Staggering 84% of world's reefs impacted by coral bleaching

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.