The world’s oceans once again hit a record high temperature in 2025, storing more heat than during any previous year since modern recording began.
That heat is so extreme that it’s calculated in zettajoules, a measurement equal to one sextillion joules. In 2025 alone, ocean heat increased 23 zettajoules—or 23,000,000,000,000,000,000,000 joules of energy.
That figure is daunting to understand. For comparison, the Hiroshima atom bomb “Little Boy” exploded with an energy of about 63,000,000,000,000 joules.
That means in 2025, the amount of heat the oceans absorbed is equivalent to more than 365 million atomic bombs—or, as thermal sciences professor John Abraham says, “like 12 Hiroshima bombs being detonated each second, for every minute, hour, and day for the entire year.”
Put another way, 23 zettajoules is about the same as 37 years of global primary energy consumption (based on 2023 figures). It’s more than 200 times the entire global use of electricity.
‘Global warming is really ocean warming’
The figure on ocean warming comes from a new analysis published in the journal Advances in Atmospheric Science, conducted by more than 50 scientists from 31 global research institutions.
Ocean heat is important to pay attention to because it’s a barometer for climate change. The ocean acts as a heat sink for our emissions.
When humans emit greenhouse gases into the atmosphere, those gases trap heat on our planet. But the ocean actually absorbs the majority of that heat—more than 90%.
“Since the vast majority of global warming heat ends up in the oceans, I like to say global warming really is ocean warming,” says Abraham, who helped conduct the analysis.
Rising ocean heat also drives climate impacts, like rising sea levels. Warmer oceans also strengthen heatwaves and worsen extreme weather like hurricanes.
Rising ocean temperatures also hurt marine life, leading to coral bleaching and disrupting food webs. As humans emit more carbon dioxide, that CO2 also dissolves into the ocean, making it more acidic.
A decades-long trend
The ocean has been warming more strongly since the 1990s. When it comes to sea surface temperatures—which specifically affect weather patterns around the world, like heavier rains and stronger tropical cyclones—2025 was the third warmest year on record.
Ocean temperatures have set a new record for each of the past nine years, notes Michael Mann, director of the Penn Center for Science, Sustainability and the Media, and another author of the analysis.
“It is indicative of the steady heating of our planet,” he says, “which will increase until fossil fuel burning and human-generated carbon emissions cease.”
The analysis about these rising ocean temperatures comes shortly after the Trump administration pulled the United States out of the United Nations Framework Convention on Climate Change, a landmark climate treaty.
“Trump cutting ties with the world’s oldest climate treaty is another despicable effort to let corporate fossil fuel interests run our government,” Jean Su, energy justice director at the Center for Biological Diversity, said in a statement. “It’s foolish and downright deadly for Trump to turn his back on the climate devastation ripping across the U.S. and the world.”
The Trump administration has also recently cut hundreds of millions of dollars from climate energy research, including for the National Center for Atmospheric Research and the National Renewable Energy Laboratory—which Trump actually renamed, in December, to the National Lab of the Rockies.
These cuts will make the U.S. even more vulnerable to climate impacts, experts say.
“Research is important to help us plan for the new climate,” Abraham says. “This research saves us money in the long term and also helps us prepare for extreme weather like hurricanes, droughts, and floods.”