A coral bleaching alert tells you about a threat to a reef. To learn what happened to the corals, you also need observations from the water. Mixing those two kinds of evidence can turn a useful warning into an unsupported claim about reef damage.
This explainer shows how to read heat-stress indicators alongside reef surveys. It is not a live bleaching bulletin for Thailand or any other destination. The featured photograph is a historical image from 2019, not evidence of present reef condition.
Three questions that need different evidence
When a reef appears in the news, separate these questions before comparing headlines:
- Was the reef exposed to unusual heat? Look for a dated temperature or heat-stress product covering the location.
- Were corals observed to be bleached? Look for a survey with a date, location, depth and description of what was examined.
- Did corals die or recover? Look for follow-up observations and a clear comparison with earlier conditions.
A report may answer the first question well while leaving the other two open. That is a reason to describe its scope accurately, rather than dismiss the warning or claim it has already measured the final damage.
What degree heating weeks measure
NOAA Coral Reef Watch uses degree heating weeks, or DHW, to combine the strength and duration of heat stress. Its calculation uses an 84-day window. It adds daily HotSpot values of at least 1°C above the local maximum monthly mean temperature, then divides that sum by seven. The resulting unit is °C-weeks, not the ocean temperature itself.
Hypothetical calculation: Assume a location has a qualifying HotSpot of 1.5°C for 14 days, with no other qualifying days in the window. Its DHW contribution is 1.5 × 14 ÷ 7 = 3°C-weeks. Extending that same HotSpot to 28 days produces 6°C-weeks. These are invented teaching examples, not measurements from Ko Rang or another real reef.
In its DHW product guidance, NOAA associates 4°C-weeks with bleaching risk and 8°C-weeks with likely reef-wide bleaching and mortality among heat-sensitive corals. These are risk indicators. A value of 8 does not mean the sea warmed by eight degrees or that eight per cent of the corals died.
Check the date and the map product
A daily temperature map, an accumulated-stress map and a multi-day alert map do not describe the same time span. NOAA’s seven-day maximum Bleaching Alert Area displays the highest alert level in each pixel during the preceding seven days. Its map date marks the end of that interval.
For anyone sharing a screenshot, the practical step is simple: keep the product name, legend and date visible. A cropped image without them makes it harder for the next reader to tell whether it depicts a recent condition, a past peak or a forecast. Record the source link as well, so others can inspect the original.
Why bleaching and live coral cover can coexist
Bleached coral can still be alive. In an explanation published on 7 August 2024, Australian Institute of Marine Science researchers described how surveys during that year’s bleaching event counted living, bleached corals as live coral cover. Follow-up surveys were needed to establish later mortality. This is a historical example of survey timing, not a current assessment of the Great Barrier Reef.
That distinction changes how to compare two reports. “High live coral cover” and “widespread bleaching” are not necessarily contradictory if the surveys counted living corals that were already stressed. Before declaring that one report disproves the other, check the survey dates and the definitions used.
Also check the denominator. Imagine a hypothetical survey examining 100 coral colonies, of which 40 are bleached. That is 40% of the colonies examined. It is not automatically 40% of the seabed area, 40% of every reef in the region, or 40% mortality. Those claims require different measurements.
What useful follow-up evidence looks like
AIMS’ Long-Term Monitoring Program combines broad reef observations with repeated fixed-site surveys. Its measurements include bleaching, disease, juvenile corals and other indicators. This illustrates why reef condition is assessed through several observations rather than a single photograph.
For a local update in Southeast Asia, ask the reporting organisation for:
- The observation period: When were the measurements made, rather than just when was the story published?
- The surveyed area: Which reefs, sites and depths were included?
- The metric: Does the percentage refer to colonies, coral cover, surveyed sites or something else?
- The comparison: Were the same places examined using comparable methods?
- The next check: Is a follow-up survey planned, and where will its results be available?
This checklist is a way to evaluate evidence, not a request to wait for mortality before taking a heat warning seriously. Alerts and surveys have complementary roles: one identifies risk; the other helps establish the ecological response.
Read the regional picture and the local evidence together
Our analysis of Asia’s ocean heat in the WMO 2025 report provides the wider context. It should not be substituted for a dated survey of a specific reef. For the broader habit of checking measurements, baselines and timescales, see our climate evidence guide.
The most informative reef story keeps three pieces of information together: the heat exposure, the observed response and what remains unknown. That makes it possible to take the risk seriously while staying precise about the evidence.