The most intense global coral bleaching event on record is still ongoing with the latest figure showing that between January 2023 and March this year, a staggering 84% of the world’s reefs had suffered from bleaching-level heat stress impact.

One year on from the official declaration made by the International Coral Reef Initiative that the fourth global coral bleaching event was unfolding, bleaching alerts have continued to develop across the world. 

From January 1 2023 to March 30 2025, bleaching-level heat stress impacted 84% of the world’s reefs, with 82 countries, territories, and economies suffering its damaging fallout. During the first global coral bleaching event in 1998, 21% of reefs experienced bleaching-level heat stress, rising to 37% in the second in 2010, and 68% during the third between 2014 and 2017.

Underscoring the severity of the most recent event, scientists have called it – the fourth global coral bleaching event on record – “unprecedented” as early as May 2024. So bad was it that the widely-used bleaching prediction platform had to add three new levels (levels three to five) to its Bleaching Alert Scale, indicating the heightened risk of mass coral mortality.

The previous highest level – Level Two – indicated risk of mortality to heat sensitive corals; Level Five indicates the risk of over 80% of all corals on a reef dying due to prolonged bleaching.

“We know coral bleaching is accelerating as our ocean warms, driven by the world’s continued reliance on fossil fuels,” said President Surangel Whipps Jr, President of Palau, an atoll nation across which damaged coral reefs have left local livelihoods threatened.

“We must urgently end the fossil fuel era and transition to a just, sustainable future powered by clean energy. Our oceans and the communities that depend on them cannot wait.”

Corals bleach when environmental stressors such as heat cause them to expel the colourful, energy-producing algae that lives inside them, leaving them white. If conditions return to normal quickly enough, coral can regain their algae and return to health. However, if the water stays too hot for too long, corals die. The main cause of large-scale coral bleaching events is higher ocean temperatures.

Last year was the hottest on record and the first to reach over 1.5°C warmer than pre-industrial times, due to human-induced climate change. This contributed to record-breaking ocean temperatures, and triple the previous record number of marine heatwaves around the world.

“There are many reasons for the demise of coral reefs; but let’s not beat around the bush. Corals are bleaching and dying primarily because the ocean is warming at an alarming rate, as evidenced by the sheer scale of this Fourth Global Bleaching Event,” said Ambassador Peter Thomson, the United Nations Secretary General’s Special Envoy for the Ocean.

“The ocean is warming primarily because of accumulating greenhouse gases emitted by humankind’s ongoing burning of fossil fuels. In short, if we want coral reefs to survive, we must drastically reduce our emissions and keep global warming to below 1.5°C.”

It’s true that around one third of all known marine life relies on coral reefs while one billion people benefit from them either directly or indirectly. Coral health has far-reaching impacts on the global economy, as reefs provide $10 trillion in benefits like food, jobs, and coastal protection. Sadly, live coral is estimated to have halved since the 1950s, with the Global Coral Reef Monitoring Network evidencing a 14% decline from 2009 to 2018.

Fiscal estimates suggest that ensuring the health of the entire ocean – not just corals – would cost less than 2% of this sum.

Losing corals, meanwhile, undermines efforts to achieve sustainable development, alleviate poverty, and ensure food security; climate change induced coral loss could cost as much as $500 billion a year by 2100.

“The fact that this most recent, global scale coral bleaching event is still ongoing takes the world’s reefs into unchartered waters,” said Dr Britta Schaffelke, coordinator of the Global Coral Reef Monitoring Network at the Australian Institute of Marine Science.

“In the past, many coral reefs around the world were able to recover from severe events like bleaching or storms. We need to continue to observe and measure if and how reefs will recover and change to help inform the combination of conservation measures most suited for a particular reef.”

While corals are in danger, it’s through a combination of rapidly addressing local and regional drivers of loss and implementing local conservation measures can still help them to survive the 21st Century.

Local solutions, including well-planned coral restoration, reducing pollution, stopping overfishing, and selective breeding of corals can increase reef health and improve the resilience of reefs to hotter ocean temperatures. 

“Coral reefs are humanity’s canary in the coal mine for much more than just climate change,” said David Obura, founding director of CORDIO East Africa. 

“What we choose to do to save them, from both climate change and from overconsumption in our economies, will determine their future and affect all life on earth, and everybody’s quality of life to the end of this century and beyond.”

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