Scientists deliver ‘starkest warning yet’ of profound changes sweeping the planet’s oceans

The rate of ocean warming has doubled over the past 20 years and sea surface temperatures reached record highs in 2023, 2024 and again in 2026.

(CNN) — The oceans are warming at an alarming pace, bringing far-reaching consequences for which the world is unprepared, according to a report by more than 100 international scientists.

The tenth Copernicus Ocean State Report, published Wednesday, draws on more than four decades of data from satellites, buoys and sensors to take the pulse of the world’s oceans — delivering its “starkest warning yet” about the profound changes they are enduring.

“The ocean is transforming before our eyes: warming, rising seas and ecosystem degradation are interconnected signs of significant and long-term change,” said Pierre Bahurel, Director General of Mercator Ocean International, the non-profit research institution that produced the report.

On top of long-term changes identified by the report, a monster El Niño is adding even more pressure. This natural climate pattern, characterized by warmer-than-average water temperatures along the equator in the Pacific Ocean, is on track to be the most intense on record.

Global oceans absorb roughly 90% of the excess heat produced by humans burning fossil fuels and they are warming at an “alarming pace,” according to the report.

The rate of ocean warming has doubled over the past 20 years and sea surface temperatures reached record highs in 2023, 2024 and again in 2026.

Last year, the world’s oceans absorbed an additional 23 zettajoules worth of heat, setting a new record for the ninth year straight, the report noted. To put that into context, it’s equivalent to the energy of 12 Hiroshima bombs going off every second for a year, or roughly 40 times the energy the entire world consumed in 2025.

All this heat has huge ripple effects, driving marine heat waves, fueling stronger and more destructive storms, damaging marine life and increasing sea level rise, both by melting ice and because warmer water takes up more space.

Global average sea levels have risen by roughly 0.15 inches a year (3.8 millimeters) since 1999 to make a total of nearly 4 inches, with rates accelerating since 2012 — threatening the hundreds of millions who live in coastal regions.

The oceans have also become around 17% more acidic since 1985, as they absorb increasing amounts of carbon pollution.

Ocean acidification is sometimes called “climate change’s equally evil twin” because of the damage it wreaks on underwater ecosystems, including making it hard for animals such as oysters, lobsters and corals to build and maintain shells and skeletons.

Some regions are particularly stressed by the changes unfolding as the world warms. European seas are under “intense pressure,” said Karina von Schuckmann, a report author and a senior advisor at Mercator Ocean International.

The Mediterranean and Black Seas both experienced record-breaking marine heatwaves in 2024, with some lasting more than 30 days and driving up water temperatures more than 8.3 degrees Fahrenheit (4.6 Celsius) above average, the report found.

“These are not isolated hot spells; they are occurring against a backdrop of sustained ocean warming that is changing the conditions marine ecosystems experience,” said Blanca Fernández-Álvarez, a report author and researcher at The Mediterranean Institute for Advanced Studies.

Ocean extremes have big impacts on people. The report pointed to Storm Louis in 2024, which brought 26-foot waves to parts of Spain’s coastline, causing one death, widespread power outages and damage to infrastructure.

That same year, a storm brought prolonged and intense winds to Bothnian Bay, between Sweden and Finland. So much water was pushed out of the bay that sea level plunged 60 inches, forcing ferries to stop operating for 18 hours.

Some of the biggest ocean changes are happening in the rapidly-warming Arctic, where sea surface temperatures have risen by around 7.6 degrees Fahrenheit (4.2 Celsius) over the past four decades. Monthly average sea ice levels fell roughly 190,000 square miles per decade between 1979 and 2025 — equivalent to losing an area around the size of Spain every 10 years.

At the other end of the world, record low Antarctic sea ice in 2022 coincided with the first recorded mass breeding failure of Emperor penguins. It’s “a sign that climate change is already reshaping the habitat of ice-dependent species,” the report noted.

Perhaps those most exposed to ocean changes are small developing island states which are on the frontline of sea level rise, increasingly intense cyclones, storm surges and coral reef die-offs.

“For island states, ocean change is not a distant environmental issue; it is an immediate economic, development and security challenge — and a warning of what many other coastlines will face in the future,” von Schuckmann said.

The report calls for sustained monitoring of the oceans to ensure that governments and communities have the information they need to prepare for what’s coming.

“It is our responsibility to build the permanent capacity to anticipate this transformation,” said Mercator Ocean International’s Bahurel.

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