The Ocean Is Absorbing Record Heat as Seas Rise and Polar Ice Shrinks

The tenth Copernicus Ocean State Report, released on 30 September 2026, describes an ocean absorbing record heat, rising faster and becoming more acidic. Its findings draw on long-running observations of the global ocean and European seas, documenting changes that affect polar habitats, marine ecosystems and coastlines. 18

The key findings at a glance

• Ocean heat: Global ocean heat content reached its highest level in the record dating to 1993 in 2025. The ocean stores up to 90% of excess heat in the climate system. 5
• Sea level: The rise in global mean sea level accelerated to an average of 4.2 millimetres per year in 2012–2025, up from 2.6 millimetres per year in 1993–2011. 45
• Polar regions: Arctic sea-ice extent declined by roughly 490,000 square kilometres per decade between 1979 and 2025. The report also describes warming polar waters and diminishing ice habitat for wildlife at both poles. 16
• European seas: The Mediterranean and Black Seas experienced record marine heatwaves; parts of Europe’s seas were more than 4.6°C above normal in 2024. 12
• Ocean chemistry: The report describes increasingly acidic waters, alongside warming and sea-level rise. 46

Record heat is building in the ocean

Ocean heat content measures heat stored below the surface as well as at it. In 2025, the global ocean reached its highest heat-content level in the record that begins in 1993. Copernicus says the ocean absorbs up to 90% of excess heat in the climate system, making its temperature a key indicator of long-term change. 5

The report’s heat finding sits alongside a faster rise in global mean sea level. The estimated rate increased from 2.6 millimetres per year in 1993–2011 to 4.2 millimetres per year in 2012–2025. These are period averages, not a prediction of the exact change in any one coastline. 45

Polar ice and wildlife face changing conditions

The report identifies substantial Arctic sea-ice loss: extent fell by roughly 490,000 square kilometres per decade from 1979 to 2025. A separate report summary says the combined surface temperature of Arctic seawater and sea ice rose by 4.2°C between 1982 and 2024. 16

The changes matter for animals that depend on sea ice as habitat. Reporting on the Copernicus findings describes habitat losses for wildlife in both the Arctic and Antarctic, though the material available here does not provide a single measure of the effect across species or locations. 6

Marine heatwaves and acidification add pressure

The Mediterranean and Black Seas saw record marine heatwaves, and parts of Europe’s seas were more than 4.6°C above normal in 2024. That figure describes specific regional conditions; it should not be read as a temperature increase across all European waters. 12

The report also flags increasing ocean acidity. Heatwaves and changes in ocean chemistry are distinct pressures, and the sources summarized here do not quantify their combined effects on marine life. The report does, however, describe broader shifts in marine ecosystems alongside warming and acidification. 46

Why long-term monitoring matters

The Ocean State Report combines satellite remote sensing, measurements taken in the water and ocean reanalysis data to track conditions across the global ocean and European regional seas. That sustained record helps distinguish long-term trends from short-term variability and provides a shared reference for understanding how ocean conditions are changing. 8

For coastal communities, rising seas and erosion make the regional detail important as well as the global trend. One report summary says around 60% of Arctic coastal infrastructure is vulnerable to erosion and rising seas; that estimate is specific to Arctic infrastructure, not all coastal settlements. 1

The report’s central message is that ocean change is measurable across heat, sea level, ice and chemistry—and that continuing observations are essential to track where those changes are happening and how they evolve. The evidence supports preparation based on local conditions, rather than assuming that every region faces the same impacts. 845