Since 2018, a group of researchers from around the world have been crunching the numbers on how much heat the oceans absorb each year. In 2025, their measurements broke records again, making this the eighth year in a row that the world's oceans have absorbed more heat than in previous years.
The study, published Friday in the journal Advances in Atmospheric Science, found that the world's oceans would absorb an additional 23 zettajoules of heat in 2025, the most in any year since modern measurements began in the 1960s. That's significantly higher than the 16 additional zettajoules they absorbed in 2024. The research comes from a team of more than 50 scientists from the United States, Europe and China.
A joule is a common way to measure energy. A single joule is a relatively small unit of measurement: it is about enough to power a small light bulb for one second, or slightly heat a gram of water. But a zettajoule is one trillion joule; numerically, the 23 zettajoules the oceans absorbed this year can be written as 23,000,000,000,000,000,000,000.
John Abraham, a professor of thermal sciences at the University of St. Thomas and one of the paper's authors, says he sometimes has trouble putting this number into contexts that laypeople understand. Abraham offers a number of options. His favorite is comparing the energy stored in the ocean to the energy of atomic bombs: 2025 warming is, he says, the energetic equivalent of twelve Hiroshima bombs exploding in the ocean. (Some other calculations he has made include equating this number to the energy required to boil 2 billion Olympic swimming pools, or more than 200 times the electricity consumption of everyone on the planet.)
“Last year was a crazy and hot year, that's the technical term,” Abraham joked to me. “The peer-reviewed scientific term is 'bonkers'.”
The world's oceans are the world's largest heat sink, absorbing more than 90 percent of the excess warming trapped in the atmosphere. While some of the excess heat warms the ocean's surface, it also flows slowly into deeper parts of the ocean, aided by circulation and currents.
