<p>The relevant literature does not explicitly address the fact that there are two fundamentally different inorganic carbon (DIC) reservoirs in seawater; (1) a long-term "background" DIC reservoir that is not in net-transfer equilibrium with the atmosphere, and (2) a short-term "atmospheric" DIC reservoir that is fed by atmospheric pCO<sub>2</sub>. In addition, we may define a third "anthropogenic" DIC reservoir that quantifies the increase in DIC since industrialization.</p> <p>We perform experiments to quantify these reservoirs. We equilibrate Aegean seawater with N<sub>2</sub>-O<sub>2</sub> (79:21) gases with variable pCO<sub>2</sub> from < 10 to 100,000 µatm, and pure CO<sub>2</sub> gas. We quantify electrochemically the changes in pH <em>and, by</em> titration and IR spectroscopy, total alkalinity (TA) and dissolved inorganic carbon (DIC) that occur with variations in pCO<sub>2</sub>. About 78 % of the Aegean DIC is “<em>background</em>“, introduced into the Aegean sea by the long-term carbon cycle, i.e. riverine input, remineralization of organic carbon, and hydrothermal CO<sub>2</sub>. In terms of concentration and in the short term, this reservoir is independent of atmospheric pCO<sub>2</sub>. About 22 % of DIC is atmospheric in origin and is in exchange equilibrium with atmospheric pCO<sub>2</sub>. The anthropogenic contribution to the atmospheric DIC reservoir is derived by measuring the increase in DIC between 280 (pre-industrial) and 410 µatm (present-day) pCO<sub>2</sub> and quantified at around 26 %.</p> <p>Our experiments also allow projections into the future. It has been suspected that increasing atmospheric pCO<sub>2</sub> lowers the CO<sub>2</sub> absorption capacity of ocean surface water. Our data confirm this assessment. When the pCO<sub>2</sub> increases, the pH and the CO<sub>3</sub><sup>2-</sup>-concentration fall, and with them the ability of seawater to hydrolyze CO<sub>2</sub>. Without measures to limit anthropogenic CO<sub>2</sub> emissions, the absorption capacity of Aegean seawater in the year 2100 will be only about one half of the absorption capacity of today.</p>