The world’s corals are bleaching. Here’s why and what it means for the ocean’s future.

Two prominent research organizations confirmed a few weeks ago what scientists had long thought: the world’s warm water coral reefs are bleaching en masse for the fourth time in 25 years.  

Corals are collections of hundreds of thousands of tiny animals. Often a riot of colours, they turn a boney white when they are under stress, which can be a precursor to their death. 

Scientists believe surging ocean temperatures, driven in ​​​​part by climate change, are responsible for the latest bleaching event, which has spanned from Panama to Australia – ​​and is getting worse.  

Experts say corals are among the most vulnerable ecosystems on the planet to climate change. These undersea cities, which support ​​​​25 per cent of marine life, could virtually disappear by the end of this century. 

“The loss of corals would be a true tragedy from a biodiversity and economic perspective,” says Leticia Carvalho, the Head of the Marine and Freshwater Branch of the United Nations Environment Programme (UNEP). “It would be devastating for one of the richest ecosystems on our blue planet and hundreds of millions of people around the world who depend on coastal fisheries.” 

Here is a closer look at the global bleaching event and what it could mean for the future of the world’s corals. 

Distribution of warm water corals. Credit: UNEP

Corals can be found throughout the ocean, from the balmy waters of the Red Sea to the frigid depths of the North Atlantic. But perhaps the best known live in the warm, shallow waters of the tropics, where they form stunning, multi-coloured reefs. 

At the heart of each individual coral is a polyp, a clear, tube-shaped animal with a ring of tentacles that it uses to catch prey. Some corals surround themselves with a limestone skeleton forged from calcium absorbed from sea water. Corals get their colour from thousands of plant-like organisms known as zooxanthellae, which live inside the corals and provide them with food. It is a symbiotic relationship that is fundamental to the health of reef ecosystems around the world.

 Despite their tough exterior, warm water corals are sensitive creatures. When water temperatures rise, they become stressed and expel their zooxanthellae, leaving them prone to disease and starvation.  

That is why researchers are so worried about a heat wave sweeping across ​​​​the ocean. Since early 2023, water temperatures have risen by as much as 5°C in some places. In February 2024, the average global sea surface temperature had passed 21°C, a record high. Just a few weeks later, perhaps the world’s most-famous collection of corals, the Great Barrier Reef, was baking under unprecedented heat. 

Ocean temperatures have been spiking for more than a year, with darkest of red sections representing waters that are at least five degrees hotter than normal. Data courtesy Allen Coral Atlas

Researchers say a combination of climate change and the El Nino weather event are driving the record sea surface temperatures.  

Like a wildfire moving through brush, the ocean heat wave has ravaged corals. Scientists recorded bleaching in 53 countries from February 2023 to April 2024, according to the United States' National Oceanic and Atmospheric Administration and the International Coral Reef Initiative, which is supported by UNEP. That number, American data suggests, has since climbed to 62 nations. 

Among the hardest hit areas is the Great Barrier Reef, where nearly 80 per cent of coral outcrops had bleached, according to a report from the Australian government. 

Corals along the Great Barrier Reef have turned a boney white as a bleaching epidemic sweeps across the UN World Heritage Site

The Great Barrier Reef has captured the most attention but researchers say similar situations are playing out in the Caribbean Sea, the South Atlantic, the Red Sea, the Gulf of Mexico, the Western Indian Ocean and the waters of East Asia.  

Reefs that were once brimming with life have been reduced to what Carvalho from UNEP called “graveyards”, filled with the grey and white carcasses of dead and dying corals. 

Researchers believe the bleaching event is continuing to gain steam and could soon become the most-widespread on record. 

“This crisis isn’t over,” says Carvalho. “We could be going from bad to worse if we don’t act now to reduce ​​the greenhouse gas emissions driving climate change​​.” 

During the last year, a wave of coral bleaching has spread across the waters of the tropics. Data courtesy Allen Coral Atlas/National Oceanic and Atmospheric Administration 

Bleaching is not always fatal for corals. If water temperatures cool quickly enough the animals can recover.  

The problem: bleachings are lasting longer and coming in rapid-fire succession. This year’s is the fourth since 1998 and second in the last decade. It follows a devastating bleaching that stretched from 2014 to 2017 that left about 9 per cent of the world’s corals dead. 

Repeated bleaching has contributed to an unmistakeable trend: corals are disappearing. Between 2009 and 2018, the world lost 14 per cent of its coral cover, according to a 2020 study from the Global Coral Reef Monitoring Network, which is supported by UNEP. 

This graph shows the percentage of tropical reefs that are covered in live corals. Coverage was stable until a series of mass bleaching events began in 1998. Credit: UNEP

It remains to be seen just how deadly this year’s bleaching will be. But a large-scale die-off would have far-reaching consequences for sea life and humans. Corals underpin coastal fisheries, which support 1 billion people. 

The long-term outlook for most warm water corals is grim. Humanity continues to churn out the greenhouse gases that have warmed the planet about 1.2°C since pre-industrial times. The vast majority of that heat is being swallowed by the ocean. In recent years, four key climate change indicators – greenhouse gas concentrations, sea level rise, ocean heat and ocean acidification – set new records. 

Even if the world manages to reach the most ambitious goal of the Paris Agreement on climate change – limiting global temperature rise to 1.5°C – 70 per cent to 90 per cent of reef-building corals are expected to die. If temperatures rise 2°C, 99 per cent will perish. 

The images show corals before, during and after bleaching events. If water temperatures remain too high for too long, corals will die, becoming a muddy brown. Credit: The Ocean Agency/The Ocean Image Bank

Researchers have found pockets of water off the coasts of Australia, Egypt, Indonesia, Kenya, Malaysia Saudi Arabia, Tanzania and where corals are resisting the ravages of climate change. In some of these safe havens, known as refugia, the upwelling of cold water from the ocean depths is keeping corals cool. In others, corals themselves have a higher tolerance for heat. 

Conservationists are focused on protecting refugia and other coral hotspots from stressors like pollution, over-fishing and coastal development. The hope: give corals the best chance possible to survive climate change. 

In recent years, governments and international organizations have stepped up efforts to safeguard corals. The Global Funds for Coral Reefs is aiming to channel up to US$3 billion towards projects that protect these underwater cities. The fund, a partnership between the public and private sectors, is designed to support a broader global effort to protect biodiversity while helping coastal communities adapt to a changing climate. The fund is co-chaired by UNEP and the government of the United Kingdom and Northern Ireland.   

Meanwhile, 45 countries, home to most of the world’s corals, have signed a pledged to protect 125,000 square kilometres of reefs, an effort known as the Coral Reef Breakthrough.  

Corals in some parts of the world, like Egypt’s Red Sea coast, have proven more resilient to climate change, leading researchers to hope some may survive on a warming planet. Credit: Ocean Image Bank/Cinzia Osele Bismarck 

Still, to save corals, the world must make a dramatic shift away from fossil fuels while reining in local threats to reefs, says Carvalho. 

​​​Equally important is understanding how little humanity still knows about the ocean, she adds. Only 20 per cent of its depths have been explored and new discoveries are constantly being made. Case in point: scientists recently found what are known as red hydrocorals in Chile and Antarctica. The corals, which form rare ecosystems brimming with life, have never been documented so far south.  

​​​The UN Decade on Ocean Science, which runs from 2021 to 2030, is designed to help fill knowledge gaps. Carvalho says the research being done under its auspices will support informed decisions on how to protect and sustainably use ocean resources. 

“The future of coral reefs is worrying,” says Carvalho. “But their fate is not sealed. We still have time to protect many of these spectacular ecosystems if we act now and if we act decisively.” 

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This story was first published by UNEP.

World Oceans Day takes place on 8 June, with the 2024 event celebrated on 7 June. An annual celebration, the day is designed to raise awareness about the threats to the ocean and drive its sustainable use. This year will focus on humanity’s relationship with the ocean, a key source of everything 

​​​The Global Fund for Coral Reefs (GFCR) was established to accelerate urgent investment and action to enhance the resilience of coastal reef ecosystems, communities and economies. The GFCR comprises two major financial vehicles: a UN-led Grant Fund and a private-sector-led large-scale private equity impact Investment Fund. The GFCR aims to promote sustainable ocean production, spur sustainable coastal development and ecotourism, forge circular economies and pollution management, and enable technologies for a sustainable future.