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Four global bleaching events, all since 1998
Reefs bleached in regional events before, but 1998 was the first time bleaching hit all three ocean basins in one year. NOAA Coral Reef Watch counts four such global events since then; the fourth was the largest on record.
How is reef heat stress measured?
Degree heating weeks add up how far, and for how long, a reef has been warmer than its usual warmest month over the past 12 weeks. NOAA’s alert levels start at 4 °C-weeks and step up every 4.
- Alert Level 1 (4 or more): risk of reef-wide bleaching
- Alert Level 2 (8 or more): risk of reef-wide bleaching with deaths among heat-sensitive corals
- Alert Level 3 (12 or more): risk of multi-species mortality
- Alert Level 4 (16 or more): risk of severe, multi-species mortality, more than half of corals
- Alert Level 5 (20 or more): risk of near-complete mortality, more than 80% of corals
Where is heat stress worst today?
Each dot is one of NOAA Coral Reef Watch’s 214 reef stations. Colour = NOAA alert level today (over the past 7 days); size = accumulated heat stress (°C-weeks over 12 weeks).
Look up a reef region
Loading reef stations…
Which reefs are under the most heat stress?
The ten stations with the most accumulated heat stress today, with the last ~90 days.
How does today compare to past global bleaching events?
Each year since 1985: the share of NOAA’s reef stations whose heat stress reached bleaching levels at some point that year. Shaded bands are the four global bleaching events.
Does El Niño raise bleaching risk?
Each global bleaching event came with an El Niño: 1997–98, 2009–10, 2015–16 and 2023–24.
What drives this
- Ocean heat →Stored ocean heat is the long-run engine of reef heat stress.
- Marine heatwaves →Acute heat spikes push reefs past their bleaching threshold.
- ENSO / El Niño →Each of the four global bleaching events came with an El Niño.
What this drives
- Coastal flood risk →Living reefs are natural breakwaters; losing them exposes coasts to surge.
How we know this
- Source: NOAA Coral Reef Watch 5 km Virtual Stations: daily degree heating weeks and the 7-day maximum Bleaching Alert Area for 214 reef regions.
- Update cadence: We download NOAA’s public station files every night; the latest values are usually one to two days old.
- Time coverage: Map and KPIs use the latest day in each station file; sparklines show the trailing ~90 days. The history chart uses each station’s full daily record back to 1985.
- Methods: 214 reef regions, each summarised by NOAA from its 5 km satellite grid, not every reef. NOAA’s station files give alert levels up to Alert Level 2; we apply NOAA’s thresholds for Alert Levels 3 to 5 (12, 16 and 20 °C-weeks) to each station’s own value. The history chart counts, per year, the stations whose peak heat stress reached 4 or 8 °C-weeks; past years need at least 300 days of data.
- Uncertainty: Satellite heat stress does not prove coral death on the seafloor — many reefs lack diver surveys.
- Limitations: “Look up a reef region” shows the station you pick; the page never detects your location. A station covers a reef region, so it can be far from a particular dive site. The ENSO panel uses our El Niño dashboard’s daily copy of NOAA’s index.
- Primary data: NOAA CRW Virtual Stations · How this page is built