What is Degree Heating Weeks (DHW)?

DHW is NOAA's heat-stress metric: temperature anomaly above the maximum monthly mean multiplied by weeks of exposure. Significant bleaching begins at DHW≥4; mass bleaching and mortality at DHW≥8.

Coral bleaching

Heat the water. Watch the reef let go.

Slide the temperature anomaly and how long it lasts. Degree Heating Weeks accumulate — the coral pales, bleaches white, then dies, and the fish leave with it.

NOAA alert ladder

No stressDHW 0

WatchDHW >0

Alert 1 · bleachingDHW ≥4

Alert 2 · mortalityDHW ≥8

Temp anomaly

+1.0°C

Degree Heating Weeks

4.3

Bleached

Mortality

Heat stress

Temperature above baseline+1.0 °C

baseline+2.5+5 °C

Days of sustained heat30 days

090180

Degree Heating Weeks4.3

4 · bleach

8 · death

DHW = anomaly °C × weeks of exposure

Reference scenarios

How to use

Slide temperature anomaly (+0 to +5 °C above the 30-year baseline) and the days of heat stress. NOAA's Degree Heating Weeks accumulate as the product of the two. Cross DHW 4 and the reef bleaches; cross DHW 8 and corals begin to die. Fish thin out as mortality climbs.

Degree Heating Weeks

NOAA's metric: DHW = (°C above max monthly mean) × weeks. DHW > 4 → significant bleaching. DHW > 8 → mass bleaching + death.

Great Barrier Reef 2016: DHW 9–16 across northern third. Mortality: 30%. Recovery for branching corals: 10–15 years if no further stress. Most reefs don't get that.

Why "bleached" ≠ "dead"

Bleaching = coral ejects its symbiotic algae under heat stress. Coral is alive but starving. Recovers in 2–3 weeks if water cools.

Beyond ~4 weeks of stress, coral dies. Algae overgrow the skeleton. Recovery requires both temperature drop AND larval recruitment from healthy reefs nearby.

Citations

NOAA Coral Reef Watch (Liu 2014) — DHW methodology. Bleaching Alert Level 1 at DHW ≥ 4 °C-weeks; Level 2 (mortality) at DHW ≥ 8.
Hughes et al. (2017) — "Global warming and recurrent mass bleaching of corals," Nature 543, 373.
Hughes et al. (2018) — GBR 2016 mortality, Nature 556, 492.
Eakin et al. (2010) — "Caribbean corals in crisis," PLoS ONE 5(11), e13969.
Heron et al. (2016) — Coral Reef Watch v3.1, Remote Sensing 8(1), 59.

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