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Measurements analyzed by an international research team led by ETH Zurich show that the global ocean absorbed significantly less COâ‚‚ than anticipated during the unprecedented marine heat wave in 2023.

The world's oceans act as an important sink for carbon dioxide (COâ‚‚). To date, they have absorbed around a quarter of human-induced COâ‚‚ emissions from the atmosphere, thereby stabilizing the global climate system.

Without this sink, the COâ‚‚ concentration in the atmosphere would be much higher and global warming would have already significantly exceeded the 1.5-degree warming limit. At the same time, the ocean absorbs around 90% of the additional heat from the atmosphere.

In the year 2023, the surface temperatures of the world's ocean rose sharply, topping record levels in various regions. The tropical Pacific was very warm due to a strong El Niño event, which reverses the currents in this ocean region so that warm surface water accumulates off the coast of South America and colder water no longer rises from deeper layers. At the same time, the ocean outside the tropics also warmed up exceptionally strongly, especially the North Atlantic.

"This sudden warming of the ocean to new record temperatures is challenging for climate research—because to date it was unclear how the marine carbon sink would respond," says Nicolas Gruber, Professor of Environmental Â鶹ÒùÔºics at ETH Zurich.

An international research team has now investigated for the first time, based on oceanic CO₂ measurements from a global observation network, whether and how the extreme temperatures recorded two years ago impacted this sink. The team was headed by ETH biogeochemist Jens Daniel Müller, who was a postdoctoral researcher in Gruber's group until recently.

In their published in Nature Climate Change, the researchers show that in 2023, the global ocean absorbed almost 1 billion tons, or about 10% less COâ‚‚ than anticipated based on previous years.

This corresponds to about half of the EU's total COâ‚‚ emissions or more than 20 times those of Switzerland. "This is not good news," Gruber notes, "but the decline is smaller than feared."

Warm water dissolves less COâ‚‚

In fact, the decline did not really surprise the researchers. By way of an everyday phenomenon, Müller explains exactly why: "When a glass of carbonated water warms up in the sun, dissolved CO₂ escapes into the air as a gas." And the same phenomenon happens in the sea.

The fact that the global ocean absorbed less CO₂ in the record-hot year of 2023 was mainly due to the high sea surface temperatures in the extratropical regions of the northern hemisphere, especially in the North Atlantic. "The high temperatures reduced the solubility of CO₂, resulting in abnormal CO₂ outgassing and reducing the strength of the ocean carbon sink," as Müller outlines.

Whether the ocean absorbs or releases CO₂, however, does not depend solely on temperature. If we consider only the reduced CO₂ solubility, the outgassing as a result of the high temperatures in 2023 should have been more than 10 times greater—this would have caused the global marine carbon sink to collapse almost completely.

The study, however, shows that the sink decreased only moderately. According to the researchers, this is due to physical and biological processes in the ocean that counteract COâ‚‚ outgassing and support the sink's strength. These processes reduce the concentration of dissolved inorganic carbon (DIC) in the surface layers.

Compensating forces stabilizing the sink

In 2023, three physical and biological processes kept DIC low in the near-surface layers. First, COâ‚‚ itself escaped, while secondly, a more stable stratification of the water column prevented COâ‚‚-rich water from rising from the deeper layers to the surface. And thirdly, the biological pump continuously transported organic carbon into the depths of the ocean: the biological pump is the process by which photosynthetic organisms in the light-flooded layers absorb COâ‚‚ and grow, subsequently die and sink to depths.

These three compensating forces—the escape of CO₂, the stratification of the water column and the biological pump—stabilized the carbon sink. "Consequently, the ocean's response to the extreme temperatures of 2023 can be understood as the result of a permanent tug-of-war between temperature-induced outgassing and the concurrent depletion of dissolved CO₂," as Gruber states.

El Niño effect overlaid

The researchers explain the influence of the 2023 El Niño on the marine carbon sink in a similar manner: during El Niño years, the circulation in the tropical Pacific weakens, preventing cold, CO₂-rich water from rising to the surface. As a result, the tropical eastern Pacific, which in normal years releases very large amounts of CO₂ into the atmosphere, emits essentially no CO₂ during El Niño years. Consequently, El Niño tends to enhance the global sink strength of the ocean—in spite of the strong warming.

This was also the case in 2023. "The strong warming of the extratropical ocean, however, has negated the El Niño effect in the tropical Pacific," Müller concludes. In fact, the temperature-driven CO₂ outgassing was so strong, especially in the North Atlantic, that it cancelled out the CO₂ uptake in the tropics. The net result in the El Niño year of 2023 was a reduction of the marine carbon sink.

In conducting their study, the researchers focused on the global ocean (excluding the Arctic Ocean and the southernmost parts of the Southern Ocean). They relied on COâ‚‚ observations from research vessels, cargo ships and measuring buoys, combined with satellite data and machine learning to establish global maps of the surface COâ‚‚ levels. This enabled them to calculate the COâ‚‚ fluxes between water and air at the sea surface.

The future of the marine sink remains uncertain

The study is one of the first to draw on actual observations as a foundation for insights into the behavior of a warming ocean. "We cannot yet say with certainty, however, how this important carbon sink will develop in the future," Müller notes.

One thing is clear: since the record-high temperatures of the year 2023, the world's ocean has hardly cooled down and Earth continues to warm up. Heat waves are becoming more frequent and more intense. "It is unclear, however, as to whether the compensating mechanisms will remain effective over the long term and limit temperature-driven outgassing," Gruber points out.

The two researchers concede that the marine carbon sink could absorb less CO₂ in the future. "For the time being, however, the global ocean is still absorbing a great deal of CO₂—fortunately," states Gruber.