SOCAT, 65 years of observations of CO2 in the surface waters of the world’s oceans

The latest version of the international data-base SOCAT (Surface Ocean CO2 Atlas) has been released on June 20 2023. SOCAT version 2023 assembles more than 40 million of carbon dioxide sea surface observations in the global ocean, marginal seas and coastal zones. The laboratory Laboratoire d’océanographie et du climat : expérimentation et approches numériques (LOCEAN/IPSL/OSU Ecce Terra, SU/CNRS/MNHN/IRD) is involved in this project that was first discussed during an international workshop organized in Paris/UNESCO in 2007.

The ocean plays a crucial role in the regulation of climate change. It absorbs more than 90% of excess heat and presently a quarter of CO2 emitted by human activities (fossil fuels and land-used changed). Since 1750, the ocean captured about 185 PgC (Peta-gramme of Carbon) of a total of 700 PgC anthropogenic emissions (Friedlingstein et al, 2022). Without this ocean carbon sink, the CO2 concentration in the atmosphere would be around 512 ppm compared to 419.3 ppm as observed in January 2023. In the context of a changing climate from decade to decade, it is important to evaluate each year the global carbon budget for a better understanding of its recent evolution, the processes that control carbon exchanges between the reservoirs, reducing the uncertainties in the climate predictions and guiding the political decisions and adaptations at international level. For this, researchers compile the inventories of anthropogenic emissions and the atmospheric and oceanic observations needed to evaluate the air-sea CO2 exchange. The oceanic and terrestrial carbon sink are also evaluated from models (Friedlingstein et al. 2022). A direct impact of anthropogenic CO2 accumulating in the oceans leads to the process called “ocean acidification” (decrease of pH). This is now relatively well observed in all oceans as evaluated from SOCAT data but understanding of the impacts of ocean acidification on marine organisms (like phytoplankton or corals) needs to be improved.

Figure 1: Left: Map of the new sea surface CO2 fugacity (fCO2, µatm) in 2022 added in the SOCAT version 2023 (color code is for month in 2022). Right: All data in SOCAT for the period 1957-2022 (7501 cruises). Squares identified CO2 probes on moorings. The atmospheric CO2 level in the atmosphere being around 420 ppm in 2022, the blue-green region (resp. red) identified ocean CO2 sink (resp. source). Note few observations available in recent years in the south Pacific and Indian Oceans that calls to use data-based approaches to extrapolate the fCO2 field, calculate air-sea CO2 fluxes at large scale (Figure 2) and estimate the ocean carbon sink over several decades (Figure 3).


To quantify the ocean carbon sink each year, it is important to maintain and integrate ocean CO2 observations from year to year in all ocean regions, including marginal seas and coastal zones where the variability of the oceanic biogeochemical properties (including CO2) is pronounced. This is the aim of the SOCAT project started in 2007 during an international workshop organized in Paris/UNESCO (Metzl et al, 2007), a data-base regularly updated since 2011 (Pfeil et al, 2013; Bakker et al, 2016). Since last year, SOCAT added 4 million of new quality controlled fCO2 observations from 333 cruises, VOS lines, moorings or drifting platforms (Figure 1). Notice that in 2022 there were few observations in the Indian and Southern Ocean. In the future, observations should be conducted in these regions in order to better evaluate ocean models and offer constraint for neural network approaches (Figure 2).

Figure 2: Annual air-sea CO2 fluxes (molC/m2/an) estimated from a neural network model constrained with SOCAT data (model CMEMS-FFNN-LSCE, Chau et al, 2022). The blue regions (resp. red) identify ocean carbon sink (resp. source).


Figure 3: The global ocean carbon sink for the period 1959-2021 using ocean models (individual in purple, model mean in black and uncertainty in grey) or from data-based approaches using SOCAT (in cyan). Source: Global Carbon Project (Friedlingstein et al, 2022). The bars (bottom right) inform on the number of fCO2 data available each year in SOCAT to constraint the diagnostic methods. Both models and diagnostic methods show an increase of the ocean carbon sink. Notice that observations suggest larger inter-annual variability not yet resolved in the models.


Since the first SOCAT version published in 2011 with 6.3 million data, SOCAT includes today 43 million sea surface fCO2 data in the global ocean and coastal zones for the period 1957-2022 (Bakker et al. 2023). Each data set available on-line is associated with quality control information (Quality Flag, Lauvset et al., 2019). SOCAT also offers gridded products (monthly scale) that could be used to validate ocean biogeochemical models and coupled climate/carbon models (CMIP6). An interactive tool (LAS Data viewer) enables to visualize the data and download them for specific region or period. The SOCAT data-base is also available in ODV format (Ocean Data View,

Figure 4: An example of sea surface pH climatology and it temporal change. Results from a neural network model constrained with SOCAT data (model CMEMS-FFNN-LSCE, Chau et al, 2023). The average pH decrease of around -0.017 per decade over 1985-2021 represents the so-called “ocean acidification” mainly due to the anthropogenic CO2 uptake.


The annual SOCAT public release contributes to UN Sustainable Development Goals (SDG) 13 and 14 (#OceanAction20464) and to the UN Decade of Ocean Science for Sustainable Development. SOCAT has been used in more than 400 publications and international reports (, in particular:

  • SOCAT is referenced in the IPCC reports.
  • SOCAT informs the Global Carbon Budget (Figure 3, Friedlingstein et al, 2022,
  • SOCAT is used to quantify and understand the seasonal to decadal variability of the ocean carbon sink in the open ocean, coastal seas or marginal seas (e.g. Chau et al, 2022; DeVries, 2022; Gruber et al, 2023; Sarma et al, 2023; RECCAP-2 project,
  • SOCAT is used to evaluate the ocean acidification (Figure 4; e.g., Jiang et al, 2019; Leseurre et al, 2022; Chau et al, 2023).
  • SOCAT is used to evaluate sensor data from Bio-ARGO floats (e.g. Bushinsky et al, 2019).
  • With the GLODAP database ( for the ocean interior, SOCAT offers important complementary knowledge on anthropogenic CO2 inventories in the oceans (Tanhua et al, 2021; Gruber et al, 2023).
  • SOCAT helps the design of new international projects (Wanninkhof et al, 2019 ; project SOCONET,
  • SOCAT data will be part of the GOA-ON database (Global Ocean Acidification Observing Network,
  • SOCAT is now integrated in the European DTO plateform (Digital Twin of the Ocean) :;

The LOCEAN/IPSL laboratory feeds regularly data in SOCAT (from observatories SO/OISO, PIRATA, SSS-CO2), contributes to the data quality control for the Tropical Atlantic, Indian and Southern Ocean regional SOCAT groups. The project is coordinated by Dorothee Bakker (East Anglia University, Norwich UK). It has been supported by international programs (SOLAS, IMBER, IOCCP), European, USA, Australian, Japan projects, and many national institutions, but the work of providing data and their quality control is mainly carried out by volunteer researchers and, in the future, ensuring the continuity of these efforts will require sustainable means (Lange et al, 2023).



Nicolas Metzl, LOCEAN-IPSL • +33 1 44 27 33 94 –

Dorothee Bakker, (UEA/Norwich/UK):



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, doi:10.5194/essd-8-383-2016

Bakker, D. C. E., et al, 2023. An alarming decline in the ocean CO2 observing capacity. Poster available online

Blunden, J. and T. Boyer, Eds., 2021: “State of the Climate in 2020”. Bull. Amer. Meteor. Soc., 102 (8), Si–S475,

Bushinsky, S. M., Landschützer, P., Rödenbeck, C., Gray, A. R., Baker, D., et al., 2019. Reassessing Southern Ocean air-sea CO2 flux estimates with the addition of biogeochemical float observations. Global Biogeochemical Cycles, 33. doi: 10.1029/2019GB006176

Canadell, J. G., P. M. S. Monteiro, M. H. Costa, L. Cotrim da Cunha, P. M. Cox, A. V. Eliseev, S. Henson, M. Ishii, S. Jaccard, C. Koven, A. Lohila, P. K. Patra, S. Piao, J. Rogelj, S. Syampungani, S. Zaehle, K.Zickfeld, 2021, Global Carbon and other Biogeochemical Cycles and Feedbacks Supplementary Material. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. Available from

Chau, T. T. T., Gehlen, M., and Chevallier, F., 2022: A seamless ensemble-based reconstruction of surface ocean pCO2 and air–sea CO2 fluxes over the global coastal and open oceans, Biogeosciences, 19, 1087–1109,

Coppola, L., J. Boutin, J.-P. Gattuso, D. Lefèvre, N. Metzl, 2021. Le système des carbonates dans la mer Ligure. In La mer Méditerranée face au changement global, Conditions de la production phytoplanctonique en mer Ligure. C. Migon, P. Nival, A. Sciandra, Eds. (ISTE Science Publishing LTD, London, UK, 2021), vol. 1, chap. 4, pp. 89-114. ISBN: 9781784057329.

Crisp, D., Dolman, H., Tanhua, T., McKinley, G. A., Hauck, J., Bastos, A., et al., 2022. How well do we understand the land-ocean-atmosphere carbon cycle? Reviews of Geophysics, 60, e2021RG000736.

DeVries, T., 2022. Atmospheric CO2 and sea surface temperature variability cannot explain recent decadal variability of the ocean CO2 sink. Geophysical Research Letters, 49, e2021GL096018. doi: 10.1029/2021GL096018

Friedlingstein, P., et al., 2022. Global Carbon Budget 2021, Earth Syst. Sci. Data, 14, 1917–2005,

Iida, Y., Takatani, Y., Kojima, A. and Ishii, M., 2020. Global trends of ocean CO2 sink and ocean acidification: an observation-based reconstruction of surface ocean inorganic carbon variables. J Oceanogr. doi:10.1007/s10872-020-00571-5

Jiang, L.-Q., Carter, B. R., Feely, R. A., Lauvset, S. K. and Olsen, A., 2019. Surface ocean pH and buffer capacity: past, present and future. Sci Rep 9, 18624; doi:10.1038/s41598-019-55039-4

Laruelle, G. G., Cai W.-J., Hu X., Gruber N., Mackenzie F. T., Regnier P., 2018. Continental shelves as a variable but increasing global sink for atmospheric carbon dioxide. Nature Communications, 9, 454, doi:10.1038/s41467-017-02738-z

Lauvset S., K. Currie, N. Metzl, S. Nakaoka, D. Bakker, K. Sullivan, A. Sutton, K. O’Brien and A. Olsen,  2019. SOCAT Quality Control Cookbook For SOCAT version 7. Int. Report.

Leseurre, C., Lo Monaco, C., Reverdin, G., Metzl, N., Fin, J., Mignon, C., and Benito, L. 2022: Summer trends and drivers of sea surface fCO2 and pH changes observed in the Southern Indian Ocean over the last two decades (1998–2019), Biogeosciences,

Lo Monaco, C., Metzl, N., Fin, J., Mignon, C., Cuet, P., Douville, E., Gehlen, M., Trang Chau, T.T., Tribollet, A., 2021. Distribution and long-term change of the sea surface carbonate system in the Mozambique Channel (1963-2019), Deep-Sea Research Part II,

Metzl, N., Tilbrook, B., Bakker, D., 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. doi: 10.1029/2007EO280005

Metzl, N., Lo Monaco, C., Leseurre, C., Ridame, C., Fin, J., Mignon, C., Gehlen, M., and Chau, T. T. T., 2022: The impact of the South-East Madagascar Bloom on the oceanic CO2 sink, Biogeosciences, 19, 1451–1468,

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, doi:10.5194/essd-5-125-2013

Rödenbeck, C., Bakker, D. C. E., Gruber, N., Iida, Y., Jacobson, A.R., et al., 2015. Data-based estimates of the ocean carbon sink variability – First results of the Surface Ocean pCO2 Mapping intercomparison (SOCOM). Biogeosciences 12: 7251-7278. doi:10.5194/bg-12-7251-2015.

Sutton, A. J., Williams, N. L., & Tilbrook, B. ,2021. Constraining Southern Ocean CO2 Flux Uncertainty Using Uncrewed Surface Vehicle Observations. Geophysical Research Letters, 48, e2020GL091748. Doi:10.1029/2020GL091748

Tanhua, T., Lauvset, S.K., Lange, N. et al. , 2021. A vision for FAIR ocean data products. Commun Earth Environ 2, 136,

Wanninkhof, R., P. Pickers, A. Omar, A. J. Sutton, A. Murata, et al., 2019. A surface ocean CO2 reference network, SOCONET and associated marine boundary layer CO2 measurements. Frontiers in Marine Science, 6, 400, DOI:10.3389/fmars.2019.00400

Watson, A. J., Schuster, U., Shutler, J.D. et al., 2020. Revised estimates of ocean-atmosphere CO2 flux are consistent with ocean carbon inventory. Nat Commun 11, 4422.

WMO/GCOS, 2018:

Nicolas Metzl