Lazy SeasNotes on waves, weather and sea life
Free tool

Coral Bleaching Watch

Pick a reef to see today's NOAA Coral Reef Watch bleaching alert level, how warm the water is compared with normal, how much heat stress has built up, and the last 12 months charted against the bleaching threshold.

Pick a reef

Loading today's data from NOAA Coral Reef Watch…

What each alert level means

NOAA Coral Reef Watch sorts heat stress into eight levels. The first three depend on how far the water is above the reef's usual summer peak, which NOAA calls the HotSpot. The five alert levels depend on how much heat has built up over the last 12 weeks, measured in Degree Heating Weeks (DHW). The levels shown here are the highest reached in the last 7 days, as on NOAA's own maps.

LevelNOAA definitionNOAA's risk wordingIn plain terms
No StressHotSpot ≤ 0No BleachingWater no warmer than the reef's usual summer peak.
Bleaching Watch0 < HotSpot < 1–Warmer than the summer peak, by less than 1 °C. Nothing is adding up yet.
Bleaching WarningHotSpot ≥ 1, 0 < DHW < 4Risk of Possible BleachingHeat stress is building. Some corals may turn pale.
Alert Level 1HotSpot ≥ 1, 4 ≤ DHW < 8Risk of Reef-Wide BleachingBleaching across the reef is likely.
Alert Level 2HotSpot ≥ 1, 8 ≤ DHW < 12Risk of Reef-Wide Bleaching with Mortality of Heat-Sensitive CoralsWidespread bleaching; branching and table corals may start to die.
Alert Level 3HotSpot ≥ 1, 12 ≤ DHW < 16Risk of Multi-Species MortalityCorals of many kinds may die.
Alert Level 4HotSpot ≥ 1, 16 ≤ DHW < 20Risk of Severe, Multi-Species Mortality (> 50% of corals)More than half the corals at risk of dying.
Alert Level 5HotSpot ≥ 1, DHW ≥ 20Risk of Near Complete Mortality (> 80% of corals)More than 80 percent of the corals at risk of dying.

HotSpot and DHW are in degrees Celsius. NOAA added Alert Levels 3 to 5 on December 15, 2023, after the heat of 2023 pushed reefs in the Caribbean and the eastern Pacific far past the old top level. Before that, anything from 8 DHW up was Alert Level 2.

What are Degree Heating Weeks?

Every reef has a usual summer peak: the average temperature of its warmest month, which NOAA calls the maximum monthly mean. Corals start to struggle about 1 °C (1.8 °F) above it, so that is the bleaching threshold. In the Florida Keys the summer peak is 29.6 °C (85.3 °F) and the threshold is 30.6 °C (87.1 °F). In the main Hawaiian Islands the threshold is 28.0 °C (82.4 °F).

Degree Heating Weeks add up how far, and for how long, the water has been at or above the threshold over the past 12 weeks. Two weeks at 2 °C above the threshold makes 4 DHW, and so does four weeks at 1 °C. At 4 DHW, significant bleaching is likely. At 8, severe bleaching and deaths of heat-sensitive corals are likely. In September 2023 the Florida Keys station passed 22 DHW. Its highest reading in the 38 years before that was under 10.

Because DHW looks back 12 weeks, it keeps rising for a while after the hottest days and only falls once the warm spell leaves the 12-week window. A reef can drop to No Stress while its corals are still white from a peak a month earlier.

Where the numbers come from

NOAA measures sea surface temperature from satellites every day on a 5 km (3 mile) grid, in a data set called CoralTemp. Its Regional Virtual Stations summarize each reef region, and this tool follows 40 of the most visited: from the Florida Keys, the Caribbean and Hawaii to the Great Barrier Reef, the Coral Triangle, the Maldives and the Red Sea. For each station and day, NOAA picks the area with the 90th-percentile HotSpot, so the numbers follow the most heat-stressed part of the region rather than its average. A given reef inside the region may be cooler.

Lazy Seas downloads NOAA's station files once a day and shows the latest day, usually one or two days behind today. NOAA's station files still record the older scale, which stops at Alert Level 2, and NOAA's station pages suggest working out the higher levels from the HotSpot and DHW numbers. This tool does that: up to Alert Level 2 it shows NOAA's own level, and above it applies NOAA's current definitions to the same numbers. Data: NOAA Coral Reef Watch, Version 3.1 Daily 5km Satellite Regional Virtual Station Time Series Data, public domain. See NOAA's station map and alert level definitions.

Does a high alert mean the corals are dead?

No. Bleaching is corals expelling the algae that live in their tissue and feed them, which leaves the white skeleton showing through. A bleached coral is starving but alive, and it can take its algae back if the water cools within a few weeks. The alert levels measure the risk, not what has happened on a given reef. Divers and scientists in the water confirm what the satellites suggest.

The risk is real, though. From early 2023 to 2025, bleaching-level heat reached about 84 percent of the world's coral reef area, in what NOAA called the fourth global coral bleaching event. How to spot coral bleaching shows what the stages look like, and climate change and coral reefs covers why the water keeps getting warmer.

What you can do

If you snorkel or dive on a reef under an alert, don't touch, kick or stand on coral. Bleached coral is fragile and slow to recover. Wear a rash guard and use sunscreen without oxybenzone or octinoxate, which Hawaii has banned from sale since 2021. The Reef-Safe Sunscreen Checker reads an ingredient list for you. Choose boat operators that tie up to mooring buoys instead of anchoring on the reef. Our guide to eco-friendly diving and snorkeling has more.

If you see bleaching, report it. NOAA Coral Reef Watch collects bleaching reports to check its satellite alerts against what is happening underwater. Florida has Mote Marine Laboratory's BleachWatch program, and the Great Barrier Reef has the Eye on the Reef app. Groups that grow and replant corals also rely on volunteers and donations; coral restoration projects explains how they work.

Questions

What water temperature causes coral bleaching?

It depends on the reef, because corals adapt to their local water. Bleaching risk starts about 1 °C (1.8 °F) above the average of the reef's warmest month. That is about 30.6 °C (87.1 °F) in the Florida Keys, 29.3 °C (84.8 °F) on the central Great Barrier Reef and 28.0 °C (82.4 °F) in Hawaii. How long the water stays that warm matters as much as the peak, which is what Degree Heating Weeks measure.

How many Degree Heating Weeks cause bleaching?

At 4 Degree Heating Weeks, NOAA expects significant bleaching across a reef (Alert Level 1). At 8, severe bleaching and the death of heat-sensitive corals are likely. Since December 2023, NOAA also warns at 12, 16 and 20 DHW, when deaths across many species, of more than half the corals and of more than 80 percent become likely.

Why does NOAA's station page show Alert Level 2 when this tool shows a higher level?

NOAA's Regional Virtual Station pages and data files still use the older scale, which tops out at Alert Level 2. NOAA says so on those pages and suggests working out the higher level from the HotSpot and DHW numbers. This tool applies NOAA's current definitions, which run to Alert Level 5, to the same numbers.

How often is the coral bleaching data updated?

NOAA publishes new satellite data every day, usually one to two days after the date it describes. Lazy Seas downloads it once a day, so the date shown on each reef is the latest NOAA has released.

Can a bleached coral reef recover?

Yes, if the heat does not last too long. Bleached corals are still alive and can regain their algae over weeks to months once the water cools. Reefs that bleach again every year or two have little time to recover, and long, severe heat such as 12 or more Degree Heating Weeks kills many corals outright.