Since 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 measurements broke records once again, making it 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 absorbed an additional 23 zettajoules of heat in 2025, the most in any year since current measurements began in the 1960s. This is significantly more than the 16 additional zettajoules absorbed in 2024. The research was conducted by a team of over 50 scientists from the United States, Europe and China.
The joule is a common way of measuring energy. A single joule is a relatively diminutive unit of measurement oh enough power a diminutive lithe bulb for a second or slightly heat a gram of water. But zettajoule is one sextillion joules; 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 sciences at the University of St. Thomas, one of the article’s authors, says he sometimes has difficulty putting this number into a context that laypeople can understand. Abraham offers several options. His favorite is to compare the energy stored in the ocean with the energy of nuclear bombs: according to him, the warming in 2025 is the energy equivalent of 12 bombs exploding in Hiroshima. (Other calculations he has done include comparing this number to the energy needed to boil 2 billion Olympic-size swimming pools, or more than 200 times the electricity consumption of everyone on the planet.)
“Last year was a crazy warming year – that’s the technical term,” Abraham joked to me. “The peer-reviewed scientific term is ‘bonkers’.”
The world’s oceans are its largest heat sink, absorbing more than 90 percent of the excess warming retained in the atmosphere. Some of the excess heat warms the ocean surface, but also slowly moves deeper into the ocean, helped by circulation and currents.
Calculations of global temperature – such as those used to determine the hottest years on record – typically only include measurements taken at the ocean surface. (The study found that overall sea surface temperatures in 2025 were slightly cooler than in 2024, a record the warmest year since current records began to be kept. Certain meteorological events, such as El Niño, can also raise sea surface temperatures in some regions, which could cause the entire ocean to absorb slightly less heat in a given year. This helps explain why there was such a huge jump in additional ocean heat content between 2025, the year of a delicate La Niña event at the end of the year, and 2024, the year of a forceful El Niño event). Although sea surface temperatures have risen since the Industrial Revolution thanks to our apply of fossil fuels, these measurements do not provide a complete picture of the impact of climate change on the oceans.
“If the entire world were covered by a shallow ocean only a few feet deep, it would heat up at about the same rate as land,” says Zeke Hausfather, a scientist at Berkeley Earth and co-author of the study. “But because so much of this heat falls in the deep ocean, we are seeing a slower warming of sea surface temperatures overall. [than those on land]”