This data layer represents surficial (0m) aragonite saturation state. Aragonite saturation state is measured as the concentration of calcium ions multiplied by the concentration of carbonate ions, and divided by apparent solubility product (Ksp). These values were extracted from a NOAA National Centers for Environmental Information (NCEI) archived data package containing gridded data of aragonite saturation state across the global oceans (spatial distributions with a resolution of 1x1 degree latitude and longitude) at depth levels of 0m, 50m, 100m, 200m, 500m, 1000m, 2000m, 3000m and 4000m. Ocean station data with at least dissolved inorganic carbon (DIC) and total alkalinity (TA) measurements were obtained from the Global Ocean Data Analysis Project (GLODAP), the Carbon Dioxide in the Atlantic Ocean (CARINA), the Pacific Ocean Interior Carbon (PACIFICA), and some recent cruise data sets. Aragonite saturation state was calculated using a Matlab version of CO2SYS from in-situ temperature, pressure, salinity, dissolved inorganic carbon (DIC), total alkalinity (TA), silicate and phosphate with the dissociation constants for carbonic acid of Lueker et al. [2000], potassium bisulfate (KHSO4-) of Dickson [1990a], boric acid of Dickson [1990b], and with the total borate concentration equations of Lee et al. [2010]. For more information about the data set, please read the below paper: Jiang, L.-Q., R. A. Feely, B. R. Carter, D. J. Greeley, D. K. Gledhill, and K. M. Arzayus (2015), Climatological distribution of aragonite saturation state in the global oceans, Global Biogeochem. Cycles, 29, 1656-1673, doi:10.1002/2015GB005198.