![\"The](\"https:\/\/www.newsbeep.com\/uk\/wp-content\/uploads\/2025\/09\/the-ocean-carbon-sink.jpg\" "\\"Sea")
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Sea surface temperatures on 29 October 2023. The colors show how much the monthly temperature deviates from the long-term average (1985\u20131993). Red indicates warmer, blue colder temperatures. Credit: Keystone<\/p>\n
Measurements analyzed by an international research team led by ETH Zurich show that the global ocean absorbed significantly less CO\u2082 than anticipated during the unprecedented marine heat wave in 2023.<\/p>\n
The world’s oceans act as an important sink for carbon dioxide<\/a> (CO\u2082). To date, they have absorbed around a quarter of human-induced CO\u2082 emissions from the atmosphere, thereby stabilizing the global climate system.<\/p>\n Without this sink, the CO\u2082 concentration in the atmosphere would be much higher and global warming<\/a> 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.<\/p>\n 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 Ni\u00f1o 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.<\/p>\n “This sudden warming of the ocean to new record temperatures is challenging for climate research\u2014because to date it was unclear how the marine carbon<\/a> sink would respond,” says Nicolas Gruber, Professor of Environmental Physics at ETH Zurich.<\/p>\n An international research team has now investigated for the first time, based on oceanic CO\u2082 measurements from a global observation network, whether and how the extreme temperatures<\/a> recorded two years ago impacted this sink. The team was headed by ETH biogeochemist Jens Daniel M\u00fcller, who was a postdoctoral researcher in Gruber’s group until recently.<\/p>\n In their study<\/a> published in Nature Climate Change, the researchers show that in 2023, the global ocean absorbed almost 1 billion tons, or about 10% less CO\u2082 than anticipated based on previous years.<\/p>\n This corresponds to about half of the EU’s total CO\u2082 emissions or more than 20 times those of Switzerland. “This is not good news,” Gruber notes, “but the decline is smaller than feared.”<\/p>\n \t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\tWarm water dissolves less CO\u2082<\/p>\n In fact, the decline did not really surprise the researchers. By way of an everyday phenomenon, M\u00fcller explains exactly why: “When a glass of carbonated water warms up in the sun, dissolved CO\u2082 escapes into the air as a gas.” And the same phenomenon happens in the sea.<\/p>\n The fact that the global ocean absorbed less CO\u2082 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 CO\u2082, resulting in abnormal CO\u2082 outgassing and reducing the strength of the ocean carbon sink,” as M\u00fcller outlines.<\/p>\n Whether the ocean absorbs or releases CO\u2082, however, does not depend solely on temperature. If we consider only the reduced CO\u2082 solubility, the outgassing as a result of the high temperatures in 2023 should have been more than 10 times greater\u2014this would have caused the global marine carbon sink to collapse almost completely.<\/p>\n The study, however, shows that the sink decreased only moderately. According to the researchers, this is due to physical and biological processes<\/a> in the ocean that counteract CO\u2082 outgassing and support the sink’s strength. These processes reduce the concentration of dissolved inorganic carbon (DIC) in the surface layers.<\/p>\n Map of the global ocean (a) with CO\u2082 fluxes and sea surface temperatures (SSTs) for the year 2023. (b) Latitudes with high temperatures (pink) and weak CO\u2082 uptake due to outgassing (turquoise). Credit: Jens Daniel M\u00fcller \/ ETH Zurich<\/p>\n \n Discover the latest in science, tech, and space with over 100,000 subscribers who rely on Phys.org for daily insights. \t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\tCompensating forces stabilizing the sink<\/p>\n In 2023, three physical and biological processes kept DIC low in the near-surface layers. First, CO\u2082 itself escaped, while secondly, a more stable stratification of the water column prevented CO\u2082-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 CO\u2082 and grow, subsequently die and sink to depths.<\/p>\n These three compensating forces\u2014the escape of CO\u2082, the stratification of the water column and the biological pump\u2014stabilized 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 CO\u2082,” as Gruber states.<\/p>\n El Ni\u00f1o effect overlaid<\/p>\n The researchers explain the influence of the 2023 El Ni\u00f1o on the marine carbon sink in a similar manner: during El Ni\u00f1o years, the circulation in the tropical Pacific weakens, preventing cold, CO\u2082-rich water from rising to the surface. As a result, the tropical eastern Pacific, which in normal years releases very large amounts of CO\u2082 into the atmosphere, emits essentially no CO\u2082 during El Ni\u00f1o years. Consequently, El Ni\u00f1o tends to enhance the global sink strength of the ocean\u2014in spite of the strong warming.<\/p>\n This was also the case in 2023. “The strong warming of the extratropical ocean, however, has negated the El Ni\u00f1o effect in the tropical Pacific,” M\u00fcller concludes. In fact, the temperature-driven CO\u2082 outgassing was so strong, especially in the North Atlantic, that it cancelled out the CO\u2082 uptake in the tropics. The net result in the El Ni\u00f1o year of 2023 was a reduction of the marine carbon sink.<\/p>\n 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 CO\u2082 observations from research vessels, cargo ships<\/a> and measuring buoys, combined with satellite data and machine learning to establish global maps of the surface CO\u2082 levels. This enabled them to calculate the CO\u2082 fluxes between water and air at the sea surface.<\/p>\n \t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\tThe future of the marine sink remains uncertain<\/p>\n 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,” M\u00fcller notes.<\/p>\n 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.<\/p>\n The two researchers concede that the marine carbon sink could absorb less CO\u2082 in the future. “For the time being, however, the global ocean is still absorbing a great deal of CO\u2082\u2014fortunately,” states Gruber.<\/p>\n![\"The](\"https:\/\/www.newsbeep.com\/uk\/wp-content\/uploads\/2025\/09\/the-ocean-carbon-sink-1.jpg\" "\\"Map")
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