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Study key findings & significance

The world's oceans are hotter than ever before, continuing their record-breaking temperature streak for the third straight year.

The warming pattern is mainly attributed to increased anthropogenic greenhouse gas concentrations, offset by the impact of aerosols.

The upper 2,000 meters (6,562 feet) of Earth’s oceans absorbed more than 227 excess zettajoules of energy, compared with the 1981-2010 average. Last year broke the previous record set in 2020 by at least 14 zettajoules.

14 ZJ is the equivalent of 440bn toasters running 24 hours a day, every day for a year.

Another way to think about this is that the oceans have absorbed heat equivalent to seven Hiroshima atomic bombs detonating each second, 24 hours a day, 365 days a year. 

Warming has significantly increased since the 1980s.

Over recent decades, portions of the Atlantic Ocean, Pacific Ocean and Indian Ocean have warmed the most.

Author quotes

“Ocean stores more than 90 percent of the Earth’s net heat gain due to greenhouse gases. Thus, ocean warming is a fundamental indicator of the climate change. The record ocean warming in 2021 is strong evidence that global warming continues.” - Lijing Cheng, lead paper author and associate professor with the International Center for Climate and Environmental Sciences at IAP CAS

"With model experiments, our study shows that the pattern of ocean warming is a result of human-related changes in atmospheric composition. 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." - Lijing Cheng

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

"Our previous work showed that scientists need less than four years of ocean heat measurements to detect a human-induced warming signal from natural variations. This is much shorter than the nearly three decades of measurements required to detect global warming using temperatures of air near the Earth's surface. Indeed, although in the top 10 warmest years, global surface temperatures for 2021 are not the highest on record because of La Niña conditions in the tropical Pacific, among other things. Ocean heat content is one of the best indicators of climate change." - John Abraham, study co-author and Professor at the of University of St. Thomas

“If you want to know how fast the Earth is warming, you have to measure the oceans. Since most of the global warming heat ends up in the oceans, we like to say that ‘global warming is ocean warming.’ ” - John Abraham, study co-author and Professor at the of University of St. Thomas

"The oceans are absorbing most of the heating from human carbon emissions. 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." - Michael Mann, study co-author and Distinguished Professor of Atmospheric Science, The Pennsylvania State University


The increased concentration of greenhouse gases in the atmosphere from human activities traps heat within the climate system and increases ocean heat content (OHC). Here, we provide the first analysis of recent OHC changes through 2021 from two international groups. The world ocean, in 2021, was the hottest ever recorded by humans, and the 2021 annual OHC value is even higher than last year’s record value by 14 ± 11 ZJ (1 zetta J = 1021 J) using the IAP/CAS dataset and by 16 ± 10 ZJ using NCEI/NOAA dataset. The long-term ocean warming is larger in the Atlantic and Southern Oceans than in other regions and is mainly attributed, via climate model simulations, to an increase in anthropogenic greenhouse gas concentrations. The year-to-year variation of OHC is primarily tied to the El Niño-Southern Oscillation (ENSO). In the seven maritime domains of the Indian, Tropical Atlantic, North Atlantic, Northwest Pacific, North Pacific, Southern oceans, and the Mediterranean Sea, robust warming is observed but with distinct inter-annual to decadal variability. Four out of seven domains showed record-high heat content in 2021. The anomalous global and regional ocean warming established in this study should be incorporated into climate risk assessments, adaptation, and mitigation.

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