How are marine heat waves caused by climate change?

Rising ocean temperatures from greenhouse gas emissions raise the baseline temperature of the ocean everywhere. This means that the same weather patterns that would have occasionally pushed temperatures above normal in the past now push them into far more extreme territory. Additionally, some heat waves form when high-pressure weather systems persist longer than usual, trapping warm air over the ocean surface.

What happened during the 2023 North Atlantic marine heat wave?

In 2023, the North Atlantic experienced historically unprecedented warmth, with temperatures reaching 3-4 degrees Celsius above historical norms in some areas during summer and fall. The warm water suppressed hurricane formation by altering atmospheric wind patterns while simultaneously raising concerns about the Atlantic Meridional Overturning Circulation (AMOC), the system of ocean currents that includes the Gulf Stream.

How do marine heat waves affect fish populations and fisheries?

Marine heat waves trigger rapid changes in ocean chemistry and oxygen levels. Fish species migrate away from warming zones, causing harvests to collapse in traditional fishing grounds while stocks shift toward colder regions or deeper water. For species that cannot adapt quickly, heat stress causes direct mortality. Shellfish beds have been wiped out entirely during extreme events.

Guide · Phenomena

Marine Heat Waves: The Underwater Fires

Q: What is the scientific definition of a marine heat wave?

A marine heat wave is defined as a period when sea surface temperatures exceed the 90th percentile of local historical temperatures for that location and time of year, sustained for five or more consecutive days. This percentile-based approach accounts for natural regional variations, since a temperature that would be extreme in the North Atlantic might be normal in the tropical Pacific.

Persistent extreme ocean temperatures that devastate fisheries, kill coral, and alter weather patterns hundreds of miles inland.

Every ocean on Earth has experienced marine heat waves—events where sea surface temperatures climb so high and stay elevated so long that entire ecosystems enter crisis. Unlike weather events that come and go in days, marine heat waves persist for weeks or months, triggering cascading biological collapses in fisheries, coral reefs, kelp forests, and seabird colonies. They are reshaping where fish live, what grows on the ocean floor, and how storms form thousands of miles away.

What defines a marine heat wave

A marine heat wave sounds simple in description: water that is unusually warm for an unusually long time. Scientists define it more precisely. A marine heat wave occurs when sea surface temperature exceeds the 90th percentile of historical temperatures for a given location and calendar day, sustained for five or more consecutive days.

This percentile-based definition matters because it accounts for natural geography. A temperature that would be extreme in the frigid waters off Alaska might be unremarkable in tropical reefs. By using the 90th percentile—the threshold that historically would be exceeded only one day in ten—researchers measure what is abnormal for each specific region. When that abnormality persists for over a week, the system has shifted into a state where normal biological and chemical processes begin to unravel.

Why 5 days, 90th percentile? The duration threshold filters out routine warming pulses. The percentile approach acknowledges that a 1-degree anomaly matters differently in cold Antarctic waters than in warm Caribbean shallows. Together, these thresholds identify events that stress local organisms beyond their adaptive capacity.

How marine heat waves form

Marine heat waves arise through multiple mechanisms, sometimes in combination. The most direct cause is a persistent high-pressure weather system anchored over the ocean. When a dome of high pressure sits in place for weeks, it suppresses winds, reduces cloud cover, and allows intense solar heating to accumulate. The ocean has no mechanism to shed that heat quickly—water conducts thermal energy slowly through mixing and currents, so warmth can build layer upon layer.

Ocean circulation patterns also contribute. Anomalously weak currents may fail to bring in cool water from deeper layers or from distant polar regions. In the North Atlantic, a slowdown in the Gulf Stream during certain years allowed warm surface waters to become isolated and intensify. In the Pacific, a shift toward a pattern that draws warm water from the tropics northward can trigger heat waves along the American coast.

The deeper cause, however, is the changing baseline. The ocean has warmed roughly 0.13 degrees Celsius per decade over the past century, and at an even faster rate in recent decades. This warming raises the floor—the starting point from which a heat wave develops. In 1970, a 2-degree anomaly might have been a major event. In 2025, similar atmospheric conditions produce a 3 or 4-degree anomaly, because the ocean was already warmer to begin with. Heat waves that would have been rare are now routine. Ones that would have been unprecedented are becoming common.

Recent major events

The 2013-2015 "Blob" in the northeast Pacific demonstrated how a single prolonged heat wave can reshape an entire regional economy. A ridge of high pressure formed off British Columbia and did not dissipate. For months, a circular patch of anomalously warm water persisted, with temperatures 2-3 degrees Celsius above normal. Fisheries managers watched in shock as kelp forests that had dominated the seafloor for decades died off. Sea urchin populations exploded in the newly barren zones. Fish species began migrating northward and eastward. The Pacific herring and Pacific sardine stocks collapsed. Seabirds starved because their prey had vanished. By the time the Blob finally released its grip, years of ecological disruption lay in its wake.

The 2023 North Atlantic marine heat wave reached into new territory—literally. Summer water temperatures ran 0.5 to 1 degree Celsius above even the hottest previous records in that ocean basin. By September, some offshore regions had warmed 3 to 4 degrees above their historical average. The heat persisted into the fall, defying predictions that cooling would arrive on schedule. Researchers debated whether a change in aerosol particles, unusual Saharan dust patterns, or an early phase of El Niño contributed, but the core driver remained the same: long-term warming raised the ocean's starting temperature, and then the right atmospheric pattern arrived to push it much further.

Days minimum to qualify as a heat wave

3-4°C

Anomaly in the 2023 North Atlantic record

90th

Temperature percentile threshold

Impacts on fisheries and marine ecosystems

When a marine heat wave strikes, the first response is migration. Fish, squid, and other mobile creatures move toward cooler water—deeper, or toward the poles, or toward freshwater inputs. This behavior makes evolutionary sense but wreaks havoc on human livelihoods. Fisheries that have operated in the same waters for generations suddenly find their target species absent. New species may arrive in their place, but commercial operations cannot pivot overnight to a species they have never fished before.

For sessile organisms—corals, mussels, kelp—migration is impossible. Heat waves trigger coral bleaching, in which heat stress causes corals to expel the algae living inside them, turning the reef white and dying rapidly if cooling does not arrive soon. Kelp forests in temperate waters show similar mortality. Mussel beds in coastal regions undergo mass die-offs. Seabirds that depend on specific prey species watch their food source vanish and starve in place. Entire food webs collapse into simplified states where only heat-tolerant species remain.

The biological impacts cascade into chemical ones. Warmer water holds less dissolved oxygen. Heat waves often coincide with reduced wind mixing, which normally ventilates the surface layer and brings oxygen down from the atmosphere. The result is hypoxia—dead zones where oxygen concentration drops so low that most animal life cannot survive. Shrimp and crabs flee toward the edges. Fish that cannot escape suffocate. Decomposing dead matter consumes the remaining oxygen, deepening the crisis.

The global pattern: Marine heat waves are increasing in frequency, duration, and intensity across nearly every ocean basin. The Eastern Boundary Currents—the Humboldt Current off Peru, the Benguela Current off Southern Africa, the California Current—have been especially hard hit, destabilizing some of the world's most productive fisheries.

Effects on weather and climate systems

The ocean does not stay isolated from the atmosphere above it. A marine heat wave transfers enormous quantities of warmth and moisture upward into the air, altering atmospheric circulation in ways that can affect weather patterns hundreds or even thousands of kilometers away.

Warm water heats the air above it, reducing atmospheric density and creating conditions that can suppress tornado and hurricane formation in some regions while potentially enhancing it in others, depending on the precise latitude and season. The 2023 North Atlantic heat wave coincided with a period of unexpectedly weak hurricane activity, though the mechanism remains debated. Other heat waves have been linked to persistent drought patterns or unusual storm tracks in continental regions far from the coast.

The atmosphere and ocean are coupled systems. Energy that accumulates in the ocean eventually radiates back to the atmosphere or drives currents that redistribute heat globally. A marine heat wave lasting months can influence seasonal weather patterns. A series of heat waves across multiple years can shift regional climate normals—what was once an extreme becomes the new baseline.

Why marine heat waves are becoming more common

The root cause is unambiguous: the ocean is warming because the atmosphere above it is warming, and the atmosphere is warming because of greenhouse gas accumulation. But the connection between rising averages and rising extremes is worth understanding.

Imagine temperature as a distribution—a range of values, most clustered near the middle, with occasional outliers toward the hot and cold extremes. As the average temperature of the ocean rises, the entire distribution shifts toward warmer values. The new average becomes the old warm extreme. What used to be a rare 100-year event becomes a common 10-year event. Events that were essentially impossible become rare. This effect is called "shifting the distribution," and it mathematically guarantees that heat waves increase in frequency even if the variability around the mean stays the same.

On top of this baseline shift, some evidence suggests that climate change may also alter the weather patterns that trigger heat waves. A slower, weaker jet stream could allow high-pressure systems to persist longer. Changes in ocean circulation might affect how warm and cool water mix. These mechanisms remain areas of active research, but the data is clear: heat waves are not simply becoming warmer by a few tenths of a degree. They are becoming far more frequent, and the intervals between successive heat waves are shrinking.

What we can expect

Under current warming trajectories, marine heat waves will continue to intensify and spread. Some projections suggest that by mid-century, conditions that today would qualify as heat waves will become the summer norm in many ocean regions. Ecosystems will face a choice: adapt, migrate, or collapse. Fisheries dependent on species vulnerable to heat will contract unless management can facilitate shifts toward heat-tolerant alternatives. Coral reefs, already bleached by recent events, will face repeated stress with fewer recovery intervals. Kelp forests and other cool-water ecosystems will shrink poleward.

The challenge is that marine heat waves are only one symptom of a larger ocean transformation. Acidification, deoxygenation, and changing nutrient cycling are occurring simultaneously, compounding the pressure on marine life. A species that might tolerate a heat wave alone may not survive a heat wave combined with hypoxia and pH change.

Tracking marine heat waves in real time — monitoring their intensity, duration, and location — is essential for understanding their impacts and preparing human communities that depend on the ocean for food and livelihoods. That is why systems like this one measure sea surface temperature anomalies continuously and make the data accessible.