as 31 degrees Celsius (most corals begin to bleach if sea surface temperatures stay at 31 degrees Celsius for more than three to four weeks).

“This is really promising, and could be combined with other interventions,” says lead researcher Patrick Buerger, who now heads an applied-biosciences lab at Macquarie University.

There’s still a lot to learn about the survival of these experimental strains in the wild, he adds. Will there be trade-offs in terms of nutrient production, coral growth, reproductive output or disease susceptibility? Can the process of experimental evolution be sped up further? For now, though, he repeats, it is a step in the right direction.

Breeding super coral

It was a mesmerising sight at Australia’s Great Barrier Reef that spurred marine biologist Peter Harrison into action, in 1981. During a night excursion there, he saw trillions of microscopic eggs and sperm being released by multiple coral species in a mass spawning event.

Most of the eggs and sperm would become fish food, he knew. And that gave him an idea: Could spawn from one healthy, hardy reef be collected, saved and used to rejuvenate it and others?

The procedure has since come to be called coral in-vitro fertilisation (IVF) or larval restoration. The spawn of heat-tolerant specimens that have survived bleaching are captured and used to rear larvae in special floating reef pools on the surface of the ocean. The larvae are then transplanted onto degraded sections of a reef, so they can replenish them.

Since 2012, this method has been successfully used in areas where reefs have been degraded by blast fishing (where explosives are used to stun or kill fish) in the Philippines. More recently, it was used on the Great Barrier Reef and in the Maldives.

“The restored populations have a higher chance of surviving future changes,” says Harrison, founding director of the Marine Ecology Research Centre at Southern Cross University, Australia. This method is also expected to prove helpful as populations dwindle, leaving fewer of a species to reproduce.

Workouts in a ‘gym’

At the Experimental Reef Lab (ERL) at the University of Miami, corals are on a unique fitness journey.

Researchers have built 16 large tanks that mirror certain projected ocean conditions, such as changes in acidity levels, temperature and light. Within these tanks, they then conduct experiments on different species, with the aim of monitoring their response and finding ways to boost immunity and resilience.

Working on autopilot, a suite of open-source robotic arms, built in-house by scientists at NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML) , injects specific doses of nutrients, diseases and chemicals such as nitrogen and phosphorus into each tank.

The simpler workouts involve being exposed to frequent pulses of warm sea water twice a day. “Think of this like going to the gym and working out really hard, then going home to relax and recover. Do that every day and it tends to make us stronger, more capable of dealing with stress,” says Ian Enochs, head of the coral programme at AOML.

Findings published last year show that resistance to heat is growing. “This ‘training’ regimen is akin to an athlete preparing for a race,” the study’s lead author, Allyson DeMerlis of the University of Miami’s Rosenstiel School, said in a statement.

Hypoxia (or low oxygen levels), ocean acidification and diseases such as Stony Coral Tissue Loss Disease (SCTLD) are the major threats that scientists are now working on.

“We can eventually identify particularly strong species and individuals that are more resilient and target them for restoration in the ocean, just like we might target more hardy crop strains for agriculture,” says Enochs. The challenge is scaling up fast enough, he adds. “We have a lot of work to do before we perfect this, and unfortunately not a lot of time to do it in.”