Heat is measured in weeks, not degrees.
Which is the single most useful thing to understand about reef stress, because it means the damage is cumulative — and anything that shortens the accumulation is worth doing.
Coral is a partnership. The animal builds the structure; algae living in its tissue supply most of its food through photosynthesis. Under sustained heat that partnership breaks down and the algae are expelled, leaving the white skeleton visible through transparent tissue. That is bleaching.
A bleached colony is starving, not dead. Bring the stress down soon enough and the partnership can re-establish. Leave it, and mortality follows — which is why the useful question at any reef is never whether it has bleached but how long it has been accumulating heat.
The scale everybody in reef management already uses
NOAA Coral Reef Watch measures that accumulation in Degree Heating Weeks. The thresholds are published and unambiguous.
| 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 above 12 that were added after the 2023 marine heatwaves.
Two practical consequences follow. A site read starts with your own DHW history rather than with a general statement about warming oceans. And a programme is scoped around the weeks where accumulation happens, which at most sites is a narrower window than people expect.
Four global events, and the trend they describe
The proportion of the world's reefs exposed to bleaching-level heat stress has risen across 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, which carries all four figures and the March 2025 total of 84 per cent across 82 countries, territories and economies. The fourth event was itself confirmed on 15 April 2024.
Against that, the loss already booked. 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 per cent of its coral after 2009.
Separating what is yours from what is everyone's
Every reef under management carries two stress loads at once. One arrives from the ocean and cannot be argued with locally. The other is generated within a few kilometres 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.
The second load is not smaller than people assume. It is simply less discussed, because it belongs to somebody with a name and an address.
Reef Flourish works on the second load and nothing else. At a site running at 12 DHW with no local component to speak of, we will say that the honest answer is monitoring and shade, not a vessel. At a site sitting at 5 DHW with a lagoon that has stopped turning over and a bloom sitting on top of it, there is real work to do.
Sources cited on this page
- 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 Programme — 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.
Asked by reef scientists
Three fair challenges
What is a Degree Heating Week, exactly?
An accumulation of heat stress rather than a temperature reading. Four flags risk, eight anticipates reef-wide bleaching with mortality among heat-sensitive species, and past twelve the expectation is multi-species mortality.
It exists because reefs answer to weeks of warmth, not to one hot afternoon.
Reference: NOAA Coral Reef Watch
If the driver is global, what is a vessel for?
Nothing at all, as far as the global signal goes.
What it can affect is whether a particular lagoon, flat or nursery piles local stress on top of that signal — a cap that will not break, oxygen falling away before dawn, turbidity cutting the light, bloom material drifting over. Those margins are what a site programme buys. At a reef already deep into a severe event, they may not be enough, and that is a legitimate finding rather than a failed sale.
Can you really move carbonate chemistry?
No. That is the short answer and it is the one we would want in writing if we were on your side of the table.
Surface acidity is up by roughly 30 per cent on pre-industrial values and aragonite saturation has fallen across a large share of surface and subsurface water. Those numbers describe the problem. They do not describe a capability, and we hold no reef data showing this platform shifts saturation state at a site. What we do is measure pH through depth and put it in the record next to everything else, so that if somebody eventually demonstrates a carbonate effect there is a baseline to demonstrate it against.
Bring the data
Start with your own DHW record.
Your temperature history, your bleaching surveys, your monitoring reports. Alarivean reads them against what is workable locally and comes back with an assessment rather than an offer.