The BFM model is online coupled to a state of the art carbonate
system module (see Fig. 2), which computes the concentration of
CO2 dissolved in water (pCO2) as a function of dissolved inorganic
carbon (DIC) and alkalinity (ALK) (Orr, 1999). DIC is the sum of
carbonate, bicarbonate, carbonic acid and dissolved CO2, and
accounts for all dissolved forms of inorganic carbon. Alkalinity is a
measure of the capacity of the sea to buffer acidification. Together,
ALK and DIC exactly determine both pH and pCO2. The relationships
among these variables are complex, but fully defined. In a
nutshell, an increase in ALK (all rest being equal) causes an increase
in pH (decrease in acidity) and a decrease in pCO2, whereas an
increase in DIC causes a decrease in pH (acidification) and an
increase in pCO2. Finally, the exchange of CO2 between the
atmosphere and the ocean depends on the difference between
pCO2 and the partial pressure of CO2 in atmosphere (pCO2air): if
the pCO2 in water is higher (lower) than in atmosphere there is an
outgassing (solubilisation) and the sea acts as a source (sink) for