The oceans are getting hotter

Since 2018, A A team of researchers from around the world have crunched the numbers on how much heat the world’s oceans absorb each year. In 2025, their measurements broke records again, making this the eighth consecutive year 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 the equivalent of an additional 23 zetajoules of heat in 2025, the most in any year since modern measurements began in the 1960s. This is much higher than the additional 16 zettajoules they absorbed in 2024. The research comes from a team of more than 50 scientists across the US, Europe and China.

The joule is a common way to measure energy. One joule is a relatively small unit of measurement, roughly enough to turn on a small light bulb for a second, or slightly heat a gram of water. But one zeitajuul Sextillion Jules. Numerically, the 23 zeta joules absorbed by the oceans 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 authors of the paper, says he sometimes has difficulty putting this number into contexts that ordinary people understand. Abraham offers two options. His favorite is to compare the energy stored in the ocean to the energy of atomic bombs: He says the temperature rise in 2025 is equivalent to the energy of 12 Hiroshima bombs exploded in the ocean. (Some of his other calculations include equating this number to the energy needed to boil two billion Olympic-sized swimming pools, or more than 200 times the electrical use of every person on the planet.)

“Last year was a crazy year and crazy warming, that’s the technical term,” Abraham joked to me. “The peer-reviewed scientific term is ‘crazy’.”

The world’s oceans are the largest source of heat, absorbing more than 90% of the excess warming trapped in the atmosphere. While some of the excess heat warms the ocean surface, it is also slowly transferred to deeper parts of the ocean, aided by circulation and currents.

Global temperature calculations — such as those used to determine the hottest years on record — typically only capture measurements taken at the ocean surface. (The study finds that total sea surface temperatures in 2025 were slightly lower than in 2024, which was recorded as the hottest year since modern records began. Some meteorological phenomena, such as El Niño events, can also raise sea surface temperatures in certain regions, potentially causing the overall ocean to absorb slightly less heat in a given year. This helps explain why there will be such a big jump in additional ocean heat content between 2025 and To the appearance of a weak La Niña at the end of 2025. (The year 2024, which came at the end of a strong El Niño year.) While sea surface temperatures have risen since the Industrial Revolution, thanks to our use of fossil fuels, these measurements do not provide a complete picture of how climate change will affect the oceans.

“If the entire world were covered by a shallow ocean just a few feet deep, it would warm almost as quickly as the Earth,” says Zeke Hausfather, a research scientist at Berkeley Earth and co-author of the study. “But because much of this heat is falling in the deep oceans, we are generally seeing a slower rise in sea surface temperatures [than those on land]”.