Heat is measured in weeks, not degrees.
Reef damage is cumulative, so anything that shortens the accumulation is worth doing.
Coral is a partnership. The animal builds the structure; algae in its tissue supply most of its food through photosynthesis. Under sustained heat the algae are expelled, leaving the white skeleton visible through transparent tissue. That is bleaching.
A bleached colony is starving, but still alive. Bring the stress down soon enough and the partnership re-establishes; leave it, and mortality follows. So the question at any reef is how long it has been accumulating heat.
The scale reef managers already use
NOAA Coral Reef Watch measures that accumulation in Degree Heating Weeks.
| Accumulated stress | What NOAA Coral Reef Watch expects |
|---|---|
| 4 DHW | Bleaching risk |
| 8 DHW | Likely reef-wide bleaching, with mortality among heat-sensitive corals |
| Above 12 DHW | Likely multi-species mortality |
Source: NOAA Coral Reef Watch — Degree Heating Week product, which also sets out the alert levels past 12.
A site read starts with your own DHW history, taken from the Coral Reef Watch pixel covering your reef and laid against your bleaching surveys from the same weeks. And a program that turns up only for the accumulating weeks meets a reef that has already spent everything it had. So the subscription runs through the cool months too.
Four global events, and the trend they describe
The share of the world's reefs exposed to bleaching-level heat stress has risen with each global event on record.
| Event | Years | Reefs exposed to bleaching-level heat stress |
|---|---|---|
| First | 1998 | 21% |
| Second | 2010 | 37% |
| Third | 2014–2017 | 68% |
| Fourth | 2023–2025 | 84% |
Source: ICRI and NOAA Coral Reef Watch, including the March 2025 total of 84 percent across 82 countries, territories and economies. The fourth event was confirmed on 15 April 2024.
The Global Coral Reef Monitoring Network's 2020 status report, built from close to two million observations at more than 12,000 sites in 73 countries between 1978 and 2019, found the world lost about 14 percent of its coral after 2009.
Separating what is yours from what is everyone's
Every managed reef carries two stress loads. One arrives from the ocean, driven from outside any fence line. The other is generated within a few kilometers of the site: dredging plumes, a marina that stopped the flushing, a wastewater outfall, an intake, a bloom, a season of anchor damage, a lagoon that no longer mixes at neap tides.
ReefHealer works on the second load, and does one thing that reaches past it. The cloud a hull lays over a reef scatters light before it reaches the coral, and shade works whichever load sent the heat. At a site running 12 DHW with nothing local to speak of, shade is the whole answer, laid from hulls working an agreed stretch of water, with nothing standing above the surface to be permitted, moored or maintained. At 5 DHW with a stalled lagoon and a bloom on top of it, both loads are in play.
None of it touches the signal itself. No fleet cools an ocean, and no program ends a global bleaching event. The covered area is set by how many hulls the program funds, a number worked out with you at the site read.
Sources
- NOAA Coral Reef Watch — Degree Heating Week product and alert levels.
- International Coral Reef Initiative — fourth global coral bleaching event, 2025 update, which carries the 1998, 2010, 2014–17 and 2023–25 figures.
- International Coral Reef Initiative — confirmation of the fourth global bleaching event, 15 April 2024.
- UNEP and the Global Coral Reef Monitoring Network — Status of Coral Reefs of the World: 2020.
- NOAA Pacific Marine Environmental Laboratory — What is ocean acidification?
- NOAA Ocean Acidification Program — Acidification is more pervasive than previously thought, on the aragonite saturation decline.
- NOAA Science On a Sphere — Ocean acidification: saturation state, on the thresholds of 3 and 1.
From reef scientists
Three questions on the numbers
What is a Degree Heating Week, exactly?
Heat added up over time. One DHW is a week spent a degree Celsius above the site's own summer maximum, and the index carries a rolling twelve weeks of those.
Coral answers to duration. A scorching afternoon barely moves the index.
Reference: NOAA Coral Reef Watch
If the driver is global, what is a vessel for?
Nothing, as far as the global signal goes.
It can affect what reaches the coral: a cap that will not break, oxygen falling away before dawn, turbidity cutting the light, bloom material drifting over, and the share of the day's sun that gets through a nanobubble cloud above the colony. The first four are local stress piled on the signal. The last is cover, which works over a reef whose only problem is the season.
A standing subscription buys those margins in the quiet months, and the fleet sets how much reef stays covered. At a reef already deep into a severe event, neither may be enough, whatever the fleet size. The site read says so before anything is signed.
Can you really move carbonate chemistry?
At a site, sometimes, where it has been measured. As a blanket promise, no.
Surface acidity is up by roughly 30 percent on pre-industrial values, and nothing on a hull reverses that. Inside one lagoon across one season, a carbonate benefit is weighed when the chemistry for a reef is chosen. Where a scope carries a pH improvement, that water was measured and the improvement can be defended for it.
No reef data held today shows this platform shifting saturation state at a site, so no proposal claims one. pH is logged through depth with everything else, giving any future carbonate effect a baseline to be tested against.
Bring the data
Start with your own DHW record.
Your temperature history, bleaching surveys and monitoring reports. Alarivean reads them against what could be held over that reef and comes back with an assessment: what the local load is made of, what holding it would take in hulls, and whether that margin is worth the money to you.