Fight for the Reef
Fight for the Reef

Coral bleaching: what is it and why is climate change to blame?

Coral bleaching on The Great Barrier Reef in 2016 - captured by Brett Monroe Garner

Coral bleaching on The Great Barrier Reef in 2016 – captured by Brett Monroe Garner

Coral bleaching is a global crisis. 

Half of our earth’s coral reefs have disappeared in the last thirty years. For those left, climate change is now their biggest threat – which could see them face extinction by 2050.

Already damaged from pollution, overfishing and other human impacts, coral reefs have been suffering a three-year global bleaching event – that only ended in 2017. As our planet warms due to climate change, ocean heatwaves cause corals to stress, bleach and die.

Unless we act now, coral reefs will go extinct by 2050. This risks not just our oceans and wildlife, but the livelihood and food security of over half a billion people worldwide.

Coral reefs are vital to life on earth. They support 25 per cent of all marine life, are valued at $1 trillion, and generate $300-400 billion every year in food, tourism, fisheries, and medicines (WWF 2015, Smithsonian Institute).

Coral reefs were previously in decline because of local issues like pollution and overfishing. But now climate change is proving to be an even deadly, global threat. Our oceans are heating higher, for longer. This change in conditions causes corals to become stressed, bleach and die. This is when mass coral bleaching events occur.

We must cut carbon pollution and take strong climate action now – or our reefs will go extinct, devastating people and planet.

Our oceans have no borders. This is a global crisis, needing global solutions.

We need to stop global warming now, so our coral reefs can recover and build resilience. We must quit dirty coal, ensure strong reef protection and transition to renewables today. If we take climate leadership and move to a coal-free future now, our Reefs – and the 25,000 species that rely on them – can recover, rebuild and thrive for generations to come.

What is coral bleaching?

Corals have a symbiotic relationship with marine algae called zooxanthellae, which live inside their tissues. This algae provides the coral with an easy food supply thanks to photosynthesis, which gives the coral energy, allowing it to grow and reproduce. It also provides their stunning colours.

When corals get stressed – from pollution, heat or acidity – they react by mistakenly expelling this algae, leaving just their ghostly, transparent skeleton behind. This is known as ‘coral bleaching’ and is coral suicide. Some corals can feed themselves, but without the zooxanthellae most corals starve.

Can coral survive a bleaching event? If the stress-caused bleaching is not severe, coral have been known to recover. If the algae loss is prolonged and the stress continues, coral eventually dies. © NOAA

Can coral recover from bleaching?

Corals can recover from bleaching – but only if conditions return to normal, for long periods. Corals can regain their algae, return to their bright colours and survive, but a bleached coral is severely damaged. It will struggle to regrow, reproduce and resist disease – so is very vulnerable.

If the stress continues, a bleached coral is likely to die. A coral reef with high a coral death rate from bleaching can therefore take years, if not decades, to recover. Reefs must be stress-free to recover.

What causes coral bleaching?

Coal kills coral. To stop coral bleaching we must quit coal – and switch to renewables now.  Global warming is causing our oceans to heat higher, for longer.

High sea temperatures cause coral to become stressed, bleach, and very likely to die. Even a rise of just 1 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 so begins to die.    

Our oceans are warming because of climate change. Increased carbon pollution in our atmosphere traps heat, which causes temperature rise on earth.

This rapidly rising carbon pollution has been unleashed by humans from burning of fossil fuels. Dirty coal is the biggest culprit, responsible for half the carbon pollution in our atmosphere. Coal is the dirtiest fossil fuel. It has the highest carbon content, and so is the most dangerous.

How is the Great Barrier Reef affected?

Climate change is killing our Great Barrier Reef. Far worse than scientific predictions, for the last two years our Reef has suffered severe mass coral bleaching.

Over 2016 and 2017 our Reef has suffered back-to-back bleaching, leaving half of corals dead. Results from extensive aerial and underwater surveys showed that a quarter of corals died from the 2016 event alone – with most perishing in the northern section, where waters are warmest.

2017’s bleaching event reached further south from Port Douglas to Townsville. To compound this, a category four cyclone also ravaged the coast south of Townsville – which led to flooding waters inundating inshore reefs with sediments and pollution from land. Around half of the corals have now died as a result of these events.


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So we don't lose the Great Barrier reef