That year, surface waters warmed dramatically worldwide, especially in the North Atlantic, alongside a strong El Nino event in the Pacific. Using global oceanic CO2 measurements, the researchers found that the ocean absorbed nearly one billion tonnes, or about ten percent less CO2 than expected, equal to half the European Union's annual emissions.
Lead researcher Jens Daniel Muller explains the process with a simple analogy: "When a glass of carbonated water warms up in the sun, dissolved CO2 escapes into the air as a gas." Warm waters in the extratropical northern hemisphere released CO2 in much the same way, weakening the sink.
Still, the study revealed that several mechanisms kept the decline smaller than anticipated. These included limited upwelling of carbon-rich deep waters, the action of the biological pump that transfers organic carbon to depth, and stratification effects. Together they partially offset the reduced solubility caused by higher temperatures.
El Nino, which normally enhances the sink by suppressing CO2 release in the tropical Pacific, was unable to counterbalance the strong outgassing from northern waters. The net effect was a weakened ocean carbon sink despite stabilizing processes at work.
The findings highlight the uncertainty of how oceans will respond to ongoing warming. "It is unclear, however, as to whether the compensating mechanisms will remain effective over the long term and limit temperature-driven outgassing," says Nicolas Gruber, professor of environmental physics at ETH Zurich.
Research Report:Unexpected decline of the ocean carbon sink under record-high sea surface temperatures in 2023.
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