Global coral reefs are facing growing pressure from rising ocean temperatures with a new assessment warning that increasingly frequent marine heat waves are leaving reefs less time to recover between bleaching events.
The “Status of Coral Reefs of the World: 2025”, released by the Global Coral Reef Monitoring Network on August 31, analysed 21.1 million observations from 87,299 surveys at 36,886 sites across 123 reef bearing countries, territories and economies between 1980 and 2024.
The assessment found that average global hard coral cover declined from 30.2% during the 1980-2009 reference period to 27.3% between 2020 and 2024. That represents a 9.5% relative decline. By 2024, hard coral cover had fallen further to 25.8%.
The report said the figures are likely conservative because monitoring began after many reefs had already experienced degradation and data coverage varies between regions.
The decline accelerated during the fourth global coral bleaching event, which began in 2023 and continued into 2025. The report recorded an 8.9% relative decline in global hard coral cover between 2023 and 2024, the largest single year reduction recorded in the assessment.
However, the report does not capture the full impact of the 2023-2025 bleaching event because its analysis ends in 2024.
Corals can recover after bleaching when temperatures return to normal and other pressures are limited. But researchers say the interval between major marine heat events is becoming shorter.
The assessment identifies four global bleaching events: 1998-1999, 2010-2011, 2014-2017 and 2023-2025. Global bleaching events that once occurred roughly once a decade are now occurring at intervals of about four to six years.
Average sea surface temperatures over coral reefs increased by 0.81 degrees Celsius between 1985 and 2025, according to the report.
Researchers warn that repeated heat stress can prevent reefs from fully recovering. Even when coral cover returns, the original mix of coral species may not necessarily return, potentially affecting biodiversity, fish habitat and the ecosystem services reefs provide.
Manuel González-Rivero, the report’s lead author and global coordinator of the GCRMN said shrinking recovery periods could have long term consequences for reef ecosystems.
Regional losses vary sharply
The global decline has not been uniform. The region covering the northern Arabian Sea, Persian Gulf and Gulf of Oman recorded a 48.7% decline in hard coral cover relative to the reference period, while the Caribbean experienced a 43.4% decline.
The Pacific recorded an 18.3% decline and Australia’s reefs declined by 10.9%.
The assessment found no clear long term change in Brazil, the East Asian Seas, the Eastern Tropical Pacific, or the Red Sea and Gulf of Aden. Hard coral cover increased in South Asia and the Western Indian Ocean, although the researchers cautioned that limited early observations and degraded starting conditions make these trends difficult to interpret.
Macroalgae, or large seaweeds, also increased globally. Coverage was 44.1% higher than during the 1980-2009 reference period, although macroalgae covered only 6.8% of coral reefs globally in 2024. The report found no evidence of a worldwide shift from coral dominated reefs to algae dominated reefs.
Millions Depend on Coral Reefs
The assessment estimates that around 104 million people now live within five kilometres of a coral reef 46% more than two decades ago.
Although coral reefs occupy less than 0.2% of the ocean floor, they support at least 25% of marine species and contribute to the livelihoods, food security and cultural traditions of nearly one billion people.
Scientists say local conservation measures can help reefs withstand stress. Sustainable fishing, improved water quality and better management of coastal development can reduce local pressures and give reefs a greater chance of recovering after heat events.
Coral restoration and efforts to develop heat tolerant corals could also provide benefits in some locations. Researchers caution, however, that such measures cannot fully offset continued increases in ocean temperatures.
The report therefore points to rapid reductions in greenhouse gas emissions, alongside protection of reefs and areas that may remain relatively sheltered from warming, as central to limiting further coral losses.
The findings come as scientists monitor another period of elevated ocean temperatures associated with El Nino raising concerns that some reefs could face additional heat stress before they have recovered from previous bleaching events.