The oceans take up more than 90% of the excess heat generated by rising greenhouse gas emissions, and research published this week shows that ocean warming during 2021 surpassed the previous year’s record to reach unprecedented levels.
Measured as ocean heat content, which describes the amount of heat stored in the upper layers of the ocean, last year’s warming is unmatched but continues an upward trend. Last year was the sixth year in a row to surpass previous record-breaking ocean temperatures, with the seven warmest years from 2015 to 2021.
The numbers are unfathomable. During 2021, the oceans absorbed about 14 zeta Joules of additional heat energy — the equivalent of 440 billion toasters running 24 hours a day, every day of the year, says John Abraham, a professor of thermal sciences at the University of St Thomas in Minnesota who contributed to the research. Another analogy is seven Hiroshima atomic bombs detonating each second, 24 hours a day, all year.
Michael Mann, the director of the Earth System Science Center at Pennsylvania State University, expects ocean warming to continue with “just about every year to be a record breaker, until we bring our carbon emissions way down and the heating stops”.
Ocean warming is unambiguous and reaches ever deeper
The research is based on two datasets, maintained by the Institute of Atmospheric Physics (IAP) at the Chinese Academy of Sciences and NOAA’s National Centers for Environmental Information, respectively.
Collectively, the datasets bring together observations from instruments deployed throughout the world’s oceans, including gliders, moorings and a fleet of ARGO floats, which have been monitoring changes in the top 2000 meters of the ocean since 2005.
The data show unambiguous ocean warming since the late 1980s, with an eight-fold increase in the rate of warming since 1986, compared to the period between 1958 and 1985. Each decade since 1958 has been warmer than the preceding decades.
“The observations do pretty clearly show a more prominent and steady increase in heat content beginning in the 1970s when the cooling effect of sulfate pollutants began to tail off, and greenhouse warming began to dominate,” Mann says.
As the top layers of the ocean take up more and more heat, the warming reaches increasingly deeper zones. “The top 500 meters [of the ocean] has clearly been warming since 1980,” says Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research in Boulder, Colorado, who now lives in New Zealand. “The 500-1000-meter layer [has been warming] since about 1987, 1000 to 1500 meters depth after 1998, and 1500 to 2000 meters after 2005.”
Regional differences, seasonal variation
The rate of warming is not the same across the world’s major ocean basins, says Abraham. “Some areas are warming faster than others. For instance, the Northern and Southern Atlantic Oceans are warming very rapidly. The Northern Pacific, the Indian Ocean just off India, and parts of the Southern Ocean are warming rapidly as well. Other areas are warming slowly or not at all.”
An explanation for the variation in warming rates lies in longer-term global climate patterns such as the oscillations between El Niño and La Niña periods (known as the El Niño Southern Oscillation or ENSO) and the Atlantic meridional overturning circulation (AMOC).
“The El Niño phenomenon can be thought of as a heat relief valve for the Pacific,” Trenberth explains. “During El Niño, heat is moved around the Pacific and pole-wards along the Americas, while losing heat to the atmosphere through evaporation. The Atlantic has a sort of relief valve in the AMOC.”
When you combine the effects of these broader climate patterns, wind circulation patterns and the oceans’ capacity to release aerosols “you have an ocean that is warming globally but with spatial variability,” Abraham says.
The study covers all seven maritime domains of the Indian, Tropical Atlantic, North Atlantic, Northwest Pacific, North Pacific, Southern oceans, and the Mediterranean Sea. The authors describe “robust warming” across all, with four of the seven domains showing record-high heat content in 2021.
Consequences for global weather patterns and heat transport
Rising sea levels represent the most direct consequence of ocean warming. As the ocean warms, it expands and lifts sea level. But warmer oceans also melt more polar ice, particularly in West Antarctica where major glaciers are already being hollowed out from below and show significant retreat.
The study shows the strongest warming during 2021 happened in the Atlantic and the Southern Ocean surrounding Antarctica. The latter is a concern for Antarctica’s ice, says Trenberth. The heat in the Southern Ocean can creep under Antarctica’s ice shelves, thinning them and reducing their buttressing effect that holds back the continent’s land-based glaciers. Any loss of land-based ice contributes to rising sea levels.
But the follow-on impacts of ocean warming go much further than sea-level rise. “Since oceans cover 70% of the planet and heat and humidify the atmosphere, they turn out to be extremely important for weather,” says Abraham.
“As the oceans warm, they dump more heat and water into the atmosphere and that makes storms more severe. Rainfall patterns are changing with more rain occurring in heavier downbursts. This leads to more flooding. But at the same time, since the Earth is warmer, water on land evaporates more quickly, drying things out in some areas.”
Another impact of ocean warming is the ongoing damage and loss of coral reefs.
Trenberth says most people consider temperature at Earth’s surface as an indicator of change, but warming in the upper parts of the ocean is a better measure of the accumulation of additional heat on the planet.