Degree Heating Weeks (DHW)

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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.

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