Average temperatures across the world’s oceans reached an all-time high in 2024, a new multi-team study shows. The temperatures surpassed even those of 2023, which themselves represented a marked uptick over any previous years on record, according to the study published Jan.10 in the journal Advances in Atmospheric Sciences.

The ocean stores about 90% of the excess heat from human-caused global warming and is therefore viewed as a measure of planetary health. The ocean data “continue to indicate unabated trends in global heating,” says the study, which had 54 authors led by Lijing Cheng of the Institute of Atmospheric Physics at the Chinese Academy of Sciences in Beijing.

“To know what is happening to the climate, the answer is in the ocean,” John Abraham, a thermal scientist at the University of St. Thomas in the U.S. and a co-author of the study, said in a statement.

The long-term rise in ocean temperatures isn’t just an effect of climate change but also a cause.

“[T]he weather is becoming more affected by warming oceans because the warmer waters add heat and moisture to the atmosphere — making our weather more wild,” Abraham told Mongabay in an email.

As hotter water evaporates, it creates more vapor in the atmosphere, contributing to heavier rains and storms, among other weather events. Water vapor is also a greenhouse gas, so it leads to still more warming over the long term.

Ocean warming also contributes to sea-level rise by helping melt ice sheets because water expands and occupies more volume as it warms. It also creates adverse effects for marine life, such as low-oxygen “dead zones” that choke out aquatic life.

Much of the rise in ocean temperatures over the last two years is due to an El Niño event that lasted roughly from April 2023 until mid-2024. El Niño is a natural, cyclical climate pattern that starts in the Pacific Ocean and can affect global temperatures. However, climate change was also a factor, the study’s authors write.

The ocean heat led to a global mass coral bleaching event, which began in 2023 and continued into 2024. It was the fourth mass bleaching event on record; the first took place in 1998. And it was the biggest. Research showed that more than three-fourths of the world’s reef areas were stressed, the most ever recorded.

Record-setting climate news has been commonplace for decades, including for ocean warming, with each decade warmer than the last. Reliable data from modern instruments dates back to the late 1950s.

Those include wire probes known as expendable bathythermographs, which are still in use. However, much of the best data for this and other studies now comes from Argo floats, which are like buoys that descend and drift underwater and are deployed at locations across the oceans. Some data also comes from sensors attached to seals, which go into areas around sea ice where other instruments aren’t feasible.

The authors used the collected data to determine the two main metrics for ocean temperatures: the global sea surface temperature (SST) for the ocean’s top 1 meter (about 3 feet) and the ocean heat content (OHC) for the upper 2,000 m (6,562 ft).

Both hit record highs in 2024.

SST was an estimated 0.07° Celsius (0.13° Fahrenheit) higher than in 2023 and 0.61°C (1.1°F) above a 1981-2010 baseline used by researchers. OHC, which the authors describe as a “robust” measure of climate change because it doesn’t experience large fluctuations like SST, was an estimated 16 zettajoules higher than in 2023. That amount of energy added to the ocean in 2024 is equivalent to about 140 times the entire world’s annual electricity generation, according to study co-author Kevin Trenberth, a distinguished scholar at the National Center for Atmospheric Research in the U.S.

The study’s release coincided with the more publicized news that 2024 was the hottest year on record overall, according to many leading government and scientific institutions. Behind those headlines is a third metric: the global marine surface temperature (GMST), which includes both land and sea figures. Some institutions found that GMST had risen to more than 1.5°C (2.7°F) above its preindustrial average in 2024, crossing a key threshold under the 2015 Paris Agreement; others found it just below. In any case, 2024 was only the second year since 2016, along with 2023, when all three metrics — SST, OHC and GMST — were the hottest on record.

Other research teams and institutions also publish annual ocean temperature findings, but this study is unique in the scope of its data and the fact that it’s peer reviewed, Trenberth told Mongabay. He and his colleagues were able to produce the vetted paper shortly after the year ended because they started the process before the data was final, he said.

Like Abraham, Trenberth expressed concern about the signals from the latest ocean data, warning that a tipping point may have been reached.

“The oceans are getting warm enough that they’re really beginning to have stronger influence,” he said. “Because up until now, the oceans have been certainly warming, but they haven’t been so warm that it has really changed the climate. And I think now the ocean feedback effects are actually changing the climate. And you know, that’s very worrisome, because it has impacts on all of the marine life.”

Annalisa Bracco, a climatologist at the U.S. university Georgia Tech, wrote in The Conversation on Dec. 9 that scientists need to investigate “whether the past two years have been a sign of sudden acceleration in global warming.”

Bracco, who was not involved in the Cheng-led study, told Mongabay in an email that 2025 could see a slight decrease in global ocean temperatures, as is normal following an El Niño event. Still, the medium-term trend is “clearly towards warming (and we’re not doing much to change that).”

She warned that ocean heating would lead to changes in cloud formation and rain patterns that are “incredibly important from a societal perspective.”

Banner image: Coral bleaching on the Great Barrier Reef in 2017. Image courtesy of The Ocean Agency / Ocean Image Bank

Ocean heating breaks record, again, with disastrous outcomes for the planet

Citation:

Cheng, L., Abraham, J., Trenberth, K. E., Reagan, J., Zhang, H. M., Storto, A., … & Gues, F. (2025). Record High Temperatures in the Ocean in 2024. Advances in Atmospheric Sciences, 1-18.

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