As of 2018, a A group of researchers from around the world have determined how much heat the world’s oceans absorb each year. In 2025, their estimates again broke records, making this the eighth year in a row that the world’s oceans absorbed more heat than in previous years.
The study, published Friday in the journal Advances in Atmospheric Science, found that the world’s oceans absorb an additional 23 zettajoules of heat by 2025, more than in any year since modern measurements began in the 1960s. That’s significantly higher than the 16 extra zettajoules they absorbed in 2024. This research comes from a team of more than 50 scientists across the United States, Europe and China.
The joule is a standard way of measuring energy. One joule is a relatively small unit of measurement—it’s about enough to power a small light bulb for a second, or heat a gram of water. But the zettajoule is another sextillion joules; In terms of numbers, the 23 zettajoules the ocean absorbed this year can be written as 23,000,000,000,000,000,000,000.
John Abraham, a professor of thermal science at the University of St. Thomas, who is also one of the authors of the paper, says that sometimes he has trouble putting this number in contexts that people understand. Abraham offers several options. He likes to compare the energy stored in the ocean to the energy of atomic bombs: The warming of 2025, he says, is the equivalent of 12 Hiroshima bombs exploding in the ocean. (Other calculations he has done include equating this number with the energy it would take to boil two billion Olympic swimming pools, or more than 200 times the electricity consumption of everyone in the world.)
“Last year was a hot, hot year—that’s the technical term,” Abraham joked to me. “The peer-reviewed scientific term is ‘bombs’.”
The world’s oceans are the largest heat sinks, absorbing more than 90 percent of the heat trapped in the atmosphere. While some of the excess heat warms the ocean, it also slowly sinks into the deeper parts of the ocean, aided by circulation and currents.
Global temperature statistics—such as those used to determine the hottest years on record—often capture only measurements taken over the ocean. (The study finds that overall ocean temperatures in 2025 were slightly lower than in 2024, which was the hottest year since modern records began. Other climate events, such as El Niño events, can also increase ocean temperatures in certain areas, which can cause the ocean as a whole to absorb less heat in a given year to explain why there was so much heat during a given year. 2025, which improved (a weak La Niña at the end of the year, and 2024, which came at the end of a strong El Niño year.) Although ocean temperatures are rising since the industrial revolution, due to our use of fossil fuels, these estimates do not give a full picture of how climate change is affecting the ocean.
“If the entire Earth were covered by a shallow ocean that was only a few meters deep, it would warm at the same rate as Earth,” said Zeke Hausfather, a research scientist at Berkeley Earth and a co-author of the study. But because most of that heat goes down deep into the ocean, we’re seeing a slight warming of ocean temperatures. [than those on land].”
