The ocean absorbed 23 zettajoules of heat in 2025, the highest annual value on record, according to the tenth Copernicus Ocean State Report. That is about 40 times global energy consumption that year. It is energy stored in ocean mass, not a temperature; the ocean takes up more than 90% of excess human-driven heat.
Sea-surface temperature in 2025 was the third highest, 0.49°C above the 1981-2010 average. It rose about 0.12°C per decade from 1982 to 2025. Surface temperature responds quickly, while heat content tracks energy stored at depth over longer periods.
Extra heat fuels marine heatwaves, thermal expansion, oxygen loss and ecosystem reorganization. The Humboldt system off Peru and Chile already has an oxygen-minimum zone, so warming can shift anchoveta, sardines, plankton and predators, affecting fisheries.
Mean sea level rose about 3.8 millimeters a year from 1999 to 2025 and 4.2 millimeters during 2012-2025. The ocean also became 17% more acidic from 1985 to 2025. Warming, rise and acidification interact.
From January 2023 to September 2025, 84.4% of global reefs experienced heat stress sufficient for bleaching. Repeated severe stress affected the equatorial Pacific and Ecuadorian and Peruvian coasts. Exposure is not identical mortality, but recurrence erodes recovery time.
All small-island developing-state exclusive economic zones have warmed since 1960, and 65% of their combined area saw accelerating sea-level rise since 1999. In Antarctica, 2025 was the fourth consecutive year below two million square kilometers of minimum sea ice.
The report combines 125 scientists, satellites, direct observations and models. Emissions cuts address the cause; local monitoring supports adaptation. The 23 zettajoules show the ocean buffering atmospheric warming while accumulating heat, sea-level and ecosystem risks.
