The ocean absorbed a stunning amount of heat in 2025

In 2025 alone, the ocean gained 23 Zetta Joules (23,000,000,000,000,000,000,000 Joules of energy) of heat. That amount of energy is roughly equal to about 37 years of total global primary energy use at 2023 levels (~620 Exa Joules per year). The findings are based on work by more than 50 scientists representing 31 research institutions across the globe.

Why the Ocean Plays a Central Role in Climate

The ocean acts as Earth's primary heat sink. More than 90% of the extra heat trapped by greenhouse gases ends up in the ocean rather than the atmosphere or land. Because of this, ocean heat content (OHC) provides one of the clearest and most reliable measures of long-term climate change, reflecting how much heat the planet has accumulated over time.

To evaluate ocean warming, researchers brought together multiple independent datasets from major international science centers. These included three observational products from the Institute of Atmospheric Physics at the Chinese Academy of Sciences, Copernicus Marine, and NOAA/NCEI, along with an ocean reanalysis known as CIGAR-RT. The data span three continents: Asia, Europe, and America.

All of these sources point to the same conclusion. Ocean heat content in 2025 reached the highest level ever observed, confirming that the oceans continue to steadily gain heat.

Uneven Warming Across the World's Oceans

Ocean warming does not occur evenly across the globe. Some regions are heating much faster than others. In 2025, about 16% of the global ocean area reached record-high heat content, while roughly 33% ranked among the three warmest years on record for their regions.

The most pronounced warming was observed in the tropical oceans, the South Atlantic, the North Pacific, and the Southern Ocean.

Long-Term Trends Show Persistent Heating

Records show that ocean warming has strengthened since the 1990s. Heat accumulation in the upper 2000m of the ocean has increased steadily over recent decades, with scientists detecting a slight rise in the rate of warming. Ocean heat content reached a new record in 2025, continuing a streak that has now lasted nine consecutive years.

Sea Surface Temperatures and Global Weather

In 2025, the global annual average sea surface temperature ranked as the third warmest year in the instrumental record. Temperatures remained about 0.5 °C (approximately 1°F) above the 1981-2010 average. Sea surface temperatures were slightly lower than in 2023 and 2024, largely because conditions shifted from El Niño to La Niña in the tropical Pacific.

Sea surface temperatures matter because they strongly influence weather around the world. Warmer ocean surfaces increase evaporation and rainfall, making storms more intense and extreme weather events more likely. In 2025, these effects contributed to severe flooding and disruption across much of Southeast Asia, prolonged drought in the Middle East, and flooding in Mexico and the Pacific Northwest.

Why Rising Ocean Heat Matters

As ocean heat continues to increase, the consequences extend across the climate system. Warmer oceans contribute to rising sea levels through thermal expansion, intensify and prolong heatwaves, and strengthen extreme weather by adding heat and moisture to the atmosphere. As long as Earth continues to absorb more energy than it releases, ocean heat content will keep rising and new records will continue to be set.

A Growing Body of Ocean Heat Research

The findings will appear in a special collection on Ocean Heat Content Changes organized by Advances in Atmospheric Sciences. The cover of the collection features cartoon images of a sad shrimp and crab, an idea proposed by the study's corresponding author, Lijing Cheng of the Institute of Atmospheric Physics at the Chinese Academy of Sciences.

"The idea comes from the 'shrimp soldiers and crab generals' guarding the underwater palace in Journey to the West," Cheng said. "We reimagined them not as mighty guardians, but as vulnerable creatures whose armor -- their shells and scales -- is under attack by ocean warming, acidification and other ocean environmental changes."

The collection will examine multiple dimensions of ocean heat change, including detailed regional studies focused on waters near China, the South Pacific, and the Indian Ocean. As climate scientist Kevin Trenberth noted in the preface, the collection is designed as an ongoing effort that reflects the evolving nature of climate science.

The Decisions That Shape the Future

While scientific understanding continues to advance, one message remains consistent. The greatest uncertainty in the climate system is how people choose to respond. By reducing emissions, preparing for future impacts, and acting collectively, it is still possible to protect a climate that allows both people and ecosystems to thrive.