OCEAN · FIELD GUIDE
How Marine Heatwaves Reshape the Ocean — Coral Bleaching, Degree Heating Weeks & the Reefs in the Firing Line
The ocean can have a heatwave just like the air can — a patch of sea that stays too warm for too long. When it sits over a coral reef, the coral starves and turns bone-white. The map shows you exactly where that heat is piling up right now, measured in 'Degree Heating Weeks.' Here is what that number means, why 4 of them is a warning and 8 is a funeral, and why the last few years have been the worst on record for reefs.
The ocean has heatwaves too
We think of heatwaves as something that happens to the air over a city. But the ocean has them just as surely — a patch of sea that warms up and then stays warm, for days, weeks, sometimes months. Scientists call these marine heatwaves, and they define one much the way they define a heatwave on land: when the sea-surface temperature climbs above the 90th percentile of normal for that place and season and holds there for at least five days running.
They matter because the ocean is where almost all of our extra heat is going. More than nine-tenths of the additional energy trapped by greenhouse gases has been absorbed by the sea, and as that heat builds, marine heatwaves are getting hotter, longer and more frequent. When one of them parks itself over a coral reef, the consequences are dramatic and visible from space.
What bleaching actually is
A coral looks like a rock but is really a colony of tiny animals, and most of them run a quiet partnership with algae called zooxanthellae that live inside their tissue. The algae photosynthesise, hand most of the resulting food to the coral, and give the reef its colour. It is a deal that works beautifully — until the water gets too hot.
Under heat stress, that partnership collapses. The coral expels the algae, and with them goes its colour: the reef turns a ghostly, uniform white. This is bleaching. A bleached coral is not dead — it is starving. Ease the heat in time and the algae return, the colour comes back, and the coral lives. Let the heat grind on, and the coral runs out of energy, falls to disease, and dies. Bleaching is the alarm bell, ringing in real time, that a reef is in trouble.
The number on the map: Degree Heating Weeks
The trouble with "the water is warm" is that warmth alone doesn't tell you how much danger a reef is in. A brief warm spell is survivable; a moderate one that never lets up is lethal. So NOAA's Coral Reef Watch built a single number that captures both how hot and for how long: the Degree Heating Week, or DHW.
It works by adding up how far the sea has risen above the local summer-maximum temperature, accumulated over the most recent 12 weeks. One DHW is about a week of water sitting one degree above that bleaching threshold. Two weeks at one degree above, or one week at two degrees above, both come to 2 DHW. The units are degree-Celsius-weeks, and the whole point is that they fold intensity and duration into one figure you can watch climb.
The thresholds are where it gets serious, and they are not arbitrary:
- Around 4 DHW — significant bleaching becomes likely, especially in heat-sensitive species.
- Around 8 DHW — reef-wide bleaching is likely, with mortality of those sensitive corals.
- 12 DHW and up — multi-species mortality becomes likely.
- 16 DHW and up — severe mortality, more than half the corals at risk.
- 20 DHW and up — near-complete mortality, over 80% of corals.
That is why the colour ramp on this layer is worth reading carefully. The jump from yellow to deep red isn't decorative — it tracks the line between a reef that will probably recover and one that won't.
Read from orbit, every day
None of this requires a diver at the reef. Coral Reef Watch blends sea-surface temperatures from a fleet of NOAA and partner satellites into one daily global map at 5 km resolution, with a consistent record stretching back to 1985. From that it works out today's HotSpot — how far above the local hottest-month average the water is right now — and accumulates it into the rolling Degree Heating Week figure. The product refreshes every afternoon, which is what lets this layer show heat stress as a live signal rather than a historical chart. This map pulls the latest published day straight from that feed.
The worst on record: 2023–2025
The reason this layer feels urgent is that the world just lived through the largest coral bleaching event ever documented. From February 2023 to about mid-2025, bleaching-level heat stress reached roughly 84% of the planet's coral-reef area, in every ocean basin that holds reefs, with bleaching reported in at least 83 countries and territories. NOAA confirmed it as the fourth global bleaching event on record in April 2024 and judged it likely over in mid-2025.
It broke the previous record — the third global event of 2014–2017, which affected about 68% of reefs — and it followed the first two global events of 1998 and 2010. The grim subtext, in the words of NOAA's Coral Reef Watch coordinator, is that we have entered an era where reefs bleach on a near-annual basis, which makes it harder and harder to even say when one event ends and the next begins. Each bleaching may not be instantly fatal, but reefs need years to recover, and they are getting less and less time between blows.
Why a sliver of seafloor matters so much
Coral reefs cover well under 1% of the ocean floor. For that tiny footprint they carry an outsized load: roughly a quarter of all known marine species depend on reefs at some point in their lives, and reefs shelter coastlines from storms, feed hundreds of millions of people, and anchor tourism and fishing economies. Put a price on all of that and you reach figures close to USD$9.8 trillion a year in ecosystem goods and services. When a reef bleaches and dies, that whole stack — the biodiversity, the coastal defence, the food, the income — goes with it.
How to read this layer
Look to the tropics, where warm-water corals live: the Caribbean and Florida, the Coral Triangle, the Great Barrier Reef, the Red Sea and Persian Gulf, the South Pacific and Indian Ocean islands. Calm, cool water is transparent; the warm colours mark reefs being cooked right now. The hot patches march with the seasons — Northern Hemisphere reefs peak in late summer, Southern Hemisphere reefs around the austral summer. Switch on Sea Surface Temp next to it to see the warm water itself, and Marine Life to see the creatures that hang their existence on these reefs. The number you're watching is exactly NOAA's Degree Heating Weeks — the ocean's fever chart, updated daily.
Frequently asked questions
What is a marine heatwave?
A marine heatwave is a prolonged spell of unusually warm water in part of the ocean — the sea's version of a heatwave on land. The usual definition is sea-surface temperature staying above the 90th percentile of what's normal for that place and time of year for at least five days in a row. They can last weeks or months and stretch across thousands of kilometres. Like heatwaves in the air, they are becoming more frequent, more intense and longer-lasting as the ocean absorbs the bulk of the extra heat trapped by greenhouse gases — the ocean has taken up more than 90% of that excess heat (IPCC; NOAA, 2021–2024).
What is coral bleaching?
Corals get their colour, and most of their food, from tiny algae called zooxanthellae that live inside their tissue and photosynthesise. When the water gets too warm for too long, that partnership breaks down: the coral expels the algae, loses its colour and turns a stark white — it 'bleaches.' A bleached coral is not yet dead; it is starving. If the heat eases soon enough, the algae can return and the coral recovers. If the heat stress is severe or drags on, the coral runs out of energy, becomes vulnerable to disease, and dies. Bleaching is the visible alarm bell of heat stress on a reef (NOAA Coral Reef Watch; Australian Institute of Marine Science).
What is a Degree Heating Week (DHW)?
A Degree Heating Week is the single number this layer maps — a way of adding up how much heat stress a reef has endured. It accumulates how far the sea-surface temperature has risen above the local summer maximum, and for how long, over the most recent 12 weeks. One DHW is roughly equivalent to one week of water sitting one degree Celsius above that bleaching threshold; two weeks at one degree, or one week at two degrees, both give 2 DHW. The units are degree-Celsius-weeks. Because it combines intensity and duration into one figure, DHW is the best available early-warning signal for when bleaching is coming, not just whether the water is warm today (NOAA Coral Reef Watch).
Why do 4 and 8 Degree Heating Weeks matter?
They are the thresholds NOAA's research has tied to real damage. At about 4 DHW, significant coral bleaching becomes likely, especially in the more heat-sensitive species. By 8 DHW, reef-wide bleaching is likely along with mortality of those sensitive corals. If the stress keeps climbing past 12 DHW, multi-species mortality becomes likely; at 16 or more, severe mortality affecting over half the corals is on the cards; and at 20 or above, near-complete mortality — over 80% of corals — becomes likely. That is why the colour ramp on the map matters: it isn't just 'warm' and 'warmer,' it's the difference between a reef that will probably recover and one that won't (NOAA Coral Reef Watch, 2026).
How does NOAA Coral Reef Watch measure all this from space?
Coral Reef Watch blends sea-surface temperature readings from a fleet of NOAA and partner satellites into a single daily global map at 5 km resolution, a record that runs back to 1985. From that it derives the HotSpot (how far above the local hottest-month average the water is right now) and accumulates it into the Degree Heating Week product over a rolling 12-week window. No instrument has to be in the water at the reef itself — the heat stress is read from orbit, updated every day at around 13:30 US Eastern time. That is what makes a live global bleaching-risk map possible at all (NOAA Coral Reef Watch).
What was the fourth global coral bleaching event?
It was the largest mass-bleaching event ever recorded. Running from February 2023 to about mid-2025, it subjected roughly 84% of the world's coral-reef area to bleaching-level heat stress, across all three ocean basins that hold reefs — the Pacific, Atlantic and Indian — with bleaching documented in at least 83 countries and territories. NOAA confirmed it as the fourth global event on April 15, 2024, and determined it had likely ended in mid-2025 after the severe Western Australia bleaching of early 2025. It surpassed the previous record, the third global event of 2014–2017, which hit about 68% of reefs. The first two global events were in 1998 and 2010 (NOAA / NESDIS, 2024–2026; ICRI, 2025).
If a reef bleaches, is it dead?
Not necessarily. Bleaching is a stress response, not death itself — a bleached coral has lost its algae but can survive and re-colour if conditions improve in time. NOAA is careful to make this distinction: corals can recuperate if the strain eases. But recovery takes years even in the best case, and reefs are now being hit so often that they get less and less time to heal between events. NOAA's Coral Reef Watch coordinator has described it plainly: we are now in an era where reefs will bleach on a near-annual basis. Repeated bleaching, even when each event isn't immediately fatal, grinds reefs down (NOAA, 2025–2026).
Why should anyone care if coral reefs die?
Because they hold up a staggering share of ocean life and human livelihoods on a tiny footprint. Coral reefs cover well under 1% of the seafloor, yet they shelter roughly a quarter of all known marine species at some point in their lives. They also protect coastlines from storms and waves, feed hundreds of millions of people, and underpin tourism and fisheries — the goods and services reefs provide have been valued at close to USD$9.8 trillion a year. When reefs collapse, that biodiversity, coastal protection and food all go with them (de Groot et al. 2012; NOAA).
Where on the map should I look?
The tropics, where warm-water corals live: the Caribbean and Florida, the Coral Triangle of Southeast Asia, the Great Barrier Reef off Australia, the Red Sea and Persian Gulf, the islands of the South Pacific and Indian Oceans. Cool, calm water shows as transparent; the warm colours mark reefs under active heat stress. The hottest patches move with the seasons — the Northern Hemisphere reefs peak around late summer (August–October), the Southern Hemisphere reefs around the austral summer (January–April). Switch on Sea Surface Temp alongside it to see the warm water itself, and Marine Life to see what depends on these reefs.
Is this layer live, and what's the catch?
It's live: the tiles come straight from NOAA Coral Reef Watch's daily 5 km Degree Heating Week product, showing the latest published day, served keyless. Two honesty notes. First, it's a 5 km satellite product, so it shows reef-scale and regional heat stress, not the condition of one individual coral head — and it reads the water, not the coral, so it's a risk signal rather than a confirmed-bleaching map. Second, the underlying grid is published in a standard lat-long projection and re-tiled onto the web map, which introduces a tiny vertical stretch far from the equator; at the tropical reef latitudes that matter here it's negligible. The number you're watching — Degree Heating Weeks — is exactly NOAA's, unmodified.
SEE IT LIVE
Everything in this guide is on the live ocean map.