In the summer of 2023, the surface waters of the North Atlantic ran so warm that scientists could scarcely believe their instruments. Off the coast of Florida, water temperatures reached the low 30s Celsius — temperatures that would be remarkable for a tropical lagoon, let alone the open ocean. Coral reefs that had survived for centuries began to bleach within weeks. Fisheries shifted, seabirds starved, and fish kills washed ashore from the Gulf of Mexico to the coasts of Europe. The heat was not confined to the Atlantic. The Mediterranean set its own records, the Pacific warmed across vast stretches, and for the first time, large areas of the global ocean ran above-average simultaneously for months on end.
This is the signature of a marine heatwave — a discrete, prolonged period of anomalously warm ocean temperatures. Like their terrestrial cousins, marine heatwaves are not new. The ocean has always had warm years and cool years. What has changed, and what makes the recent events so alarming, is the pace at which they are intensifying. Research published in the journal Nature has shown that the frequency of marine heatwaves has roughly doubled since 1982, and their average duration has grown by more than two-thirds. Events that once occurred once in a generation now arrive every few years, and the temperature anomalies they carry are being stacked on top of an ocean that is itself steadily warming under the weight of greenhouse gas emissions.
What Exactly Is a Marine Heatwave?
A marine heatwave is typically defined as a period of at least five consecutive days in which sea surface temperatures in a given region exceed the local 90th percentile of the historical record for that time of year. This is a relative definition — it measures how unusual conditions are for a specific place at a specific time. A temperature of 26°C off the coast of Norway is a marine heatwave; the same temperature in the Gulf of Thailand is a normal summer afternoon. What matters is the departure from the expected local baseline, and the persistence of that departure.
Categories and Severity
Scientists classify marine heatwaves on a scale that mirrors the category systems used for hurricanes. A Category 1 event is a moderate heatwave, the kind that marine ecosystems can often withstand with temporary stress. Category 2 is strong, Category 3 is severe, and Category 4 is extreme — the kind of event that can trigger mass mortality, shift entire species ranges, and cause cascading damage to fisheries and coastal economies. In some recent events, ocean regions have pushed into a notional Category 5, exceeding the scale entirely. The North Pacific "Blob" of 2013–2016, the Mediterranean events of 2015 and 2018, and the global ocean heat of 2023–2024 all reached the upper tiers of this scale, and each left a trail of ecosystem damage in its wake.
The definition matters because it helps researchers track trends and compare events across regions and decades. But it also matters because it shapes how we think about attribution. When scientists ask whether a particular marine heatwave was made more likely by climate change, they are comparing the event's intensity and duration against what would be expected in a world without human-caused warming. For most recent events, the answer has been emphatic: the odds of such extremes have increased dramatically, and their intensity has been amplified by the background warming of the ocean.
What Drives a Marine Heatwave?
Marine heatwaves are the product of a handful of physical mechanisms working alone or in combination. The first is simple heat exchange with the atmosphere. When the ocean surface is heated by sunlight and warm air, it stores that heat. If atmospheric conditions — a lingering high-pressure system, for example — suppress clouds and wind, the ocean surface can warm rapidly because the normal cooling mechanisms of evaporation and mixing are suppressed. High-pressure systems act like a lid, allowing the sun to beat down on calm, still water.
The Role of Ocean Currents
The second mechanism is oceanic. Ocean currents transport heat around the globe, and when those currents shift, they can deliver unusually warm water to regions that normally stay cooler. The 2023 North Atlantic event, for example, was influenced by a pattern of weakened trade winds and altered ocean circulation that allowed warm water from the tropics to surge northward. El Niño plays a starring role as well. During El Niño years, the redistribution of warm water across the tropical Pacific sends temperature anomalies rippling across the ocean, and marine heatwaves in the Pacific are strongly tied to the state of the El Niño–Southern Oscillation.
The third mechanism is the interaction between heatwaves and the deeper ocean. During a heatwave, the warm surface layer becomes stratified — it forms a barrier that prevents mixing with cooler water below. This stratification is self-reinforcing: the warmer the surface, the more stable the layering, and the more resistant the system becomes to the mixing that would normally bring up cool water. This is why some heatwaves persist for months even after the atmospheric conditions that triggered them have passed. The ocean has stored the heat, and it takes time — sometimes a very long time — to release it.
Marine Heatwaves at a Glance
2x: Increase in the frequency of marine heatwaves since 1982
5+ days: Minimum duration for an event to qualify as a marine heatwave
90th percentile: The temperature threshold defining a heatwave for a given region
2013–16: The North Pacific "Blob", one of the largest marine heatwaves on record
50%+: Share of global ocean heat that comes from human-caused warming in recent decades
The Great Heat: Case Studies of Recent Events
To understand what marine heatwaves do, it helps to look at what they have already done. Each major event of the past decade has been a natural experiment — unplanned, uncontrolled, and revealing.
The North Pacific Blob (2013–2016)
In the winter of 2013–2014, an enormous patch of anomalously warm water formed in the Gulf of Alaska and spread across the northeastern Pacific. Dubbed "the Blob," it persisted for more than two years, with temperature anomalies of 2 to 3°C above normal across a region spanning thousands of kilometres. The ecological consequences were staggering. The Gulf of Alaska's ecosystem, one of the most productive fisheries on Earth, collapsed in stages. Young cod starved, sea lion pups washed up dead on the beaches, and the common murre — a seabird that depends on forage fish — suffered one of the largest documented seabird die-offs in history, with an estimated one million birds killed. The Blob also primed the West Coast for one of its most destructive harmful algal blooms on record, a toxic event that shut down crabbing and shellfish fisheries across California, Oregon, and Washington.
The North Atlantic and Mediterranean Events (2015, 2018, 2023)
The Mediterranean Sea, a semi-enclosed basin that warms faster than the global ocean, experienced severe marine heatwaves in 2015 and 2018 that killed corals, seagrasses, and shellfish across large areas. Then, in 2023, the entire basin ran hot simultaneously with the North Atlantic, where average sea surface temperatures in June exceeded the previous record by a margin that startled oceanographers. The 2023 Atlantic event was driven by a combination of an emerging El Niño, weakened trade winds, and record-low aerosol pollution over the ocean — a reminder that even "natural" drivers interact with human influence in complex ways. The heat fueled marine mortality from Florida's coral reefs to the coasts of Ireland, and it set the stage for the global ocean heat record that continued through 2024.
The 2023–2025 Global Ocean Heat
Beginning in early 2023 and continuing through 2024 and into 2025, the global ocean ran consistently warmer than any period in the instrumental record. Sea surface temperatures around the world broke daily records for more than a year in a row. The global ocean absorbed more than 90 percent of the excess heat from climate change, and that heat is now circulating through marine ecosystems, atmospheric circulation, and coastal communities. The events of this period demonstrated that marine heatwaves are no longer regional anomalies — they can span ocean basins and persist across years.
Impacts on Ecosystems and Fisheries
The ocean is not a passive container of heat; it is alive. When marine heatwaves strike, they cascade through the entire web of life, from microscopic phytoplankton to apex predators, and from wild ecosystems to the industries and communities that depend on them.
Coral Bleaching and Reef Death
Corals are among the most sensitive organisms to heat. When water temperatures exceed a reef's tolerance for even a few weeks, the coral expels the symbiotic algae that give it colour and energy — the process known as bleaching. A bleached coral is not dead, but it is starving and vulnerable. If the heat persists, it dies. Marine heatwaves have driven repeated mass bleaching events on the Great Barrier Reef, in the Caribbean, in the Indian Ocean, and across the Pacific. The 2015–16 El Niño, amplified by background warming, triggered the third global bleaching event on record. The 2023–24 heat extended bleaching to reefs in places that had never bleached before. Some researchers now project that coral reefs could lose the majority of their living cover by mid-century under continued warming — a loss that would remove the habitat of roughly a quarter of all marine species.
Fisheries, Food Webs, and Coastal Communities
When warm water arrives, species move. Fish that depend on cooler water shift poleward or to greater depths, disrupting the food webs that predators — including humans — depend on. In the northeastern United States, the prized Atlantic cod fishery collapsed in part because cod's core habitat shifted north and the fish became scarcer within traditional fishing grounds. On the other side of the Atlantic, the warming North Sea has seen species like anchovy and sardine appear in waters where they were once absent, altering ecosystems and forcing fishing communities to adapt. The economic stakes are enormous: the world's fisheries employ and feed hundreds of millions of people, and marine heatwaves strike hardest at the small-scale fishers and coastal nations that have the fewest alternatives.
Marine Heatwaves and the Weather
The ocean and atmosphere are coupled. When the ocean surface runs hot, it transfers heat and moisture to the atmosphere, fueling storms, shifting rainfall patterns, and amplifying heatwaves on land. The record-breaking land heatwaves of recent summers, from Europe to Asia to the Americas, have in several cases been intensified by warm offshore waters that supplied extra heat and moisture. Warm ocean water also fuels hurricanes, which is why the most active Atlantic hurricane seasons in recent history — 2005, 2017, 2020, and 2023 — were associated with unusually warm tropical Atlantic waters. A marine heatwave is not merely an ocean story; it is a global weather story.
Attribution: How Much Is Climate Change?
Attribution science has become one of the most powerful tools for understanding marine heatwaves. The method is straightforward in principle: run climate models with observed greenhouse gas levels, run them again with pre-industrial levels, and compare how often and how intense marine heatwaves occur in each world. The difference is the human fingerprint.
For the 2023 North Atlantic heatwave, attribution studies found that a similar event was essentially impossible in a world without climate change — the observed conditions sat far outside the range of natural variability. Even in the 2013–16 Blob, once attributed primarily to natural variability, subsequent analyses showed that background warming added roughly a degree of heat to the event, pushing it from unusual to record-breaking. The general finding across the literature is stark: almost every marine heatwave of the past decade has been made more likely and more intense by climate change. Natural variability provides the spark; human-caused warming provides the fuel.
Can We Predict Marine Heatwaves?
Forecasting marine heatwaves is a growing field, and it has already begun to pay dividends. Forecast systems run by agencies like NOAA and the European Centre for Medium-Range Weather Forecasts can now predict the likelihood of a marine heatwave weeks to months in advance, using coupled ocean-atmosphere models that track the same heat that drives seasonal weather forecasts. These forecasts give fisheries managers, aquaculture operators, coral reef managers, and coastal planners valuable lead time to prepare — moving vulnerable stock, altering harvest quotas, deploying coral restoration teams, or triggering emergency response plans.
Prediction is not perfect. Marine heatwaves that are driven by abrupt atmospheric events — a sudden collapse of the trade winds, an unexpected high-pressure ridge — remain hard to anticipate. But the trend is clear: as the ocean warms, the baseline against which heatwaves are measured keeps rising, which means that today's extreme event may be tomorrow's normal. This is the crux of the challenge. Even if the science of prediction improves, it cannot compensate for the relentless upward drift of the ocean's temperature. The only durable solution is to stop adding heat to the system — which means cutting the greenhouse gas emissions that are warming the ocean from the top down and the bottom up.
What the Future Holds
Under current emissions trajectories, marine heatwaves are projected to become even more frequent, more intense, and longer-lasting. Under a high-emissions scenario, some ocean regions could be in a state of quasi-permanent heatwave by the end of the century — the "new normal" would be a perpetual marine heat event. Even under strong mitigation, the ocean's thermal inertia means that heatwaves already locked into the system will continue for decades. The ocean remembers the heat we have already released, and it will keep returning it to the surface for a long time.
The response, therefore, has two tracks. The first is adaptation: building resilience into marine ecosystems and coastal economies so that they can withstand the heat that is coming. This includes marine protected areas, sustainable fisheries management, coral restoration, and diversification of coastal livelihoods. The second is mitigation: cutting emissions fast enough to keep the ocean's temperature rise within bounds that ecosystems and people can survive. Neither track is optional. The ocean is burning, and the fire is ours to put out.
Frequently Asked Questions
What is a marine heatwave?
A marine heatwave is a period of at least five consecutive days in which sea surface temperatures in a specific region exceed the 90th percentile of historical records for that location and time of year. It is a measure of how unusually warm the ocean is for a particular place, not an absolute temperature threshold.
What causes marine heatwaves?
Marine heatwaves are caused by a combination of factors: direct heating from the atmosphere, high-pressure systems that suppress wind and clouds, shifts in ocean currents that transport warm water, and interactions with climate cycles like El Niño. Climate change amplifies all of these by raising the background temperature of the ocean.
How does climate change affect marine heatwaves?
Climate change increases the frequency, intensity, and duration of marine heatwaves by warming the ocean's baseline temperature. Attribution studies have shown that many recent events — including the 2023 North Atlantic heatwave — would be virtually impossible without human-caused global warming.
What are the impacts of marine heatwaves?
Marine heatwaves cause coral bleaching, mass die-offs of fish and seabirds, harmful algal blooms, shifts in species ranges, disruption of fisheries and aquaculture, and intensified extreme weather on land, including stronger storms and hotter terrestrial heatwaves.
Can marine heatwaves be predicted?
Yes, to a degree. Forecast systems can now predict the likelihood of marine heatwaves weeks to months in advance using coupled ocean-atmosphere models. This allows fisheries managers, aquaculture operators, and coastal planners to prepare, though abrupt events driven by sudden atmospheric changes remain harder to forecast.
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