Unexpected decline in the ocean carbon sink under record-high sea surface temperatures in 2023

nature climate change Article https://doi.org/10.1038/s41558-025-02380-4 Unexpected decline in the ocean carbon sink under record-high sea surface temperatures in 2023 Received: 3 October 2024 Accepted: 16 June 2025 Jens Daniel Müller 1 , Nicolas Gruber 1, Aline Schneuwly1, Dorothee C. E. Bakker 2, Marion Gehlen 3, Luke Gregor 1, Judith Hauck Peter Landschützer 6 & Galen A. McKinley 7 , 4,5 Published online: 2 September 2025 Check for updates In 2023, sea surface temperatures (SSTs) reached record highs, partly due to a strong El Niño. Based on historical responses to elevated global mean SSTs, oceanic CO2 uptake in 2023 should have increased (−0.11 ± 0.04 PgC yr−1), driven by reduced outgassing in the tropical Pacific Ocean. However, using observation-based estimates of ocean CO2 fugacity, we show here that the global non-polar ocean absorbed about 10% less CO2 than expected (+0.17 ± 0.12 PgC yr−1). This weakening was caused by the anomalous outgassing of CO2 in the subtropical and subpolar regions, especially in the Northern Hemisphere, driven primarily by elevated SSTs reducing the solubility of CO2. In most regions, this SST-induced outgassing was mitigated by the depletion of dissolved inorganic carbon in the surface mixed layer. Such negative feedbacks caused an overall muted response of the ocean carbon sink to the record-high SSTs, but this resilience may not persist under long-term warming or more severe SST extremes. The ocean currently removes about a quarter of the annual anthropogenic CO2 emissions from the atmosphere1–3. However, how further global warming4 and the increasing occurrence of anomalously high sea surface temperatures (SSTs)5–7 might affect the functioning of this sink remains unclear. Given that most parts of the ocean experienced record-high SSTs in 20238–10, this particular year provides a unique opportunity to study this impact. Without global warming, this anomalous state of the surface ocean would have been virtually impossible11. Even accounting for the linear trend in SSTs over the past 34 years, the annual mean anomaly of +0.21 ± 0.02 °C was the largest observed between 50° S and 65° N (Fig. 1a). In addition to global warming, a strong El Niño was an important contributor to this unprecedented SST anomaly8,10,12. The spatial pattern of the SST anomalies represented in many parts the typical response to this phenomenon (Fig. 1c), but unusually high temperatures in the North Atlantic Ocean made 2023 distinct13,14. It is well established that warming reduces the solubility of CO2 in seawater, favouring increased outgassing of CO2 to the atmosphere15. Under isochemical conditions, that is, when the dissolved inorganic carbon (DIC) concentration and alkalinity (TA) remain constant, each 1 °C rise in temperature increases the fugacity of CO2 (fCO2) by ~4% (ref. 16). Thus, in the absence of any compensating mechanism, the 2023 SST anomaly of +0.2 °C would have raised fCO2 by 4 µatm. Such an increase in the oceanic fCO2 would largely eliminate the mean sea–air fCO2 gradient (ΔfCO2) over the non-polar global ocean17 and cause the uptake of CO2 from the atmosphere to cease. However, non-thermal processes, such as changes in ocean circulation, mixing and biogeochemical processes, can compensate for the Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland. 2Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich, UK. 3Laboratoire des Sciences du Climat et de l’Environnement, LSCE-IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, Gif-sur-Yvette, France. 4Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung, Bremerhaven, Germany. 5Universität Bremen, Bremen, Germany. 6Flanders Marine Institute (VLIZ), Ostend, Belgium. 7Columbia University and LamontDoherty Earth Observatory, Palisades, NY, USA. e-mail: 1 Nature Climate Change | Volume 15 | September 2025 | 975–982 975 Article https://doi.org/10.1038/s41558-025-02380-4 a b 22.6 FCO2 (PgC yr−1) SST (°C) 22.4 22.2 22.0 Warm 21.8 21.6 21.4 Cold 1990 EN EN 2000 2010 –1.0 Weak carbon sink –1.5 –2.0 Strong carbon sink –2.5 EN 1990 2020 Expected 2023 anomaly EN EN 2000 2010 Year EN 2020 Year c d –1.6 –0.8 0 0.8 1.6 SST anomaly (°C) –0.6 –0.3 0 0.3 0.6 FCO2 anomaly (mol m–2 yr–1) Fig. 1 | Diagnosis of the record-high SSTs in 2023 and their impact on sea-toair CO2 fluxes. a,b, Time series of mean SSTs (a) and FCO2 (b) over the region between 50° S and 65° N based on the ensemble mean of four fCO2 products (Extended Data Table 1). Annual (black lines) and monthly mean (grey lines) values are shown. Annual mean anomalies relative to the linear long-term trend from 1990 to 2022, representing the baseline of our analysis, are shaded in red and blue (the meaning of the shading is indicated in the respective panel). Error bars indicate the standard deviation of the annual anomalies across the four fCO2 products (the annual anomalies for each fCO2 product are shown individually in Supplementary Fig. 1). EN indicates years with a strong El Niño. In b, the green bar for 2023 indicates the FCO2 range expected from the linear relationship between global mean FCO2 and SST anomalies between 1990 and 2022 (see ‘Expected FCO2 anomaly in 2023’ in Methods). c,d, Maps of the SST (c) and FCO2 (d) anomalies for 2023 relative to the extrapolated long-term trend. Stippling indicates regions where the ensemble standard deviation is higher than the absolute anomaly. SST-driven (thermal) effects by modifying the DIC and TA concentrations18. Thermal and non-thermal drivers are often in a delicate balance, which is well documented for the seasonal cycle of surface ocean fCO2 (refs. 16,19,20). In some cases, non-thermal processes even overcompensate the direct temperature effect. This was observed during previous El Niño years, when the oceanic uptake of CO2 became unusually strong (Fig. 1a) despite anomalously high global SSTs. This strengthening during El Niño results from the reduced outgassing of CO2 in the eastern equatorial Pacific Ocean of roughly −0.1 to −0.2 PgC yr−1 due to reduced upwelling of cold and CO2-rich waters21. Note that here we report sea-to-air CO2 fluxes: an (anomalous) oceanic CO2 uptake is negative and outgassing is positive. In contrast, fCO2 in the subtropics tends to be thermally controlled, so that exceptionally warm SSTs are associated with enhanced outgassing of CO2 (refs. 22–24). Hence, the overall response of the ocean carbon sink to unusual warming depends sensitively on the regional distribution of the SST anomalies and the outcome of the ‘tug of war’ between the thermal and non-thermal drivers of the surface ocean carbon cycle. To quantify the impact of 2023’s record-high SSTs on the oceanic uptake of CO2, we employed four observation-based fCO2 products25–29. These products are machine learning-based statistical (...truncated)