How we track coral bleaching

What the dashboard shows, where the numbers come from, how current they are, and what they can’t tell you.

The short answer

When the water around a reef stays too warm for too long, corals expel the algae that live in their tissue, feed them and give them colour, and turn white. That is bleaching. A bleached coral is not dead yet, and it can recover if the heat eases, but if the heat goes on it starves and dies. This dashboard shows, every day, how much heat stress NOAA’s satellites see at 214 reef regions around the world, and how this year compares with the four global bleaching events since 1998.

How we know

Heat relative to normal. Corals are adapted to their local waters, so what matters is not the temperature itself but how much warmer the water is than the reef’s usual warmest month. NOAA Coral Reef Watch measures sea-surface temperature from satellites on a 5-kilometre grid. Whenever the water is at least 1 °C above a reef’s usual warmest-month average, it counts the excess; added up over the past 12 weeks, that gives degree heating weeks, in °C-weeks. One °C-week is one week at 1 °C above normal; two weeks at 2 °C above make 4.

Alert levels. NOAA turns that number into alert levels. At 4 °C-weeks (Alert Level 1) there is a risk of reef-wide bleaching; at 8 (Level 2), of bleaching with deaths among heat-sensitive corals. After the extreme heat of 2023, NOAA added three more levels in December 2023: 12 °C-weeks (Level 3, risk of multi-species mortality), 16 (Level 4, severe mortality, more than half of corals) and 20 (Level 5, near-complete mortality, more than 80%).

Our stations. We use NOAA’s 214 “virtual stations”, each a summary of the satellite grid around one reef region, with a daily record back to 1985. NOAA’s station files carry alert levels up to Level 2, so for Levels 3 to 5 we apply NOAA’s own thresholds to each station’s heat-stress value. The history chart is our own count: for each year, the share of stations whose heat stress reached 4 and 8 °C-weeks.

Global events. NOAA counts four global bleaching events, when significant bleaching is documented in all three major ocean basins: 1998, 2010, 2014–2017 and 2023–2025. The fourth was the largest on record: from January 2023 to September 2025, bleaching-level heat stress reached about 84% of the world’s reef area, against 68.2% in the third event, and mass bleaching was documented in at least 83 countries and territories. In June 2026 NOAA said the fourth event likely ended in mid-2025, and warned that El Niño could bring widespread bleaching back.

How current each part is

• Heat stress at each station: NOAA updates its files daily; we download them every night. The latest values are usually one to two days old.
• History since 1985: rebuilt every night from the same files; the current year is the year so far.
• Global events: NOAA Coral Reef Watch’s status page and announcement of 2 June 2026; we update them when NOAA does.
• El Niño: from our El Niño dashboard, which downloads NOAA’s index daily.

What the dashboard can’t tell you

Satellite heat stress is a strong warning, not a diver’s eyes on the reef. A high value means the conditions for bleaching or coral deaths are there; it does not prove that a particular reef has bleached or died. That depends on the coral species, depth, water clarity, currents and shading, which a satellite cannot see. Confirming bleaching still takes in-water surveys, and most reefs have none.

Each station covers a reef region, not every reef, so a dive site can be far from its nearest station. And the weekly alert level only looks at the past seven days: a reef that has started to cool can show a low level while its 12-week heat stress is still high, so read the two together.

Further reading

• Hughes et al. (2017), Nature: global warming and recurrent mass bleaching of corals. doi.org/10.1038/nature21707
• Hughes et al. (2018), Science: spatial and temporal patterns of mass bleaching of corals in the Anthropocene. doi.org/10.1126/science.aan8048
• Eakin et al. (2019), Coral Reefs: the 2014–2017 global-scale coral bleaching event, by the Coral Reef Watch team. doi.org/10.1007/s00338-019-01844-2
• Skirving et al. (2020), Remote Sensing: CoralTemp and the Coral Reef Watch heat-stress products this dashboard uses. doi.org/10.3390/rs12233856
• Heron et al. (2016), Scientific Reports: warming trends and bleaching stress of the world’s coral reefs, 1985–2012. doi.org/10.1038/srep38402

Credits

The data come from NOAA Coral Reef Watch, whose scientists built and run the satellite heat-stress products this page depends on, and from the reef scientists and survey teams around the world whose observations in the water tell us what the satellite numbers mean. We only download and present NOAA’s public files.