How we measure coral bleaching — and how a satellite knows a reef is in trouble

The dashboard's data, the choices behind it, and the limits of what it can tell you.

The question this tool answers

When water around a coral reef stays too hot for too long, the coral expels the symbiotic algae that feed it and give it colour, turning bone-white. That is bleaching. A bleached coral is not yet dead — but if the heat persists, it starves. This dashboard answers, day by day, where reef heat stress is dangerous right now, and puts today's numbers against a sobering frame: mass coral bleaching across whole oceans was unknown to science before 1998, and there have been four global-scale events since.

How we know

The numbers come from NOAA Coral Reef Watch (CRW), a program that has turned satellite sea-surface temperature into an early-warning system for reefs since the late 1990s. The core idea is elegant. Corals are adapted to their local climate, so what matters is not absolute temperature but heat relative to the warmest part of a normal year. CRW defines each reef's "bleaching threshold" as 1°C above its historical warmest-month mean. Whenever the satellite sees water above that threshold, it banks the excess as "HotSpot" heat; summed over a rolling twelve weeks, that accumulation is the Degree Heating Week (DHW), measured in °C-weeks.

The thresholds are calibrated against decades of reef observations: around 4 °C-weeks, significant bleaching becomes likely; around 8, widespread bleaching and some mortality; past 12, catastrophic mortality. Those guideposts are why a single DHW number can carry so much meaning. The temperatures themselves come from CRW's CoralTemp record, a blended satellite product on a 5-kilometre grid. This dashboard reads CRW's Virtual Stations — roughly 150 fixed reef and coastal points — and shows each one's current DHW and its 7-day-maximum Bleaching Alert Area (the 0–5 scale you see as dot colours on the map).

The choices we made

Stations, not the full grid. CRW publishes a global 5-kilometre map; we use its Virtual Station point files instead. That keeps the page fast and legible, and the stations are placed at reefs people care about — but it means your favourite dive site may not have its own dot, and the geolocation feature picks the nearest station, which can be hundreds of kilometres away. We say so on the panel rather than implying full coverage.

We also lead with the historical frame rather than a bare count. "Twenty-one stations in alert" is a statistic; "the fourth global bleaching event, the largest ever by reef area, and here is today's worst-hit reef by name" is news. Named places land. And we keep the four-event timeline as a promoted panel because the progression — from a once-unthinkable event in 1998 to a recurring one with shrinking gaps — is, on its own, one of the clearest stories in this portfolio.

What this tool cannot tell you

Satellite heat stress is a strong predictor of bleaching, but it is not a diver's eyes on the reef. A high DHW means conditions for bleaching are met; it does not prove that a particular colony has bleached or died, because that depends on species, depth, water clarity, local currents, and shading that a satellite cannot see. Some hardy corals survive heat that kills their neighbours. Confirming mortality still requires in-water surveys, which exist for only a fraction of the world's reefs.

The composite stress index on the recent-trend chart is a snapshot from our daily compile, not a satellite-era global reconstruction — treat it as a short-horizon view, with the long history carried by the events timeline. And because we download daily files rather than stream a live feed, the readings typically lag real conditions by about a day.

What's coming next

The natural next steps are wider station coverage toward the full CRW network, longer per-station DHW histories so every sparkline shows the trailing season, and an integration with in-water bleaching reports (such as those compiled after major events) so the page can show, where data exist, how satellite stress translated into observed mortality.

Further reading

• Hughes et al. (2017), Nature — "Global warming and recurrent mass bleaching of corals." Shows the interval between bleaching events collapsing. doi.org/10.1038/nature21707
• Hughes et al. (2018), Science — maps the 2016 Great Barrier Reef bleaching and links DHW to mortality. doi.org/10.1126/science.aan8048
• Eakin et al. (2019), Coral Reefs — the definitive account of the 2014–2017 third global bleaching event, by the CRW team. doi.org/10.1007/s00338-019-01844-2
• Skirving et al. (2020), Remote Sensing — describes CoralTemp and the CRW heat-stress product suite this dashboard relies on. doi.org/10.3390/rs12233856
• Heron et al. (2016), Scientific Reports — warming trends and bleaching stress across the world's reefs, 1985–2012. doi.org/10.1038/srep38402

Credits

The reef heat-stress warning system exists because of the scientists at NOAA Coral Reef Watch — among them Mark Eakin, who led the program for years, and William Skirving and Gang Liu, who built the satellite products — together with the reef ecologists, including Terry Hughes and the survey teams who dived the Great Barrier Reef in successive bleaching summers, whose field observations calibrated what the satellites mean. This dashboard fetches from CRW's public files. We are downstream of their work.