With its capacity to absorb around 25% of anthropogenic CO₂ emissions and over 90% of excess heat every year, the ocean plays a crucial role in regulating climate change. Of the approximately 750 PgC (Peta-gramme of Carbon) injected by human activities since 1750, it is estimated that the ocean has absorbed 200 PgC (Friedlingstein et al., 2025), limiting the impact of anthropogenic CO₂ emissions on climate change. Without this oceanic carbon sink, the concentration of CO₂ in the atmosphere would be around 526 ppm today, compared with the global average of 426 ppm observed in January 2025. However, it should be noted that the acceleration in atmospheric CO₂ is not following the trajectory recommended to limit warming to 2°C; the increase in CO₂ was +3.77 ppm in 2024 (Lan et al., 2025), much higher than the records of +2.8 ppm/year identified during the El Niño years in 1987 or 1998. In order to quantify the oceanic carbon sink, it is necessary to have accurate observations of oceanic CO₂ and, if possible, in all oceanic sectors and at different seasons, because the oceanic carbon cycle is highly variable in time and space, and depending on whether we are in offshore or coastal areas. This is the aim of the SOCAT ocean CO₂ database, which was launched at a workshop in Paris (Metzl et al., 2007) and has been regularly updated since 2011 (Bakker et al., 2016).

The 2025 version has been enriched with new qualified data from 495 oceanographic campaigns, ships of opportunity, sensors on moorings or drifting platforms (Figure 1). Note the low number of data recently collected in the Indian Ocean or the ocean areas of the southern hemisphere. Efforts must be made in the coming years to better document these areas and better constrain the CO₂ field reconstruction models using the SOCAT database (Figure 2). Since the first version in 2011, which gathered 6.3 million data (Pfeil et al., 2013), SOCAT now contains almost 50 million observations of CO₂ fugacity in the surface waters of the global ocean and coastal areas, covering the period 1957-2024.

Figure 1. Left: distribution of new observations integrated into the SOCAT-v2025 database (the colour code represents the year, 2024 in yellow). Right: all CO₂ fugacity data (fCO₂, µatm) at the ocean surface in the SOCAT database over the period 1957-2024. The squares symbolise CO₂ sensors on moorings. With CO₂ levels in the atmosphere currently approaching 420 ppm, the blue-green (resp. red) areas indicate that the ocean acts as a CO₂ sink (resp. source). Note the absence of recent observations in certain sectors, notably the northern Indian Ocean and the southern oceans, which means that extrapolation models need to be developed to assess large-scale air-sea exchanges of CO₂ (Figure 2) or the distribution of pH to study ocean acidification (Figure 3). D. R.

Figure 2. The global ocean carbon sink assessed over the period 1959-2024 using ocean models (individual models in violet, mean black line and uncertainty in grey) or reconstruction methods based on the SOCAT database (in cyan). The histograms show the number of CO₂ observations in the SOCAT database used each year to constrain the reconstruction methods. Source: Global Carbon Project (Friedlingstein et al., 2025). One thing is certain: both models and observation-based methods show that the ocean carbon sink is increasing over time. However, the models underestimate the carbon sink in recent years. D. R.

In addition to the data accessible online and accompanied by evaluation comments (Quality Flag, Gkritzalis et al., 2024), the database also offers gridded products at different resolutions for the open ocean and coastal domain, which can be used to build climatologies, initialise and validate ocean biogeochemical models and coupled climate/carbon models (CMIP6) or constrain atmospheric inversion models. An interactive display tool (LAS Data viewer) provides easy access to the data, which can be extracted by region, period, vessel or platform (buoys or moorings). Matlab reading codes for data files and gridded products, as well as the ODV format (Ocean Data View, https://odv.awi.de/) are also available online.

The new database will be used, among other things, to assess the global carbon balance for 2024, for which the next report from the Global Carbon Project (ww.globalcarbonproject.org) is due to be published before COP-30 (10-21 November 2025, Belem). The SOCAT database also provides valuable information on the ocean carbonate system: a direct consequence of CO₂ emissions and its absorption by the oceans is the phenomenon of acidification (reduction in pH), which can be estimated using the SOCAT database (Figure 3) and whose impact on marine ecosystems has yet to be assessed.

Figure 3. Example of pH evolution in the surface ocean over time. Results obtained from selected SOCAT data in the South Indian Ocean around New Amsterdam Island (data from historical campaigns and OISO since 1998). The decrease in pH, of the order of -0.019 per decade over the period 1962-2024, reflects the acidification of the oceans due to the increase in anthropogenic CO₂. D. R.

IPSL’s LOCEAN laboratory regularly feeds this database (SNO/OISO, PIRATA, SSS-CO₂ observatories), contributes to data quality control, and participates in the coordination of the SOCAT groups (Tropical Atlantic, Indian and Southern Oceans). The SOCAT project is coordinated by Dorothee Bakker (University of East Anglia, UK). It has been supported by international programmes (SOLAS, IOCCP) and numerous national institutes. It should also be noted that the work of making data available and checking its quality is mainly carried out by volunteer researchers and, in the future, ensuring the continuity of these efforts will require sustainable resources (Bakker et al., 2023; IOCCP, 2024), in the hope that these efforts will not be limited by the new American administration. This is all the more true given that the SOCAT database, like other databases such as GLODAP (www.glodap.info) and SNAPO-CO₂, has been integrated into a recent synthesis of databases dedicated to oceanic CO₂ (Jiang et al., 2025).

Contact

Nicolas Metzl, LOCEAN-IPSL •

References

– Bakker, D. C. E., Pfeil, B., Landa, C. S., Metzl, N., O’Brien, K. M., et al., 2016.: A multi-decade record of high-quality fCO2 data in version 3 of the Surface Ocean CO₂ Atlas (SOCAT), Earth Syst. Sci. Data, 8, 383-413, https://doi:10.5194/essd-8-383-2016.
– Bakker, D., R. Sanders, A. Collins, M. DeGrandpre, T. Gkritzalis, S. Ibánhez, S. Jones, S. Lauvset, N. Metzl, K. O’Brien,  A. Olsen, U. Schuster, T. Steinhoff, M. Telszewski, B. Tilbrook,  D. Wallace, 2023. Case for SOCAT as an integral part of the value chain advising UNFCCC on ocean CO₂ uptake http://www.ioccp.org/images/Gnews/2023_A_Case_for_SOCAT.pdf
– Bakker, D. C. E., S. R. Alin, T. Aramaki, L. Barbero, N. R. Bates, T. Gkritzalis, S. D. Jones, A. Kozyr, S. K. Lauvset, V. Macovei, N. Metzl, D. R. Munro, S.-i. Nakaoka, K. O’Brien, A. Olsen, D. Pierrot, T. Steinhoff, K. F. Sullivan, A. J. Sutton, C. Sweeney, C. Wada, R. Wanninkhof, and all >100 SOCAT contributors. SOCAT version 2025: Open ocean CO₂ data submissions stabilise. Available at www.socat.info
– Friedlingstein, P., et al.: Global Carbon Budget 2024, Earth Syst. Sci. Data, 17, 965–1039, https://doi.org/10.5194/essd-17-965-2025, 2025.
– Gkritzalis, T., D. C.E. Bakker, S. K. Lauvset, T. Steinhoff, S. Alin, N. Gruber, S. Jones, L. Kamb, V. Macovei, N. Metzl, C. Neil, K. O’Brien, L. Olivier, A. Olsen, D. Pierrot, A. Sutton, M. Telszewski, B. Tilbrook, 2024. SOCAT Quality Control Cookbook – For version 2025 of the Surface Ocean CO₂ Atlas. Available at www.socat.info
– Lan, X., Tans, P. and K.W. Thoning: Trends in globally-averaged CO₂ determined from NOAA Global Monitoring Laboratory measurements. Version Monday, 05-May-2025 16:38:58 MDT https://doi.org/10.15138/9N0H-ZH07
– IOCCP, 2024. Declaration on Operationalising the Surface Ocean Carbon Value Chain. http://www.ioccp.org/images/Gnews/Declaration_on_Operationalising_the_Surface_Ocean_Carbon_Value_Chain.pdf
– Jiang, L.-Q., et al., Synthesis of data products for ocean carbonate chemistry, Earth Syst. Sci. Data Discuss. [preprint], https://doi.org/10.5194/essd-2025-255, in review, 2025.
– Metzl, N., Tilbrook, B., Bakker, D. C. E., Le Quéré, C., Doney, S., Feely, R., Hood, M., Dargaville, R., 2007. Global Changes in Ocean Carbon: Variability and Vulnerability. Eos, Transactions of the American Geophysical Union 88 (28): 286-287. https://doi:10.1029/2007EO280005
– Pfeil, B., Olsen, A., Bakker, D. C. E., et al., 2013. A uniform, quality controlled Surface Ocean CO₂ Atlas (SOCAT), Earth Syst. Sci. Data, 5, 125-143, https://doi:10.5194/essd-5-125-2013