Ocean "stratification" means less deep water is rising towards the surface carrying oxygen and nutrients, while the water at the surface absorbs less atmospheric carbon dioxide to bury at depth
PARIS - The world's oceans, which have absorbed most of the excess heat caused by humanity's carbon pollution, continued to see record-breaking temperatures last year, according to research published Wednesday.
Climate change has increased surface temperatures across the planet, leading to atmospheric instability and amplifying extreme weather events such as storms.
Oceans absorb about 90 percent of the excess heat from greenhouse gas emissions, shielding land surfaces but generating huge, long-lasting marine heat waves that are already having devastating effects on underwater life.
The study, by researchers in China, the US, Italy and New Zealand, said that 2022 was "the hottest year ever recorded in the world's oceans".
Heat content in the oceans exceeded the previous year's levels by around 10 Zetta joules -- equivalent to 100 times the electricity generation worldwide in 2021, according to the authors.
"The oceans are absorbing most of the heating from human carbon emissions," said co-author Michael Mann, a professor at the University of Pennsylvania.
"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," he said. "Better awareness and understanding of the oceans are a basis for the actions to combat climate change."
Records going back to the late 1950s show a relentless rise in ocean temperatures with almost continuous increases going back to around 1985.
- 'Nightmare for marine life' -
Scientists have warned that climbing temperatures have wrought major changes to ocean stability faster than previously thought.
The research, published in the journal Advances in Atmospheric Sciences, was based on observations from 24 scientists across 16 institutes worldwide.
It also found other indications suggesting that ocean health is deteriorating.
Increasing water temperatures and ocean salinity -- also at an all-time high -- directly contribute to a process of "stratification", where water separates into layers that no longer mix.
This has wide-ranging implications because it affects the exchange of heat, oxygen and carbon between the ocean and atmosphere, with effects including a loss of oxygen in the ocean.
"Deoxygenation itself is a nightmare for not only marine life and ecosystems but also for humans and our terrestrial ecosystems," the researchers said in a statement.
Updated data released this week showed that average global atmospheric temperatures across 2022 made it the fifth warmest year since records began in the 19th century, according to Europe's Copernicus Climate Change Service.
Countries across the world have faced a cascade of unprecedented natural disasters made more likely and deadly by climate change.
Many of these impacts can be linked to a fast-warming ocean and the related changes in the hydrological cycle.
"Some places are experiencing more droughts, which lead to an increased risk of wildfires, and other places are experiencing massive floods from heavy rainfall, often supported by increased evaporation from warm oceans," said co-author Kevin Trenberth, of the US National Center for Atmospheric Research and the University of Auckland.
By Kelly Macnamara