Marine heatwaves spread across key oceans, from the Mediterranean to the Southern Ocean

In the first week of March, as late-winter storms raked Europe’s coasts, the sea was telling a different story. From the western Mediterranean—where surface waters between Sicily and the French Riviera are locked in a “severe” marine heatwave—to a band of abnormally warm water stretching across the North Pacific and deep into the Southern Ocean, large swaths of the world’s oceans are running a sustained fever.

Those conditions are laid out in a Marine Heatwave Bulletin dated March 7 from Mercator Ocean International, which operates the European Union’s Copernicus Marine Service. The bulletin, based on satellite measurements and operational ocean models, maps sea surface temperature anomalies and classifies marine heatwaves around the globe for the week of March 1–7, with projections for the weeks ahead.

The findings show multiple marine heatwaves persisting or intensifying in key basins at the same time, rather than isolated hot spots that come and go in a few days. Scientists say that pattern is consistent with a broader shift in the global ocean, which has absorbed most of the excess heat from human-driven climate change and is now exhibiting longer, more frequent, and more intense warm episodes.

What counts as a marine heatwave

Marine heatwaves are defined as periods when sea surface temperatures at a given location exceed a seasonally adjusted threshold—typically the 90th percentile of historical conditions—for at least five consecutive days. Researchers rank them in four categories: moderate, strong, severe, and extreme, depending on how far temperatures climb above that local threshold.

Mediterranean: a severe event expands in the west

In the western Mediterranean, the March bulletin reports that “the marine heatwave in the western side of the basin is increasing,” with the area classified as severe now stretching “from Sicily to the French coast.” Sea surface temperatures across the basin were running up to about 2 degrees Celsius above long-term averages during the week, with the most intense anomalies concentrated in that western arc.

The eastern Mediterranean also remains unusually warm, with a marine heatwave ranging from moderate to severe but described as “stable overall” in extent and intensity.

That matters in a basin already warming faster than the global ocean. Studies over the past decade have found that Mediterranean surface waters are heating at roughly 0.3 to 0.5 degrees Celsius per decade. Previous marine heatwaves there have triggered mass die-offs of corals, sponges, and seagrasses; shifts in fish communities; and the spread of warm-water and invasive species such as tropical fish and jellyfish.

The economic stakes are high. The Mediterranean supports dense coastal tourism, small-scale fisheries, and expanding aquaculture industries. Warmer seas are associated with increased risk of harmful algal blooms, jellyfish swarms, and disease outbreaks in farmed fish and shellfish—factors that can force beach closures, trigger health advisories, and cut into local incomes.

North Pacific: multiple warm pools persist

Across the globe in the North Pacific, the bulletin highlights an intensifying marine heatwave east of the Philippines that now extends into the central basin. That event is “increasing in intensity,” with areas classified as strong expanding, although the overall footprint remains roughly stable.

Farther north, a broad patch of warm water near 50 degrees north and close to the date line continues, classified mostly as moderate with pockets of strong to extreme heat, but shrinking in area compared with late February.

Along North America’s west coast, the bulletin notes a marine heatwave “west of the North American coast” that is stable in both area and intensity, with moderate categories prevailing. That description aligns with separate monitoring by the U.S. National Oceanic and Atmospheric Administration, which has warned that West Coast waters have been in a prolonged warm phase.

NOAA Fisheries said the current event has “raised the temperature of waters along the West Coast roughly 3 to 4 degrees Fahrenheit above normal,” conditions that have persisted in some form since mid-2025. Officials have linked similar past events—including the 2013–2015 “Blob” in the Northeast Pacific—to mass seabird and marine mammal deaths, northward shifts in fish species, and large harmful algal blooms that shut down shellfish and crab fisheries.

Southern Ocean: warming in near-freezing waters

In the Southern Ocean—a region critical for global climate regulation—the Mercator bulletin points to a marine heatwave between roughly 60°E and 120°E longitude that is “increasing in intensity,” with the surface area of strong categories growing. While basin-wide sea surface temperature anomalies in the Southern Ocean are lower—generally up to about 1.5 degrees Celsius above average—the ecological significance is magnified in waters that are normally near freezing.

That sector, south of the Indian Ocean, includes important habitat for Antarctic krill, a keystone species that underpins food webs for penguins, seals, and whales and supports a small but economically valuable fishery. The Southern Ocean also plays an outsize role in absorbing both heat and carbon dioxide from the atmosphere. Warming and changing circulation there could affect how much heat and carbon the global ocean can continue to take up.

Other hot spots: Atlantic and South Pacific

The March update also notes other, less intense but still notable heat anomalies:

• Tropical North Atlantic: A marine heatwave stretching from the Antilles toward the center of the basin is shrinking but remains moderate.
• South Pacific: A heatwave from eastern Papua New Guinea to northern New Zealand is weakening and now classified as moderate, while a separate event in the central South Pacific is expanding and intensifying, with moderate to strong categories.

A warmer baseline for the global ocean

These snapshots come against the backdrop of an ocean that has been running unusually warm for several years. An assessment of global ocean conditions found that about 90% of the world’s ocean surface experienced at least one marine heatwave in 2023, with some regions logging more than 300 heatwave days in a single year. Another review of 2025 conditions reported that 89% of the ocean between 60°S and 60°N was affected by at least one marine heatwave, and nearly the entire Mediterranean—99.6%—saw such events, with 93% of its area exposed to intense categories.

Global average sea surface temperature reached a record of about 21 degrees Celsius in the spring of 2024, highlighting what European scientists described as a “step change” in ocean heat.

Research indicates that human-driven warming is the main factor behind the rising frequency, duration, and intensity of marine heatwaves, while natural climate cycles such as El Niño and La Niña help determine where they flare up in a particular year. One recent study concluded that the climate crisis has roughly tripled the length of the most dangerous marine heatwaves compared with pre-industrial times.

Beyond short-term heatwaves, scientists warn that chronic ocean warming is eroding marine life over the long term. An analysis of nearly 33,000 marine populations published this year found that fish levels decline by about 7.2% for each additional 0.1 degrees Celsius per decade of seabed warming. “To put it simply, the faster the ocean floor warms, the faster we lose fish,” one of the study’s authors said.

That has direct implications for food security. The ocean provides a key source of protein for more than 3.3 billion people, and fisheries and aquaculture support tens of millions of jobs worldwide. As fish and other species move in response to changing temperatures, governments face growing pressure to adapt management plans and resolve disputes over shifting stocks.

How the bulletin is produced—and why it matters

The data underlying the March 7 bulletin come from the Copernicus Marine Service’s Global Ocean Physics Analysis and Forecast product, which combines satellite observations and in-situ measurements from buoys, floats, and ships with numerical models to generate daily maps and short-term forecasts of temperature, currents, and other variables.

European officials have promoted these systems, along with emerging “digital twins” of the ocean, as critical tools for climate adaptation. Governments, fishery managers, and coastal industries are increasingly using marine heatwave information to plan fishing seasons, adjust aquaculture operations, anticipate coral bleaching risks, and prepare for potential storm impacts.

As March progresses, forecasters expect some of the current hot spots to wane while others persist or shift. But the bulletin’s global overview underscores that marine heatwaves, once considered rare extremes, now blanket large regions of the ocean year after year. The question scientists and policymakers are grappling with is how societies will respond to forecasts of a warmer, more volatile sea that is no longer an occasional outlier, but the new operating environment.