OceanSODA-ETHZ: a global gridded data set of the surface ocean carbonate system for seasonal to decadal studies of ocean acidification

Ocean acidification has profoundly altered the ocean's carbonate chemistry since preindustrial times, with potentially serious consequences for marine life. Yet, no long-term, global observation-based data set exists that allows us to study changes in ocean acidification for all carbonate system parameters over the last few decades. Here, we fill this gap and present a methodologically consistent global data set of all relevant surface ocean parameters, i.e., dissolved inorganic carbon (DIC), total alkalinity (TA), partial pressure of CO2 (pCO(2)), pH, and the saturation state with respect to mineral CaCO3 (Omega) at a monthly resolution over the period 1985 through 2018 at a spatial resolution of 1 degrees x 1 degrees. This data set, named OceanSODA-ETHZ, was created by extrapolating in time and space the surface ocean observations of pCO(2) (from the Surface Ocean CO2 Atlas, SOCAT) and total alkalinity (TA; from the Global Ocean Data Analysis Project, GLODAP) using the newly developed Geospatial Random Cluster Ensemble Regression (GRaCER) method (code available at https://doi.org/10.5281/zenodo.4455354, Gregor, 2021). This method is based on a two-step (cluster-regression) approach but extends it by considering an ensemble of such cluster regressions, leading to improved robustness. Surface ocean DIC, pH, and Omega were then computed from the globally mapped pCO(2) and TA using the thermodynamic equations of the carbonate system. For the open ocean, the cluster-regression method estimates pCO(2) and TA with global near-zero biases and root mean squared errors of 12 mu atm and 13 mu mol kg(-1), respectively. Taking into account also the measurement and representation errors, the total uncertainty increases to 14 mu atm and 21 mu mol kg(-1), respectively. We assess the fidelity of the computed parameters by comparing them to direct observations from GLODAP, finding surface ocean pH and DIC global biases of near zero, as well as root mean squared errors of 0.023 and 16 mu mol kg(-1), respectively. These uncertainties are very comparable to those expected by propagating the total uncertainty from pCO(2) and TA through the thermodynamic computations, indicating a robust and conservative assessment of the uncertainties. We illustrate the potential of this new data set by analyzing the climatological mean seasonal cycles of the different parameters of the surface ocean carbonate system, highlighting their commonalities and differences. Further, this data set provides a novel constraint on the global- and basin-scale trends in ocean acidification for all parameters. Concretely, we find for the period 1990 through 2018 global mean trends of 8.6 +/- 0.1 mu mol kg(-1) per decade for DIC, 0.016 +/- 0.000 per decade for pH, 16.5 +/- 0.1 mu atm per decade for pCO(2), and 0.07 +/- 0.00 per decade for Omega. The OceanSODA-ETHZ data can be downloaded from https://doi.org/10.25921/m5wx-ja34 (Gregor and Gruber, 2020).

Automated summary

This study presents a global dataset of surface ocean parameters, including dissolved inorganic carbon, total alkalinity, partial pressure of CO2, pH, and saturation state with respect to mineral CaCO3, from 1985 to 2018. The dataset, named OceanSODA-ETHZ, was created by extrapolating surface ocean observations using a newly developed method.