What is the CO2 content in the air in a compartment of air and water, as a function of temperature?
Simple, partial pressure of $\ce{CO2}$ is misleading.

$\ce{CO2}$ actually reacts with sea water:
$$\ce{H2O + CO2 -&gt; H2CO3}$$
which then ionizes:
$$\ce{H2CO3 -&gt; H+ + HCO3-}$$
which lowers the pH of the sea water (making it more acid).

In previous high $\ce{CO2}$ periods, $\ce{CaCO3}$ came out of sea water 
making limestone and marble rock formations geologically. $\ce{CO2}$ at high pressure is responsible for other side effects, also killing 
fish, and microbes, damaging coral reefs and generally messing up ocean life.