Before discussing model simulated ocean chemistry in the context of climate change, it is illustrative to examine the effect of changes in carbon and temperature on ocean chemistry. We used the chemistry routine from the OCMIP project (available at http://www.ipsl.jussieu.fr/OCMIP/phase3/simulations/NOCES/HOWTO-NOCES.html) to calculate ocean pH and the state of aragonite saturation (Ωaragonite) as a function of surface CO2 pressure (pCO2), the concentration of dissolved inorganic carbon (DIC), and sea surface temperature (SST). It is shown that at constant pCO2 both pH and Ωaragonite increase with increased temperature, but their dependences on temperature are weaker than on pCO2 for the range of typical ocean surface temperature (−2 to 32°C) and atmospheric CO2 concentrations (200 to 1200 ppm) experienced in the past and likely to be seen in the future (Figures 1a and 1b). This is especially true for pH. Our calculations show that at present-day conditions with a global sea surface temperature of about 16°C and atmospheric CO2 concentration of about 380 ppm (corresponding to a DIC concentration of 2066 μmole kg−1), the surface ocean has a globally averaged pH and Ωaragonite of 8.07 and 2.68, respectively. With a doubling of atmospheric CO2 and a climate sensitivity (ΔT2x, global mean temperature change as a result of doubling of atmospheric CO2) of 2.5°C and 4.5°C (corresponding to a DIC concentration of 2168 and 2158 μmole kg−1), the pH decreases by 0.27 and 0.26 respectively, with almost all effects originating from increased CO2 concentration. The Ωaragonite decreases from 2.68 to 1.75 and 1.87 respectively (increased temperature increases Ωaragonite by 0.23 and 0.44 for ΔT2x = 2.5°C and 4.5°C, respectively, while increased CO2 concentration decreases Ωaragonite by 1.09 in both cases. The difference between the sum of these effects and the total change in Ωaragonite is due to nonlinearity in the carbonate system).