The report draws on more than four decades of observations from satellites, buoys, sensors and other monitoring systems. It found that the total amount of heat stored in the ocean reached a record high in 2025. The oceans absorbed about 23 zettajoules of additional heat during the year, nearly 40 times the amount of energy consumed worldwide in 2025.
The report also found that the rate of ocean warming has doubled over the past two decades. Sea levels have risen by about 3.8 millimeters per year since 1999, with the rate accelerating to about 4.2 millimeters annually between 2012 and 2025.
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At the same time, ocean acidity increased by about 17% between 1985 and 2025 as the seas absorbed more carbon dioxide.
Mediterranean under growing pressure
European seas are among the areas facing particularly intense changes. The Mediterranean and Black seas experienced record marine heatwaves in 2024, with some events lasting more than 30 days and temperatures in some areas reaching more than 4.6 degrees Celsius above average.
The Mediterranean has also recorded an increase in marine heatwave days over the past two decades, while sea-level changes have varied between its eastern and western basins.
These changes are already affecting marine ecosystems. Around 5% of the global ocean has experienced persistent shifts in the seasonal patterns of phytoplankton over the past three decades. These microscopic organisms form the base of many marine food webs and play an important role in the ocean carbon cycle.
The report also points to growing risks in the Arctic. Monthly average sea ice extent fell by roughly 490,000 square kilometers per decade between 1979 and 2025, equivalent to losing an area about the size of Spain every 10 years.
In Antarctica, record-low sea ice in 2022 coincided with the first recorded large-scale breeding failure of emperor penguins.
Rising risks for coastal communities
The changes are also affecting people and infrastructure. Rising seas, marine heatwaves and stronger storms expose coastal communities to greater risks, particularly in small island developing states.
The report found that 65% of the exclusive economic zones of small island developing states have been directly exposed to accelerating sea-level rise since 1999. Ocean warming has also increased tropical cyclone heat potential in roughly 56% of these countries since 1960.
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Scientists behind the report said continued ocean monitoring will be essential as governments and coastal communities prepare for increasingly changing marine conditions.
“The ocean is transforming before our eyes,” Pierre Bahurel, director general of Mercator Ocean International, said in the report. “Warming, rising seas and ecosystem degradation are interconnected signs of significant and long-term change.”
The Copernicus Ocean State Report is an annual reference assessment of the global ocean and European regional seas, combining satellite observations, measurements collected at sea and numerical ocean models.
