Ocean Warming Report Warns Our Seas Are Changing Faster Than Ever

A new ocean warming report, compiled by more than 100 scientists and released this week, is being described as the starkest warning yet about the state of Earth's seas. The tenth edition of the Ocean State Report, produced by Mercator Ocean International through the European Union's Copernicus Marine Service, draws on four decades of satellite, buoy and sensor data to show an ocean that is absorbing record heat, rising faster and becoming more acidic all at once.

"The ocean is transforming before our eyes," said Pierre Bahurel, the institution's director general. The annual assessment puts hard numbers on that transformation. Last year, the world's seas absorbed an additional 23 zettajoules of heat — roughly 40 times the energy the entire world consumed in 2025, or the equivalent of a dozen Hiroshima-sized bombs going off every second for a year, according to CNN. The ocean warming report notes it was the ninth year in a row that ocean heat hit a new record, and the overall rate of ocean warming has doubled over the past 20 years.

All that stored heat is driving concrete damage. Global average sea levels have risen about 3.8 millimetres a year since 1999, and the pace quickened to 4.2 millimetres a year from 2012 onward — accelerating rise that threatens the hundreds of millions of people living in coastal regions. Warmer water also takes up more space, which adds to the flooding risk. Meanwhile the seas have become roughly 17 percent more acidic since 1985, as they soak up growing amounts of carbon pollution, a change scientists sometimes call climate change's equally evil twin because it strips oysters, lobsters and corals of the ability to build and maintain their shells and skeletons.

What the ocean warming report found at the poles

The ocean warming report's polar chapter is among its grimmest. The Arctic's combined sea surface and sea-ice surface temperature has climbed about 4.2 degrees Celsius since 1982 — roughly a degree a decade, more than eight times the global average rate of 0.12 degrees a decade. In the Kara Sea north of Siberia, temperatures have jumped 6.7 degrees over the same period. Arctic ice cover has shrunk by nearly half a million square kilometres every decade since 1979, the equivalent of losing an area the size of Spain every ten years, and the retreat is exposing coastlines to waves that chew away at shorelines and infrastructure, as reported by AFP.

At the other end of the planet, record-low Antarctic sea ice in 2022 coincided with the first recorded mass breeding failure of emperor penguins, birds that depend on stable ice to live, hunt and raise chicks. In April the species was upgraded from near threatened to endangered. The assessment warns that more than nine in ten colonies could face near extinction by the end of the century if ice keeps disappearing at the current rate.

Coastal communities are living these numbers

For small island developing states, the numbers translate into everyday economic pressure. Every one of their ocean territories has warmed since 1960, and nearly two-thirds of their combined ocean area has seen accelerating sea-level rise since 1999. About a quarter of the threatened coral reefs in those waters endured mortality-level heat stress for at least a month between 1982 and 2025, according to Inter Press Service. "For island states, ocean change is not a distant environmental issue; it is an immediate economic, development and security challenge," said Karina von Schuckmann, the assessment's director.

That pressure is visible in the daily grind of Zanzibar's seaweed farmers. Fatuma Makame learned to farm seaweed in the shallow water where her parents worked, but warming shallows have made the crop more vulnerable to poor growth and disease, so she has moved her lines farther from shore — a shift that demands a boat, gear and deep-water skills she did not have. She joined a cooperative union that pooled resources for both, and her harvests have since become larger and more reliable. Researchers have documented similar failures in shallow farming sites farther south, and some farmers are testing new income sources like sea cucumber cultivation while authorities explore climate-risk insurance for the sector.

European seas are also under what the report calls intense pressure. The Mediterranean and Black seas suffered record-breaking marine heatwaves in 2024, with some lasting more than 30 days and pushing water temperatures 4.6 degrees above average. The same year, Storm Louis sent 26-foot waves crashing into parts of Spain's coastline, killing one person and knocking out power and infrastructure, while a separate storm pushed so much water out of the Bothnian Bay between Sweden and Finland that ferries stopped running for 18 hours.

A monster El Niño — on track to be the most intense on record — is now piling onto these long-term trends, and warming seas supercharge the storms and marine heat waves that follow. Still, the ocean warming report argues the future is not unknowable. It calls for sustained ocean monitoring so governments and communities can prepare, and points to adaptation already underway, from relocated seaweed lines to new drying equipment and training programs on Zanzibar's coasts. We'll keep tracking how that response unfolds on our Climate Emergency beat — and if you want to push back on misleading eco-labels yourself, start with The EU Greenwashing Ban: How to Read Labels Like a Pro.