Comparison of ten packages that compute ocean carbonate chemistry

Marine scientists often use two measured or modeled carbonate system variables to compute others. These carbonate chemistry calculations, based on well-known thermodynamic equilibria, are now available in a dozen public packages. Ten of those were compared using common input data and the set of equilibrium constants recommended for best practices. Current versions of all 10 packages agree within 0.2 mu atm for pCO(2), 0.0002 units for pH, and 0.1 mu mol kg(-1) for CO32- in terms of surface zonal-mean values. That represents more than a 10-fold improvement relative to outdated versions of the same packages. Differences between packages grow with depth for some computed variables but remain small. Discrepancies derive largely from differences in equilibrium constants. Analysis of the sensitivity of each computed variable to changes in each constant reveals the general dominance of K-1 and K-2 but also the comparable sensitivity to K-B for the A(T)-C-T input pair. Best-practice formulations for K-1 and K-2 are implemented consistently among packages. Yet with more recent formulations designed to cover a wider range of salinity, packages disagree by up to 8 mu atm in pCO(2), 0.006 units in pH, and 1 mu mol kg(-1) in CO32- under typical surface conditions. They use different proposed sets of coefficients for these formulations, all of which are inconsistent. Users would do well to use up-to-date versions of packages and the constants recommended for best practices.