Key points
- The global ocean absorbed approximately 1 billion tons less CO₂ in 2023 than expected, due to record-high sea surface temperatures.
- This reduction is equivalent to about half of the EU’s total CO₂ emissions.
- Warmer water dissolves less CO₂, leading to increased CO₂ outgassing from the ocean.
- While the reduction was less severe than initially feared, compensating ocean processes helped mitigate the effect.
Record-breaking ocean temperatures in 2023 resulted in a significant reduction of the ocean’s capacity to absorb carbon dioxide (CO₂), according to a new study published in Nature Climate Change. An international team, led by ETH Zurich, analyzed global ocean CO₂ measurements and discovered a shortfall of nearly 1 billion tons—approximately 10% less than anticipated based on data from previous years.
This reduction is substantial, equivalent to roughly half of the European Union’s annual CO₂ emissions. The findings highlight a concerning trend impacting the Earth’s climate regulation system.
The ocean acts as a vital carbon sink, absorbing about a quarter of human-induced CO₂ emissions. This absorption helps mitigate the effects of climate change; without it, atmospheric CO₂ levels and global warming would be drastically higher.
The reduced absorption capacity in 2023 was primarily caused by higher-than-average sea surface temperatures, particularly in the North Atlantic. Warmer water, similar to a warm glass of carbonated water, releases more dissolved CO₂ into the atmosphere.
However, the decrease in CO₂ absorption wasn’t as drastic as the temperature increase alone might suggest. Several natural processes within the ocean counteracted the temperature-induced outgassing. These included the escape of CO₂ itself, stable water column stratification preventing CO₂-rich water from rising to the surface, and the continuous transport of organic carbon to deeper ocean layers by the biological pump.
These counterbalancing mechanisms helped to stabilize the overall carbon sink, although it still absorbed considerably less CO₂ than in previous years. The El Niño effect, which typically enhances the ocean’s CO₂ absorption, was largely negated by the extratropical warming.
The study utilized a comprehensive dataset, integrating CO₂ observations from research vessels, cargo ships, and buoys, combined with satellite data and machine learning. While the research provides valuable insights based on real-world observations, the long-term future of the ocean’s carbon absorption capacity remains uncertain.
With ongoing global warming and increasingly frequent and intense heatwaves, the effectiveness of the compensating mechanisms is questionable. The potential for further reductions in the ocean’s CO₂ absorption capacity presents a significant challenge for climate change mitigation efforts.
