World's oceans were warmest on record, again, in 2021: study

For the sixth straight year, the world's oceans were warmer in 2021 than at any time before, according to a new study.

Researchers found that in 2021 the upper 2,000 metres of the oceans absorbed 14 more zettajoules than in 2020. For context, the annual global energy consumption by humans is estimated to be half a zettajoule. A zettajoule is equal to one joule, a unit of energy, plus 21 zeros.

"The ocean heat content is relentlessly increasing globally, and this is a primary indicator of human-induced climate change," Kevin Trenberth, distinguished scholar at the U.S. National Center for Atmospheric Research and co-author of the study, said in a news release. "In this most recent report, we updated observations of the ocean through 2021, while also revisiting and reprocessing earlier data."

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The study, published Tuesday in the peer-reviewed Advances in Atmospheric Sciences, summarizes data from the Institute of Atmospheric Physics at the Chinese Academy of Sciences and the National Centers for Environmental Information of the U.S. National Oceanic and Atmospheric Administration.

The research looked into the role of natural weather variations, such as the warming and cooling phases El Nino and La Nina, which greatly affect regional temperature changes. In spite of these phenomena, analyses indicated significant ocean warming across the globe since the 1950s.

"With model experiments, our study shows that the pattern of ocean warming is a result of human-related changes in atmospheric composition," said Lijing Cheng, associate professor with the International Center for Climate and Environmental Sciences and lead author of the study. "As oceans warm, the water expands and sea level rises. Warmer oceans also supercharge weather systems, creating more powerful storms and hurricanes, as well as increasing precipitation and flood risk."

Ocean heat content is one of the best indicators of climate change, the researchers say.

"The oceans are absorbing most of the heating from human carbon emissions," said Michael Mann, distinguished professor of atmospheric science at The Pennsylvania State University and co-author of the study. "Until we reach net zero emissions, that heating will continue, and we'll continue to break ocean heat content records, as we did this year. Better awareness and understanding of the oceans are a basis for the actions to combat climate change."

The researchers also say rising ocean temperatures can affect the amount of carbon dioxide in the air.

"As well as absorbing heat, currently, the ocean absorbs 20 to 30 per cent of human carbon dioxide emissions, leading to ocean acidification," Cheng said. "However, ocean warming reduces the efficiency of oceanic carbon uptake and leaves more carbon dioxide in the air."