# How do the oceans keep carbon dioxide levels in atmosphere at a lower level than they would otherwise be?

carbon dioxide is a few soluble in water, Furthermore, over $\text{pH} = 6.5$ it becomes mostly ${\text{HCO}}_{3}^{-}$, which is much more soluble. It can react with ions ${\text{Ca}}^{2 +}$ and ${\text{Mg}}^{2 +}$ to give insoluble carbonates that settle down on the bottom of the sea.
Pay attention, instead, this effect decreases with an increase in temperature (the solubility of gases decreases at highter temperatures) and with a decrease of $\text{pH}$ that dissolves the carbonates. Hence it is a very delicate equilibrium.