Degree Heating Weeks (DHW)

Assembled from 26 blocks · 1 evidence · 18 related

1. Story
2. The unit in which reef thermal stress is measured, and the reason bleaching can be forecast rather than only observed. It is built from two ideas. First, a HotSpot: the amount by which satellite-measured sea surface temperature exceeds the local maximum monthly mean — the warmest month of the site's own climatology, not a global number, so a reef in the Persian Gulf and a reef in Fiji are each judged against their own summer. Second, accumulation: HotSpot values of one degree Celsius or more are summed over a rolling twelve-week window, giving a figure in degree Celsius-weeks. Roughly four DHW is the level at which significant bleaching becomes likely; roughly eight is where widespread bleaching with significant mortality becomes likely. What the unit encodes is that heat stress is a dose, not a temperature: two weeks at two degrees over and four weeks at one degree over are the same accumulated stress, and neither is captured by a daily maximum.
3. Knowledge
4. Coral bleaching
5. Degree Heating Weeks (DHW)
6. Connections
7. Coral bleaching
8. NOAA Coral Reef Watch
9. The fourth global bleaching event is confirmed
10. Reef-building corals (Scleractinia)
11. Marine heatwave
12. Degree Heating Weeks (DHW)
13. Symbiont shuffling and thermal tolerance
14. The first global mass bleaching event
15. A published projection that reefs would bleach faster than they could recover
16. The second global bleaching event
17. The third global bleaching event
18. Mass bleaching on the Great Barrier Reef
19. A second consecutive year of mass bleaching on the Great Barrier Reef
20. The interval between severe bleaching events is measured, and it has collapsed
21. Mass bleaching on the Great Barrier Reef during a La Nina
22. The fourth global bleaching event is confirmed
23. The fifth mass bleaching on the Great Barrier Reef since 2016
24. The current state of the fourth global bleaching event is not established in this package
25. Evidence
26. Reef thermal stress is measured as HotSpot (sea surface temperature above the local maximum monthly mean) accumulated over a rolling twelve-week window into Degree Heating Weeks, with roughly four DHW indicating likely significant bleaching and roughly eight indicating likely widespread bleaching and mortality. V55 verification basis: the product documentation was NOT retrieved — coralreefwatch.noaa.gov was refused by the egress proxy — and these definitions and thresholds are stated from the producing model's working knowledge of the products. Any alert levels added above Level 2 are not described here.