Since 2018, a group of researchers from around the world have crunched the numbers on how much heat the world’s oceans are absorbing each year. In 2025, their measurements once again broke the record, making it the eighth consecutive year that the world’s oceans have absorbed more heat than in prior years.
The study, published Friday in the journal Advances in Atmospheric Science, found that the world’s oceans could absorb an additional 23 zettajoules of heat in 2025, the most in any year since modern measurements began in the 1960s. This is significantly more than the 16 additional zetajoules they will absorb in 2024. The research comes from a team of more than 50 scientists from the United States, Europe and China.
Joules are a common way to measure energy. A joule is a relatively small unit of measurement – it is enough to power a small lightbulb for one second or slightly heat one gram of water. but zetajoule is one sextillion Joule; Numerically, the 23 zettajoules absorbed by the oceans this year can be written as 23,000,000,000,000,000,000,000.
John Abraham, professor of thermal science at the University of St. Thomas and one of the paper’s authors, says he sometimes has trouble putting these numbers into context that laypeople understand. Abraham offers some alternatives. His favorite is comparing the energy stored in the ocean to the energy of nuclear bombs: warming by 2025, he says, is the energetic equivalent of 12 Hiroshima bombs exploding in the ocean. (Some other calculations he made include equating this number to the energy it would take to boil 2 billion Olympic swimming pools, or more than 200 times the electrical usage of everyone on the planet.)
“Last year was a weird, crazy summer year — that’s the technical term,” Abraham joked to me. “Peer-reviewed scientific term is ‘bonkers’.”
The world’s oceans are its largest heat reservoir, absorbing more than 90 percent of the excess heat trapped in the atmosphere. While some of the extra heat warms the ocean surface, it also gradually moves into deeper parts of the ocean with the help of circulation and currents.
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