Degree Heating Weeks has a memory
NOAA Coral Reef Watch’s daily 5 km Degree Heating Week product accumulates qualifying heat stress over the most recent 12 weeks. It is a rolling exposure measure in °C-weeks, not today’s sea-surface temperature. A reef can therefore remain under accumulated stress after an individual hot day has ended.
In a simplified teaching example, a qualifying HotSpot of 2°C lasting 14 days contributes 2 × 14 ÷ 7 = 4°C-weeks. A qualifying 1°C HotSpot lasting 28 days also contributes 4°C-weeks. The daily patterns differ even though the accumulated measure is the same. This illustrates the time integration; it is not a reconstructed NOAA record for a real reef.
What the thresholds support
NOAA describes bleaching risk when DHW reaches 4°C-weeks, with reef-wide bleaching and mortality of heat-sensitive corals likely by 8°C-weeks. These are risk relationships, not counts of observed dead coral. Local susceptibility, prior exposure and conditions below the water surface can differ within a satellite grid cell.
The distinction matters when reading a before-and-after pair. A declining DHW value may mean older hot days have aged out of the 12-week window. It does not demonstrate that coral recovered. Likewise, a persistent high value does not specify the percentage of live coral remaining.
Separate the thermal timeline from the ecological timeline
To assess an event, first locate the reef in the dated stress product and follow its time series. Then compare field observations before, during and after the stressful period, retaining survey depth, site and sampling method. The thermal series describes exposure; bleaching surveys describe visible response; later surveys are needed to assess mortality and recovery.
Do not subtract a habitat-area map from a heat-stress map. They have different units and represent different objects. A consistent habitat classification can measure mapped habitat change, while repeated biological observations are needed to attribute a change in coral condition.
NOAA Coral Reef Watch, Daily Global 5 km Degree Heating Week, version 3.1 (released 1 August 2018). The product documentation supplies the 12-week window and risk thresholds. The two arithmetic examples are hypothetical and assume qualifying HotSpot values throughout the stated days. Source checked 30 October 2026; analysis updated on that date.
Start with the mapped variable
Optical imagery can classify shallow-water habitats where water clarity and depth allow. A “coral reef” polygon may represent geomorphology, live coral, mixed benthos or a modeled habitat probability. Read the legend and validation report before calculating area.
Layer-by-layer evidence
| Layer | Supports | Does not establish alone |
|---|---|---|
| Habitat classification | Mapped class under a stated scheme | Live-coral percentage or health |
| Degree Heating Weeks | Accumulated satellite heat stress | Observed bleaching or mortality |
| Diver/transect survey | Condition at sampled sites and dates | Unsampled regional condition |
| pH or carbonate chemistry | Chemical state at stated depth and time | A unique biological response |
Scale controls comparison
A satellite pixel can mix coral, algae, sand and water. Field transects capture finer detail but sparse coverage. When comparing years, keep classification, tide, season, sensor and spatial unit as consistent as possible.
A heat alert is not a damage map
Thermal stress raises bleaching risk. Confirmation requires observations, and ecological impact requires follow-up on mortality and recovery.
Evidence task
For one reef-change claim, record map class, dates, pixel size, depth or cloud limits, field-validation sample and uncertainty. State whether the claim concerns extent, cover, stress, bleaching or mortality.
Sources and limits
- NOAA Coral Reef Watch: heat-stress products and methods.
- Allen Coral Atlas: habitat mapping and monitoring products.
- IPCC AR6 WGII Chapter 3: ocean and coastal ecosystem assessment.
Sources accessed 10 October 2026. This page is not a current bleaching alert or navigation chart.