In 2023, the world’s oceans presented a startling phenomenon: a significant decline in their role as a carbon sink amid record-breaking sea surface temperatures. For decades, the oceanic carbon sink has been a critical buffer against climate change, absorbing approximately 25% of anthropogenic carbon dioxide emissions. This remarkable function has helped stabilize atmospheric CO₂ levels, thereby limiting the severity of global warming. However, the unprecedented thermal anomalies observed last year have raised compelling questions about the ocean’s capacity to continue this vital service under the pressures of a warming climate.
Ocean surface temperatures soared to new heights across multiple regions in 2023, with the tropical Pacific witnessing a pronounced warm phase linked to an intense El Niño event. This phenomenon altered oceanic current patterns, suppressing the upwelling of colder, CO₂-rich waters off South America and leading to the accumulation of warm surface waters. Concurrently, extratropical regions, particularly the North Atlantic, experienced exceptional warming, marking a global shift in marine thermal regimes that challenged existing climate models and carbon cycle projections.
Researchers, led by ETH Zurich’s biogeochemist Jens Daniel Müller and environmental physicist Nicolas Gruber, embarked on an unprecedented investigation to dissect how these temperature extremes might have influenced the ocean’s carbon uptake. Utilizing a robust dataset compiled from a worldwide network of oceanic CO₂ measurements—incorporating data from research vessels, cargo ships, autonomous buoys, satellite observations, and advanced machine learning algorithms—the team created comprehensive global surface CO₂ maps. This integrative approach enabled precise calculations of air-sea CO₂ fluxes, revealing a near one billion tonne reduction in oceanic carbon absorption compared to expectations derived from prior years.
This decline equates to approximately 10% less CO₂ uptake, corresponding to about half of the total CO₂ emissions of the European Union or over twenty times Switzerland’s emissions, thus signaling a substantial setback for the ocean’s climate regulation capacity. Intriguingly, scientists were not entirely surprised by this decline. The core chemical principle of gas solubility—demonstrated in everyday scenarios such as the escape of carbon dioxide from a warmed glass of carbonated water—applies at ocean surfaces as well. Elevated sea temperatures reduce the solubility of CO₂, facilitating its escape back into the atmosphere, a process known as outgassing.
In particular, the thermal anomalies outside the tropical Pacific, especially in the higher latitudes of the Northern Hemisphere, suppressed CO₂ solubility sufficiently to induce notable outgassing. However, based on solubility changes alone, the expected magnitude of outgassing should have been far more dramatic, potentially collapsing the ocean carbon sink. The observed moderate decline indicates the presence of compensatory oceanic processes that temper the expected losses in carbon absorption.
These compensatory mechanisms involve a complex interplay of physical and biological factors. Firstly, the direct outgassing of CO₂ depletes dissolved inorganic carbon (DIC) levels in the near-surface ocean layers. Secondly, the stronger stratification of the water column during these warm periods inhibits the vertical mixing that normally transports CO₂-rich deep waters to the surface, effectively limiting surface CO₂ concentrations available for outgassing. Lastly, the ocean’s biological pump remains an active actor, with photosynthetic plankton in sunlit surface waters assimilating CO₂, converting it to organic matter, which subsequently sinks, sequestering carbon in the ocean’s depths.
This nuanced tug-of-war between temperature-driven CO₂ release and the physical-biological processes that mitigate surface carbon concentrations allowed the carbon sink to maintain a considerable fraction of its former strength, despite record heat. Moreover, the study highlighted the countervailing effects of the 2023 El Niño event on oceanic CO₂ fluxes. While El Niño’s suppression of Pacific upwelling typically reduces CO₂ outgassing in the eastern tropical Pacific, thereby enhancing global oceanic carbon uptake, this beneficial effect was essentially neutralized by the intensified CO₂ release in extratropical waters, particularly the North Atlantic.
The research team’s robust methodology excluded the Arctic Ocean and the extreme southern reaches of the Southern Ocean, focusing instead on the global ocean due to data reliability and representativeness considerations. This comprehensive spatial coverage allowed for a high-confidence assessment of marine CO₂ fluxes during an anomalously warm year, marking a significant advancement over model-based studies that often lack observational grounding.
Despite these insights, significant uncertainties remain regarding the future trajectory of the ocean’s carbon sink. The persistence of elevated sea surface temperatures beyond 2023, coupled with increasing frequency and intensity of marine heatwaves, casts doubt on the long-term resilience of the physical and biological processes stabilizing the sink. It is unclear whether processes like stratification and the biological pump will continue to effectively counterbalance the thermally induced decline in CO₂ solubility or whether the sink will progressively weaken.
This study underscores the delicate balance in ocean carbon dynamics and the ocean’s integral role in mitigating anthropogenic climate change. It serves as an imperative call for ongoing monitoring and enhanced mechanistic understanding of ocean carbon processes under changing climate conditions. Although the ocean continues to absorb substantial quantities of CO₂, researchers caution against complacency, emphasizing the need for accelerated decarbonization efforts globally.
The findings from this landmark study not only provide a vital empirical benchmark for climate modeling but also raise awareness about potential tipping points in the ocean-atmosphere carbon exchange system. As the climate crisis deepens, understanding whether the ocean carbon sink is nearing such a threshold is critical for developing effective mitigation strategies and safeguarding the planet’s climate stability.
In conclusion, while the ocean’s response to the extreme sea surface temperatures in 2023 manifested as a significant yet moderated decline in its carbon uptake capacity, the interplay of chemical, physical, and biological compensatory processes reveals a complex and dynamic system. The ocean remains a vital buffer against climate change, though its future efficacy is subject to evolving environmental stresses and requires vigilant scientific scrutiny to navigate the uncertain waters ahead.
Subject of Research: Ocean carbon sink dynamics under record-high sea surface temperatures in 2023
Article Title: Unexpected decline of the ocean carbon sink under record-high sea surface temperatures in 2023.
News Publication Date: 2-Sep-2025
Web References:
References:
Müller JD, Gruber N, Schneuwly A, Bakker DC, Gehlen M, Gregor L, Hauck J, Landschützer P, McKinley GA: Unexpected decline of the ocean carbon sink under record-high sea surface temperatures in 2023. Nature Climate Change, 2 September 2025, doi: 10.1038/s41558-025-02380-4
Keywords: ocean carbon sink, sea surface temperature, carbon dioxide absorption, El Niño, ocean stratification, biological pump, dissolved inorganic carbon, marine heatwave, climate change mitigation
