This reduces the saturation state ( Ω) with regard to the mineral calcium carbonate (CaCO 3), where an Ω of 1 due to a shift in energetic requirements for shell formation ( Orr et al., 2005 Pörtner and Farrell, 2008).įor example, it is well known that corals start to decrease their calcification already at saturation states well above 3 ( Gattuso et al., 1998). While some of the added CO 2 stays as CO 2, the majority of it is titrated away by the ocean's carbonate ion ( Sarmiento and Gruber, 2006), leading to a substantial reduction in its concentration. In fact, surface ocean pCO 2 tends to track the increase in atmospheric pCO 2 rather closely (e.g., Bates et al., 2014) owing to the ∼ 1-year timescale for the equilibration of CO 2 across the air–sea interface ( Sarmiento and Gruber, 2006), which is smaller than the decadal timescale increase in atmospheric CO 2 ( Friedlingstein et al., 2019). Some of the anthropogenic CO 2 taken up from the atmosphere remains in the seawater as dissolved CO 2, thus increasing its partial pressure ( pCO 2). This decrease in pH equates to a ∼ 30 % increase in the concentration of H + ions. The uptake of anthropogenic CO 2 over the last 150 years has made the surface ocean more acidic with a decrease in global mean pH from ∼ 8.2 around 1850 to ∼ 8.1 today ( Feely et al., 2009 Jiang et al., 2019). However, this buffering of anthropogenic climate change by the ocean comes with a substantial cost, i.e., ocean acidification ( Doney et al., 2009). The oceans have taken up roughly one-quarter of the anthropogenic CO 2 that has been released into the atmosphere since the start of the industrial era ( Sabine et al., 2004 Gruber et al., 2019), modulating the increase in atmospheric CO 2 substantially. The OceanSODA-ETHZ data can be downloaded from ( Gregor and Gruber, 2020). Concretely, we find for the period 1990 through 2018 global mean trends of 8.6 ± 0.1 µmol kg −1 per decade for DIC, −0.016 ± 0.000 per decade for pH, 16.5 ± 0.1 µatm per decade for pCO 2, and −0.07 ± 0.00 per decade for Ω. Further, this data set provides a novel constraint on the global- and basin-scale trends in ocean acidification for all parameters. 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. 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 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 µmol kg −1, respectively. Taking into account also the measurement and representation errors, the total uncertainty increases to 14 µatm and 21 µmol kg −1, respectively. 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 µatm and 13 µmol kg −1, respectively. Surface ocean DIC, pH, and Ω were then computed from the globally mapped pCO 2 and TA using the thermodynamic equations of the carbonate system. 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. 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 CO 2 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, Gregor, 2021). 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 CO 2 ( pCO 2), pH, and the saturation state with respect to mineral CaCO 3 ( Ω) at a monthly resolution over the period 1985 through 2018 at a spatial resolution of 1 ∘ × 1 ∘. 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. Ocean acidification has profoundly altered the ocean's carbonate chemistry since preindustrial times, with potentially serious consequences for marine life.
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