Coral Bleaching Tracker
Daily ocean heat stress for 214 coral reef regions worldwide, from NOAA Coral Reef Watch’s satellite monitoring. See which reefs are at risk of bleaching today, how this year compares with every year since 1985, and NOAA’s four-month outlook for any reef - with the El Niño link behind every global bleaching event so far.
Current State - 26% of Reef Regions at Alert Level 1 or Higher
55 of 214 reef regions monitored by NOAA Coral Reef Watch were under bleaching-level heat stress on 3 Oct 2026. Click any region on the map to see its charts below.
At Alert 1+ Now
26%
55 of 214 Regions
Above Threshold
24%
of Regions Today
Warmer Than Normal
+1.2°C
Average Reef Today
Heat Stress
3.7 DHW
Average Reef
2026 to Date
67%
at Alert 1+ So Far
Record Year
94%
in 2024
Heat Stress by Region
Each region's alert level on the latest day of data. Selected - Maldives
Current Alert Levels214 Regions
A Fifth Global Event?
Heat Stress in All Three Oceans in 2026
NOAA calls a global event when bleaching is confirmed in all three reef ocean basins, now from field reports rather than heat stress alone. NOAA warned in June 2026 that the El Niño could bring a fifth. Outlook
29 Likely by 31 Jan
57 Likely by 31 Jan
22 Likely by 31 Jan
Outlook
Outlook - Bleaching-Level Heat Forecast in All Three Oceans
NOAA’s Four-Month Coral Bleaching Heat Stress Outlook for all 209 reef regions, issued 27 Sep 2026. “Likely” means a 50% or better chance - half or more of NOAA’s forecast runs.
Regions Likely at Alert Level 1+, Week by Week
39 regions are likely at Alert Level 1+ in the first week, rising to 74 in the week of 8 Feb 2027. NOAA’s outlook runs on a half-degree forecast grid, so its starting count differs from today’s satellite figure in Current State above.
Likely to Reach Alert Level 1+ by 31 Jan 2027
NOAA calls a global event when bleaching is confirmed in all three oceans - it now uses field reports, so this is a forecast of the heat, not of the event.
Still to Come
The outlook runs to 14 Feb 2027. The Indian Ocean’s hottest months - March to May for reefs like the Maldives and Seychelles - come after that, and are when El Niño’s delayed warming usually reaches them. El Niño
Most at Risk - 90% or Higher Chance of Alert Level 2
And 25 moreAnd 19 more - switch the map above to Outlook to see every region.
Source: NOAA Coral Reef Watch Four-Month Coral Bleaching Heat Stress Outlook (CFSv2, v5), issued weekly. Each region takes the highest value within half a degree of its centre.
Reef Regions
Maldives - Bleaching Watch
Latest data 3 Oct 2026. Bleaching threshold 30.7°C - this region’s warmest-month average (29.7°C) plus 1°C. Daily records since 1985.
Alert Level
Watch
Today
Sea Temperature
30.1°C
-0.5°C vs Threshold
Anomaly
+1.6°C
vs Normal Today
Heat Stress
0.0 DHW
Last 12 Weeks
2026 Peak
3.2 DHW
Warning
Record Peak
10.9 DHW
2016
Worst years on record - 2016 (10.9 DHW), 2024 (9.1 DHW), 1998 (8.8 DHW). All came during or just after an El Niño.
Sea Temperature - Last Two Years
Temperature in the most heat-stressed part of the region (NOAA's 90th-percentile HotSpot pixel).
Source: NOAA Coral Reef Watch daily 5km Regional Virtual Stations (v3.1), Maldives, daily since 1985.
Heat Stress (DHW) - Last Two Years
4 °C-weeks brings a risk of reef-wide bleaching and 8 a risk of coral deaths, while the water is still above the threshold. DHW is NOAA's index in °C-weeks.
Source: NOAA Coral Reef Watch daily 5km Regional Virtual Stations (v3.1), Maldives, daily since 1985.
2026 Against the Worst Years
Heat stress (DHW) through the year. Southern-hemisphere reefs peak around March, northern ones around September.
Source: NOAA Coral Reef Watch daily 5km Regional Virtual Stations (v3.1), Maldives, daily since 1985.
Peak Heat Stress Every Year Since 1985
Each bar is the year's highest DHW, coloured by the highest alert level reached. 7 of 42 years reached Alert Level 1 or higher.
Source: NOAA Coral Reef Watch daily 5km Regional Virtual Stations (v3.1), Maldives, daily since 1985.
Outlook - Maldives
NOAA’s Four-Month Coral Bleaching Heat Stress Outlook, issued 27 Sep 2026 from the CFSv2 seasonal forecast. Chance of each alert level, week by week to 14 Feb 2027.
Over the Four Months
Some Heat Stress Possible
Chance of reaching each level at some point between 11 Oct 2026 and 31 Jan 2027. The headline is our plain-English summary of NOAA’s figures.
Beyond This Outlook
Maldives’ water is usually warmest in April, after this outlook ends. NOAA issues a new one every week, so that season should come into range from around December.
The chance is the share of forecast runs reaching each level - 112 runs for the first five weeks, fewer further ahead - in steps of 10%. It forecasts heat stress, not bleaching itself - how much coral bleaches also depends on cloud, currents and the corals’ own history.
Source: NOAA Coral Reef Watch Four-Month Coral Bleaching Heat Stress Outlook (CFSv2, v5), issued weekly. Each region takes the highest value within half a degree of its centre.
Since 1985
Every Year Since 1985
Share of the 214 reef regions that reached Alert Level 1 or higher at any point in each year. The gold bands are NOAA's four global bleaching events.
Worst Years
2024, 2016, 2025
94%, 79%, 68%
1985-1994 Average
14%
of Regions Reached Alert 1 Each Year
Last 10 Years Average
60%
2016-2025
This counts regions, not reef area - NOAA’s figure of 84% for the 2023-25 event is the share of the world’s reef area. The figure for 2026 is the year to date.
Source: NOAA Coral Reef Watch daily 5km Regional Virtual Stations (v3.1), all 214 regions, 1985 to date. Global events - NOAA Coral Reef Watch.
Day by Day
Share of reef regions under Alert Level 1+ and Alert Level 2+ each day. Northern reefs peak around August to October and southern reefs around February to April, so the line rises twice a year.
Source: NOAA Coral Reef Watch daily 5km Regional Virtual Stations (v3.1), all 214 regions, daily.
El Niño
El Niño and Bleaching
The share of reef regions reaching Alert Level 1 each year, with the El Niño / La Niña index (RONI) below - its December-February value each year, every season since, then NOAA’s official forecast (issued September 2026). Dashed lines mark each moderate or stronger El Niño’s peak.
ENSO (RONI)
Sources: NOAA Coral Reef Watch daily 5km Regional Virtual Stations (v3.1) and NOAA RONI (Relative Niño 3.4 index, ERSSTv5) with the NOAA CPC RONI outlook - the same ENSO series and forecast shown on the ENSO Tracker.
Where ENSO Is Heading
How El Niño Drives Bleaching
Every Event Came With El Niño
The Timing Differs by Ocean
Bleaching Without El Niño
Global Events
Global Bleaching Events
Each event is also in the Climate History register on the Global Climate Update, where it sits on the temperature record.
1997-1998
First Global Event
The first recorded global-scale coral bleaching event, during the strong 1997-98 El Niño (peak ONI +2.3) followed by a strong La Niña that brought further warm-water stress to the Pacific. [NOAA Coral Reef Watch]
See It in Climate History2010
Second Global Event
The second global-scale coral bleaching event, in 2010, when a moderate-to-strong El Niño pushed tropical sea temperatures above the level corals can tolerate across the Indian Ocean, South-East Asia and the Caribbean. [NOAA Coral Reef Watch]
See It in Climate History2014-2017
Third Global Event
The third and (at the time) most severe global bleaching event, spanning 2014-2017 and peaking during the record 2015-16 El Niño - 68.2% of the world's reef area experienced bleaching-level heat stress. [NOAA / AGU]
See It in Climate History2023-2025
Fourth Global Event
The fourth and most severe global bleaching event on record - bleaching-level heat stress impacted 84% of the world's coral reef area across the Pacific, Atlantic and Indian Oceans, confirmed by NOAA 15 April 2024. [NOAA NESDIS]
See It in Climate HistoryA Fifth?
In June 2026, a year after the fourth event ended, NOAA warned that the developing El Niño could trigger a fifth global event [CBS News]. NOAA now plans to use field reports of bleaching, not heat stress alone, to decide when a global event is under way. Each event so far has been more widespread than the last (Climate History).
The First Tipping Point
The Global Tipping Points Report 2025, by around 160 scientists, judged that warm-water coral reefs are passing their tipping point now - the first part of the Earth system to do so. Its best estimate of the threshold is 1.2°C of global warming (range 1 to 1.5°C), and the world is already about 1.4°C warmer than before industrialisation [Global Tipping Points Report 2025].
Past that point the report expects widespread reef die-off, with how far it goes depending on how much more warming there is. Holding warming down gives more reefs the time to recover between heatwaves.
How It Works
What Bleaching Is
Reef-building corals live in partnership with tiny algae inside their tissue. The algae photosynthesise and pass most of the food they make to the coral, and they give corals their colour.
When the water stays too warm for too long, the partnership breaks down and the coral expels the algae. Its tissue turns see-through and the white skeleton shows through - it has bleached.
A bleached coral is not dead, but it is starving. If the water cools within a few weeks the algae can return. If the heat lasts, corals die from starvation and disease, and a reef can take a decade or more to recover - longer than the gap between heatwaves is now leaving.
How Heat Stress Is Measured
NOAA Coral Reef Watch sets a bleaching threshold for each place - its warmest month’s average temperature plus 1°C. Corals are adapted to their local summer, so the threshold ranges from about 23°C off southwest Australia to over 34°C in the Persian Gulf.
Degree Heating Weeks (DHW) add up how far, and for how long, the water has been at or above that threshold over the last 12 weeks. Four weeks at 1°C over the warmest-month average is 4 DHW, and so is two weeks at 2°C over. At 4 DHW reef-wide bleaching becomes likely, and at 8 heat-sensitive corals start to die.
Because DHW looks back 12 weeks, it falls slowly after a heatwave ends. The alert level also checks the water is still hot, so a region can show high DHW with only a Watch.
The Alert Levels
NOAA Coral Reef Watch's daily Bleaching Alert Area levels (version 3.1), with NOAA's own wording for the risk at each level.
| Level | When | Risk |
|---|---|---|
| No Stress | HotSpot 0 or below | No heat stress |
| Bleaching Watch | HotSpot between 0 and 1°C | Low-level heat stress |
| Bleaching Warning | HotSpot 1°C+, DHW below 4 | Risk of Possible Bleaching |
| Alert Level 1 | HotSpot 1°C+, DHW 4 to 8 | Risk of Reef-Wide Bleaching |
| Alert Level 2 | HotSpot 1°C+, DHW 8 to 12 | Risk of Reef-Wide Bleaching with Heat-Sensitive Coral Mortality |
| Alert Level 3 | HotSpot 1°C+, DHW 12 to 16 | Risk of Multi-Species Mortality |
| Alert Level 4 | HotSpot 1°C+, DHW 16 to 20 | Risk of Severe, Multi-Species Mortality (over 50%) |
| Alert Level 5 | HotSpot 1°C+, DHW 20 or more | Risk of Near Complete Mortality (over 80%) |
HotSpot is how far the water is above the warmest-month average. Levels 3 to 5 were added in December 2023, for heat stress far beyond the old top level [NOAA Coral Reef Watch].
Explore
Explore Climate Data
Data Sources
NOAA Coral Reef Watch
Sea temperature, anomaly, HotSpot, Degree Heating Weeks and alert level for each reef region, daily since 1985, from NOAA’s CoralTemp satellite record. Refreshed here every day.
Weekly probabilistic outlook from NOAA’s CFSv2 seasonal forecast, on a half-degree grid. Each region here takes the highest value within half a degree of its centre, since reefs sit close to coasts the grid can miss.
Data courtesy of NOAA Coral Reef Watch (coralreefwatch.noaa.gov). ENSO episodes from NOAA CPC’s Relative Oceanic Niño Index. Global events from the site’s Climate History register, each with its own source.
FAQs
Coral Bleaching - Common Questions
What is coral bleaching?
Reef-building corals live in partnership with tiny algae inside their tissue, which make food for the coral and give it its colour. When the water stays too warm for too long, the partnership breaks down and the coral expels the algae, leaving its tissue see-through over a white skeleton. A bleached coral is not dead, but it is starving - it can recover if the water cools within a few weeks, but long or repeated heat kills it.
What are Degree Heating Weeks (DHW)?
Degree Heating Weeks measure accumulated heat stress. NOAA Coral Reef Watch counts how far the water has been 1°C or more above the local warmest-month average, and for how long, over the last 12 weeks. Four weeks at 1°C above is 4 DHW, as is two weeks at 2°C above. NOAA rates 4 DHW a risk of reef-wide bleaching and 8 DHW a risk of reef-wide bleaching with heat-sensitive coral mortality. Levels 12, 16 and 20 bring risks of multi-species, severe and near-complete mortality.
What do the bleaching alert levels mean?
NOAA Coral Reef Watch's daily alert levels run from No Stress, Bleaching Watch (water above the warmest-month average) and Bleaching Warning (at least 1°C above, under 4 DHW) to Alert Levels 1 to 5, which start at 4, 8, 12, 16 and 20 Degree Heating Weeks while the water is still 1°C or more above the warmest-month average. Levels 3 to 5 were added in December 2023 for extreme heat stress beyond the old top level.
Why does a reef show high heat stress but only a Watch?
Degree Heating Weeks look back over 12 weeks, so they fall only slowly after a heatwave ends. The alert level also checks whether the water is still at least 1°C above the warmest-month average. Once the water cools, the level drops to Watch or No Stress even though the accumulated heat stress, and any bleaching it caused, is still there.
What is a global coral bleaching event?
NOAA calls a global bleaching event when mass bleaching is confirmed in all three ocean basins with coral reefs - the Atlantic, Pacific and Indian Oceans. There have been four - 1998, 2010, 2014-2017 and 2023-2025. The fourth was the largest on record, with bleaching-level heat stress across 84% of the world's reef area and mass bleaching in at least 83 countries and territories. NOAA now plans to rely on field bleaching reports to decide when events start and end.
Will there be a fifth global bleaching event?
In June 2026 NOAA warned that the developing El Niño could trigger a fifth global bleaching event, just a year after the fourth ended. The El Niño has since grown strong. Every global event so far has come with a strong El Niño, but only field surveys can confirm one is under way. The Outlook section on this page counts, from NOAA's latest weekly outlook, how many reef regions in each of the three oceans are likely to reach bleaching-level heat stress over the next four months. NOAA's Coral Reef Watch coordinator says reefs are now in "the era where reefs will bleach on a near-annual basis".
How does El Niño cause coral bleaching?
During El Niño, heat stored in the western Pacific spreads east and the tropical oceans warm well beyond the Pacific. Long-term warming has lifted ocean temperatures close to corals' limits, so the extra warmth of El Niño pushes many reefs over the bleaching threshold at once. The central and eastern Pacific heat up as El Niño builds, while the Indian Ocean warms a few months after the peak - which is why the Maldives' worst years, 1998, 2016 and 2024, all peaked in May or June after a strong El Niño.
When is the Maldives or the Great Barrier Reef most at risk?
Each reef is most at risk in its own warm season. The Great Barrier Reef's water is warmest in February, so its risk is greatest from January to March. The Maldives' water is warmest in April, with the worst heat stress usually building through May and June. NOAA's Four-Month Outlook only looks about four months ahead, so a season further off appears in it as the weeks go by - this page notes when a reef's warm season is beyond the current outlook.
How accurate is the bleaching outlook?
NOAA's Four-Month Coral Bleaching Heat Stress Outlook uses the CFSv2 seasonal climate forecast - 112 forecast runs for the first five weeks, 49 for weeks 6 to 12 and 28 beyond. The chance shown is the share of runs reaching each alert level, in steps of 10%. It forecasts heat stress, not bleaching itself, and like all seasonal forecasts it is less certain further ahead. It is updated every week.
Are coral reefs past a tipping point?
The Global Tipping Points Report 2025 judged that warm-water coral reefs are passing their tipping point now, making them the first part of the Earth system to do so. Its best estimate of the threshold is 1.2°C of global warming (range 1 to 1.5°C), and the world is already about 1.4°C warmer than pre-industrial times. The report expects widespread reef die-off, with how far it goes depending on how much more warming there is.