Oceans Held More Heat in 2025 Than Ever Recorded, European Report Finds

The world’s oceans stored more heat in 2025 than in any year since measurements began, according to a report released Wednesday by the European Union’s Copernicus Marine Service. The 10th edition of its annual Ocean State Report, compiled by a network of researchers across Europe and beyond and published through Mercator Ocean International, describes oceans that are growing warmer, higher and more acidic at the same time.

Heat is the headline. The oceans have absorbed the great majority of the extra energy trapped by greenhouse gases, and the report finds that the total heat stored in seawater reached its highest level on record last year. That warmth does not stay offshore: The Guardian, in its account of the findings, reported that hot coastal waters can intensify heat waves on land by keeping nights from cooling down.

The report also tracks chemistry. Over roughly the past four decades, from 1985 to 2025, the global ocean has become about 17 percent more acidic as it takes up carbon dioxide from the atmosphere. More acidic water makes it harder for corals, shellfish and some plankton to build their shells and skeletons.

Europe’s enclosed seas stood out. The Mediterranean and the Black Sea both experienced record-breaking marine heat waves in 2024, some lasting more than 30 days. The authors found that the Mediterranean has logged the most marine heat wave days of any region they examined over the past two decades, and that sea level there has risen unevenly, faster in some parts of the basin than others.

Living systems are shifting too. Over the past 30 years, about 5 percent of the global ocean has shown a lasting change in the seasonal timing and patterns of phytoplankton, the microscopic organisms at the base of marine food webs that also play a large role in moving carbon into the deep sea. In Antarctica, record-low sea ice in 2022 coincided with the first recorded large-scale breeding failure among emperor penguin colonies.

The Copernicus reports pull together years of ocean monitoring into a single yearly assessment rather than a set of policy prescriptions. People who favor a slower energy transition often argue that the costs of rapid cuts in emissions fall on households now while the benefits arrive later; the report does not weigh those costs. What it does provide is a measured baseline showing that the changes scientists have long projected are visible in the water today.

The release comes ahead of the next United Nations climate summit, known as COP31, where governments will again debate how fast to cut emissions. For coastal communities, the findings about warming water and rising seas are less a forecast than a description of conditions they already face.