Marine heatwaves

When the sea stays too warm for too long

Sixteen coral reef regions followed daily by satellite. Colour: NOAA Coral Reef Watch bleaching alert level; size: heat stress accumulated over twelve weeks.

No stress
Warning
Alert Level 1
Alert Level 2
Alert 3 to 5

Reefs under alert

Alert Level 1 or higher, out of 16 regions

Marine heatwaves in progress

as defined by Hobday (90th percentile, 5 days)

Last day

2026-10-23

NOAA Coral Reef Watch

Coral Reef Watch 5 km Regional Virtual Stations, daily heat stress of 16 reef regions (NOAA) · U.S. Government work, public domain; cite NOAA Coral Reef Watch and Skirving et al. (2020) · 10/25/26

Alert levels of the day; clicking a reef opens its sheet.

Chagos -6.20°, 71.75° · Indian Ocean

Marine heatwave in progress since 2026-09-21 (33 days), category II strong, up to +1.6 °C above the normal of the day.

Sea surface temperature

30.10 °C

2026-10-23

departure from the normal of the day: +2.40 °C

Coral Reef Watch 5 km Regional Virtual Stations, daily heat stress of 16 reef regions (NOAA) · U.S. Government work, public domain; cite NOAA Coral Reef Watch and Skirving et al. (2020) · 10/25/26 Current

Heat stress (DHW)

0.2 °C-weeks

Warning

Highest stress

17.9 °C-weeks

record of the region, in 2016

Marine heatwaves since 1985

strongest: 2012-08-20, 43 days, II strong

Sea surface temperature, last two years

Show the data as a table

Daily temperature of the region (pixel at the 90th percentile of the HotSpot, CoralTemp), 1991–2020 normal of the day and marine heatwave threshold (90th percentile of the day). Above the threshold five days in a row: a marine heatwave.

Accumulated heat stress (Degree Heating Weeks)

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Twelve-week sum of the excesses of at least 1 °C above the mean of the warmest month (°C-weeks). Around 4: significant bleaching likely; around 8: widespread bleaching and mortality likely; 12 and above: multi-species mortality likely.

The 16 regions on 2026-10-23

Region	Alert level	DHW (°C-weeks)	Anomaly (°C)	Heatwave in progress	DHW record (year)
Great Barrier Reef, north	No stress	0.0	+0.45	—	13.6 (2017)
Great Barrier Reef, centre	No stress	0.0	+0.11	—	12.0 (2024)
Great Barrier Reef, south	No stress	0.0	+0.10	—	14.6 (2024)
Ningaloo (W. Australia)	No stress	0.0	+0.61	—	28.6 (2025)
Fiji	No stress	0.0	-0.76	—	16.7 (2026)
Palau	Watch	0.0	+0.59	—	10.4 (2017)
Southern Ryukyu (Japan)	No stress	1.7	+1.26	—	18.8 (2024)
Hawaii	Watch	0.1	+0.49	—	13.6 (2019)
Galápagos	Alert Level 3	13.0	+6.88	144 d, III severe	40.1 (1998)
Florida Keys	Watch	14.5	+1.44	73 d, II strong	22.3 (2023)
Puerto Rico	Alert Level 2	10.6	+1.71	10 d, II strong	21.3 (2024)
Belize	Watch	14.5	+1.21	—	20.9 (2024)
Maldives	Watch	0.0	+1.60	63 d, II strong	10.9 (2016)
Chagos	Warning	0.2	+2.40	33 d, II strong	17.9 (2016)
Kenya	No stress	0.0	+1.44	—	13.5 (2024)
Egypt (Red Sea)	No stress	13.3	+1.72	—	26.2 (2024)

Regions under alert each year, out of 16

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Number of regions whose heat stress reached at least 4 °C-weeks (Alert Level 1) or 8 (Alert Level 2) during the year. The great bleaching events stand out: 1998, 2010, then almost every year since 2014, up to the fourth global event of 2023–2025. Current year dashed.

Marine heatwave days per year, mean of the regions

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Mean number of marine heatwave days (Hobday et al., 2016) per region and per year. Means per complete decade: 1985–1994 : 9 · 1995–2004 : 13 · 2005–2014 : 18 · 2015–2024 : 69 days. With a fixed 1991–2020 normal, the underlying warming shows as a rise in the number of days.

Method and sources

Data. NOAA Coral Reef Watch regional virtual stations (version 3.1, 5 km): each region gathers the reef pixels; the value kept each day is that of the pixel at the 90th percentile of the HotSpot, so that an isolated pixel cannot trigger the alert. CoralTemp temperature since 1985.

Heat stress (DHW). HotSpot = temperature − mean of the warmest month of the climatology; only HotSpots of at least 1 °C are summed, over a running twelve weeks (°C-weeks). Alert levels of the file: watch, warning, Alert Level 1 (DHW ≥ 4), Alert Level 2 (DHW ≥ 8); Alert Levels 3 to 5 (DHW ≥ 12, 16, 20) added by NOAA in 2023 and rebuilt here from the DHW.

Marine heatwaves. Definition of Hobday et al. (2016): at least five days in a row above the 90th percentile of the day (11-day window, 1991–2020, smoothed over 31 days); two episodes separated by two days or less are merged. Category (Hobday et al., 2018) from the peak departure from normal, in multiples of the difference between threshold and normal: I moderate, II strong, III severe, IV extreme. Computed by the observatory on the daily temperature of each region.

Reading this result

DHW measures the stress the corals undergo, not bleaching itself: underwater surveys are needed to confirm it, and sensitivity varies with species and the history of the reef. A marine heatwave (defined against the normal of the place) can also occur in winter, harmless to corals. Sixteen regions are not a global inventory: NOAA declared the fourth global bleaching event in April 2024 from all its pixels. Ever closer repetitions of the events leave reefs less time to recover (Hughes et al., 2017).

SOURCES OF THIS PAGE

Coral Reef Watch 5 km Regional Virtual Stations, daily heat stress of 16 reef regions (NOAA): NOAA Coral Reef Watch (2018, updated daily). NOAA Coral Reef Watch Version 3.1 Daily 5km Satellite Regional Virtual Station Time Series Data. College Park, Maryland, USA. Skirving, W., et al. (2020), CoralTemp and the Coral Reef Watch Coral Bleaching Heat Stress Product Suite Version 3.1, Remote Sensing, 12(23), 3856. · doi:10.3390/rs12233856 · source page ↗
Licence : U.S. Government work, public domain; cite NOAA Coral Reef Watch and Skirving et al. (2020) · retrieved on Oct 25, 2026 · SHA-256 01618b76ecc5…
Hobday et al. (2016): Hobday, A. J., Alexander, L. V., Perkins, S. E., Smale, D. A., Straub, S. C., et al. (2016). A hierarchical approach to defining marine heatwaves. Progress in Oceanography, 141, 227–238. · doi:10.1016/j.pocean.2015.12.014
Licence : Published literature; cited
Hobday et al. (2018): Hobday, A. J., Oliver, E. C. J., Sen Gupta, A., Benthuysen, J. A., Burrows, M. T., et al. (2018). Categorizing and naming marine heatwaves. Oceanography, 31(2), 162–173. · doi:10.5670/oceanog.2018.205
Licence : Published literature; cited
Skirving et al. (2020): Skirving, W., Marsh, B., De La Cour, J., Liu, G., Harris, A., et al. (2020). CoralTemp and the Coral Reef Watch Coral Bleaching Heat Stress Product Suite Version 3.1. Remote Sensing, 12(23), 3856. · doi:10.3390/rs12233856
Licence : Published literature; cited
Liu et al. (2014): Liu, G., Heron, S. F., Eakin, C. M., Muller-Karger, F. E., Vega-Rodriguez, M., et al. (2014). Reef-scale thermal stress monitoring of coral ecosystems: new 5-km global products from NOAA Coral Reef Watch. Remote Sensing, 6(11), 11579–11606. · doi:10.3390/rs61111579
Licence : Published literature; cited
Hughes et al. (2017): Hughes, T. P., Kerry, J. T., Álvarez-Noriega, M., Álvarez-Romero, J. G., Anderson, K. D., et al. (2017). Global warming and recurrent mass bleaching of corals. Nature, 543, 373–377. · doi:10.1038/nature21707
Licence : Published literature; cited

Marine heatwaves Heat stress of coral reefs, day by day

When the sea stays too warm for too long

Chagos -6.20°, 71.75° · Indian Ocean

The 16 regions on 2026-10-23