Comparison of ten packages that compute ocean carbonate chemistry

Abstract. 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 μatm for pCO₂, 0.0002 units for pH, and 0.1 μmol 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 That represents more than a 10-fold improvement relative to outdated versions of the same packages.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 K₁ and K₂ but also the comparable sensitivity to K_B for the A_T–C_T input pair. Best-practice formulations for K₁ and K₂ are implemented consistently among packages. Yet with more recent formulations designed to cover a wider range of salinity, packages disagree by up to 8 μatm in pCO₂, 0.006 units in pH, and 1 μmol 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.