Despite their beauty and importance, coral reefs around the world are disappearing fast. Water pollution, overfishing and coastal development are taking their toll on coral reefs at the local level, while carbon pollution threatens reefs worldwide and remains their biggest threat.
Record-breaking marine heatwaves are causing mass coral bleaching on the Great Barrier Reef and coral reefs globally. Since 2016 the Great Barrier Reef has suffered five mass bleaching events. In 2024, some regions in the southern reef experienced elevated water temperatures that broke all previous records for the duration of weeks. In March 2022, 91% of the Reef bleached for the first time during a La Nina weather event, which typically creates cooler, cloudier conditions.
When the Reef’s waters stay too hot for too long, the coral becomes stressed and expel the marine algae living inside their tissues, called zooxanthellae. Zooxanthellae provide the coral with food and energy from the sun (due to a process called photosynthesis), allowing them to grow and reproduce.
When corals get stressed, from things such as heat or pollution, they react by expelling this algae, leaving a white skeleton behind. This is known as ‘coral bleaching’. Some corals can feed themselves, but without the zooxanthellae, most corals starve. A healthy Reef can recover from coral bleaching, but it needs time and the right conditions. Coral reefs take around a decade to fully recover.
Stages in mass coral bleaching
In some instances corals can recover from bleaching. If heat stress is not prolonged and conditions return to normal, corals can regain their algae, return to their natural colours and survive. However prolonged elevated temperatures, such as those seen in the 2024 bleaching, cause more severe and extreme bleaching that increases coral mortality.
If coral is impacted by more than heat stress, like damage from tropical cyclones and floods, and poor water quality, these cumulative impacts can leave coral living in a weakened state. It can struggle to recover, reproduce and thrive, becoming even more vulnerable to coral diseases and mortality.
It can take decades for coral reefs to fully recover from a bleaching event, so it is vital that these events do not occur frequently. If we continue burning fossil fuels at our current rate then severe bleaching events are likely to hit reefs annually by the middle of the century. This would be devastating for coral reefs as they would have no chance to recover.
We need to urgently reduce our carbon pollution and limit the global temperature increase to 1.5 degrees, or the world’s reefs will face a bleak future.
If you’re interested in diving deeper, watch our YouTube video with Professor Morgan Pratchett from ARC Centre of Excellence for Coral Reef Studies at James Cook University. The Coral reef expert answers audience questions about the 2020 coral bleaching event.
The mining and burning of fossil fuels releases carbon pollution into the air, which is warming our oceans and causing more frequent and intense marine heatwaves. The ocean absorbs excess heat and this creates above average sea surface temperatures. Since 1900 the ocean’s temperature has warmed by 1°C. This may seem insignificant, however corals are very sensitive to temperature changes. Corals start to get stressed if temperatures increase by 1°C or more.
Marine heat waves are dangerous for coral reefs because the hotter than average ocean temperatures stress corals and can lead to mass coral bleaching events. Marine heatwaves can last weeks, months or even years. If ocean temperatures remain high for eight weeks or longer, the coral cannot recover and begin to die.
A before and after photo of coral bleaching on Lizard Island, Great Barrier Reef, Queensland. Photo credit: The Ocean Agency.
Scientists tell us that once we hit 1.5C of warming coral reefs will struggle to survive. If we don’t keep 1.5C of global warming within reach this decade, the harsh reality is we will see more mass bleaching events and the rapid decline of our global icon.
In just seven years, our Reef has suffered four severe mass coral bleaching events, faster and more severe than scientists predicted. If we don’t act to halt this pollution, we risk the future of our precious Reef.
We have the solutions. With real leadership we can stop carbon pollution and allow our coral reefs to recover and rebuild. But to do this, we must quit dirty coal, and transition to renewable energy today. If we take climate leadership then our coral reefs, and the thousands of species that rely on them (which includes us), can rebuild and thrive for generations to come.
We must commit to:
To stop coral bleaching we must stop carbon pollution that is heating our planet and cooking our oceans. We must quit coal and switch to renewables now.
To stop coral bleaching we must stop carbon pollution that is heating our planet and cooking our oceans. We must quit coal and switch to renewables now.
Mining and burning coal pollutes the atmosphere and causes global warming. Increased carbon pollution in our atmosphere traps heat, which causes the temperature to rise on Earth.
Mining and burning coal pollutes the atmosphere and causes global warming. Increased carbon pollution in our atmosphere traps heat, which causes the temperature to rise on Earth.
The higher and longer the temperature, the worse it gets. Ocean heatwaves cause stress for corals.
The higher and longer the temperature, the worse it gets. Ocean heatwaves cause stress for corals.
Even a rise of just one degree celsius for only four weeks can trigger bleaching in a coral. If ocean temperatures stay high for eight weeks or longer, the coral cannot recover and begins to die.
2024 – The Great Barrier Reef suffers the most severe and widespread mass bleaching event ever recorded. Extreme bleaching affects all regions of the reef while parts of the southern reef experience elevated water temperatures that break previous records.
The Australian Government reports that the Reef has suffered its worst summer on record after being impacted by two tropical cyclones, severe flooding, crown-of-thorns starfish outbreaks and its fifth mass bleaching event in under a decade.
2022 – For the fourth time since 2016, our Reef is hit with a devastating mass coral bleaching event, and the first in a La Nina year, which is usually characterised by cooler, cloudier conditions. On the other side of the country, the world famous Ningaloo Reef Marine Park also showed signs of bleaching.
Image: Ningaloo Marine Park, WA, and Stanley Reef, Great Barrier Reef, QLD.
2020 – our Reef suffered a third mass coral bleaching event in just five years. This was the most widespread bleaching event, with reefs impacted from the top to the bottom of the Great Barrier Reef. Fortunately most of the prime Reef tourism destinations were spared in 2020 and surveys in the southern Great Barrier Reef indicate that a late cool weather change meant that many bleached corals were able to survive.
Over 2016 and 2017, our Reef suffered back-to-back bleaching, leaving half of the shallow water corals dead. Results from extensive aerial and underwater surveys showed that 29% of corals died from the 2016 event alone – with most perishing in the northern section, where waters are warmest.
The bleaching event in 2017 reached further south from Port Douglas to Townsville. A further estimated 20% of corals died. To compound this, a category four cyclone, with wind speeds up to 263 km/h that violently ravaged the Whitsundays region – decimating coral reefs in its path.
The combined impact of the two severe bleaching events, Cyclone Debbie and current severe outbreaks of coral-eating Crown of Thorns starfish since early 2016 have significantly impacted our Great Barrier Reef.
If we lose coral reefs we also lose a vast number of fish and other animals that rely on them. This will have terrible consequences right up the food chain impacting larger creatures, like dolphins and sharks that rely on reef fish for their diet.
Coral bleaching risks not just our oceans and wildlife, but the livelihood and food security of over half a billion people worldwide.
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