Ocean Warming, Strongest Since 1990

In 2025, approximately 16% of the global ocean surface reached record temperatures, and about 33% ranked among the three warmest in their historical records, according to research published today in Advances in Atmospheric Sciences.

The study notes strong warming in the Southern Ocean, the North Atlantic, the Mediterranean Sea, and the Indo-Pacific warm pool. In contrast, cooling occurred in the equatorial Pacific, the western Indian Ocean, and the tropical Atlantic, reflecting in part large-scale basin dynamic adjustments largely associated with a transition to La Niña conditions.

Ocean warming is not uniform. The warmest zones in 2025 were the Southern Ocean, the North Atlantic, the Mediterranean Sea, and the Indo-Pacific warm pool. The study emphasizes that this warming has been strongest since the 1990s.

The global annual average sea surface temperature in 2025 was the third warmest on record and remained about 0.5°C above the 1981-2010 reference average, although it was slightly lower than in 2023 and 2024 due to the transition from El Niño to La Niña conditions in the tropical Pacific.

The ocean absorbs over 90% of the excess heat trapped by greenhouse gases, making it the primary heat reservoir of the climate system.

The new calculations were conducted by an international team of over 50 scientists from 31 institutions worldwide.

The Ocean Heat Content (OHC), an index reflecting the accumulation of heat stored in the ocean and one of the best indicators of long-term climate change, reached its highest level ever recorded, confirming the ongoing increase in ocean heat.

The study combines data from leading international data centers and independent research groups, including the Institute of Atmospheric Physics of the Chinese Academy of Sciences; Copernicus Marine (the European Earth observation program) and NOAA/NCEI (the US environmental monitoring program); and an ocean meta-analysis (CIGAR-RT) spanning three continents: Asia, Europe, and America.

The research estimates that in 2025, heat increased by 23 zettajoules—a figure equivalent to 37 years of global primary energy consumption (natural sources like oil, coal, or natural gas used to produce electricity or gasoline). The study notes that rising ocean temperatures cause sea level rise and intensify and prolong heatwaves and extreme weather events.

Warmer surface temperatures promote greater evaporation and more intense rainfall, leading to tropical cyclones and more extreme weather phenomena like the floods and widespread disruptions across much of Southeast Asia, drought in the Middle East, and floods in Mexico and the Pacific Northwest in 2025.