Ocean heat has become the main threat to coral reefs: the 2023–2025 global bleaching event exposed 84% of the world's reef area to bleaching-level heat stress, global hard coral cover has fallen, and 44% of reef-building coral species are now threatened with extinction.
Updated October 15, 2026. Written from the sources listed below, not independently peer-reviewed.
During the global coral bleaching event of 2023–2025, what share of the world's coral reef area was exposed to heat stress strong enough to cause bleaching?
84%
Reef area hit by bleaching-level heat stress
Share of the world's coral reef area exposed to heat stress strong enough to cause bleaching between early 2023 and mid-2025, the fourth and most extensive global bleaching event on record. This is a satellite-based heat measure; mass bleaching was confirmed in at least 83 countries and territories.
Data year 2025. Source: NOAA National Environmental Satellite, Data, and Information Service, 2026
44%
Reef-building coral species threatened with extinction
Share of 892 warm-water reef-building coral species assessed as threatened on the IUCN Red List in 2024, up from about one third in the previous assessment in 2008. Climate change is named as the main threat.
Data year 2024. Source: International Union for Conservation of Nature, 2024
25.8%
Global hard coral cover
Average share of the reef floor covered by living hard coral in 2024, according to GCRMN's 2025 report. The average for 2020–2024 (27.3%) was about 9.5% lower, in relative terms, than in 1980–2009 (30.2%). Many reef regions have few surveys, so global averages carry uncertainty.
Data year 2024. Source: Global Coral Reef Monitoring Network, 2026
Unit: % of reefs
Each point is one global bleaching event, as reported by GCRMN from NOAA Coral Reef Watch data: 1998, 2010, 2014–2017 (shown at 2017) and early 2023 to mid-2025 (shown at 2025). Values measure satellite-estimated heat stress over each event, not observed bleaching or coral death. The years between events are not plotted.
SDG target 14.2 called for marine and coastal ecosystems to be sustainably managed and protected by 2020, including by strengthening their resilience and restoring them. Since then, coral cover has continued to fall and the share of reefs exposed to bleaching-level heat stress has risen to 84%. GCRMN's 2025 report concludes that whether reefs persist this century depends largely on how far warming is limited, pointing to the Paris Agreement goal of keeping warming well below 2°C and pursuing 1.5°C, and finds that 16 of the 23 targets of the Kunming-Montreal Global Biodiversity Framework are highly relevant to reefs.
Hard coral cover fell by a relative 48.7% in the ROPME Sea Area around the Gulf and by 43.4% in the Caribbean compared with 1980–2009, while it rose by 43.0% in the Western Indian Ocean and 16.2% in South Asia.
Source: Global Coral Reef Monitoring Network, 2026The number of people living within 5 km of a coral reef rose by 46% between 2000 and 2020, to around 104 million, adding local pressure on reefs that also protect them from storms.
Source: Global Coral Reef Monitoring Network, 2026During Florida's record 2023 marine heatwave, 97.8% to 100% of elkhorn and staghorn coral colonies died in the Florida Keys and Dry Tortugas, which researchers describe as the functional extinction of these reef builders there.
Source: Manzello D.P. et al., Science 390(6771): 361–366, 23 October 2025, 2025Mass bleaching between 2023 and 2025 was documented in at least 83 countries and territories across the Atlantic, Pacific and Indian Oceans.
Source: NOAA National Environmental Satellite, Data, and Information Service, 2026Instead of one “percent solved” score, DO GOOD rates three separate dimensions. A problem can have mature solutions that still reach too few people. These ratings are an editorial reading of the sources on this page, not a measured index.
Has the situation improved in the real world?
LowAverage hard coral cover fell from 30.2% in 1980–2009 to 27.3% in 2020–2024 and stood at about 25.8% in 2024, and the share of reef-building coral species threatened with extinction rose from about a third in 2008 to 44% in 2024.
How mature and validated are the available solutions?
ModerateCutting emissions and reducing local stressors are well understood, and GCRMN finds that protection and adaptive management are linked to better reef condition. Restoration methods work at small scales, but few have shown potential beyond about one hectare.
How widely are effective solutions actually used?
LowHeat stress keeps reaching more reefs, and restoration remains small: a review of 362 projects found a median restored area of 100 m². GCRMN calls for more monitoring and funding, especially in developing nations.
Satellite heat-stress data cover all reefs, and GCRMN's 2025 report draws on 87,299 surveys at 36,886 sites, but survey coverage is uneven and thin in regions that hold a large share of the world's reefs, and data end in 2024, before the full effect of the latest event.
Each global bleaching event has reached more reefs than the last, from 21% in 1998 to 84% in 2023–2025, and NOAA says reefs are entering an era of near-annual bleaching, which leaves less time for recovery.
Coral reefs cover less than 0.2% of the ocean floor, yet they support more than a quarter of marine species, and close to one billion people depend on them for food, income or coastal protection. Reefs reduce up to 97% of wave energy, shield an estimated 5.3 million people and US$109 billion in assets from storms each year, and are linked to about US$36 billion a year in tourism. They are built by corals that live in partnership with microscopic algae. When the water stays too warm for too long, corals expel these algae and turn white, which is called bleaching. Bleached corals can recover if temperatures fall in time, but long or repeated heat stress kills them. Climate change is the main threat to reef-building corals, with pollution, runoff from farms, disease and unsustainable fishing adding to the pressure.
Heat stress has become more widespread with each global bleaching event: it reached 21% of the world's reefs in 1998 and 84% in the event that ran from early 2023 to mid-2025. The Global Coral Reef Monitoring Network found that average hard coral cover fell from 30.2% in 1980–2009 to 27.3% in 2020–2024, with sharp drops after each global event and only partial recovery in between, while seaweed-like macroalgae increased. Losses are uneven: the Caribbean and the ROPME Sea Area around the Gulf lost more than 40% of their coral cover relative to the reference period, while the Western Indian Ocean gained. In Florida, a record marine heatwave in 2023 killed almost all remaining elkhorn and staghorn corals in the Florida Keys. The International Union for Conservation of Nature (IUCN) now classes 44% of reef-building coral species as threatened with extinction, up from a third in 2008.
No single number captures the health of coral reefs. Global reporting relies on satellite measurements of ocean heat, field surveys of what covers the reef floor and assessments of extinction risk for individual coral species. The main measures are:
Bleaching-level heat stress
NOAA's Coral Reef Watch uses satellite sea surface temperatures to estimate where heat has built up enough to cause bleaching. A bleaching event counts as global when significant bleaching occurs in the Atlantic, Pacific and Indian Oceans within a year. The headline share of reef area is a heat-stress measure: it shows where bleaching was likely, not how many corals bleached or died.
Hard coral cover
Divers and scientists record the share of the reef floor covered by living hard coral. The Global Coral Reef Monitoring Network (GCRMN) combines these surveys into global and regional trends; its 2025 report drew on 87,299 surveys at 36,886 sites in more than 120 countries, territories and economies, covering 1980 to 2024. Declines are given relative to the 1980–2009 average.
Macroalgae cover
When corals die, seaweed-like macroalgae can take over the space. GCRMN tracks their cover as a second sign of reef condition. It rose by a relative 45% over four decades to 6.8% globally in 2024, which GCRMN does not yet read as a sign of irreversible change.
Extinction risk of coral species
The IUCN Red List assesses each species against criteria such as population decline. Its 2024 reassessment of 892 warm-water reef-building coral species found 44% threatened with extinction. Species that are not yet threatened can still be declining locally.
What works
Ocean warming is the main cause of mass bleaching, so limiting it is the only way to reduce heat stress on reefs everywhere. IUCN calls cutting emissions the main solution to prevent coral extinctions, and GCRMN ties reef survival this century to the Paris temperature goals.
Limits
Emissions cuts act slowly on ocean temperatures, depend on action by every major economy, and cannot undo warming already reached, at which reefs are entering an era of near-annual bleaching.
What works
Limiting pollution, runoff from farms and land clearing, and unsustainable fishing removes stresses that weaken corals and slow recovery after bleaching. GCRMN finds that reducing local stressors is consistently linked to better reef condition and recovery.
Limits
Local action cannot prevent bleaching in extreme heatwaves; in 2023, almost all elkhorn and staghorn corals in the Florida Keys died when the water became too hot.
What works
Protected areas that restrict fishing and damaging activities, combined with management that adjusts to new threats, give reefs better conditions to recover. GCRMN's 2025 report associates stronger protection and adaptive management with improved reef condition.
Limits
Protection does not stop ocean heat at a reef's borders, and many protected areas lack the staff and funding to enforce their rules.
What works
Fragments of coral are grown in nurseries and planted back onto damaged reefs. A systematic review of 362 restoration cases in 56 countries found that on average 66% of restored corals survived during monitoring.
Limits
The median restored area was only 100 m², and the median monitoring period was 12 months, so survival through later heatwaves is often unknown. Few methods have shown potential to scale beyond about one hectare, and the authors stress that restoration is no substitute for action on climate change.
What works
Satellite heat-stress alerts from NOAA Coral Reef Watch and field surveys pooled by GCRMN show where bleaching is likely and how reefs respond, helping managers time protective measures and target recovery efforts. GCRMN's 2025 report pooled 87,299 surveys from more than 120 countries, territories and economies.
Limits
Field monitoring is patchy in many developing countries and in regions that hold a large share of the world's reefs, and GCRMN calls for more funding to fill these gaps.
Promising, but not proven at scale. The stage shows how far each approach has come; it is not a recommendation.
Researchers are identifying coral colonies and genotypes that survive heatwaves better and testing whether breeding or planting them can make restored reefs more resilient. IUCN's assessors call for more research into whether and how corals can adapt to warmer water; how much extra heat tolerance can be gained, and whether it lasts in the wild, is still unclear.
Instead of planting fragments, some projects collect coral spawn, rear larvae and settle them on reefs, which could reach larger areas and add genetic diversity. Survival of corals raised from larvae is naturally low, and methods to deliver them at large scale are still being tested.
Frozen coral sperm and larvae and living coral collections in aquariums and nurseries keep genetic material of species that are disappearing from the wild, such as the Caribbean Acropora corals lost from much of Florida's reef. They preserve options for future restoration but do not by themselves restore reefs.
Coordination gap
Global bleaching events now occur every few years, and NOAA expects near-annual bleaching. GCRMN finds that recovery between events has been only partial, leading to stepwise decline; reversing this depends on global emissions cuts that no reef manager controls.
Technology gap
Across 362 restoration cases the median restored area was 100 m², and few methods have shown potential beyond about one hectare, tiny compared with the area of the world's reefs.
Knowledge gap
In the restoration review, 60% of projects were monitored for less than 18 months, and 45% of projects with ecological goals measured nothing that could show whether those goals were met, while failed projects are rarely published.
Implementation gap
Large reef areas, especially in developing countries, have few or irregular surveys. GCRMN calls for wider monitoring coverage and more funding for monitoring in developing nations.
Implementation gap
About 104 million people lived within 5 km of a reef in 2020, 46% more than in 2000, increasing pollution, runoff, coastal construction and fishing pressure where enforcement capacity is often limited.
Coordination gap
Reef protection depends on climate, biodiversity, fisheries and water-quality policies run by different agencies and treaties; GCRMN counts 16 of the 23 global biodiversity targets as highly relevant to reefs and says effective protection needs coordinated local, national and global action.
Unresolved questions, not vetted breakthroughs. Answering any of them would change what is possible.
Can natural selection, symbiont shifts or assisted breeding raise corals' heat tolerance quickly enough to keep pace with warming, and what trade-offs in growth or disease resistance come with it?
As bleaching becomes near-annual, what interval between heatwaves allows reefs to regrow, and at what point do they shift to lasting dominance by algae or other organisms?
Some reefs, such as those in the Western Indian Ocean, gained coral cover over the past decades. Which reefs are likely to escape the worst heat, why, and can protecting them help reseed others?
The latest global figures measure heat stress and coral cover only up to 2024. How much coral died during the fourth global event, and how does mortality vary between regions and species?
Heat is the main threat to reefs. Reducing your own emissions where you can and asking representatives about your country's climate commitments addresses the root cause, although the effect of any single action is small.
When snorkelling or diving, choose operators that brief guests on reef etiquette, avoid touching or standing on corals, and do not buy coral souvenirs. This reduces local damage but does not protect reefs from heat.
Volunteer programmes that train divers and snorkellers to survey reefs feed data into regional and global monitoring. Taking part or donating to established monitoring groups helps fill data gaps; the value depends on the programme's training and data standards.
If you live near a reef or in a river basin that drains to one, ask local authorities how they limit sewage, farm runoff and coastal construction, which add stress to corals.
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NOAA National Environmental Satellite, Data, and Information Service (NESDIS), news feature, 2 June 2026, published 2026
NOAA's announcement that the fourth global coral bleaching event (confirmed on 15 April 2024) likely ended in mid-2025. Source of the 84% of the world's coral reef area exposed to bleaching-level heat stress from early 2023 to mid-2025, mass bleaching in at least 83 countries and territories, and NOAA's statement that reefs are entering an era of near-annual bleaching.
Global Coral Reef Monitoring Network (GCRMN), news post, 8 June 2026, published 2026
GCRMN summary of NOAA's finding that the fourth global event has ended. Source of the shares of the world's reefs exposed to bleaching-level heat stress in each global event (21% in 1998, 37% in 2010, 68% in 2014–2017, 84% in 2023–2025), used for the trend.
Global Coral Reef Monitoring Network (GCRMN) and International Coral Reef Initiative (ICRI), August 2026, published 2026
Seventh global status report, based on 21.1 million observations from 87,299 surveys at 36,886 sites in more than 120 countries, territories and economies (1980–2024). Source of hard coral cover (30.2% in 1980–2009, 27.3% in 2020–2024, about 25.8% in 2024, relative decline about 9.5%), macroalgae (+45%, 6.8% in 2024), regional changes, coastal population within 5 km (about 104 million, +46% from 2000 to 2020), reef values (less than 0.2% of the ocean floor, more than a quarter of marine species, up to 97% of wave energy, 5.3 million people and US$109 billion protected, US$36 billion tourism, close to one billion people), the 16 of 23 relevant biodiversity targets and the recommendations on protection, management and monitoring.
International Union for Conservation of Nature (IUCN), press release, Baku, 13 November 2024, published 2024
Release on the global reassessment of 892 warm-water reef-building coral species. Source of the 44% threatened (about one third in 2008), climate change as the main threat, the other threats (pollution, agricultural runoff, disease, unsustainable fishing), and the call for research into coral adaptation.
Boström-Einarsson L. et al., PLOS ONE 15(1): e0226631, 2020, published 2020
Systematic review of 362 coral restoration case studies in 56 countries. Source of the average survival of restored corals (66%), the median restored area (100 m²), the median monitoring period (12 months; 60% of projects under 18 months), the 45% of projects with ecological goals that measured no relevant metric, low survival of corals raised from larvae, and the finding that few methods have shown potential beyond about one hectare.
Manzello D.P. et al., Science 390(6771): 361–366, 23 October 2025 (record via USGS Publications Warehouse), published 2025
Study of the 2023 marine heatwave on Florida's Coral Reef. Source of the 97.8% to 100% mortality of elkhorn (Acropora palmata) and staghorn (A. cervicornis) colonies in the Florida Keys and Dry Tortugas, and the conclusion that the heatwave marks their functional extinction on Florida's reef.