The world’s oceans are warming at an alarming pace, with consequences for which the world is unprepared, more than 100 international scientists said in a report published on Wednesday.

The tenth Copernicus Ocean State Report said the rate of ocean warming has doubled over the past 20 years, and that sea surface temperatures reached record highs in 2023, 2024 and again in 2026. ““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, which produced the report.

The oceans absorb roughly 90% of the excess heat produced by humans burning fossil fuels. Last year they absorbed 23 zettajoules of additional heat, a record for the ninth straight year, the report said — equivalent to about 40 times the energy the world consumed in 2025.

That heat drives marine heatwaves and stronger storms, harms marine life and raises sea levels. Global average sea levels have risen about 3.8 millimetres a year since 1999, a total of nearly 4 inches, with the rate accelerating since 2012. Oceans have also become about 17% more acidic since 1985 as they absorb carbon pollution, making it harder for oysters, lobsters and corals to maintain shells and skeletons.

The report said a powerful El Niño is adding further pressure, with the Pacific warming pattern on track to be the most intense on record.

European seas are under intense pressure, said Karina von Schuckmann, a report author and senior adviser at Mercator Ocean International. The Mediterranean and Black Seas saw record-breaking marine heatwaves in 2024, some lasting more than 30 days and pushing water temperatures more than 4.6 degrees Celsius above average. “These are not isolated hot spells,” said Blanca Fernández-Álvarez, a report author at the Mediterranean Institute for Advanced Studies, citing sustained background warming.

The report cited Storm Louis in 2024, which brought 26-foot waves to parts of Spain’s coast, killed one person and caused power outages and infrastructure damage. That year, winds in Bothnian Bay between Sweden and Finland pushed so much water out of the bay that the sea level fell 60 inches, halting ferries for 18 hours.

Arctic sea surface temperatures have risen about 4.2 degrees Celsius over four decades, and average sea ice fell by roughly 190,000 square miles a decade from 1979 to 2025, about the area of Spain every 10 years. Record low Antarctic sea ice in 2022 coincided with the first recorded mass breeding failure of emperor penguins, which the report called a sign that climate change is already reshaping the habitat of ice-dependent species.

Von Schuckmann said small developing island states face the greatest exposure. “For island states, ocean change is not a distant environmental issue; it is an immediate economic, development and security challenge,” she said.

The report called for sustained ocean monitoring so governments can prepare. “It is our responsibility to build the permanent capacity to anticipate this transformation,” Bahurel said.