An international study led by Lijing Cheng finds that global ocean heat content down to 2,000 metres reached a record high in 2025, rising by 23 quadrillion megajoules since 2024. The data, published in Advances in Atmospheric Sciences and extending back to 1958, mark the ninth consecutive year of ocean warming. While 2025 was the third-warmest year at the surface, a weakened El Niño helps explain the difference; OHC is considered a more robust indicator of long-term planetary warming.
Oceans Hit Record Heat in 2025 — Ninth Straight Year of Warming, Study Shows
A southern right whale (Eubalaena australis) in the Atlantic Ocean. Maxi Jonas/dpa
Global ocean heat content down to 2,000 metres reached a new record in 2025, marking the ninth consecutive year of warming, an international team of scientists reported.
Key Findings
The researchers, led by Lijing Cheng of the Chinese Academy of Sciences in Beijing, calculated that the ocean's thermal energy increased by 23 quadrillion megajoules in 2025 compared with 2024. Their analysis, published in the journal Advances in Atmospheric Sciences, draws on measurements that extend back to 1958.
Surface vs. Deep Ocean Temperatures
Although average global sea-surface temperatures ranked 2025 as the third-warmest year on record, behind 2024 and 2023, the deeper ocean layers set a new heat record. The authors attribute much of this difference to a significant weakening of the El Niño phenomenon in the Pacific during the past year. Because ocean heat content (OHC) includes temperatures well below the surface, it is less sensitive to short-term surface variations and provides a clearer signal of long-term warming.
Why Ocean Heat Content Matters
"The ocean absorbs more than 90% of the excess heat trapped by greenhouse gases, making it the main heat reservoir of the climate system," the study authors wrote. "Because ocean heat content reflects the accumulation of heat stored in the ocean, it provides one of the best indicators of long-term climate change."
Rising OHC has wide-ranging implications: it contributes to sea-level rise as water expands, fuels more frequent and intense marine heatwaves, and increases stress on marine ecosystems such as coral reefs and fisheries.
What Comes Next
The record 2025 OHC underscores the long-term trend of planetary warming driven by greenhouse gas emissions. Scientists say continued monitoring of ocean heat is critical for understanding future changes to climate, sea level, and marine life.
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