Carbon dioxide, like other gases, is soluble in water.  However, unlike many other gases (oxygen for instance), it reacts with water and forms a balance of several ionic and non-ionic species (collectively known as dissolved inorganic carbon, or DIC).  These are dissolved free carbon dioxide (CO ), carbonic acid (HCO), bicarbonate (HCO) and carbonate (CO), and they interact with water as follows :
The balance of these carbonate species (which ultimately affects the solubility of carbon dioxide), is dependent on factors such as pH, as shown in a Bjerrum plot.  In seawater this is regulated by the charge balance of a number of positive (e.g. Na, K, Mg, Ca) and negative (e.g. CO itself, Cl, SO, Br) ions.  Normally, the balance of these species leaves a net positive charge. With respect to the carbonate system, this excess positive charge shifts the balance of carbonate species towards negative ions to compensate.  The result of which is a reduced concentration of the free carbon dioxide and carbonic acid species, which in turn leads to an oceanic uptake of carbon dioxide from the atmosphere to restore balance.  Thus, the greater the positive charge imbalance, the greater the solubility of carbon dioxide.  In carbonate chemistry terms, this imbalance is referred to as alkalinity.
In terms of measurement, four basic parameters are of key importance: Total inorganic carbon (TIC, T or C), Total alkalinity (T or A), pH, and pCO.  Measuring any two of these parameters allows for the determination of a wide range of pH-dependent species (including the above-mentioned species).  This balance can be changed by a number of processes.  For example, the air-sea flux of CO, the dissolution/precipitation of CaCO, or biological activity such as photosynthesis/respiration.  Each of these has different effects on each of the four basic parameters, and together they exert strong influences on global cycles.  The net and local charge of the oceans remains neutral during any chemical process.