The extent of coral reefs worldwide has played a key role in Earth’s climate in the past 250 million years – but not in the way you might expect.

Coral reefs are net producers of carbon dioxide because the greenhouse gas is a by-product of the formation of calcium carbonate, which makes up corals’ skeletons.

Some types of plankton also build shells out of calcium carbonate, and when they die, this mineral is buried in the seabed. When large areas of shallow marine environments are covered in coral, calcium and carbonate ions that would normally be taken up by deep-sea plankton is no longer available.

 at the University of Sydney, Australia, and his colleagues modelled the interplay between shallow-water corals and deep oceanic plankton over the past 250 million years by integrating reconstructions of plate tectonics, climate simulations and changes in sediment flowing into the sea.

They found that the balance between corals and plankton has been disrupted when plate tectonics and geomorphology lead to periods when there are extensive areas of shallow continental shelf, which is the ideal habitat for reef-building corals.

When coral reefs are less extensive, calcium and alkalinity build up in the ocean, plankton become more productive and more carbonate is buried in the deep sea, which helps to lower CO2 levels and bring temperatures down.

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There were three major periods when the carbon cycle was severely disrupted in the past 250 million years, the researchers found. These events, in the mid-Triassic, the mid-Jurassic and the late Cretaceous, involved extensive coral reefs using huge amounts of calcium carbonate, leading to big rises in sea temperatures.

When the balance between shallow-water coral reefs and deep-sea plankton breaks down, says Salles, it can take hundreds of thousands to millions of years to re-establish equilibrium.

“So, even if the system manages to recover from a huge crisis, the rebalancing is going to happen over a really long time period that is much longer than human timescales,” says Salles.

On the positive side, says Salles, if planktonic nutrient blooms are ever out of control, corals are great at taking up excess nutrients to build coral reefs.

Now, human CO2 emissions are causing global warming and ocean acidification at an unprecedented rate, which is killing both corals and plankton, says Salles. The consequences are unknown, but are likely to be ecologically catastrophic, he says.

“The deep-time feedbacks we modelled do not apply today – the pace of modern change is far too rapid for carbonate-platform feedbacks to matter in any comparable way.”

at the Australian National University in Canberra says the study shows there is a “deeply intertwined feedback cycle between life and climate”.

He says people often think that species evolve and adapt in response to Earth’s climate, which is governed by “immutable physical and chemical processes”.

“However, more and more often we are seeing examples where biological species directly influence the climate itself, creating a co-evolving feedback loop,” says Skeels. “Not just corals, but also more ancient microbial colonies like stromatolites have played a key role in modulating atmospheric carbon.

“We know that carbon warms our climate, as it is doing so rapidly today, and reefs may have contributed to this process over very deep timescales, explaining oscillating warm and cold intervals.”

A recent surge in ocean temperatures has caused the widespread bleaching and death of warm-water corals around the world, officially triggering the first climate tipping point for one of Earth’s ecosystems, scientists have declared.

The collapse of one of the world’s most diverse and fragile ecosystems poses a “human health and security hazard” that governments are unprepared for, warns at 91ɫ��Ƭy Reefs for 91ɫ��Ƭy People, a conservation programme operating in Central America.

Warm-water coral reefs support up to and provide food, coastal protection and a source of income for up to a billion people around the world. Reef services .

Yet corals are highly sensitive to changes in water temperature. Record-breaking global temperatures documented since 2023 have pushed ocean heat levels to new highs, triggering a mass bleaching event that has affected more than 80 per cent of all the world’s corals. Bleaching is when corals expel the algae living in their tissues in response to high water temperatures, which turns them white. This leaves corals vulnerable to disease, and prolonged bleaching can kill them off altogether by depriving them of their primary food source.

The latest bleaching event has been a “different order of magnitude” to anything scientists have previously witnessed, says McField. “We’re in the tipping point,” she confirms. This is generally defined as a critical threshold that, if passed, could cause dramatic and probably irreversible changes in the climate system.

McField was one of the authors of the chapter on corals in the , released today. The report, the first update since 2023, is compiled by 160 scientists from around the world and co-ordinated by the University of Exeter in the UK and campaign group WWF. It warns that warm-water corals are the first Earth system to cross over into their tipping point and are now in the throes of an “unprecedented crisis”.

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Central academic estimates suggest that the thermal limit of warm-water corals is reached when global atmospheric temperatures hit 1.2°C above pre-industrial levels, with an upper threshold of 1.5°C. In 2024, global average temperatures exceeded 1.5°C above pre-industrial levels for the first time in recorded human history, an event that has pushed the world’s coral reefs beyond the limits of their endurance, according to at the University of Exeter, who led the report.

“We’ve taken a sample of the 1.5°C world, and we have seen the consequences,” he told reporters at a press briefing ahead of the report’s launch. “A majority of coral reefs are under risk of extensive dieback [or bleaching] and tipping into the alternative seaweed-dominated, algal-covered state.”

The best hope of saving the world’s warm water corals from almost complete extinction now lies in bringing global average temperatures down to 1.2°C above pre-industrial levels as soon as possible, he says. Whether or not such an ambitious goal – which goes well beyond the demands of even the 1.5°C temperature target – is feasible to achieve is a separate question, says Lenton.

at James Cook University in Australia warns there are now “almost no unbleached reefs left anywhere in the world”. But the situation can still be mitigated. “Where coral reefs end up in the next few decades is under our control, if global greenhouse gas emissions are rapidly curtailed,” he says.

Is climate change accelerating and is it worse than we expected?

With temperature records tumbling, it is only natural to worry about cascading tipping points, but the reality is far more nuanced

Often the point at which climate tipping points could be triggered is highly uncertain, but researchers warn the widespread decline of the Amazon rainforest, the melting of polar ice sheets and the collapse of the crucial AMOC ocean current could all happen at warming levels below 2°C.

But people can also trigger “positive tipping points” to mitigate the risk, Lenton stresses, highlighting the exponential growth of renewable energy over the past decade and the rapid take-up of electric vehicles. Rapid adoption of cleaner technologies has the potential to deliver emissions cuts at the scale needed to keep warming below 2°C, the report notes.

In a statement, Lenton said urgent action is needed from world leaders at the upcoming COP30 summit in Brazil to accelerate emissions cuts across the global economy and minimise the amount of time global temperatures spend above 1.5°C. “We are rapidly approaching multiple Earth system tipping points that could transform our world, with devastating consequences for people and nature. This demands immediate, unprecedented action from leaders at COP30 and policy-makers worldwide,” he said.

There might be an upside to the loss of coral reefs. Their decline would mean oceans can absorb up to 5 per cent more carbon dioxide by 2100, researchers estimate, slowing the build up of this greenhouse gas in Earth’s atmosphere.

“It is a beneficial effect if you only care about the concentration of CO2 in the atmosphere,” says at Sorbonne University in Paris, France. But the decline of corals will also reduce biodiversity, harm fisheries and leave many coasts more exposed to rising seas, he says.

How much the world will warm depends mainly on the level of CO2 in the atmosphere. So far the land and oceans have been soaking up around half of the extra CO2 we have emitted. Any factors that increase or decrease these so-called land or ocean carbon sinks could therefore have a significant impact on future warming.

It is often assumed that corals remove CO2 from seawater as they grow their calcium carbonate skeletons. In fact, the process, also known as calcification, is a net source of CO2.

“You’re taking inorganic carbon in the ocean, generally in the form of carbonate and bicarbonate ions, turning it into calcium carbonate and that process releases CO2 into the seawater, some of which will be lost to the atmosphere,” says Kwiatkowski.

This means that if reef formation around the world slows or even reverses, less CO2 will be released by reefs and the oceans will be able to absorb more of this greenhouse gas from the atmosphere – a factor not currently included in climate models.

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Observations suggest coral reef calcification is already declining as rising seawater temperatures cause mass coral bleaching and die-offs. The higher level of CO2 is also making oceans more acidic, which can make it harder to build carbonate skeletons and even lead to their dissolution.

Kwiatkowski and his team took published estimates of how corals will be affected by warming and ocean acidification and used a computer model to work out how this might change the ocean sink in various emission scenarios. They conclude that the oceans could take up between 1 and 5 per cent more carbon by 2100, and up to 13 per cent more by 2300.

This doesn’t take account of other factors that can cause reef decline such as overfishing and the spread of coral diseases, says Kwiatkowski, so might even be an underestimate.

On the other hand, the work assumes that corals aren’t able to adapt or acclimatise, says at the University of Hawai’i at Manoa, who wasn’t involved in the study.

“If the worst-case or even medium-case scenario in this study comes to pass, it means the near-total destruction of coral reefs globally,” says Jury. “I think that with consideration of realistic levels of adaptation and acclimatisation by corals and other reef organisms, the authors might come to different conclusions under a low to moderate level of climate change.”

If Kwiatkowski’s team is correct, it means that the amount of emitted CO2 that will lead to a given level of warming – the so-called carbon budget – is a little larger than currently thought.

“I think we would like our budgets to be as accurate as possible, even if we’re blowing through them,” says Kwiatkowski.

In the south-west Pacific, off the coast of one of the tropical Solomon Islands, a giant structure beneath the water’s surface has just been identified as the world’s largest known coral.

Visiting the remote site in mid-October, a team of scientists and film-makers from National Geographic thought the object was so large, it must be the remains of a shipwreck.

But when underwater cinematographer jumped into the water to take a closer look, he was astonished by what he saw.

“I remember perfectly just jumping and looking down, and I was surprised,” he told reporters during a briefing. Instead of a shipwreck, San Félix had stumbled upon the largest coral ever discovered. “It is enormous,” he said. “The size is close to the size of a cathedral.”

The coral, which lies a few hundred metres off the eastern coast of Malaulalo Island, has been identified as the species Pavona clavus. It measures 34 metres by 32 metres, making it larger than a blue whale, and is thought to be 300 years old.

The discovery was a “happy accident”, says of National Geographic’s Pristine Seas project, which aims to inspire governments to protect ocean ecosystems through exploration and research. It is by far the largest single coral colony ever discovered, easily beating – a giant Porites colony found in American Samoa in 2019, which was 22.4 metres in diameter and 8 metres in height.

Over the past two years, record-breaking ocean temperatures have triggered a wave of coral bleaching events across the world. But while other reefs around the Solomon Islands are showing signs of bleaching, Sala says the huge P. clavus coral is looking healthy. It is a vital habitat for ocean life, he says, providing shelter and food for fish, shrimp, worms and crabs. “It’s like a big patch of old growth forest.”

But the coral isn’t immune from ecological threats, from local pollution and overfishing to global climate change. Sala says he would like to see more marine protected areas (MPAs) established to shield marine life from local pollution, alongside global action to tackle climate change. “Protecting the reef cannot make the water cooler, cannot prevent the warming of the ocean,” he says. “We need to fix that, we need to reduce carbon emissions. But MPAs can help us buy time by making the reefs more resilient.”

Here’s how coral reefs might survive hotter, more acidic oceans

Coral reefs around the world are at risk of collapse due to rising temperatures and acidifying oceans, but the natural adaptability of some species may help buy time