Karmactive Staff
The world’s oceans absorbed 10% less carbon dioxide (CO2) than expected during the record-breaking marine heatwave of 2023, according to new research published in Nature Climate Change.
A team led by ETH Zurich, with involvement from the University of East Anglia, found that extreme ocean temperatures significantly weakened one of our planet’s most important climate stabilizers. The oceans typically absorb about 25% of human-caused CO2 emissions, but that buffer is showing signs of strain.
“This sudden warming of the ocean to new record temperatures is challenging for climate research,” said Nicolas Gruber, Professor of Environmental Physics at ETH Zurich.
The drop in CO2 absorption represents nearly one billion tonnes of carbon dioxide – roughly equivalent to half the European Union’s annual emissions. This decline was primarily driven by record high sea temperatures in the North Atlantic and other northern regions.
Warmer water simply can’t hold as much CO2, similar to how a carbonated drink loses its fizz when left in the sun. “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,” explained ETH biogeochemist Dr. Jens Daniel Müller, who led the research team.
Interestingly, while high temperatures in the North Atlantic caused abnormal CO2 release, this was partially offset by changes in the tropical Pacific. During the 2023 El Niño event, the eastern Pacific, which normally releases large amounts of CO2, emitted almost none. However, this positive effect wasn’t enough to counterbalance the reduced absorption elsewhere.
The researchers found the situation could have been much worse. If considering only the reduced CO2 solubility caused by warm water, the ocean carbon sink might have collapsed almost completely. Instead, three natural processes helped limit the damage:
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First, as CO2 escaped from the ocean surface, it reduced the concentration of dissolved carbon in those areas. Second, warmer surface waters created more stable stratification, preventing CO2-rich deep water from rising. Third, the “biological pump” – where marine organisms absorb carbon, die, and sink – continued transporting carbon to ocean depths.
Professor Dorothee Bakker from UEA’s School of Environmental Sciences, who chairs the Surface Ocean CO2 Atlas (SOCAT) used in the study, emphasized the importance of ongoing measurement efforts: “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 research team gathered CO2 measurements from ships, cargo vessels, and buoys, combining this data with satellite information and machine learning to create global maps of CO2 exchange between the ocean and atmosphere.
While the ocean carbon sink showed some resilience during the 2023 heatwave, scientists remain concerned about its future capacity as ocean temperatures continue to rise. The global ocean has hardly cooled since the record-setting temperatures of 2023, raising questions about whether these natural compensating mechanisms will continue to function effectively under prolonged warming.
For now, Gruber notes, “The global ocean is still absorbing a great deal of CO2 – fortunately.”
