- Ocean temperatures set a record high in 2025, according to a new study.
- The authors found that the heat content of the ocean increased by about 23 zettajoules between 2024 and 2025. That’s roughly the equivalent of 210 times humanity’s annual electricity generation.
- The ocean has warmed significantly in recent decades largely because it absorbs roughly 90% of the excess heat trapped in the atmosphere by human-caused greenhouse gases. That makes the ocean a key indicator of global warming.
- Warming ocean temperatures contribute to sea-level rise and to extreme weather events, which were frequent in 2025.
Every calendar year since 2019, ocean temperatures have reached new record highs. 2025 was no exception, according to a new study.
The study, published Jan. 9 in the journal Advances in Atmospheric Sciences, found that the ocean heat content (OHC) in the upper 2,000 meters (6,562 feet) of the water column had increased by a larger amount than in any year since 2017.
“Holy shit, the oceans are hot,” John Abraham, a professor of thermal sciences at the University of St. Thomas in the U.S. and a coauthor of the study, told Mongabay.
“I would say it’s an exceptionally large [heat] increase, and it’s surprisingly large and it’s alarmingly large,” he added.
The study was undertaken by 55 scientists in 10 research teams located all over the world and led by Lijing Cheng of the Institute of Atmospheric Physics (IAP) at the Chinese Academy of Sciences (CAS) in Beijing. The authors found that OHC increased by about 23 zettajoules between 2024 and 2025. That’s roughly the equivalent of 210 times humanity’s annual electricity generation, Cheng told Mongabay. From 2023 to 2024, OHC had increased by about 13 zettajoules, according to IAP/CAS data.
The ocean has warmed significantly in recent decades largely because it absorbs roughly 90% of the excess heat trapped in the atmosphere by human-caused greenhouse gases, the study states. That makes the ocean a key indicator of global warming.
“Traditionally, the air temperatures have gotten more press, but now the scientific community has realized oceans are really the key,” Abraham said.
OHC is a particularly strong metric for assessing long-term warming because it’s not as subject to short-term fluctuations as other metrics.
In fact, the study showed the 2025 story was different for the other main metric used for ocean warming: Sea surface temperature (SST), which accounts for only the top meter (about 3 ft), went down slightly. In 2025, SST was the third highest ever recorded, just below 2023 and 2024. The slight decrease was due to a move toward La Niña conditions in 2025, the authors said. La Niña, part of the El Niño-Southern Oscillation cycle that drives weather patterns around the world, occurs when cool surface temperatures develop in the eastern tropical Pacific Ocean.
Though it may at first glance seem paradoxical, the slightly cooler SST in fact added “extra juice” to the increase in OHC, Abraham said. Study coauthor Michael Mann, a professor of earth sciences at the University of Pennsylvania in the U.S., agreed.
“Cooler ocean surface temperatures lose less heat to the atmosphere through convection or infrared radiation,” Mann told Mongabay in an email. “So the ocean retains that heat instead.”
Techniques for measuring ocean temperatures have vastly improved in recent decades. Argo floats, cylindrical instruments that drift in the ocean, have become the main source of data; they cover much of the world’s seas, spitting out data in real time. Satellites are often used to determine surface temperatures. In hard-to-reach places, marine mammals fitted with temperature sensors do the data gathering.
The researchers used statistical methods to estimate temperatures in locations with observation gaps.
“Let’s say I have two temperature sensors separated by 500 miles [805 kilometers]. What is the temperature between those sensors?,” he said. “We have to do what’s called curve fitting, and we use AI techniques to best take temperature sensors to stitch them together to give us a complete picture of the ocean.”
Abraham added that even using slightly different techniques, the research teams reached similar conclusions.
The human and environmental impacts of the trends the new study described are severe. Warming ocean temperatures contribute to sea-level rise and increase the likelihood of extreme weather events, which were frequent in 2025. For example, in the U.S., scientists estimate that hundreds of people died from the wider effects of wildfires in California in January, while at least another 135 were killed by flooding in Texas in July. In South and Southeast Asia, catastrophic flooding killed more than 1,350 people in November.
A separate study published in November in the journal Nature Climate Change, which Cheng coauthored, found that over the last six decades, warming, along with other changes to the ocean, was having “profound effects on Earth system cycles, marine ecosystems and human well-being.”
Annalisa Bracco, a senior scientist at the Euro-Mediterranean Center on Climate Change, a research center in Italy, who wasn’t involved with the new study on 2025 ocean temperatures, said its findings are “alarming” because of the impact warming will have on future generations. For one thing, much of the excess heat the ocean is storing will eventually be released back into the atmosphere, she said.
“It’s a huge engine, the ocean, and we are just filling it up with energy,” Bracco told Mongabay.
Banner image: A crew prepares the ground for electric poles near homes destroyed by the Pacific Palisades Fire in Malibu, California, U.S., in January 2025. Image by AP Photo/Richard Vogel.
Citations:
Pan, Y., Cheng, L., Abraham, J., Trenberth, K. E., Reagan, J., Du, J., … & Chen, L. (2026). Ocean Heat Content Sets Another Record in 2025. Advances in Atmospheric Sciences, 1-23. doi:10.1007/s00376-026-5876-0
Tan, Z., Von Schuckmann, K., Speich, S., Bopp, L., Zhu, J., & Cheng, L. (2025). Observed large-scale and deep-reaching compound Ocean State changes over the past 60 years. Nature Climate Change, 16(1), 58-68. doi:10.1038/s41558-025-02484-x
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