The heat content of the world's oceans up to a depth of 2,000 meters (6,561.68 feet) reached a new record in 2025, the highest level since measurements began and the ninth consecutive year of warming, according to an analysis by an international group of scientists.
They found in a study released on Friday that the thermal energy increased by 23 quadrillion megajoules in 2025 compared to 2024.
A team led by Lijing Cheng from the Chinese Academy of Sciences in Beijing published their research findings with data since 1958 in the journal "Advances in Atmospheric Sciences."
In terms of average global sea surface temperatures, 2025 was the third warmest year after 2024 and 2023.
The researchers explain the difference in depth and surface temperatures by stating that the climate phenomenon El Nino in the Pacific Ocean weakened significantly over the past year.
The El Nino phase is regularly associated with higher global temperatures at the sea surface.
When considering heat content, temperatures in deeper layers of the oceans are also included. Therefore, the increase in heat content is comparatively less influenced by surface climate phenomena and is considered a more important indicator of planetary warming.
"The ocean absorbs more than 90% of the excess heat trapped by greenhouse gases, making it the main heat reservoir of the climate system. Because ocean heat content (OHC) reflects the accumulation of heat stored in the ocean, it provides one of the best indicators of long-term climate change," the study authors wrote.
They cite a flash flood in the U.S. state of Texas, an extreme monsoon with up to 800 millimeters of rain in five days in South and South-East Asia and flash floods in West Africa as examples of extreme weather events in the past year.
Additionally, they said that the west and south of Europe suffered under a heat dome with temperatures up to 48 degrees Celsius (118.4 degrees Farenhiet) and numerous wildfires.
Based on three databases with observational data, the team found that the increase in ocean heat content has intensified since around 1990.
On average, from 1960 to 2025, the heat content in the upper 2,000 metres of the world's oceans increased by 0.14 watts per square meter over 10 years. From 2005 to 2025, it was 0.32 watts per square metre over 10 years.
In 2025, the global sea surface temperature was 0.49 degrees Celsius above the average for the period 1981 to 2010.
As with heat content, surface temperatures are also distributed differently regionally.
In the North-West Pacific, they exceeded the long-term average by more than 3 degrees Celsius in some areas. In the North Atlantic and the Mediterranean, they were up to 1.5 degrees Celsius above the average, also more than the global average.
According to the study, the North Atlantic and the Mediterranean are also hot spots related to climate change, experiencing salinization, oxygen deficiency and acidification. This indicates a profound, complex state change in the North Atlantic and the Mediterranean, making their ecosystems and the organisms living within them more fragile, the study states.
As measurements by the Federal Maritime and Hydrographic Agency (BSH) show, there was also a record value in the North Sea in 2025: The average surface water temperature was 11.6 degrees Celsius. This was the highest value in the BSH data series since 1969. The North Sea was thus 0.9 degrees Celsius warmer compared to the long-term average from 1997 to 2021.
In the Baltic Sea, the BSH recorded an average surface temperature of 9.7 degrees Celsius. This is an increase of 1.1 degrees Celsius compared to the long-term 1997-2021 average.
Since the beginning of the corresponding BSH data series in 1990, 2025 was the second warmest year for the Baltic Sea.
