Reconciliation of pH 25 and pH insitu acidification rates of the surface oceans: A simple conversion using only in situ temperature

Seawater pH is frequently measured at 25°C (pH 25 ), and can be converted thermodynamically to pH at the in situ temperature ( T ), (pH insitu ) using an additional carbonate chemistry parameter, which is the total alkalinity (TA), dissolved inorganic carbon (DIC), or the partial pressure of CO 2 (pCO 2 ) of seawater. Although rates of temporal change of pH insitu (βp H i n s i t u) and pH 25 (βp H 25) are both extensively used in studies of ocean acidification, the difference betweenβp H i n s i t uandβp H 25has not yet been quantified. This study deducts from 816 sets of data of the surface oceans over wide ranges of T (1–31°C) from six time series to reveal that the difference between calculated pH insitu and pH 25 is a 1 ( T − 25°C), where a 1 is a nearly constant of −0.0151 pH unit °C −1 . We illustrate thatβp H i n s i t uequals (βp H 25 +  a 1β T), whereβ Tis the rate of temporal change of T . We further show that uneven distributions of sampling points significantly widen the difference betweenβp H i n s i t uandβp H 25, making the degree of ocean acidification unclear. Distributions of a 1 values are modeled for the surfaces of the global oceans at various pCO 2 levels, and they closely match the observations from the studied time series. Without the use of an additional carbonate chemistry parameter, the pH insitu and pH 25 , as well asβp H i n s i t uandβp H 25can now be converted into each other using only T , facilitating the study of the changing carbonate chemistry of seawater under the influences of increasing atmospheric CO 2 concentration.