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What counts as a marine heatwave?
A marine heatwave is when the sea stays warmer than in 90% of years (1991–2020) at that place and time of year for at least five days. How strong it is depends on how far the water is above normal, measured in multiples of the gap between normal and that 90% line. Today’s count of reef regions in each category:
Look up a reef region
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Is El Niño warming the Pacific?
Monthly sea-surface temperature in the four Niño regions of the tropical Pacific, compared with normal (NOAA CPC).
The last 90 days in the strongest heatwaves
Orange: how much warmer than normal for the date. Dashed: the heatwave threshold, the 90% line for that place and time of year. Above the dashed line for five days or more is a marine heatwave.
Where are marine heatwaves today?
Each dot is one of NOAA Coral Reef Watch’s 214 reef regions, coloured by marine heatwave category: blue none, yellow Moderate, orange Strong, red Severe, dark red Extreme.
How many regions in each category?
Marine heatwaves that made history
Are marine heatwaves becoming more common?
Average number of marine heatwave days per reef region each year since 1985.
What does a marine heatwave do?
- Kills seafloor life: in 2003 a Mediterranean heatwave killed at least 25 species of sponges, gorgonians and other invertebrates along thousands of kilometres of coast; in the worst-hit areas almost 80% of gorgonian colonies were affected (Garrabou et al. 2009).
- Turns ecosystems over: after the 2011 Western Australian heatwave, kelp forests retreated by 100 km and seaweed turfs and tropical species took their place (Wernberg et al. 2016).
- Starves wildlife: during the 2014–2016 northeast Pacific heatwave about 62,000 starving common murres washed ashore; total deaths are estimated at close to a million (Piatt et al. 2020).
- Hits fisheries and farms: the 2015–16 Tasman Sea heatwave brought new disease outbreaks in farmed shellfish and deaths of wild molluscs (Oliver et al. 2017).
- Bleaches coral: on reefs, the same heat builds up as degree heating weeks, NOAA’s measure for bleaching risk; see the coral bleaching dashboard.
What drives this
- Ocean heat →The ocean heat bank raises the baseline that regional spikes build on.
- ENSO / El Niño →Pacific warm phases tilt the odds toward basin-wide marine heat.
What this drives
- Coral bleaching →Reefs bleach when marine heat persists long enough to accumulate DHW.
- Hurricane risk →Warm surface water can provide fuel when storms pass over it.
How we know this
- Source: NOAA Coral Reef Watch 5 km Virtual Stations: daily sea-surface temperature since 1985 for 214 reef regions; NOAA CPC monthly Niño-region indices.
- Update cadence: We download NOAA’s files every night; the latest day is usually one to two days old. The Niño indices are monthly.
- Time coverage: Each region’s daily record from 1985; the climate baseline is 1991–2020.
- Methods: The standard definition of Hobday et al. (2016) and the categories of Hobday et al. (2018), applied to each region’s own daily temperature: a heatwave is at least five days above the local 90th percentile for that time of year; categories are multiples of the gap between that percentile and the average.
- Uncertainty: Satellite surface temperature only; heat below the surface is not seen. Each region is summarised by NOAA from its 5 km grid.
- Limitations: Tropical and subtropical reef seas only, not the whole ocean (no North Atlantic, North Pacific or polar seas). You pick the region; the page never detects your location.
- Primary data: NOAA CRW Virtual Stations · NOAA CPC Niño indices · How this page is built