A new international study published Friday in Advances in Atmospheric Sciences reports that the world’s oceans took in more heat in 2025 than ever before, making it the ninth straight year of record-high ocean temperatures.

The upper 2,000 meters of the global ocean gained approximately 23 zettajoules of heat energy compared to 2024, an amount equivalent to 37 years of global energy consumption at 2023 levels, or the explosive force of 12 Hiroshima atomic bombs detonating every second throughout the entire year.

New highs have been recorded every year since 2017. 

More than 50 scientists from 31 research institutions across Asia, Europe, and the Americas collaborated on the analysis, which synthesized data from multiple independent observational systems including the Chinese Academy of Sciences, NOAA’s National Centers for Environmental Information, and the European Union’s Copernicus Marine Service.​​

“The ocean is the hottest on record,” said Dr. Kevin Trenberth, co-author of the study and honorary academic at the University of Auckland. “We’re looking at creating a very different planet—do we really want to do that?”

Ocean heat content serves as one of the most reliable indicators of long-term climate change because the world’s oceans absorb more than 90% of excess heat trapped by greenhouse gases. Unlike atmospheric temperatures, which fluctuate significantly due to short-term weather patterns and natural climate cycles like El Niño and La Niña, ocean heat measurements provide a more stable signal of the planet’s energy imbalance.

The warming is not uniform across the planet’s oceans. In 2025, approximately 14% of the global ocean area reached its warmest conditions on record, while 33% ranked among the top three warmest years in their historical records.​​

The most pronounced warming occurred in the tropical and South Atlantic, Mediterranean Sea, North Indian Ocean, and Southern Ocean. The Southern Ocean exhibited particularly dramatic increases, with ocean heat content rising by 0.11 gigajoules per square meter, more than four times the region’s average heating rate over the past two decades.​​

The Mediterranean Sea warmed noticeably as well, with new data from research transects showing that mid-depth waters hit their highest temperatures since records started. An independent review of readings from the CNR-ISMAR mooring system in the Sicily Channel also found that temperatures kept climbing throughout 2025.

Meanwhile, the Northwest Pacific, including seas around China and the Kuroshio Current, recorded the fourth-highest ocean heat content values since 1958, despite surface cooling in the region.

Even as heat in the deep ocean climbed to record levels, conditions at the surface looked a bit different. The average global sea surface temperature in 2025 was the third highest ever recorded, coming in about 0.5°C (0.9°F) above the 1981–2010 average.

That was a slight drop of roughly 0.08–0.12°C from 2024, mainly because the strong 2023–2024 El Niño faded and gave way to La Niña in the tropical Pacific. La Niña strengthens the easterly trade winds, which pull cooler water up from below in the eastern Pacific and temporarily hold down surface temperatures.​​

Scientists stress that this brief dip in temperature doesn’t mean global warming is slowing down. Warmer oceans add more heat and moisture to the atmosphere, fueling stronger tropical storms and heavier rainfall.

In 2025, those effects were evident as devastating floods swept across South and Southeast Asia. Some areas saw up to 800 millimeters of rain in just five days in July, and in November, flooding across Indonesia, Sri Lanka, Thailand, and Vietnam claimed more than 1,350 lives and forced millions from their homes.

The United States experienced a devastating flash flood in Central Texas over the Fourth of July weekend that claimed at least 138 lives. Europe suffered extreme heat waves and extensive wildfires fueled by a persistent summer heat dome that drove temperatures above 48°C. Canada recorded over 6,127 fires burning 8.9 million hectares nationally in 2025.

Rising ocean heat also contributes directly to sea-level rise through thermal expansion, accounting for approximately one-third of sea-level rise since 2004. Higher ocean temperatures accelerate melting where glaciers and ice sheets meet the sea in polar regions.​

Marine ecosystems face mounting stress as well. Prolonged marine heat waves cause widespread coral bleaching and mortality, disrupt food chains, and force species to migrate to cooler waters. If global temperatures rise to 1.5°C above preindustrial levels, 70-90% of coral reefs are predicted to die off.