A marine heatwave is a period when ocean temperatures in a specific area rise well above normal and stay there for at least five days. The standard definition, established by researchers in 2016, sets the threshold at the 90th percentile of historical temperatures for that location and time of year, based on a 30-year baseline. In other words, the water has to be hotter than 90% of what’s been recorded there during that same season. These events can last weeks, months, or in extreme cases, years.
How Marine Heatwaves Are Classified
Not all marine heatwaves are equal. Scientists classify them on a scale from Category 1 to Category 5, similar to how hurricanes are ranked. Category 1 is “Moderate,” meaning temperatures exceed the 90th percentile but stay below the next threshold. Category 2 is “Strong,” Category 3 is “Severe,” Category 4 is “Extreme,” and Category 5 is “Beyond Extreme.” Each step up means the temperature anomaly is roughly double the distance above the historical average compared to the previous category.
A Category 1 event might stress some marine species. A Category 4 or 5 event can reshape entire ecosystems.
What Causes Them
Two forces primarily drive marine heatwaves: atmospheric conditions pushing heat into the ocean surface, and ocean currents carrying warm water into regions where it doesn’t normally belong. Of these, heat exchange between the air and sea is the more common trigger, with reduced evaporation (latent heat flux) playing a particularly important role during the onset of events.
The atmosphere sets the stage in a few ways. High-pressure systems can park over an ocean region for weeks, creating clear skies that let solar radiation pour in while suppressing the winds that would normally mix and cool surface waters. These “blocking” patterns are especially effective at triggering heatwaves during spring and summer. In autumn and winter, low-pressure cyclonic systems can also drive events through different heat exchange dynamics.
How deep the warm surface layer extends matters enormously. When the mixed layer of the ocean is shallow, the same amount of incoming heat warms a smaller volume of water, so temperatures spike faster. This shallow layering is a critical factor in how quickly a heatwave develops. Once the atmospheric forcing weakens, ocean processes like mixing and currents gradually dissipate the built-up heat, but that can take a long time.
They Don’t Just Happen at the Surface
Some marine heatwaves intensify below the surface rather than at it. Research in the East/Japan Sea found that the strongest temperature anomalies during summer heatwaves occurred between 20 and 200 meters deep, with peak intensity around 70 meters. At that depth, maximum temperature departures reached 4°C (about 7°F) above normal, compared to smaller anomalies at the surface.
These subsurface heatwaves can extend down to roughly 300 meters and pose a particular threat to bottom-dwelling species and deep-water ecosystems that surface measurements alone would miss. Events in the East/Japan Sea from 2016 through 2023 showed this pattern consistently, with the strongest warming signals at depth rather than at the top of the water column.
The Blob: A Case Study
The most studied marine heatwave in recent history began in the fall of 2013, when a stubborn ridge of high pressure settled over the northeastern Pacific and suppressed normal winter winds. The sun heated an expanding patch of ocean that researchers nicknamed “the Blob.” By fall 2014, it stretched from the Gulf of Alaska to Baja California, with sea surface temperatures running as much as 7°F above average.
The ecological fallout was extraordinary. The food web shifted from nutrient-rich tiny crustaceans to gelatinous organisms like jellyfish and pyrosomes (translucent, pickle-shaped creatures never before recorded off the Pacific Northwest). These gelatinous species vacuumed up food that fish like anchovy and sardine depended on, while offering far less nutrition to predators in return. Fish that normally spawned in midsummer began spawning in winter off Oregon. Salmon migration routes changed. Subtropical species like tuna and swordfish appeared hundreds of miles north of their usual range, and an estimated 70,000 young endangered loggerhead turtles were spotted riding warm currents off Southern California in 2015.
The consequences cascaded upward. A record toxic algal bloom shut down West Coast Dungeness crab fisheries worth millions of dollars. Massive seabird die-offs followed. Record numbers of whales became entangled in fishing lines that had been repositioned because of shifting catch patterns. Starving California sea lion pups washed up on beaches, and salmon returns crashed. Even after much of the northeast Pacific cooled in 2016, conditions never entirely returned to normal.
Damage to Coral Reefs and Coastal Ecosystems
Coral reefs are among the most vulnerable ecosystems. When water temperatures stay elevated, corals expel the symbiotic algae living in their tissues, turning white in a process called bleaching. If temperatures don’t drop soon enough, the corals die. During recent heatwaves in the Caribbean and tropical Pacific, coral reef ecologist Sophie Dove at the University of Queensland noted that “corals are literally dying before they even have a chance to bleach,” meaning some are being killed outright by the heat rather than going through the usual stress response first.
High levels of mortality in staghorn and elkhorn corals have been reported in Florida, Puerto Rico, and Mexico. The damage extends beyond reefs to mangrove forests and seagrass meadows, both of which support nursery habitat for commercially important fish species and protect coastlines from storm surge.
Economic Toll
The financial damage from marine heatwaves is already measured in billions. Individual events have caused more than $800 million in direct losses, with indirect losses from degraded ecosystem services exceeding $3.1 billion over multiple years, according to a review published in Science. These costs stem from fisheries closures, aquaculture die-offs, harmful algal blooms that contaminate shellfish, and the collapse of tourism tied to healthy marine environments. Clear economic consequences have been linked to at least 34 separate marine heatwave events across all major ocean basins.
Getting Worse Over Time
Marine heatwaves are not new, but they are becoming more common and lasting longer. An analysis of ocean temperature records from 1925 to 2016 found that global average marine heatwave frequency increased by 34% and duration increased by 17%, resulting in a 54% rise in the total number of marine heatwave days each year. The primary driver is straightforward: as greenhouse gas emissions warm the atmosphere and oceans, the baseline temperature creeps upward, making it easier to cross heatwave thresholds.
Projections from the IPCC’s Special Report on the Ocean suggest the trend will accelerate dramatically. Under a low-emissions scenario, marine heatwaves are projected to become 20 times more frequent by the end of the century compared to pre-industrial conditions. Under a high-emissions scenario, they become 50 times more frequent, with intensity increasing roughly tenfold. The Arctic and tropical oceans face the steepest increases. Under that same high-emissions pathway, the ocean would absorb five to seven times more heat by 2100 than it accumulated between 1970 and today, with cascading effects on marine animal biomass (projected to decline 15% globally) and fisheries productivity (projected to drop 20%).