Marine heatwave resulted in world’s ocean absorbing less CO₂
By: Communications
New research involving the University of East Anglia has found that the global ocean absorbed significantly less carbon dioxide (CO₂) than anticipated during the unprecedented marine heatwave in 2023.
Until now, the ocean has reliably absorbed CO₂ from the atmosphere – but if warming continues, less absorption may occur.
Published in the journal Nature Climate Change, the study involved an international research team led by ETH Zurich.
The world’s oceans act as an important sink for CO₂. To date, they have absorbed around a quarter of human-induced CO₂ emissions from the atmosphere, stabilising the global climate system.
Without this sink, the CO₂ 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 per cent of the additional heat from the atmosphere.
In 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,” said Nicolas Gruber, Professor of Environmental Physics at ETH Zurich.
The team investigated for the first time, using oceanic CO2 measurements from a global observation network, whether and how the extreme temperatures recorded two years ago impacted this sink.
One of these datasets is the Surface Ocean CO2 Atlas (SOCAT), updated annually and led by UEA. Prof Dorothee Bakker, of the School of Environmental Sciences, is chair of the SOCAT global group and a co-author on the paper. She said: “This study uses surface ocean CO2 measurements, made available via the Surface Ocean CO2 Atlas. This underlines the importance of such measurements and SOCAT.”
The researchers show that in 2023, the global ocean absorbed almost one billion tonnes or around 10 per cent less CO₂ than anticipated based on previous years. This corresponds to about half of the EU’s total CO₂ emissions or more than 20 times those of Switzerland.
Warm water dissolves less CO₂
In fact, the decline did not really surprise the researchers. “When a glass of carbonated water warms up in the sun, dissolved CO₂ escapes into the air as a gas,” explained ETH biogeochemist Dr Jens Daniel Müller, who led the team. 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,” added Dr Müller.
Whether the ocean absorbs or releases CO₂, however, does not depend solely on temperature. If considering 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 CO₂ outgassing and support the sink’s strength. These processes reduce the concentration of dissolved inorganic carbon (DIC) in the surface layers.
Compensating forces stabilising the sink
In 2023, three physical and biological processes kept DIC low in the near-surface layers. First, CO2 itself escaped, while secondly, a more stable stratification of the water column prevented CO2-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 CO₂ 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 – stabilised the carbon sink. As a result, 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.
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. 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 CO₂ observations from research vessels, cargo ships and measuring buoys, combined with satellite data and machine learning to establish global maps of the surface CO2 levels. This enabled them to calculate the CO₂ 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 behaviour of a warming ocean. However, it is not possible to say with certainty how this important carbon sink will develop in the future.
One thing is clear: since the record-high temperatures of the year 2023, the world’s ocean has hardly cooled down and the earth continues to warm up. Heat waves are becoming more frequent and more intense. Whether the compensating mechanisms will remain effective over the long term and limit temperature-driven outgassing is unclear.
It is possible 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,” said Prof Gruber.
‘Unexpected decline of the ocean carbon sink under record-high sea surface temperatures in 2023’, Müller JD, Gruber N, Schneuwly A, Bakker DC, Gehlen M, Gregor L, Hauck J, Landschützer P, McKinley GA, was published in Nature Climate Change on September 2.
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