We are excited to announce the release of Surface Ocean CO₂ Atlas (SOCAT) version 2021 which is now available via www.socat.info. SOCAT documents the increase in surface ocean CO₂ for the global oceans and coastal seas. Quantification of the global ocean CO₂ uptake to better than 0.2 Pg C yr-1 requires an accuracy of < 2 μatm for in situ surface ocean fCO₂ (fugacity of CO₂) measurements. Quality-controlled, synthesis and gridded products in SOCAT version 2021 contain 30.6 million in situ surface ocean fCO₂ measurements with an estimated accuracy of < 5 μatm. A further 2.1 million fCO₂ values with an accuracy of < 10 μatm are also available. Data collection has continued during the pandemic, albeit at a reduced rate.

SOCAT-based data products are used for quantification of ocean CO₂ uptake and ocean acidification and for evaluation of sensor data and climate models. The community-led SOCAT synthesis products are a key step in the value chain based on in situ inorganic carbon measurements of the oceans, which provides policy makers in climate negotiations with essential information on ocean CO₂ uptake. Some regional data collection is variable and at risk for lack of long-term funding. The global need for accurate knowledge of ocean CO₂ uptake and its (future) variation makes sustained funding for in situ surface ocean CO₂ observations imperative.

We want to thank all >100 data contributors, quality controllers and other contributors who made SOCAT version 2021 possible, and acknowledge the continued leadership of Dorothee Bakker (UEA, UK) in producing the annual SOCAT release.

Fig. 1. All in situ surface ocean fCO2 values (colour coded, µatm) with an accuracy of < 10 μatm in version 2021. Squares indicate moorings.

Fig. 2. Number of surface ocean fCO2 values with an accuracy of < 10 μatm for years in SOCAT versions 1 to 2021.